DEVELOPMENT Diesel Engines

12 MTZ worldwide 4/2003 Volume 64

Diesel engines are not only used in automobiles. In this way,the engine manufacturer Dr. Schrick GmbH was contracted todesign a small, high-speed diesel engine for the use in smallaeroplanes. Because of the targets for the propeller drive, it

was not possible to use a conventional design or a public light-weight construction of a diesel engine. This article containssome of the detailed developments that lead to the successfulachievement with a two-cylinder diesel engine with direct

injection and 600 cm3 displacement.

1 Introduction

In 1997, the engine manufacturer Dr.Schrick GmbH located in Remscheid wascontracted in a project to design a small,high-speed diesel engine for the use in anunmanned aircraft (drone), motivated bythe goal to use diesel or kerosene as a fuel[1]. Because of the targets for the propellerdrive, it was not possible to use a conven-tional design or a public lightweight con-struction of a diesel engine. Taking an un-conventional design approach and with asubsequent intensive development pro-gramme, one was able today to achieve therequired targets with the high performancediesel engine TKDI 600 [2]. This article con-tains some of the detailed developmentsthat lead to the successful achievement ofthe targets with this high-speed diesel en-gine with direct injection.

2 Specifications

The key data of the specifications for thepropeller drive are summarized in Table 1.The unusual operating conditions requiredfor this diesel engine are a result of the avia-tion application. The extremely high maxi-mal speed of 10.000 rpm for example resultstherefore from the diving performance ofthe aircraft. The small space for the engineis influenced by the size of the fuselage. Fig-ure 1 shows the comparison of productionseries and racing diesel engines with the de-manded weight-to-power ratio and there-fore indicates the requirements of this en-gine and the engine concept. The requiredtargets of the small high-performancediesel engine TKDI 600 represent, comparedto the trade-off of the weight-to-power overthe engine power output, an extreme mini-mum. Each component had to be conse-

By Barna Hanula,

Stephan Tafel,

Andreas Mück and

Christian Schlüter

Schnelllaufender

Hochleistungs-Dieselmotor

für kleine Flugzeuge

You will find the figures mentioned in this article in the German issue of MTZ 4/2003 beginning on page 286.

High-Speed andHigh PerformanceDiesel Engine for Small Aeroplanes

13MTZ worldwide 4/2003 Volume 64

quently made out to light-metal engineer-ing in order to fullfill the weight specifica-tions. The light-metal engineering of thecrank train, valve gear and cylinder headare going to be shown to be exemplarily.

3 Engine ConceptThe drive concept for reaching the demand-ed flying performance was not defined atthe beginning of the project. The recipro-cating engine had a fuel consumption ad-vantage over the gas turbine and a weightadvantage over the Wankel engine with itsdiesel combustion process. The followingparameters had to be fixed for the recipro-cating machine:■ number of cylinders (1, 2 or 3 cylinders)■ arrangements of cylinders (inline, boxeror 90° V)■ operating method (two-stroke or four-stroke)■ diesel injection (direct or indirect)■ boosting (mechanical, with exhaust gasturbocharger, piston- or g-loader)■ displacement■ cooling (air or liquid)■ selection charge air-cooler and/or oilcooler.

Weight and package space were key fac-tors in determining the engine configura-tion. There is a dependence of weight to thenumber of cylinders and their arrangementrelating to the air-cooling. The disadvan-tage concerning the weight of the liquidcooling can be compensated with an in-creasing number of cylinders within an in-line engine. However an inline engine irre-spective of the number of cylinders is notpossible in the available space. The air-cooled two-cylinder V-engine with an in-cluded cylinder-row angle of 90° and a jointcrankpin was chosen ahead of the boxer-engine because of the above mentioned as-pects as well as the mass balance and thefavourable form of the crankshaft.Extensive studies and calculations had tobe undertaken before a decision betweenthe two-stroke and four-stroke cycles couldbe made. The piston controlled two-strokeengine has the lowest weight. The badscavenging efficiency and the non-optimalcharge motion lead to the selection of an in-direct injection combustion process with afuel consumption disadvantage relative tothe direct injection combustion process. Avalve controlled, two-stroke engine withuniflow scavenging, possesses the architec-ture of a four stroke engine. Componentscan be designed more easily through thelow peak pressure however, the mechani-cal scavenging pump which is necessaryfor the start could not be accommodatedbecause of the weight as well as for spacereasons.

