Report No. 556-1193-18National P. 0. No. D AK781-D-0157, Task 0018Technicllsvetem, Date Septerber 1984

37 Page Report

THE EFFECTS OF LONG TERM HIGH IDLE

OPERATION ON DIESEL ENGINES

S"N CCNT: DAAK 70-81-D-0157 - T.O. 0018

FINAL REPORT

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. PREPARED FOR: BELVCIR RESEARCH AND DEVELOPMENT CENTERTactical Enera'v Systems Labcratory

S {Fort Belvoir, VA 22060

PREPARED BY: NATIONAL TECH'ICAL SYSTEMS/ Testin DiviSion c

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Report No. 556-1193-18P. 0. No. DAAK70-81-D-0157, Task 0018

tIC~7~NationalTechnical Date Septerber 1984.,eSys1ems

PREFACE

This work was carried out with the assistance of personnel

from a number of organizations identified in the report.

Their efforts were largely responsible for making the timely

completion of the program possible.

Particular thanks are due to Mr. James P. Lucas of theBelvoir Research and Development Center. His guidance and

assistance were critical in directing the course and

maintaining the progress of the investigation.

Finally, thanks are due to Mr. E. W. Spannhake fcr his

technical review of this work, as well as for the dieselbackground information he provided. Mr. Spannhake began his

career as a Diesel Injection Engineer for American Bosch, and

progressed through assignments as Director of Engineering and

Research for Le Tourneau-Westinghouse and Vice-President of

Zngineering for White Consolidated industries. He is

presently an independent consultant in Naples, Florida.

rITe views, cpinions and findings contained in this report

are those of the author. They should not be construed as an

official Department of the Army position, policy or decision

- o- ent:.fied by other dccxT.entation.

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4. TITLE (and Subtitle) S. TYPE OF REPORT 6 PERIOD COVEREDThe Effects of Long Term High Idle Operation

oI on Dliesel Engines6. PERFORMINO ORG, REPORT NUMBER

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* Alan E. Surosky, P.E. DAAK70-81-D 0157Task 0018

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ELECTRIC POWER GENERATION ELECTRIC POWER SYSTEMSDIESEL WET STACKTNGDIESEL SLOBBERINGGENERATOR SELECTION

20. A11TN ACT (Can ue an ,evet ie H neeay md Id. _,y ammbff

There is a cnrr:rion belief among users of DoD diesel-driven

(DED) generators that extended operation of diesel engines at

rated sproeds and light loads (high idle) results in.

deterioration of performance and engine damae. ...

AFO2 1473 I[DfTION Or I NOV 65I O1BSOLSTEDOU I JCAA 7OSECURITY CLASSIFICATION Olt THIS PAGE (WN Data Rnfere')

Paxrt No. 55fi-1193-±eNational P. 0. No. DAA70-81-D>-0157, Task 0011Technical Date September 1984Syglems I

SUMMARY

There is a common belief among users of DoD diesel driven

(DED) generators that extended operation of diesel engines at

rated speeds and light loads (high idle) results in

deterioration of performance and engine damage.

This investigation was directed at defining the nature and

extent of the damage potential. It included a comprehensive

literature survey, and conferences with users and suppliers of

diesel engines and DED generators. Quality Deficiency Reports

and Equipment Improvement Recommendations concerning the DoD

family of tactical DED generators were reviewed for the period

from 1970 to the present.

Information developed indicates that the ill effect of

high idle on most military diesel engines is limited to the

nuisance of carbon buildup in the exhaust system and discharge

of unburned fuel (wet stacking or slobbering) in the enginevicinity when operating at low temperatures. The 15 kW and 30

kW set engines are an exception in that some of these engines

wet stack lubricating oil at moderate temperatures.

E.J. Kates (12) reaches the same conclusion in his

standard text, "Diesel and High Compression Gas Engines", with

the statement "no serious harm follows underloading, but it is

uneconomical ... overloading causes combustion problems and

overheating."

Techniques to avoid the nuisance effects of high idle

oceration are ;resented. Recommendations are made with the

goal of i.-.:Drc'*-ng cperating efficiencies and reducing life

e costs.

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Raport No. 556-1193-18N,,ional P. o. No. Dr-AK0-81-D-0157, Task 00181L.t, ~Date Beptanber 1984

TABLE OF CONTENTS

PAr.raph pag

1.0 INTRODUCTION . . . . . . . . . . . . . . . . . 11.1 Background . . . . . . . . . . . . . . . . . . 11.2 Scope of Investigation .. . . . . . . . . . . 11.3 Sources of Information . . . . . . . . . . . . 1

2.0 DIESEL ENGINES - DESIGN VARIABLES . . . . . . 22.1 General . . . . . . . . . . . . . . . . . . . 22.2 Military Diesel Engines . . . . . . . . . . . 3

3.0 DIESEL ENGINES - COMBUSTION PROCESSES . . . . 33.1 General . . . . . . . . . . . . . . . . . . . 33.2 Exhaust Composition ... . . . . . . . . 43.2.1 Carbonization . . . . . . . . . . . . . . . . 43.2.1.1 Fuel and Oil Effects on Carbonization .... 43.2.2 Wet Stacking . . ......... . . . . . . 53.2.2.1 Lubricating Oil Effects on Wet Stacking .... 53.2.2.2 Engine Cycle Effects on Wet Stacking . . ... 63.3.3 Combustion Under Light Loads . . . . . . . . . 63.3.4 High Idle vs. Low Idle . . . . . . . . . . . . 73.3.5 Decarbonization . . . . . . . . . . . .*. . . 8

4.0 HIGH IDLE OPERATION - BACKGROUND SURVEY. . . . 94.1 General . . . . . . . . . . . . . . . . . . . 94.2 Air Force DED Generator Sets . . . . .. . . . 94.3 DoD DED Generator Sets - User Assessment . . . 104.3.1 Documented User Reports . . . . . . . . . . . 104.3.1.1 System Assessment Reports . . . . . . . . . . 104.3.1.2 Field Reports . . . . . . . . . 0 . . . . . 0 114.3.2 Verbal Reports ........... . . . . . . . 114.4 Diesel Manufacturers - High Idle Assessment • 124.5 QDR's and EIR's . . . . . . . . . . . . . . . 134.5.1 198ER / 298ER Engines - High Idle Problems . . 134.6 Engine Oriented Problem Scenarios . . . . . . 14

5.0 HIGH IDLE - DAMAGE ANALYSIS . . . . . . . . . 155.1 Genera1 . . . . . . . . . . . . . . . . . . . 155.2 High Idle Prob ernts . . .. ..... 155.2.1 Effects of Wtt Stacking ........... 155.3 Alternate Problem Sources .......... 165.4 Assessment - Military Diesel Engines . . . . . 17

Idle Mode Sytpom Alle.iation . . . . . . . . 175.5.1 increased Eh~ust Back Pressure .. . . . . . . 7

C a5ai ,cd 1aticn . . . . . . . . . . . . . 18.ntake Air Preheat . . . . . . . . . . . . . . 18

:. .4 "...r. t Gas ?ecir la ic . . . . . . . . . . 19

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Report No. 556-1193-18National P. 0. No. DAAK70-81-D-0157, Task 0018Tochnical

sv--E, Date September 1984

Paracra-h Pace

6.0 THE 198ER AND 298ER ENGINES .. ........ 196.1 General ....... ................. 196.2 Manufacturer Assessment ... .......... .. 196.3 Analysis - Manufacturer's Position ...... . 20

7.0 SUMMARY OF FINDINGS ... ............. 217.1 The Effects of High Idle .. ........... . 217.2 Military Engines ........ ............... 2.

