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COMBUSTION IN

COMPRESSION-

IGNITION

ENGINES

R. Adhitya Gugan (08M130)

Rohan Bayan (08M133)

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BASIC DIFFERENCES BETWEEN SI AND CI ENGINES

Description SI Engine CI Engine

Basic Cycle

Otto cycle (heat addition at

constant volume)

Diesel cycle (heat addition at

constant pressure)

Fuel

Gasoline, a high volatile fuel.

Self-ignition temperature is

high.

Diesel oil, a non-volatile fuel.

Self-ignition temperature is

comparatively low

Introduction of

fuel

Fuel introduced during suction

stroke. A carburetor and an

ignition system are necessary.

Modern engines have gasoline

injection.

Fuel introduced at end of

compression stroke. A fuel

pump and an injector are

necessary.

Load Control Throttle controls the quantity

of fuel-air mixed introduced.

The quantity of fuel is

regulated. Air quantity is not

controlled.

Ignition

Requires an ignition system

with spark plug in the

combustion chamber. Primaryvoltage is provided by either a

battery or a magneto.

Self-ignition occurs due to high

temperature of air because of

the high compression. Ignitionsystem and spark plug are not

necessary.

Compression Ratio 6 to 10 (Upper limit is

constrained by antiknocking

property of the fuel).

16 to 20 (Upper limit is limited

by weight increase of the

engine).

Speed High speed due to

homogeneous combustion.

Low speed due to

heterogeneous combustion.

Thermal Efficiency Maximum value of efficiency

that can be obtained is lower.

Maximum value of thermal

efficiency that can be obtained

is higher.

Weight Lighter due to lower peak

pressures.

Heavier due to higher peak

pressures.

Remains close to

stoichiometric value from no

Irrespective of load, at any

given speed, an approximately

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Air Fuel Ratio load to full load. constant supply of air enters

the cylinder.

The overall air-fuel ratio varies

from about 18:1 at full load to

about 80:1 at no load.

The air-fuel ratio should be as

close to stoichiometric as

possible while operating at full

load.

Always designed to operate

with excess air of 15% to 40%

depending upon the

application.

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FUEL JET AND COMBUSTION PRODUCTS

CHARRACTERISTICS

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PRINCIPLE OF COMBUSTION

Only air is compressed through a high compression ratio raising its

temperature and pressure to a high value.

Fuel is injected through one or more jets into this highly compressed air in

the combustion chamber.

Here, the fuel jet disintegrates into a core of fuel surrounded by a spray

envelope of air and fuel particle.

This spray envelope is created both by the atomization and vaporization of

the fuel.

The turbulence of the air in the combustion chamber passing across the jet

tears the fuel particles from the core.

A mixture of air and fuel forms at some location in the spray envelope and

oxidation starts

In CI engine fuel-air mixture is heterogeneous, so if the air within the cylinder

were motionless under these conditions, there will not be enough oxygen in

Shadow graph

Backlit Photo

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the burning zone and burning of the fuel would be either slow or would totally

fail as it would be surrounded by its own products of combustion.

Hence an orderly and controlled movement must be imparted to the air and

the fuels so that a continuous flow of fresh air is brought to each burning

droplet and the products of combustion are swept away.

This air motion is called the air swirl.

Due to this swirl intermixing of the burned and unburned portions of the

mixture also takes place.

A glow plug may be used to aid cold starting.

WORKING OF A CI ENGINE

The diesel internal combustion engine differs from the gasoline powered Otto cycle by usinghighly compressed, hot air to ignite the fuel rather than using a spark plug (compression ignitionrather thanspark ignition).

In the true diesel engine, only air is initially introduced into the combustion chamber. The air isthen compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar(4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa) (about 200 psi) in thepetrol engine. This high compression heats the air to 550 C (1,022 F).

At about the top of the compression stroke, fuel is injected directly into the compressed air in thecombustion chamber. This may be into a (typically toroidal) void in the top of the piston or apre-chamberdepending upon the design of the engine. The fuel injector ensures that the fuel is broken down into small droplets, and that the fuel is distributed evenly. The heat of thecompressed air vaporizes fuel from the surface of the droplets. The vapor is then ignited by theheat from the compressed air in the combustion chamber, the droplets continue to vaporize fromtheir surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. The startof vaporization causes a delay period during ignition, and the characteristic diesel knocking

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sound as the vapor reaches ignition temperature and causes an abrupt increase in pressure abovethe piston. The rapid expansion of combustion gases then drives the piston downward, supplyingpower to the crankshaft.

Model aeroplane engines use a variant of the Diesel principle but premix fuel and air via acarburation system external to the combustion chambers.

As well as the high level of compression allowing combustion to take place without a separateignition system, a high compression ratio greatly increases the engine's efficiency. Increasing thecompression ratio in a spark-ignition engine where fuel and air are mixed before entry to thecylinder is limited by the need to prevent damaging pre-ignition. Since only air is compressed ina diesel engine, and fuel is not introduced into the cylinder until shortly before top dead centre(TDC), premature detonation is not an issue and compression ratios are much higher.

Diesel engines have several advantages over other internal combustion engines:

They burn less fuel than a petrol engine performing the same work, due to theengine's higher temperature of combustion and greater expansion ratio. Gasolineengines are typically 25 percent efficient while diesel engines can convert over 30percent of the fuel energy into mechanical energy.

They have no high-tension electrical ignition system to attend to, resulting in highreliability and easy adaptation to damp environments. The absence of coils, spark

plug wires, etc., also eliminates a source of radio frequency emissions which caninterfere with navigation and communication equipment, which is especiallyimportant in marine and aircraft applications.