The four-stroke principle with specificfuel consumption, low thermal applicationof the components, and the possibility ofturbo charging proved the highest poten-tial including experimental data from aone-cylinder four stroke engine and uni-flow scavenging engine. The pre- and swirl-chamber engine have the shortest ignitiondelay during the diesel combustion process,as well as a low load application on thecomponents which results from thesmooth combustion with lower cylinderpeak pressures. In relation to the superiorfuel consumption of up to 20 % the decisionwas made in favour of the diesel direct fuelinjection inhibiting the low combustion-airdemand and the low thermal loading of thecylinder head.

At the high demanding power outputper litre it is necessary to supercharge theengine. The possible boosting units wereanalyzed as described within Table 2. In re-lation to the good high performance as wellas the favourable weight in combinationwith the ideal isentropic efficiency the de-cision was made in favour for the tur-bocharger. In this application the waste-gate was deleted, as the engine is opti-mized at a point of rated power which isotherwise driven on a propeller curve. Thesmallest possible cylinder displacementwas fixed at 300 cm3 in relation to the avail-able injection equipment.

4 Crankshaft Drive

The crankshaft within a piston engine hasto be laid out to a high oscillating torque be-cause of the discontinued operating meth-ods with charge exchange and combustioncycle. The oscillating torques will increaseby lowering the number of cylinders. Fur-thermore it should be noticed that the oscil-lating torques are higher with the direct in-jecting diesel combustion process resultingfrom the high peak pressures, rather thanwith the spark-ignition cycle.

Engine torques of approximately 750 Nmwithin the crankpin arise through the gaspressure. The discontinued firing sequenceof the V2 engine reduces the oscillation ef-fect within the crankshaft, yet there arepeak torques at the drive of about 1500 Nm.

The crankshaft and the conrod bearingjournals have to be drilled hollow in orderto get a stiff and also light crankshaft. Thecrankshaft is adapted to the force applica-tions, profiled from the inside and addition-ally profiled at the sides of the crank web inorder to optimize the weight ideally. It is tosay that only those areas were removedthat were researched as being not relevant-ly stable within the FEM calculations. Thehollow crankpin is closed up via two lids.

The design and positioning of these lids areoptimised in such a way that the powerfrom the piston rod to the crankshaft bear-ing is conducted and therefore restrains theovalisation of the crankpin. The balancingweights are cut out of a wolfram alloy(18 g/cm3 density). The imbalance neces-sary for the mass balancing can thereforebe generated on a great radius with a corre-sponding low weight in the given space.

A special engine ventilation had to beimplemented next to the oil system in or-der to realize the demand of the specifica-tions of a fully position-independent air-craft engine. This was possible through theintegration of the engine ventilation intothe crankshaft. The blow-by gas is lead intothe hollow propeller wave via a centrifu-gal-operated valve within the balancingweight, Figure 2. The valve opens up at anengine speed of 400 rpm and prevents anoil leakage by means of the rotation of thecrankshaft.

The optimization of the given space andthe engine weight is crucially dependenton the connecting rod and the stroke. Thereis an optimum at the stroke with which thegiven mean pressure is connected to thepoint of a nominal displacement. Thedrilling will have to be enlarged when thestroke at the given displacement is down-sized. The force of the thrust rods increasewith the greater piston surface whichmeans that the conrod bearing and thecrankpin also have to be enlarged. Thisleads via greater counterbalance andlonger conrods to the fact that the engineheight can only be minimalized in a minorway. The cylinder body is the main compo-nent within the space available relating tothe determined conrod length at a 90°-V-engine. The optimization took place withan analysis on the path of every compo-nent in the conrod area. The outline of thecrankcase was determined via the motionof the piston rod itself. The conrod isweight-optimized and implemented into atitanium drop-forge construction boltedfrom the top.

The piston is made of a forged design inorder to keep up the high load power out-put of the material. The forged design doesnot allow a ring-carrier. The external con-tour of the piston was hard-anodized in or-der to avoid a ring clogging caused throughpressure weld at the ring slot. The pistonwas equipped with thin walls; plus the bot-tom of the piston was assimilated to theform of the piston relief in order to reducethe weight. This resulted in an invariablewall thickness of 5 mm within the wholepiston area. With regards to the compactcrank gear, the piston body had to be car-ried out as short as possible. Nevertheless it

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results in a nominal distance of 1,5 mm tothe balancing weights of the piston withinthe lower dead centre.