8.0 ENGINE SCREENING - HIGH IDLE OPERATION .... 228.1 General ....... ................... 228.2 Test Methods ...... ................. 22

9.0 CONCLUSIONS AND RECOMIMENDATIONS ....... 239.1 Conclusions ...... ................. 239.2 Recommendations ..... ............... 24

10.0 BIBLIOGRAPHY ...... ................. 25

I1.0 APPENDIX ....... ................... 27a. Tactical Generator Drivers . ........ 28b. Diesel Combustion - Theory and Effects . 30

Accession For

NTIS- GRA &IDTIC TABUnannounced

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Report Jo. 556-1193-18N,,in P. 0. No. DAA70-81-D-0157, Task 0018

h/1LJ hIa Date Soptutir 1984

1.0 INTRODUCTION

1.1 Background - There is some feeling among diesel enginedriven (DED) generator users that sustained operation oflightly loaded diesel engines at rated operating speedsresults in deterioration of performance, significantlyincreased maintenance requirements, and ultimately in damageto the engines. This concept is expressed both verbally andoccasionally in engine generator literature.

Most engine driven generator sets function in a uniqueengine operating environment in that they usually run atspeeds dependent only on the desired output frequency andvirtually independent of the electrical load. Under the lightloads frequently encountered in military service thiscondition may be described (approximately) as high idle.

This operating mode is somewhat exaggerated in DoD setssince they are powered for full performance at elevatedtemperatures and altitudes, as well as for emergency overloadconditions. This results in excess engine power availabilityat low altitudes and moderate temperatures.

1.2 Scope of Investigation - This program is concerned withthe extent and nature of the potential problems describedabove as they pertain to the DoD family of tactical DEDgenerator sets from 5 kW to 200 kW.

The conclusions reached here are based on an extensivesurvey of the literature including documented user complaints.In addition personal and telephonic conferences were held withboth users and suppliers of this equipment to determine thesources and the extent of any problems.

1.3 Sources of Information - Data bases were accessed at theNational Technical Information Service (NTIS), Library ofCongress, University of Central Florida, and the Society ofAutomotive Engineers (SAE). Additionally Quality DeficiencyReport (QDR) and Equipment Improvement Recommendation (EIR)files were searched at TROSCOM, St Louis, MO for any failuremode patterns.

other Government documents (standards, specfications,.manuals, drawings and internal memoranda) were also examinedan-d, where applicable, are identified in the bibliographysection of this report.

A larce number of references were retrieved by the data- search, and were reviewed for information relating to

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Report No. 556-1193-18Natonsg P. 0. No. DAAK70-81-D-0157, Task 0018"Technical Date September 1984

this program. Those considered applicable are discussed inthis report and identified in the bibliography. In certaincases the authors were contacted for clarification of thepublished information. Any communications with authors thatplayed a part in forming our opinions are also identified.

Technically oriented personnel from the followingGovernment and Industrial organizations actively involved indiesel engine technology were contacted. They providedbackground and "state of the art" information as viewed byboth suppliers and users.

DoD Project Manager (MEP) Springfield, VAAMC (AMCDE-SSV) Alexandria, VABelvoir R & D Center (STRBE-EE) Fort Belvoir, VABelvoir R & D Center (STRBE-TE) Fort Belvoir, VABelvoir R & D Center (STRBE-VF) Fort Belvoir, VATROSCOM (DRSTR-MES) St. Louis, MOAllis-Chalmers Engine Division Harvey, ILCaterpillar Tractor Company Peoria, ILCummins Engine Company, Inc. Columbus, INDeutz Corporation (Engine Div.) Atlanta, GALister Diesels, Inc. Olathe, KAOnan Corporation Minneapolis, MNTechnology Research Corporation Clearwater, FLWhite Engines, Inc. Canton, OH

The timely completion of this work was made possible in alarge part by the excellent cooperation and informationprovided by personnel from all of the organizations identifiedabove.

2.0 DIESEL ENGINES - DESIGN VARIABLES

2.1 General - The diesel cycle is based on the atomization ofa jet of fuel in a combustion chamber containing air heated bycompression above the ignition temperature of the fuel-airmixture. All diesel engines share this basic concept. Themethods of creating this scenario are widely diverse. Thedifferences that exist among diesel engines are much moredraratic than those in spark ignition engines.

- rng -e cn histor-of diesel engine develcsmentand refnment, there are an incredible number cf deslan

Sr-i- n production. There are manor

Paport No. 556-1193-18:TNeflo. P. No. DAAKJ70-81-D-0157, Tak 0016Date September 1984

diffirences in the engine cycle, cooling, breathing, fuelsystems, injector and nozzle design, combustion chamberdesign, lubrication systems, injection timing methods, andperipheral devices. These design differences exist not onlybetween manufacturers, but also between different engines fromthe same manufacturer.

In 1953 C.C. Pounder (18) wrote that "no amount of theorycan ever replace test bed development in the final stages ofthe evolution of a new diesel engine". Some 25 years laterLipkea (16) paraphrased that concept with the statement that"...regardless of the results of fundamental research andtheoretical analysis, both engine manufacturers and fuelsuppliers work largely by empiricism and experiment ratherthan by direct application of the fundamentals of combustion".

While it is true today that some use is made of computersimulation in diesel engine design and development, the finaldesign of these engines is at best semi-empirical. Eachsuccessful engine is a workable combination of the abovefactors developed by "cut and try" techniques.

2.2 Military Diesel Engines - The military engines within thescope of this program are rated from 9 to 340 horsepower at1800/2000 RPM and are the product of a number of differentmanufacturers. They include most of the inter-enginedifferences identified above. The design of each of themilitary engines is described briefly in appendix a.

The engines involved are all four cycle models currentlyprocurable for the DoD family of DED generator sets. Alsodescribed for comparison purposes is the two cycle DetroitDiesel 4-71 engine used by the Air Force on a 72 kW generatorset. The comparison is made because this Detroit Dieselengine ii the subject of the most complete investigation of ahigh idle problem and solution reported in the literature.

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3.0 DIESEL ENGINES - COMBUSTION PROCESSES

3.1 General - While inter-engine differences dictate analysisof high. idle operation on an engine by engine basis, thecombuszion processes are co..on to all of the engines. Thisapplies to the rapid reactions within the combustion chamberas well as to the much slower combustion processes occurring

Report No. 556-1193-18National P. 0. No. DAAK70-81-D-0157, Task 0018Technical Date September 1984Systems

in the exhaust system. Thus, the nature of combustion both in

the combustion chamber and exhaust system affords ageneralized basis for a consideration of high idle phenomena.

The effect on combustion processes of high load conditionswill also be considered for two reasons. First, high loadoperation is currently used for engine "cleanup". Second,some of the effects of high load operation are occasionallyattributed to high idle operation.

A brief (and simplified) review of diesel combustion andcarbonization theory and processes is presented in appendix b.of this report. This theory serves as the basis for the

following discussion.

3.2 Exhaust Composition - The diesel exhaust constituentsimportant to this work are soot, bound carbon, and the various

oxides of sulfur. The bound carbon consists of unburned fueland lubricating oil, in addition to high molecular weighthydrocarbons formed during combustion.

3.2.1 Carbonization - Characteristics of the various forms ofcarbon generated during combustion may be summarized asfollows.

Soot is essentially soft powdery carbon. When depositedin the combustion chamber, it normally migrates to the oilfilter due to the detergent action of the oil. When depositedin a dry exhaust system, it is removed by high flow (highload) exhaust gases.

Tar formation is due to heat induced reactions of bothfuels and lubricants, and is primarily a high temperaturephenomenon. Extended exposure of tar to high temperatures ina combustion chamber can result in transformation to a"lacquer", with possible resultant ring sticking.

Vitreous carbon is a hard shiny black deposit that isformed most rapidlv on surfaces in the active "coking range"of 50C to 700 dearees F. The phenomenon usually occurs incombusticn chambers under heavy loads, and exhaust systemsunder light loads.

3.2..1 Fuel and oil Effects on Carbonization - Carbon buildupo sme extent by bhue an lubricatng oil.

Az=..dx d cetails the fuel and lubricant characteristics. . :..... c .. .. i-.ibit carbcn for. -i

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National 1; , 0, Nc1, 1 J1V '()"U.'1":-1 57, Task 001iTechnical 1:t a)tter 1984

It is important to note that carbon Ln alI of Lts formscan be copiously generated in any engine and in an c|:eratingmode when improper fuels or lubricants are used. rt i3ho3idalso be noted that combustion chamber carbonizati, i adlacquer formation are characteristic of nigh coml)u.3ticrchamber wall and ring temperatures (hig], loads).