They can deliver much more of their rated power on a continuous basis than a petrolengine.

The life of a diesel engine is generally about twice as long as that of a petrol enginedue to the increased strength of parts used. Diesel fuel has better lubricationproperties than petrol as well.

Diesel fuel is considered safer than petrol in many applications. Although diesel fuelwill burn in open air using a wick, it will not explode and does not release a largeamount of flammable vapor. The low vapor pressure of diesel is especially

advantageous in marine applications, where the accumulation of explosive fuel-airmixtures is a particular hazard. For the same reason, diesel engines are immune tovapor lock.

For any given partial load the fuel efficiency (mass burned per energy produced) of adiesel engine remains nearly constant, as opposed to petrol and turbine engines whichuse proportionally more fuel with partial power outputs.

They generate less waste heat in cooling and exhaust.

http://en.wikipedia.org/wiki/Vapor_lockhttp://en.wikipedia.org/wiki/Vapor_lock

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1 00 0 1 40 0 1 80 0 2 20 0 2 60 0 3 00 00

1

2

3

4

5

6

1 00 0 1 40 0 1 80 0 2 20 0 2 60 0 3 00 00

1

2

3

4

5

6

1 00 0 1 40 0 1 80 0 2 20 0 2 60 0 3 00 00

1

2

3

4

5

6HC/CO

With a diesel, boost pressure is limited only by the strength of the enginecomponents, not predetonation of the fuel charge as in petrol engines.

The carbon monoxide content of the exhaust is minimal; therefore diesel engines areused in underground mines.[29]

Biodiesel is an easily synthesized, non-petroleum-based fuel (through

transesterification) which can run directly in many diesel engines, while gasolineengines either need adaptation to run synthetic fuels or else use them as an additive togasoline (e.g., ethanol added to gasohol), making diesel engines the clearly preferredchoice forsustainability.

COMBUSTION DEVELOPMENT

LOW TEMPERATURE COMBUSTION

1000 1400 1800 2200 2600 3000

Temperature, K0 1 2 3 4 5 6Equivalence Ratio

Toward LTCConventionalCombustionLow NOx/Soot Reg

FUEL SPRAY FUEL VAPOURREVERSE SQUISH

SOOT

SWIRL

DIRECTION

FUEL FILM ON WALL

SPRAY ON BOWL LIP

http://en.wikipedia.org/wiki/Biodieselhttp://en.wikipedia.org/wiki/Transesterificationhttp://en.wikipedia.org/wiki/Synthetic_fuelhttp://en.wikipedia.org/wiki/Ethanolhttp://en.wikipedia.org/wiki/Gasoholhttp://en.wikipedia.org/wiki/Sustainabilityhttp://en.wikipedia.org/wiki/Biodieselhttp://en.wikipedia.org/wiki/Transesterificationhttp://en.wikipedia.org/wiki/Synthetic_fuelhttp://en.wikipedia.org/wiki/Ethanolhttp://en.wikipedia.org/wiki/Gasoholhttp://en.wikipedia.org/wiki/Sustainability

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TYPES OF DIESEL COMBUSTION SYSTEMS

Direct Ignition System

Engine cylinder has a single open combustion chamber into which fuel is directly

injected

Used for the largest sized engines, where mixing rate requirements are least

stringent

Momentum and energy of the inlet fuel are sufficient to achieve adequate fuel

distribution and rates of mixing with the air

Additional organized air motion is not required

As engine size reduces, increasing amounts of air swirl are used to achieve

faster air-fuel mixing rates

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Indirect Ignition System

Cylinder chamber is divided into two regions and the fuel is injected into the

pre-chamber which is connected to the main chamber (located above the

piston crown) via a nozzle, or one or more orifices.

Used in the smallest sized engines such as automobile engines, where fuel-air

mixing rates have to be high.

The vigorous charge motion required during fuel injection is generated duringcompression stroke.

During compression, air is forced from the main chamber into the auxiliary

through the nozzle or orifice(s). Thus, a vigorous flow in the auxiliary chamber is

set up toward the end of compression stroke.

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Fuel is injected into the auxiliary chamber at lower injection-system pressure

than in DI systems through a pintle nozzle as a single spray.

Combustion starts in the auxiliary chamber: the pressure rise associated with

combustion forces the fluid back into the main chamber where the jet issuing

from the nozzle entrains and mixes with the main chamber air.

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COMBUSTION PROCESS

Direct Injectionquiescent chamber

Direct Injectionmulti-hole nozzleswirl in chamber

Direct Injectionsingle-hole nozzleswirl in chamber

Indirect injectionswirl pre-chamber

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CHARACTERISTICS OF A CI PROCESS

Graph showing the fuel injection flow rate, net heat release rate and cylinder

pressure for a direct injection CI engine.

0.4 ms after ignition 2.5 ms after ignition

3.2 ms after ignition Late in combustion process

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Shaded area represents the objectionable smoke area at approximate region

of A/F ratios

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COMPARISON OF FUEL CONSUMPTIONDIESEL & GASOLINE @ 2500 r/min

BSFC vs BMEP

150

200

250

300

350

400

450

500

550

600

0 2 4 6 8 10 12 14 16

BMEP, bar

BSFC,g/kWh

Gasoline

DI-Diesel

MAXIMUM SPECIFIC POWER & BM

COMPARISONGASOLINE vs DIESEL

0

5

10

15

20

25

30

35

40

45

50

0 1000 2000 3000 4000 5000 6000 7000

SPEED, r/mi

POWER,kW/L&

BMEP,bar

BMEP-GAS

POWER-GAS

POWER-DIESEL

BMEP-DIESEL