Figure 3 mediates an impression of thewhole crank gear. The FEM calculations ofthe deformation through inertia forces arerepresented here, too. At all FEM calcula-tions crankshaft, crankcase, conrod, pistonand head were calculated in a compoundway. Therefore it was possible to introducethe stiffening effect of the total system intothe calculation in a realistic way.

5 Valve Gear

For space reasons a valve gear with a lowercamshaft, which is matched with an en-gine speed of 10.000 rpm via a light weightand oscillation optimization is required.Figure 4 it can be recognised that thecamshaft lies in between the cylinders. Fur-thermore it is demonstrated that the move-ment of the camshaft is transmitted via thethin walled barrel tappets, valve pushrodsand rock arms to the valves. The oscillatingmass is kept low via the titanium valves.The total length of the intake valve is only66 mm with a head diameter of 33 mm anda skirt diameter of 5,5 mm. The oil return ofthe cylinder head was achieved throughthe pushrod tube and the overload drillingwithin the barrel tappets.

6 Cylinder Head in MonoblocDesign

The cylinder head was carried out in mono-bloc design, Figure 5. The weight can besaved through the abandonment of longcylinder head screws and it enables a betterdisposition of channels, injection techniquesand fining at the head. The thickness of thecylinder wall was adapted to the gas pres-sure process, and a grey cast iron liner wasapplied instead of a Nicasil-coated barrel inorder to win further weight savings.

For achieving a reduction of the tensionat the transition from cylinder barrel to thecylinder head this area was filleted. Theform of the piston is matched to the fillet toreduce the dead volume. For space reasonsthe outlet lies at the side of the cylinderwhich is turned away from the air. Thismeans that the layout of the ribbing be-comes greatly significant within this air-cooled cylinder head. To avoid overheating,following actions were taken:■ The cooling fins at the outside of the en-gine were laid out asymmetrically. There-fore it is possible to flush the outlet withfresh air. The stagnant area of the air-coolant stream shifts into the direction ofthe cylinder side and therefore away fromthe hot outlet.

■ On the inside of the engine an outletarea was installed with approximately30 % increased ribbing surface. This wasmade possible with the cambered design ofthe cooling fins. This design allows addi-tional cooling of the very hot stagnant area.■ The cooling fins were designed to be ex-tremely long. The cambered design enablesthe cooling fins inspite of their extremelength-thickness-rate of 36 not to break.■ The area in between the inlet and outletchannel was set up as cooling channel.Cooling fins at the front lead the coolant airdirectly to this cooling channel. The add-onparts (such as an injection valve withclamp for example) were integrated intothe air duct.■ The design of cooling fins is optimizedwithin the engine cowling.A simulation demonstrating the stream ofthe cooling air made by a tool of Fluent isshown in Figure 6. The temperature withinthe valve bridge and the outlet area can in-crease up to 290 °C at a high coolant airtemperature. As this temperature is far toohigh for normal cylinder head-materials,the head has to be cast from a high-temper-ature resistant piston alloy.

7 Lubrication System

A crucial aspect of the development was anindependent lubrication system, in relationto its positioning and acceleration, whichalso fulfils the low weight targets. A drysump system with a suction pump, cen-trifuge and pressure pump, as known fromthe motor sport, was achieved. Via the opti-mization of the pressure ratio it is possibleto dispense a system-inherent catch-tank.The crankcase is designed with a minimaldistance from the counter weights to theoutline of the piston rod motion. The dy-namic which is produced through thecrankshaft within the crankcase is used todrive the oil within the suction channel ofthe suction pump underneath the oil plane,Figure 7. The suction pump supplies the oilinto the centrifuge, where the oil is de-gassed. The deposited air is led into thegearbox via the wave of the suction pumpand the centrifuge. The gas-free oil flowsinto the suction pump and furthermore ispumped via the oil cooler into the engine.The integral crankcase and gearbox castingcontain cast holes to provide a pressure bal-ance. The design of these holes is critical incontrolling the foaming of the oil due to thering gears in the tightly designed gearbox.Furthermore it is crucial that the crankcasedoes not lead the oil into the gearbox wherean overpressure would be generated. Thisdeteriorates the degassing, increasing thefoaming of the oil up to the point at which

the system would collapse. Great attentionwas paid to the analysis of the requiredquantity of oil. A too low amount of oilwould lead to a drop in oil pressure where-as a much too large amount of oil would in-fluence the consumption in a negative wayand would add to the foaming of the oil.The oil amounts to a total of 0,7 kg and iscompletely pumped through the engine atthe point of the rated speed within 2 s.