3.2.2 Wet Stacking - Diesel engines ope::ating at colder tnannormal temperatures have always exhibited a charac.eristiccommonly known as "slobbering". In recent years -. is term hasbeen replaced in some quarters by the more euphemt3tic term,"wet stacking".

The phenomenon in a properly operating engine isattributable to the incomplete combustion of fuel andlubricating oil because of low combustion chambertemperatures. It is normally a characteristic of a coldoperating diesel engine rather than a symptom of an engineproblem . However, there are conditions under which it can bea symptom of potential trouble, as well as conditions underwhich the symptom itself is sufficiently severe to become anuisance.

In most cases the liquid discharged through the exhaustsystem is primarily fuel oil mixed with some lubricating oiland condensed hydrocarbons. In certain cases the effluent isprimarily lubricating oil. In any case the lubricating oilhas a more significant impact on diesel exhaust than itsrelative proportion in the exhaust stream would indicate.

3.2.2.1 Lubricating Oil Effects on Wet Stacking - Theproperties of lubricating oil that make it a satisfactorycylinder lubricant also make it difficult to eliminate fromdiesel exhaust by combustion.

Lubricating oil will typically have a much higher flashpoint and molecular weight, and much poorer ignition qualitythan fuel oil. Additionally it is usually introduced into thecombustion chamber in the form of a film or droplets ratherthan as an atomized spray. Thus any lubricating oil (and itscombustion products) in the exhaust stream will tend togenerate significantly more liquid and solid particulates thanwill the same quantity of fuel oil.

A recent General Motors Research Laboratories study (17)indicated that lubricating oil contributed from 50 to 280ti.-es as much material to particulate emissions (solid andlz.id) as would be expected on the basis of its consumptionrea-ive to fuel. The oil contribution to particulate exhaust

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Report No. 556-1193-18National P.O . No. DAAK70-81-D-0157, Task 0018Technicol Date September 1984Systems

was further shown to increase with increasing engine speed,and to decrease with increasing engine load.

3.2.2.2 Enaine Cycle Effects on Wet Stacking - Two cycleengines appear more likel to exhibit wet stacking phenomenathan 4 cycle engines. Oil consumption is inherently higher ina 2 cycle engine because of combustion chamber and exhaustport geometry. This is substantiated by emission data thatindicate 2 cycle exhaust to be considerably higher inhydrocarbons than 4 cycle exhaust.

A test program at Southwest Research Institute (9)compared the exhaust of a Detroit Diesel 6L-71T (2 cycle)engine with that of a Cummins NTC-290 (4 cycle) engine. Theinvestigators found the 2 cycle engine particulates to beprimarily hydrocarbons with a small amount (17%) of excesscarbon or-soot. The 4 cycle engine showed hydrocarbonmaterial with substantial (160 %) excess carbon.

This would indicate a tendency toward liquid rather thansolid exhaust particulate for the 2 cycle engine, and aresultant tendency toward wet stacking.

A comparison of a Caterpillar 3208 with a Detroit Diesel6V-71 (8) yielded similar results. It was observed that the6V-71 engine exhaust was more "oily" than the 3208 exhaust,and that the hydrocarbon content of the 6V-71 engine exhaustparticulate was about four times that of the 3208.

3.3.3 Combustion Under Liaht Loads - A desirable condition inoperating diesel engines at light loads is the maintenance ofcombustion chamber temperatures adequate to support fullcombustion. This temperature is influenced by both enginedesign parameters (e.g. compression ratio) and byenvironmental parameters (e.g. intake air temperature).

A naturally aspirated diesel engine ingests a quantity ofair that is independent of the amount of fuel being injected.Under light load conditions (low exhaust flow) this is alsotrue of turbocharged engines.

Any diesel engine will exhibit wet stacking symptoms ifthe intake air is cold enough and if the fuel required forpower is insufficient to maintain an appropriate combustionchamber temperature. Carried to an extreme, this conditioncan result in one or more cylinders ceasing to fire.

Acn effe t of : h oa4 operation is -he depositionof oarticu-ates in the exhaust syste because of relativei,

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Report No. 556-1193-18Technical P. 0. No. DAAK70-81-D-0157, Task 0018JSsts, Date September 1984

low exhaust gas flow. This phenomenon has been documented byDanielson (2) among others, who found that diesels exhibitedlower than normal emission characteristics for some time afterclearing the exhaust system by wide cp-en throttle operation.

Thus, light load operation can be enhanced by maintainingmoderate air and engine temperatures through the use ofenclosures and cooling system modulation.

Overfuelling at light loads can also contribute to wetstacking and exhaust carbonization. This can be avoided bymeans of fuel injectors designed to operate properly over theentire engine load range.

3.3.4 High Idle vs. Low Idle - Engines used to drive militarygenerator sets typically operate at idle speeds in the rangeof 1500 to 2000 RPM rather than at low idle speeds in therange of 800 to 1100 RPM. The effect of the idle speedincrease on combustion adequacy will vary with engine design,and may be either detrimental or beneficial.

Atomization of fuel will improve at high idle sinceinjection rate and chamber turbulence generally increase withengine speed. The combination of more finely dispersed fueland faster injection (better preignition mixing) will lead tomost of the fuel being consumed during the period of rapidcombustion (see appendix b).

Further the tendency of fuel particles to overpenetrateand impinge on cylinder surfaces is reduced at hich idle.Finally an engine will normally operate at a somewhat highertemperature at high idle than at low idle because of the addedfuel requirement due to increased windage and friction.

On the other hand the total quantity of exhaustparticulate generated per unit time at high idle will increaseapproximately in proportion to the increase in speed.Furthermore the time available for combustion at high idle isdecreased approximately in proportion to the increase in speed(depending on ignition lag).

4 The relative effect of these factors are determined by theengine design, the operating conditions, and the nature of theeXau t effluent. There is sce evidence that hich idie ingeneral may be a less severe ooeratinc condition -han low idle• t_. ren'ect to wet stackin. it can be a rcre

-eratina ccnz:icn in t e a=e cf any sce: f::encin.se e eo4a lc '. .

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Report No. 556-1193-18Niona P. O. 1o. DAAK70-81-D-0157, Task 0018NtiohnalTSchnica Date September 1984Systems

3.3.5 Decarbonization - It has been stated that carbon formedas soft soot is easily removed from both the combustionchamber and exhaust system. A mixture of carbon and liquidhydrocarbons that has been exposed to coking temperatures isconsiderably more difficult to eliminate.

The dissipation of coked carbon can be accomplished by thesame technique used in the modern self-cleaning oven. Thisrequires heating the carbon deposits to a temperature abovetheir ignition temperature in the presence of adequate oxygenfor a sufficient period of time. The temperature and timewill depend on the nature and ratio of carbon and hydrocarbonconstituents in the coked material, and on the amount ofmaterial present.

The self-cleaning oven typically requires four hours at800 degrees F to remove carbonaceous material. Ignitionpoints of engine-formed hydrocarbons range from approximately500 to 900 degrees F, and the ignition point of carbon isapproximately 925 degrees F. A normally operating dieselengine ingests 15% or more excess air at maximum power.

The surfaces of the coked areas in the exhaust system of afour c'cle diesel engine under load can easily reachtemperatures in excess of 925 degrees F, due both to highexnaust temperatures and to the ignition of exhaust deposits.Thus, carbon removal by high load operation is feasible.

Removal of carbon based combustion chamber deposits bythis technique is not possible. Unfortunately, the same loadconditions that promote carbon removal from the exhaust systemare thcse that tend to promote carbon formation in thecombustion chamber. This occurs because some of thecombus-in chamber surfaces can reach coking temperatures athich loads, but will not norally reach the ignitiontemperatures of the deposits.

The heat transfer and combustion processes that occur inan exhaust cleaning operation are too complex to be treated onan analytical basis for determination of optimum cleaningtimes and (load-induced) temzeratures. This can only be doneempirically for each specific engine generator system, andw l -d -nd to a larce exten- on the condition of the systemprior uo cleaning.

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Report No. 556-1193-18National P.O0. No. MMA70-81-D-01571 Task 0018Technil Date September 1984Symmsrn

4.0 HIGH IDLE OPERATION - BACKGROUND SURVEY

4.1 General - The term high idle is used here to designate anoperating mode for.DED generator sets at rated (synchronous)speeds from no load to approximately 25% of rated load. Thereis a considerable difference of opinion with regard to thedamage potential of this mode of operation. The origins anddevelopment of these opposing points of view are highlightedbelow.

4.2 Air Force DED Generator Sets - The concept of high idle asa problem operating mode was apparently first addressed in themilitary by the Air Force. The prime mover for most of the AirForce flight line DED generator sets is a Detroit Diesel 2cycle engine in any one of a number of configurations.

We have seen earlier that this engine design isparticularly prone to a wet exhaust. For some years thesegenerator sets were operated with load banks to yield higherengine temperatures and decrease wet stacking tendencies.

The Air Force in 1981 and 1982 attempted to find analternative to load banking in an evaluation of the Stewartand Stevenson DRYXAUST system. The work was performed byStewart and Stevenson Services, Inc., and the results werereported in a 1983 Air Force report (24)..

Wet stacking was indicated in that report to cause"increased fuel consumption, accelerated engine wear, anddeterioration of exhaust system components". Air Forceengineering and maintenance personnel identified in the report(5'(7) were contacted to determine the nature of the engineand exhaust system damage.

These personnel stated that the problem was not one ofengine damage, but rather an unacceptable housekeeping andaccident hazard situation involving ground surfaces coveredwith black oily liquids.

Stewart and Stevenson states (in the Air Force report)that when diesel engines are operated at high idle, "the fuelinjectors supply more fuel than the engine is capable ofburning.... This unburned fuel collects in the exhaust systemand causes a fuel sludge buildup commonly referred to asslobbering or wet stacking".

The Stewart and Stevenson "fix" consisted of substantiallyreducing exhaust back pressure by means of a chance in mufflerdesign, slightly increasing back pressure by means of a

Report No. 556-1193-18Netional P. 0. No. DAAK70-81-D-0157, Task 0018Technical Date September 1984

gravity operated exhaust damper valve, and substitutinginjectors less likely to overfuel at low loads.

The theory of using an exhaust brake to increase backpressure and reduce wet stacking tendencies is valid and willbe discussed later. However, based on the data presented byStewart and Stevenson it would appear that the mostsignificant change to the engine wet stacking tendency in thiscase was the result of installing injectors better suited tolow load operation than the original injectors.

It would also appear (in the absence of more completeinformation) that the reportedly improved fuel consumption wasprimarily a result of better injection characteristicscombined with lower exhaust back pressure from a moreappropriate muffler design. It is not obvious from theinformation presented that the DRYXAUST valve played asignificant role in the "fix".

4.3 DOD DED Generator Sets - User Assessment - As previouslystated, there is a perception on the part of many users ofthese sets that high idle is a highly deleterious operatingmode for all DED generators. The result has been a movementtoward integral load banking of sets to avoid light loadoperation. It is useful to examine the background anddevelopment of this perception in order to assess itsvalidity.

4.3.1. Documented User Reports - While much of the evidence ofuser dissatisfaction with high idle operation is verbal, thereare several documents which fairl 'yreflect that generalposition. Quality Deficiency Reports (QDR's) and EquipmentImprovement Recommendations (EIR's) will be discussedseparately since they refer to specific generator sets.

4.3.1.1 System Assessment Reports - The 1981 system assessmentreport (15) for the 30 kW 60 Hz generator sets makes thestatement that "users are operating the generator sets underlow-output load requirements ..... load bank kits wouldeliminate engine carbonization". The 1982 system assessmentreport (3) states that "use of load banks when the operationalloads .... are low will contribute to a longer life for thegenerator engine".

No evidence was presented in either of these reports tosubstantiate the position that light loading caused engini"carbonization" or shor:ened engine life. An author of theiC82 re.dcrt (4) was contacted to determine the basis for theh.ih idle damage scenario.

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It was stated that a thorough search of both nilitary andcivilian data bases by these authors had provided no data uponwhich to assume reduced engine life or decreased reliability.It would appear that these judgements either were based onintuition, cr reflect opinions voiced by others during thefield survey portion of that work.

4.3.1.2 Field Reports - Recent sample data collection activitysummarizes in an unpublished trip report some of the feelingsabout high idle operation held by field personnel. Thatreport essentially stated that all 10 kW sets in a tacticalunit were inoperative. The problem was attributed to thesystem usually operating at less than 25% of maximum ratedload (high idle).

It is significant that 7 out of 9 reported generatorfailures occurred in the period from November to January in arelatively cold climate. It is also significant thatsimilarly equipped tactical units operating in the same areaconcurrently reported no major problems.

In light of these facts it is probable that a lack offamiliarity with these air cooled units on the part ofoperating personnel rather than engine deficiency was thecause of the problem. Nevertheless supervisory personnelattributed the failures to extended high idle operation.

Another unpublished field report stated with regard to 30kW DED generator sets that "it is evident that these sets mustnot be operated for any length of time without a load, or wetstacking will result". The writer evidently considered thewet stacking to be a problem rather than a symptom. Again noevidence was presented that any damage or deterioration of theengines had occurred.

4.3.2 Werbal Recorts - Discussions were held with operatingand maintenance personnel from a number of organizationsvarying both in location and function. A substantialproportion of these personnel expressed the opinion that highidle was a problem operating mode. Their position parallelsthat expressed in the previous documented reports.

In scme few instances specific problem sets wereidentified as experiencing failures due to high idleoneraticn. In fact, most of these failures were of a natureinc.re ..-e to be the result of other circumstances than high:ce. The its which recortedly failed due to high ia.eCzer=:-_:=n z been rezaireC, so that no -irst hand*czserva-'ns zcud ze made.

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4.4 Diesel Manufacturers High Idle Assessment - Enginebuilders and those associated with the industry consistentlyexpress a viewpoint somewhat contrary to that evidenced byusers. The concensus of builder cnr4ion was that hich idleoreration dictated careful attention to specified routineservice procedures, but caused no decrease in engine life orreliability. This proved to be true regardless of whether ornot they were current suppliers of engines to the military.

All sources contacted provided some narrative to supporttheir position, although once again documentation was lacking.The almost unanimous manufacturer perception of high idle as a(usually) non-problem operating mode is best exemplified bythe following case histories.

Several manufacturers report that diesel constructionequipment in very cold climates operates almost continuouslyat low idle during winter months because of the difficulty oflow temperature starting.

The equipment is stopped only for routine maintenance, andnever long enough for engines to cold soak. The only specialprovision recommended for this usage is the increase of lowidle speed by 200 to 300 RPM in order to increase the engineidle operating temperature.

Two sources also report that cold climate diesellocomotives are started in round houses daily, and then movedoutside. in the case of yard engines they are operated atidle almost continuously, and rarely experience high loads.The only detrimental effects reported were of a housekeepingnature, namelv the spraying of unburned diesel fuel uponapplying load.

One manufacturer reports the current use of a diesel• generator engine in the 50 kW class which at present has

operated without problem for over 12,000 hours. This engineis in marine service as a primary power unit and operatesalmcst continuously at high idle. It has had no unscheduledservice, and apparently does not exhibit any tendency to wetstack.

Final-/, the a.mcs continuous use of 100 kW Dri-ar- Dowersets to meet a demand typically in the 5 to 15 kW range wasreorted. These sets were sa-o to have operated f-or a n er

"ears witnout an-,- unusual enoine service orc.ie.s.

0...

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4.5 QDR's and EIR's - The TROSCOM DED generator maintenancefiles were examined for evidence of high idle problemsexperienced by any of the DoD standard family of tacticalengine generators. The review included all prcblems reportedfor these sets from 1970 until the present.

With the exception of the 15 kW and 30 kW sets, there weretwo reports concerning problems that might be considered highidle associated. The first, EIR (control no. Y77992)referenced exhaust carbon buildup and incomplete combustion in5 kW DED generator sets. The problem was finally traced tofaulty fuel injection nozzles.