8 Crankcase

The crankcase is built out of four compo-nents, which are as follows: two housinghalves, the pre-bolted gearbox and thecamshaft bearing-cap. Each of the compo-nents are manufactured through a process ofprecision casting and show a nominal wallthickness of 1,8 mm. Via the vertical parti-tioning it was possible to save on the boltingof the main bearing. Furthermore it was pos-sible to optimize the power flow between thecylinder base screws and the crankshaftbearing. The oil pump body and the oil chan-nels are integrated into the crankcase as arethe suspension of the gear drive for thecamshaft and the suction pump.

The bearing of the camshaft was designedto minimize the stresses in the crankcase.The bearing positions follow the crankshaft-deformation, which is unavoidable regard-ing the extreme lightweight design.

9 Integral Engine Bearer

The necessity to use the extreme light-weight design led to a highly integratedcomponent, which merges different func-tions within one component. The integralengine bearer absorbs the charge air andthe oil coolant as well as the vibration de-coupling of the engine in the aircraft, con-trols the air duct between the turbochargerand the two cylinder heads, takes up thepreheating of the inlet air and forms theplenum, which connects both sides.

The engine bearer forms the outer limitof the engine in wide areas and is thereforethe component that follows the space spec-ifications of the aircraft with the greatestintensity. The mainly symmetrical de-signed carrier contains two sideway-arranged oil-intercooling sets. These arefurthermore supplied with the necessarycooling air via lateral air inlets. The airducts finish evenly with the coolers in or-der to achieve a sufficient back pressure.The charge air stream flowing from the tur-bocharger distributes itself to both chargeair coolers and flows further to the cylinderheads. The engine bearer is fastened atthree supporting points connecting thebearer with the frame and the engine. Ele-

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ments for the vibration suppression are at-tached in between the engine and the en-gine bearer.

10 Development of the Combustion Process

The high rated speed of 6000 rpm wasachieved via the desired propulsion powerand the possible space. In this applicationthe optimization of the engine is reduced tothe rated power point and the correspond-ing propeller curve. Considering the back-ground of the direct injection diesel en-gines in the automobile industry, which de-liver a rated power between 3800 and 4200rpm, achieving 6000 rpm was question-able. It was only possible to clarify whethersuch high speed is viable with a direct in-jection (DI) diesel combustion process viafundamental one-cylinder analyses as wellas with gas exchange, injection and com-bustion process simulation calculations.

The pump-line-injector system (PLI-sys-tem) is preferred due to several reasons.Both of the injection unit pumps are coordi-nated from the camshaft. A similar proce-dure to the valve gear was chosen in orderto increase the speed limit. The weight ofthe pump elements was optimized throughthe new construction and the choice of ma-terial supported through the dynamic sim-ulation. The cam profile was defined after-wards. The resulting compromise enables asufficiently high injection pressure for thespecifications towards the combustionprocess regarding the visible black smokeas well as the maximal performance andsufficient stability at the maximum speedof 10.000 rpm.

The piston bowl is tuned to the high rat-ed speed in regards to the development ofthe combustion process. In addition, theswirl of the combustion air within thecylinder, which is produced through the in-take port, also has to be tuned to the highrated speed. This was optimized with theaid of injection parameters obtained fromthe one-cylinder engine. The data of the fullload and the course of combustion of theengine at the nominal power output is rep-resented in Figure 8.

The engine achieves the expected nomi-nal power output of 34 kW at 6000 rpm. Itwas accomplished to decrease the con-sumption under the specifications in spiteof the unusually high rotational speedwithin a diesel engine. The injection pres-sure is approximately 800 bar, which at fullload leads to an injection period of 34 de-grees crank angle. The combustion startsafter an ignition delay of merely 10 degreescrank angle with a high homogeneous quo-ta although the injection holes for the bowl

diameter have a greater diameter.The increase in pressure of 7 bar per de-

gree crank angle is moderate, yet it leads incombination with the air-cooled cylindersto a definite combustion noise. The soundof the engine is insignificant relative to thepropeller noise. The cylinder peak pressurewas limited to 140 + 5 bar for the limitationof the component load.