The second, QDR (control no. 670997) referenced a 100 kWDED generator set showing signs of oil leakage and "excessiveslobbering" at the exhaust. This was eventually determined tobe caused by faulty piston ring installation in a singlepiston of the problem engine.

All other complaints and recommendations were in referenceto the 15 kW and 30 kW DED generator sets powered by the 198ERand 298ER engines described in appendix a.4.5.1 198ER / 298ER Engines - High Idle Problems - These

engines were the subject of a number of EIR's, QDR's andmemoranda concerning reportedly unsatisfactory high idleoperation. The "exhaust problem" correspondence for theseengines begins in 1979, and continues to the present. Theyhave recently been the subject of a review with regard totheir application in both the Pershing and Patriot systems.The problems as perceived by operating personnel are bestsummarized by the following case histories.

Fort Stewart, GA reported in 1979 that one 30 kW and four15 kW sets were "pumping fuel and oil, mostly oil, out theexhaust". They also reported oil leaking at the manifold tomuffler flanged joint. The sets were run at 75% load for 16hours without improvement.. Teardown inspection, cylinderhoning, and the addition of valve seals failed to solve theproblem.

The Illinois National Guard reported in 1980 that when a30 kW set was operated at "low power (10% of capacity), oilleakage was noted at the manifold gasket and the exhaust gaseswere very dirty". They also reported (but did not define)excessive oil consumption.

Replacing the manifold gasket solved the oil leakageproblem. Load banking cleared up the "dirty exhaust" problemtemporarily, but "exhaust gases continued dirty at low -cwer".

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" i 7 National P.0. No. DAAK70-81-D-0157, Task 0018 018 L8Sysem Date September 1984

Teardown inspection of the engine indicated the combustionchamber and associated components to be in "excellent visualcondition".

The Penns':-ania National Guard repor-:ed in 1980 that a 15kt 3enerator set wet stacked and used excessive oil. It wasstated that "the engine does not miss and seems to operatenormally". Oil consumption from 14 to 93 hours (on therunning time meter) averaged one quart of oil every sevenhours.

Recently this writer had the opportunity to examine eight30 kW sets at the Belvoir R an D Center in temperate weather.These sets were powered by 298ER engines manufactured betweenJune 1981 and February 1982. The sets had between 10 and 15hours of running time on the meters.

Four of the sets were dribbling oil at the flanged jointbetween the exhaust manifold and the muffler at high idle.The result was a coating of oil down the side of the engineand on the set structure.

The wet stacking and oil leakage was stopped by loadbankin a: full load for about 15 minutes. When the load wasdropped to hich idle, oil dribbling and wet stacking resumed.After running the sets at full load overnight, return to highidle acain resulted in oil dribbling after approximately onehour of operation.

4.6 Enzine Oriented Problem Scenarios - It becomes apparent-* that we are concerned with the effects of two different

syndromes generated by high idle operation of DED generatorsets.

The first is characterized by the generic "slobbering" ofunburned fuel in all diesel engines operating under lightloads in a cold environment. The second is characterized bythe "slobbering" of lubricatino oil in some 198ER and 298ERengines under light loads regardless of environmentalconditions.

The remainder of this report will treat these phenomena astwo separate and distinct areas for consideration.

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5.0 HIGH IDLE - DAMAGE ANALYSIS

5.1 General - While there is no apparent evidence that highidle operation of diesel engines in general causes acute orcatastrophic damage, there are some chronic "nuisance" effectswhich must be considered. It should be noted at this pointthat any of these chronic nuisances can also occur for reasonsother than light load operation.

5.2 High Idle Problems - Most diesel engines in good repairexhibit undesirable symptoms attributable to a high idleoperating mode only when conditions are such that thecombustion chambers becomes too cold for good combustion tooccur. This usually occurs only with a combination of lightloads and low environmental temperatures. The noticeableeffects are some carbon buildup in the exhaust system andoccasional wet stacking.

while carbon buildup in the combustion chambers, valves,and injectors has occasionally been attributed to high idleoperation, combustion theory and some experimental dataindicate that this is not the case. We have previouly notedthat teardown inspection of 198ER and 298ER engines in severalcases after wet stack operation showed clean combustionchambers.

The Caterpillar Tractor Research Department (14) performedsome experiments with their diesel engines at low temperaturesbecause, they said, "many commercial users idle our enginescontinuously between working periods; and in some instances,the engines are not stopped long enough to get cold during theentire winter".

A D2 Caterpillar engine was operated continuously for 4days (96 hours) at 700 RPM idle in an ambient temperature of-40 degrees F. The only engine protection consisted of paperin front of the radiator. Teardown inspection immediatelyafter cold idle operation revealed no evidence of nozzleplugging, and showed an engine reported to be in excellentcondition. The only effect noted was some soft carbon sludgein the exhaust valve ports.

5.2.1 Effects of Wet Stacking - Wet stacking (by fuel oil)tends to result in cylinder wall washing. This is usuallyaccompanied by increased blow-by of combustion gases into thecrankcase with resultant oil dilution and occasional sludging.

"Cold sludge" (crankcase sludge) is defined by Stinson(21; as a colloiJal mixture of oil, water, and combustion!.rcd' cts. He further states that "hot sludge" can be formed

Report No. 556-1193-18

National P. 0. No. DAAK70-81-D-0157, Task 0018T.h.,W Date Septenber 1984

in the exhaust system as a result of continuous exposure ofhydrocarbon and carbon mixtures to high temperatures duringwet stacking. Hot sludge is reported to resemble coffeegrounds in appearance.

It should be deposited in combustion chambers (withsubsequent "lacquer" formation) only under the high wall andring temperatures associated with heavy loads. Sludgeformation in the combustion chamber is thus not a problemrelated to high idle operation.

Sludges can cause corrosion of crankcase and exhaustsystem components when high sulfur fuels and lubricants whichdecompose to form inorganic and organic acids are used.D.L. Raymond (19) states (with regard to cold weatheroperation of diesel engines) that prolonged idling causescondensation and sludging of crankcase oils, promotescrankcase corrosion, and interferes with lubrication. Toavoid these problems he advises raising the idling speed to1000 RPM to maintain higher engine temperatures.

Finally, severe wet stacking can create a housekeepingproblem by coating surrounding equipment and ground surfaceswith a fuel, oil, and carbon mixture. In the case oflocations requiring personnel access, this can create apotential safety hazard in the form of slippery footing areas.

5.3 Alternate Problem Sources - The nuisance symptoms of lowtemperature high idle operation are also caused by a number ofother more serious conditions. Among these are overfuellingdue to improper injector sizing, after dribble caused by leakyor corroded injectors, low compression, improper timing, andpoor quality fuel or lubricating oil.

When high idle type symptoms are found in certainindividual engines rather than in a class of engines, it isprobable that the cause is one of these engine specificproblems rather than operation in the high idle mode.

occasionally other problems such as the failure to acceptload or rough running have been attributed to extended highidle operation. These kinds of failures are not typical ofextended low temperature light load operation unless thetemperatures are low enough to cause one or more cylinders tobe inoperative. Again it is more likely to be peculiar to anindividual engine not in good repair than to a class ofengines.

,LNati al. 0. N:-. .'K'O-E.--:-0157, Tas. 0018Technical ::ate k3pt(rr.r 1.984

5.4 Assessment - Military Diesel Engines - witi the possibleexception of the 198ER and the 298ER engines wiich wil bediscussed separately, there is no indication that high idleoperation is a problem mode for any cf the engines used as DoDtactical generator drivers. Occasional reports of heavyexhaust carbon or failure to accept load in these engines areprobably due to poor engine condition, poor fuel, or poorlubricating oil.

The directed maintenance regimen for the military enginesis conservative with regard to inspection, operation, andservicing. Adherence to the service guidelines and use ofspecified fuels and lubricants should prevent engine damagefrom any of the possible effects of high idle operationdescribed above.

5.5 Idle Mode Symptom Alleviation - Several techniques otherthan load banking are available to reduce the symptoms ofextended cold operation (in either the low or high idle mode)when the symptoms become an unacceptable nuisance. Thesetechniques are all based on increasing the combustion chamberand exhaust temperatures.

They have been used primarily in cases where the sprayingof unburned fuel has been unacceptable to the user. There areno reports of these techniques being used to enhance engineoperation, extend engine life, or prevent engine damage.

5.5.1 Increased Exhaust Back Pressure - Three instances ofavoiding unburned fuel discharge by increasing exhaust backpressure have been reported.

The Stewart and Stevenson DRYXAUST system apparentlyintends their gravity operated exhaust valve to serve thispurpose. Cummins Engine Co. (21) reported the installation ofan air operated "guillotine" valve in the exhaust of alocomotive diesel engine to increase back pressure and reducespraying of fuel. Lister Diesels (6) reported that unmuffledair cooled diesel engines in irrigation service dischargedunburned fuel until back pressure was increased by theinstallation of mufflers.

Increasing exhaust back pressure (exhaust braking) acts toincrease engine temperatures by increasing the fraction ofresidual gas in the combustion chamber at the start of theintake cycle. It also requires additional fuel (and thushigher cylinder temperatures) to discharge the exhaust againstthe higher pressure.

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On tho negative side, increasing exhaust back pressurewill tend to increase crankcase oil dilution, and may increasesoot formation.

5.5.2 Capacity Modulation - This technique involves cuttingoff fuel to one or more cylinders in either V or in-lineengines. This is generally feasible only in cases where lowpressure fuel distribution is used e.g. the Cummins PT system.The result of this procedure is to cause the working cylindersto carry more load, use more fuel, and thus operate hotter.It is reported that engines operating in this mode do not runnoticeably rougher than with all cylinders firing.

While this approach is not suitable for the current lineof military diesels, it does point out the possibility of DoDDED generators appearing to run normally while in fact thereare one or more inoperative cylinders. This would provide alikely scenario for a set in apparent good repair *failing topick up load".

5.5.3 Intake Air Preheat - Absolute air temperature at the endof the compression cycle (without combustion) varies almostlinearly with absolute intake air temperature. It also variesas a fractional power of the compression ratio, the powerbeing typically of the order of 0.33 (22).

If we assume an intake air temperature of 0 degrees F(460 degrees absolute), a compression ratio of 15 to 1, and acompression ratio factor of 0.33, we would arrive at acompression temperature of 664 degrees F (1124 degreesabsolute). Intake air temperature of 100 degrees F wouldresult in a compression temperature of 909 degrees F. Thus anincrease of 100 degrees F in intake air temperature wouldresult in an increase of 245 degrees F in compressiontemperature.

This indicates that increasing of intake air temperaturewould result in an amplified increase in compressiontemperature. Thus intake air preheat would appear to be apromising avenue for extending the non-symptomatic temperaturerange for high idle operation.

Intake air heating might also be accomplished byelectrical heating of the intake manifold. This would havethe dual effect of preheating intake air and increasinggenerator load. Alternately, heating could be provided byexhaus- gas heat exchange, or (under scme conditions) bye- .ine intercoolers, aftercoolers, or radiators.

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.cshnial Date September 1984

5.5.4 Exnaust Gas Recirculation - The technique of exhaust gasrecirculation (EGR) consists of mixing some of the exhaustwith the intake air at light loads through the application ofwaste gate technology. The effect of this technique is toincrease intake gas temperature, and thus combustion chamberand exhaust temperature.

On the negative side oil dilution will be somewhatincreased as in the case of the exhaust brake. Additionallythere is some evidence (10)(22) that EGR increases sootformation by extending combustion duration (decreasing flamespeed). This could result in less complete soot combustion atthe time of exhaust valve opening.

6.0 The 198ER and 298ER Engines

6.1 General - Some of the 198ER and 298ER engines have atendency to wet stack at high idle regardless of ambienttemperature. Contrary to the ususal wet stacking phenomenon,the effluent appears to be primarily lubricating oil.

This might indicate combustion chamber temperatures highenough to permit essentially complete combustion of the finelydispersed fuel, but not high enough for combustion of the lessdispersed and lower cetane value lubricating oil.Alternately, it might result from an excessive amount oflubricating oil reaching the cylinders.

No information concerning lube oil consumptionspecifically under high idle conditions has been found forthese engines. There is an indication from field reports thathigh idle oil consumption in some cases may be more than onequart every eight hours. It would' seem on the basis ofavailable information that this may not be typical of thisdesign, but may be limited to certain sets.

6.2 Manufacturer Assessment - White Engines, Inc. (1)essentially states with regard to the 198ER and 298ER enginesthat :

1. continuous operation at light or no load will notaffect engine reliability, or shorten engine life.

2. lubr~cating oil wet stacking will not cause injectorfouling, or cause failure of the engine t respond to sudden

full load application.

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3. oil consumption will not be appreciably different athigh idle than at the same speed under load.

4. excessive cylinder oil in these engines is primarilya problem of the state of repair of the engine, or the absenceof valve guide seals.

5. oil leakage from manifold gasketing is not anabnormal condition, and oil discharge from the exhaust stackor manifold gasketing is a purely "cosmetic" problem.

6.3 Analysis - Manufacturer's Position - Based on informationpreviously reported, the assessment by the manufacturer seemsvalid with several reservations.

There is an indication in the field literature that someof these engines consume excessive oil when operating at lightloads. It has been reported previously that in at least onecase the installation of oil seals in accordance with t.emanufacturer's recommendations did not correct the oilthrowing problem. It is possible that oil in excess of thatrequired for good lubrication is reaching the cylindersthrough the valve guides or through faulty oil ring action insome of the 198ER and 298ER engines.

Oil leakage past the exhaust manifold gasketing results inan oil covered engine and enclosure. This causes thecollection of dirt on the engine and requires a cleanupoperation before maintenance is performed. In addition theslobbering of oil down the sides of the engine creates aquestion in the mind of the user about the possibility ofengine damage through continued use.

Since keeping oil contained in the engine envelope iswithin the state of the art, it would be desirable to correctthis problem once its scope and cause are determined.

Finally, whether the problem of oil discharge is cosmeticor safety related depends on the application. In those caseswhere the generators are mounted on trailers or other

0 structures, and where personnel access to these areas isrequired, the presence of oily surfaces can be a personnelhazard. This is rarticularly true in hich stress situationss• Ucn as mign- occur in a combhat scenario.

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KSystem

7.0 SUMMARY OF FINDINGS

7.1 The Effects of High Idle - This investigation has revealedno instances in which reduced engine life, reducedreliability, engine damage, or failure to accept load could bedirectly traced to high idle operation. Under most conditionsfor most engines no unusual symptoms result.

There is no indication that high idle is in general a moresevere operating mode for diesel engines than low idle, andsome indication that it may be less severe. This may bededuced from the typical recommendation to increase low idlespeed by several hundred RPM for extended low temperatureidle. Whether or not the recommendation can be safelyextrapolated to high idle is problematical.

The wet stacking symptoms exhibited by the 198ER and 298ERengines do appear to be aggravated by an increase in speed, asmight be expected in the case of excessive lube oil reachingthe cylinders. Some of the 198ER and 298ER engines have atendency to slobber oil out of the exhaust stack as well asout of exhaust manifold gasketed joints. It may becharacteristic of the engine design, or may be a qualityproblem with part of the production.

This trait may cause excessive oil consumption,housekeeping, and safety problems, but it is not likely toprcduce engine damage. In fact, excessive cylinder oilingshould not create the oil dilution problems occurring in theoverfuel-ing situation more typical of diesel wet stacking.

7.2 Military Engines - Except as noted, the military enginesas a class do not exhibit any unusual tendency toward highidle problems. Reported excessive carbon buildup in theexhaust at certain geographical locations is probablyattributable to fuel, oil or filter problems.

Carbonized or wet exhaust can be eliminated (if considerednecessary) by the occasional load banking of sets inaccordance with the provisions of the 1983 TSARCOM directive

* (25). T-is document recommends load banking lightly loadedsets at full load for 6 hours every 30 days, checking oilweek-. for signs of oil d;lution, and uslnc smaller sets whereocssible to provide operation at a higher percentace of rated

-T..r.is no. c available urcn which to i...rvet -e abc.e schedule in terms o' freauency, duration, or level

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Report No. 556-1193-18National P.O . No. DAAK70-81-D-0157, Task 0018Technical Date September 1984Systems

of load application. In the absence of hard data that mightpermit some reduction in these factors, this empiricallyspecified procedure appears effective.