The maximal speed of the combustionprocess is 8500 rpm, at which a balance be-tween the indicated performance and thefrictional power arises. The pressure andthe heating course at different speeds witha constant injection rate at a constant injec-tion begin, are shown in Figure 9.

The boost pressure increases simultane-ously with speed because of the tur-bocharger, which runs without exhaust gaspressure regulation (waste gate). Besidesthe higher exhaust back pressure, the timeof the mixture formation also shortenswith increasing speed. The realisable injec-tion rate decreases and the efficiency drops.It becomes clear that, dependant on the giv-en mixture formation, the direct injectionprocedure attains an optimal speed. Thisrotational speed can only be increased fur-ther through the loss of efficiency.

With the help of modern injection tech-nology it is possible to shift the decline ofefficiency towards a higher rotationalspeed. For a DI diesel engine the highspeeds of the spark-ignition engines arestill unattained which is lead back to the in-ternal mixture formation and the herewithlinked time. Furthermore it has to be point-ed out that the charging pressure andtherefore the rotational speed, typical of adiesel, within the lower speed ranges is notavailable with a conventional single-phased forced induction.

11 Cold Start

The compression ratio has to be lowered forthe high power density with the great boost-ing rate regarding the stability of the com-ponents. This leads to a massive reduction inthe cold-starting ability, which had to becompensated with the suitable actions.

The starting speed for these applicationslies far above the general speed for dieselengines because of the external starter. Thetemperature dependent mixture forma-tion-time prevents an ignition through thehigh starting speed. This can neither becompensated with the increase of the com-pression temperature, the heating of theengine, the low heat loss nor with the low-er leakage losses. It was only possible tostart the engine at an environmental tem-perature of 0 °C by decreasing the startingspeed to 500 rpm.

The evaporation of the fuel takes placenear the bowl-wall during the cold start.The liquid injection spray hits the bowl-wall with full impact where it is atomizedthrough the splashing. The shifting of theheating element from the conventional po-sition near to the injection nozzle to thespray cloud at the bowl-wall enabled thecrucial decrease of the cold cranking tem-perature. The heating element stands in aflat angle within the combustion chamberso it neither disturbs the charge motion northe combustion itself. The spray cloud getsinto direct contact with the hot zone of theheating element. The geometrical position-ing and the optimization for the minimalcold cranking temperature is demonstratedin Figure 10 in a qualitative way. The coldcranking temperature could finally bebought down to the target value of –32 °Cthrough a further increase of the heatingelement’s temperature and by using a heat-ing spiral within the air-supply.

12 Summary

The engine manufacturer Dr. SchrickGmbH has demonstrated the feasibility ofdesigning and developing a high-speed andhigh performance V2 engine with the smalldiesel engine TKDI 600. The engineeringteam worked in close collaboration withthe design, simulation, prototype manufac-turing and engine testing departments toachieve the targets in a short time span.The speed requirements for this applicationwere in excess of typical diesel engine val-ues. These high speeds were obtained bycareful attention to the mechanical andthermal design and optimizing the com-bustion system for higher speeds. In thiscase the high performance had to be real-ized over the rotational speed, which is rep-resented through the reduced efficiency.

The targets of performance, weight andconsumption were accomplished with aconsequent design of the engine regardingbest possible thermal and mechanical effi-ciency. The most important and essentialtechnical data of the engine with 600 cm3

displacement as well as the consumptionmap are summarized in Figure 11 andTable 3. The curve in Figure 11 correspondswith the configuration curve of the pro-pellers during the flight.

References

[1] Weinzierl, S.; Wildemann, R.; Hanula, B.: The Design and Development of a Light-Weight High-Speed Diesel Engine forUnmanned Aerial Vehicles. SAE 2002-01-0160

[2] Tafel, S.; Hanula, B.: Der kleine schnell-laufende Hochleistungs-Dieselmotor. HdT-Tagung Hochleistungs- und Rennmotoren,Essen, November 2002

MATERIALSTitanium