The recommendation to use sets well sized to theirapplication to minimize the nuisance factors of high idleoperation is well taken for several reasons.

First, initial set cost is reduced for smaller sets.* *. Second, brake specific fuel consumption decreases and

efficiency increases dramatically with load for diesel enginesuntil approximately the 50% load point is reached. Thirdthere is evidence (11) that combustion quality with regard tototal exhaust particulates (carbonizing and wet stackingcontributors) is optimum at about 50% load.

Thus it appears that sets sized to operate at generatorloads of 50% to 100% will operate more efficiently and cleanlythan lightly loaded sets. Additionally sets thus sized canundergo exhaust clearing by the occasional application oftheir operating load rather than by load banking.

8.0 ENGINE SCREENING - HIGH IDLE OPERATION

8.1 General - It has been shown that problems associated withhigh idle operation are primarily related to exhaust systemreactions, and secondarily to crankcase oil contamination. Ifit is desired to screen new engine types to assuresatisfactory operation in this mode, these two areas can beevaluated by the following techniques.

8.2 Test Methods - Exhaust analysis can be performed under no*e load and rated speed conditions for the test engine using U.S.

Environmental Protection Agency procedures for diesel engineexhaust testing (26). Analysis should be made for dry sootand soluble organic fraction (SCF). The dry soot value will.dicate exhaust carbonizing tendencies, and the SOF will

indicate the wet stacking propensities of the engine.

C

0

Crankcase cil can be ex&7,ined for viscosity, total basen mber (TBN), and insolubles. These tests will serve to

-.Measur_ the extent cf the lubricating ci! detericration.

in cr4er t- eszaz:-. :ass fail criteria, a baseline stud:*wcoul- e rebe .s uina enz,--i'. no' In service that arec nEidered a ccer-taCe. Az:ro:riate values for all of the

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Technical Date September 1984Systems

above parameters could be established by the application ofreasonable tolerances to the performance of the currentlyacceptable engines.

9.0 CONCLUSIONS AND RECOMMENDATIONS

9.1 Conclusions - Based on the foregoing analysis thefollowing conclusions may be drawn.

a. Diesel engines designed in accordance with goodcommercial practice, in good repair, and properly serviced canoperate at high idle speeds indefinitely without sufferingdecreased life, decreased reliability, or engine damage. Theprimary potential source of damage from high idle operation ascontrasted to high load operation involves possible excessivecrankcase oil dilution and contamination.

The military specification for diesel fuels and lubricantstogether with conservative servicing procedures are sufficientto insure safe operation of military diesels with regard tooil dilution.

b. The nuisance effects of high idle operation are somecarbon buildup in the exhaust, and occasional slobbering of aprimarily diesel fuel effluent at low temperatures. There isno significant effect on the combustion chamber from extendedhigh idle operation, and consequently no effect on theengine's ability to respond to load application.

These effects can be ameliorated (if desired) by properorientation of the exhaust system, and by occasional loadbanking in accordance with the previously cited 1983 TSARCOMdirective.

c. The 198ER and 298ER engines are exceptions to thisconclusion to the extent that at least some of these enginesslobber lubricating oil rather than fuel oil, and theslobbering is relatively independent of ambient temperature.

No damage to the 198ER and 298ER engines is expected fromthis .. de of oDeration, although cortinucus housekeeping andoccas1o.al safety proDlems are created by the e-ection of

lucr c=__ing oil..

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Report No. 556-1193-18National P. 0. NO. DAAK70-81-D-0157, Task 0018Technicalst.,,. Date September 1984

9.2 Recommendations - The following suggestions are intendedto help answer some unresolved questions, to decreasemaintenance costs, and ultimately to help provide morereliable tactical power with lower life cycle costs.

a. The DoD DED tactical generator drivers in general areadequate for extended high idle operation. The concept ofmonthly clearing of the systems by high load operation, andweekly oil inspection in accordance with the 1983 TSARCOMdirective should be maintained until improved procedures canbe developed.

b. A refinement of the TSARCOM procedures should bedeveloped to optimize the load and operating time requirementsfor high load clearing operation. This should be done as afunction of set design, operating hours, service application,and climatic conditions. Where possible load clearing of thesystem should be by normal operating load rather than by loadbanking.

c. The 198ER and 298ER engines should be tested andevaluated to determine whether lube oil slobbering is a designor production quality characteristic. Worst case enginesshould be tested for lubricating oil consumption under highidle and full -i conditions.

The leakage of oil at exhaust manifold gaskets on some198ER and 298ER engines is not typical of good design andworkmanship. It presents an unacceptable housekeeping problemand should be corrected.

Where the 198ER and 298ER engines are used in a fieldapplication such that the oil discharge presents a personnelsafety hazard, it is suggested that the exhaust systems bereoriented to minimize the problem.

d. The future selection of engines for DED generatorapplication should include screening for fuel and lubricatingoil control by application of tests similar to those outlinedin paragraph 8.2 of this report.

e. An effort should be made to properly size tacticalgenefrator sets for particular applications to increaseeff:iency and mobility, to decrease the need for load

an ..... ,anz to reduce life cycle costs.

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10.0 BIBLIOGRAPHY

1. Caruso, E.J. and T. Bednar: White Engine Co., Meeting

Notes PM-MEP, Springfield, VA, 2 May 1984.

2. Danielson, E.: EPA Technical Report SDSB 79-05.

3. Evins, G.L. et. al.: "TSARCOM System Assessment No. 56",August 1982.

4. Evins, G.L.: Private Communication, June 1984.

5. Elliot, D.: SA-ALC/MMPR, Private Communication, May1984.

6. Frayling, P.: Lister Diesels, Inc., PrivateCommunication, June 1984.

7. Gaucher, E.: SM/ALC/MMIR, Private Communication, May1984.

8. Hare, C.T. and R.L. radow: "Characterization of HeavyDuty Diesel Emissions", SAE Paper 790490, February 1979.

9. Hare, C.T., et. al.: "Fuel and Additive Effects onDiesel Particulate", SAE Paper 760130, February 1976.

10. Hiroyasu, H. et. al.: "Soot Formation and Oxidation inDiesel Engines", SAE Paper 800252, February 1980.

11. Kageyama, K and N. Kinehara: "Characterization ofParticulate Emission", SAE Paper 820181, February 1982.

12. Kates, E.J. and W.L. Luck: "Diesel and High CompressionGas Engines", American Technical Publishers, Alsip IL, 1974

13. Khan, I.M.: "Formation and Combustion of Carbon in aDiesel Engine", Proc. Instn. Mech. Engrs., London, October1969.

14. Kiltv, D.E. et. al.: "Starting Cold Diesels", SAE Paper52795, September 1952.

15. Kirsc.man, R.H. and H.L. Riser: "TSARCOM System.ssessmen- .c. 50", Seztember 1981.

"-' - y ica an Ch- _c.-_.-kea, "';. 7 - nt a :a.

aracter c: -e: -- i.ssicns", SAE atr. 7S0!CE,

. . . . . .

Report No. 556-1193-18NTial P. 0. No. DAAK7-81-D-0157, Task 0018svaeh Date September 1984

17. Mayer, W.J. et. al.: "The Contribution of Engine Oil toDiesel Exhaust Particulate Emissions", SAE Paper 800256,February 1980.

18. Pounder, C.C.: "Diesel Engine Principles and Practice",Philosophical Library, New York, 1953.

19. Raymond, D.L.: "A Synopsis of Cold Weather OperationProblems of Engine Powered Equipment", SAE Pape 650935,January, 1964.

20. Ricardo, H.: "Combustion in Diesel Engines", Proc. IAE,London, March 1930.

21. Schisler, H.E.: Cummins Engine Co., PrivateCommunication, May 1984.

22. Stinson, K.W.: "Diesel Engineering Handbook", BusinessJournals, Inc., Norwalk CT, 1983.

23. Taylor, C.F.: "The Internal Combustion Engine in Theoryand Practice", Vol. 2, M.I.T. Press, Cambridge MA, 1982.

24. Timan, R.D. and E.A. Schwerman: "DRYXAUST DieselEngines", USAF PRAM Report, Project 35480-07.

25. TSARCOM Service Message, Maintenance Instructions for"Wet Stacked" Generators, St. Louis MO, January 1983.

26. U.S. Federal Register, Vol. 45, No. 45, March 1980.

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11.0 APPENDIX

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Appendix a.

TACTICAL GENERATOR DRIVERS

2.2.1 Onan Model 100-1344 - This engine is two cylinder aircooled, displaces 70 cubic inches, and develops 9 brakehorsepower (BHP) at 1800 RPM. It is naturally aspirated(non-turbocharged) with a compression ratio of 19 to 1. Ithas an AMBAC fuel system with indirect injection into aprecombustion chamber equipped with glow plugs. Cold weatherstarting is also aided by electrical induction-air heaters inthe intake air manifold. It powers the 5 kW 60 Hz DEDgenerator set.

2.2.2 Onan Model 100-1345 - This engine is a four cylinderversion of the Onan 100-1344, displacing 140 cubic inches anddeveloping 18 BHP at 1800 RPM. It drives the 10 kW 60 Hz DEDgenerator set and (with minor modificaticns) the 10 kW 400 HzDED set.

2.2.3 White Engines, Inc. Model 198ER - This engine is fourcylinder, liquid cooled, and naturally aspirated. It has adisplacement of 198 cubic inches based on a bore of 3.75inches and a stroke of 4.50 inches. It has a compressionratio of 17.5 to 1, and is rated 41 BHP at 1800 RPM. It usesa Stanadyne/Hartford fuel system in a direct injectioncombustion chamber. It powers the current production of 15 kW50/60 Hz and 400 Hz DED generator sets.

2.2.4 White Engines, Inc. Model 298ER - This engine is a sixcylinder version of the White 198ER, displacing 298 cubicinches and developing 57 BHP at 1800 RPM. It has acompression ratio of 17 to 1, and drives the 30 kW 50/60 Hzand 400 Hz DED generator sets.

2.2.5 Allis-Chalmers Model 3500 - This engine is six cylinder,licuid cooled with a displacement of 426 cubic inches. It isturbccharged and rated 120 BHP at 1800 RPM. It uses aStanadyne/Hartford fuel system with direct injection of fueli n-c the combustion chamber. It has a compression ratio of 16- !, and drives the 50 kW 50,/60 Hz and 400 Hz DED generator

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2.2.6 Caterpillar Model D-333T - This engine is six cylinder,liquid cooled with a displacement of 638 cubic inches (bore4.75 inches, stroke 6.0 inches). It is turbocharged , rated221 BHP at 1R00 RPM, and has a 17.5 to 1 compression ratio.It uses a Caterpillar designed fuel system, and aprecombustion chamber with a single hole nozzle delivering afairly coarse spray. It drives the 100 kW 50/60 Hz and 400 HzDED generator sets.

2.2.7 Caterpillar Model D-343T - This engine is similar to theCaterpillar D333T except that the bore is increased to 5.4"and the stroke to 6.5". The compression ratio is lowered to16 to i. This provides a six cylinder displacement of 893cubic inches and a continuous power rating of 344 BHP at 1800RPM. It powers the 200 kW 50/60 Hz DED generator set.

2.2.8 Detroit Diesel 4-71 - This engine is two cycle, fourcylinder, naturally aspirated (Roots pump scavenged) with adisplacement of 284 cubic inches. It has a compression ratioof 17 to 1, and is rated 118 BHP at 1800 RPM. It is a directinjection engine using General Motors unit injectors. Itpowers an Air Force version of the 72 kW Hobart enginegenerator set used by the commercial airlines.

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Appendix b.

DIESEL COMBUSTION - THEORY AND EFFECTS

Combustion Theory - The three stage combustion theorypostulated by Ricardo (20) in 1930 still governs dieselcombustion research. Taylor (23) states that stage one (thedelay period or ignition lag) is long enough so that, atignition, there is a considerable quantity of gaseous andfinely divided liquid fuel well mixed with the air.

The ignition lag will depend on the temperatures andpressures in the combustion chamber as well as the walltemperatures on which the injected spray impinges. Thesefactors are a function of the engine design and operatingconditions

The second Ricardo stage is the period of rapidcombustion. This results from multiple ignition pointscombined with high combustion chamber temperatures at thestart of ignition. The rapid combustion period consumes fuelwhich has had a chance to evaporate and form a combustiblemixture with air during the period of ignition lag.

Taylor further points out that the nature of fuelinjection in diesel engines is such that there will befuel/air ratios in the chamber from zero (no fuel) to infinity(no air in fuel droplets) until the fuel is completelyevaporated. It follows from this that the quantity of fuelconsumed in stage two is a function of the duration ofignition lag, of injection spray characteristics, and ofchamber turbulence.

During the third Ricardo stage unburned fuel from stagetwo together with any fuel still being injected burns at arate controlled by the local availability of air. This finalstage begins approximately at the time of maximum cylinderpressure. It ends when combustion is essentially complete oris interrupted by the action of the exhaust cycle.

When stage three combustion extends into the exhaustUyce, inccmplete combustion results, with effects thatinclude wet stacking and exhaust carbonization.

C.rbonization - Accepted theor-: i' s that exhaust soot is the_ - Fs-C n d ri. c tustnion,, a.n a ne

arial com'bustion of the soct s formied durl.nc the remainder

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of the combustion cycle. Factors affecting soot formation aredetailed by I.M. Khan (13) and restated in the later work byLipkea. Khan indicates that soot formation is partiallyengine design related, depending essentially on i.njectiontiminc, in3ectin rate, and nozzle design.

Tar results primarily from liquid phase pyrolysis, and isa semisolid form of high polymeric carbonaceous material.When deposited in the combustion chamber and exposed to highcylinder wall and ring temperatures it can become a source ofthe "lacquer" occasionally found in those areas.

Vitreous carbon is a hard shiny black deposit that isformed on hot surfaces. When found in the combustion chamber,it is most likely the result of mixtures of carbon, highmolecular weight hydrocarbons, and lubricating oil beingexposed for extended periods of time to surface temperaturesin the active coking range of 500 to 600 degrees F. Thesetemperatures are typical of ring and liner temperatures foundin small and medium 4 cycle engines under heavy loadsaccording to Stinson.

The same material and temperature considerations apply tohard carbon formation in the exhaust system. However, in theexhaust system temperatures in excess of 500 degrees F occurat low load as well as high load conditions

Fuel and Oil Effects on Carbonization - Carbon buildup issignificantly affected by both fuel and lubricating oil aswell as engine operating loads. Taylor indicates thatcarbon-based deposits tend to increase with increasingviscosity, decreasing volatility, and decreasing cetane rating(fuel igniticn quality). He also notes that sulfur in fuelpromotes both deposits and corrosion.

Stinson states that oils vary in their wall and pistontemperature reactions. Naphthenic based mineral oils tend toform soft soot which (by means of oil detergent action) ispicked up by the oil filter or escapes through the exhaustsystem. Paraffin based oils form harder deposits which aremore likely to lead to ring sticking.

lHe further noints out that even the naphthenic based oils

wil. cause abnormal deposits in the combustion chambers and.-aVe %,hen en_..es are run at hich loads for long periods of

Mie .These dez:s ts take the form o piston lacquers andstk rinc d_:,s--ts. This condition increases blow-c'v o:

c _zs-onc c -E -_ "- crankcase, a n mav resut in ci".. r n.-e o -L as -uffic tnt cxdatinc_ n stazilitv.

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Kates (12) notes that the (Conradson) test for carbonresidue in oils does not indicate whether :he oil will formsoft fluffy carbcn which will be blown cut, or the harddeposits that cause stickinq rinas.

It is noteworthy that combustion chamber carbonizaticn,sludging and lacquer deposition depend to a large extent onthe qualitv of fuels and lubricants. It is also significantthat these effects are typical of heavy rather than light loadoperation.

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