COMPRESSION IGNITION ENGINE OPERATION

1.8 COMPRESSION IGNITION ENGINE

OPERATION

In compression-ignition or diesel engines, air alone is drawn into the

cylinder during intake. The fuel (in most applications a light fuel oil, though

heated residual fuel is used in large marine and power-generating diesels) is

injected directly into each cylinder just before the combustion process is

required to start. Load control is achieved by varying the amount of fuel

injected each cycle; the airflow at a given engine speed is not directly

controlled, and in naturally-aspirated engines is essentially unchanged. There

are a great variety of CI engine designs in use in a wide range of applications

—automobile, truck, locomotive, marine, power generation. Both naturally aspirated

engines where atmospheric air is inducted, and boosted engines

where the inlet air is compressed by a turbocharger—an exhaust-driven

turbine-compressor combination—are common. Turbocharging increases

engine output by increasing the air mass flow per unit displaced volume,

thereby allowing an increase in fuel flow. These devices are used to reduce

engine size and weight for a given power output. In large engine sizes, the

two-stroke cycle is competitive with the four-stroke cycle because in these

large low-speed engines, the cylinder can be scavenged more effectively

and, with the diesel’s direct fuel injection, only air is lost in the scavenging

process.

The operation of a typical four-stroke naturally-aspirated CI engine is

illustrated in Fig. 1.24.

The compression ratio of diesels is much higher than

typical SI engine values, and is in the range 14 to 22, depending on the type

of diesel engine and whether the engine is naturally aspirated or

turbocharged. The valve timings used are similar to those of SI engines. In a

naturally-aspirated engine, air at close-to-atmospheric pressure is inducted

during the intake stroke and then compressed to a pressure of about 5 MPa

(50 atm) and temperature of about 900 K (600°C) during the compression

stroke. At about 20 crank angle degrees before TC, fuel injection into the

engine cylinder commences; a typical rate of injection profile is shown in

Fig. 1.24 b. Usually there are four to eight or more liquid fuel jets; each jet

exiting the injector nozzle atomizes into drops and entrains air to form a set

of sprays that penetrate into the bowl-in-piston combustion chamber, as

shown in Fig. 1.25. In each spray, the liquid fuel drops evaporate, and the

fuel vapor then mixes with the entrained air. The fuel-vapor air mixture

temperature and pressure are above the ignition point where fuel oxidation

chemistry can occur. Thus, after a short delay period, spontaneous ignition

(autoignition) of parts of the nonuniform fuel-air mixture within these sprays

initiates the combustion process, and the cylinder pressure (solid line in Fig.

1.24 c) rises above the non firing engine level as fuel chemical energy is

released. A diffusion flame then spreads rapidly to surround each fuel spray,

with partly reacted fuel-air mixture in the spray on the inside of the flame,

and the additional air in the cylinder on the outside. As the expansion process

proceeds, mixing between fuel vapor, air, and burning gases continues,

accompanied by further combustion ( Fig. 1.24 d). At full load, the mass of

fuel injected is about 5% of the mass of air in the cylinder. At higher fueling

levels, increasing amounts of black smoke in the exhaust limit the quantity of

fuel that can be burned efficiently. The exhaust process is similar to that of

the four-stroke SI engine: a rapid outflow or blowdown of burned gases as

soon as the exhaust valves start opening, followed by displacement of the

remaining burned gases from the cylinder during the exhaust stroke. At the

at the end of the exhaust stroke, the cycle starts again.

Figure 1.24 Sequence of events during compression, combustion, and

expansion processes of a naturally-aspirated compression-ignition engine

operating cycle. ( a) Cylinder volume/clearance volume V/Vc, ( b) rate of

fuel injection , ( c) cylinder pressure p (solid line, firing cycle; dashed

line, motored cycle), and ( d) rate of fuel burning (or fuel chemical energy

release rate) are plotted against crank angle.

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Figure 1.25 Schematic of diesel engine fuel sprays, formed from liquid

fuel jets injected at high pressure through individual injection nozzle holes,

penetrating the diesel bowl-in-piston combustion chamber.

In this diesel combustion process, the fuel is injected directly into the

engine cylinder at a pressure of between several hundred and more than 2000

bar. 25 The diesel fuel-injection system consists of a low-pressure pump, and

a high-pressure injection pump, delivery pipes, and fuel injector nozzles.

Several different types of injection pumps and nozzles are used. In one

common system shown in Fig. 1.26, an in-line pump containing a set of camdriven

plungers (one for each cylinder) operate in closely fitting barrels.

Early in the stroke of the plunger, the inlet port is closed and the fuel trapped

above the plunger is forced through a check valve into the injection line. The

injection nozzle ( Fig. 1.27) has one or more holes through which the fuel

sprays into the cylinder. A spring-loaded valve closes these holes until the

pressure in the injection line, acting on part of the valve surface, overcomes

the spring force and opens the valve. Injection starts shortly after the line

pressure begins to rise. Thus, the phasing of the pump camshaft relative to the

engine crankshaft controls the start of injection. Injection is stopped when the

inlet port of the pump is uncovered by a helical groove in the pump plunger,

because the high pressure above the plunger is then released ( Fig. 1.27,

bottom). The amount of fuel injected (which controls the load) is determined

by the injection pump cam design and the position of the helical groove. Thus

for a given cam design, rotating the plunger and its helical groove varies the

load.

Figure 1.26 Diesel fuel system with in-line fuel-injection pump.25

(Courtesy Robert Bosch GmbH and SAE.)

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Figure 1.27 Details of fuel-injection nozzles and fuel-delivery control.26

(Courtesy Robert Bosch GmbH and SAE.)

In smaller diesel engines, distributor-type fuel pumps are often used.

These have one pump plunger and barrel, which meters and distributes the

fuel to all the injection nozzles. The unit contains a high-pressure injection

pump, an overspeed governor, and an injection timer. High pressure is

generated by the plunger, which is made to describe a combined rotary and

stroke movement. This rotary motion distributes the fuel to the individual

injection nozzles. Distributor pumps can operate at higher speed and take up

less space than in-line pumps. They are normally used on smaller three- to

six-cylinder diesel engines. In-line pumps are used in larger, midsize

engines.

An alternative fuel-injection approach uses individual single-barrel

injection pumps, close mounted to each cylinder with an external drive.

These unit injector systems (UIS) combine the pump and injector into a single

unit. Figure 1.28 illustrates how this system operates. The unit is driven by

the engine camshaft, and the start of injection and injected fuel quantity are

controlled by a solenoid valve in the injector. With this type of system, very

high injection pressures (some 2000 bar) are achieved along with precise

control of the amount and timing of injection.

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Figure 1.28 Diesel fuel-injection system with unit injector (for passenger

cars). (1) Fuel tank with fuel supply pump; (2) Fuel cooler; (3) Electronic

control unit; (4) Fuel filter; (5) Fuel feed line; (6) Fuel return line; (7)

Tandem pump; (8) Fuel-temperature sensors; (9) Glow plug; (10) Injector. 26

( Courtesy Robert Bosch GmbH and SAE.)

Common rail (or fuel accumulation) injection systems allow greater

freedom to control the fuel-injection process, and thus combustion. The

functions of fuel pressure generation and fuel injection are separated by an

accumulator or common rail. Figure 1.29 shows the system layout. The high pressure pump feeds the common rail, which feeds each of the injectors.

Control of this system can readily be integrated with other engine parameters

as indicated. Injection pressures of 1400 to 1800 bar can be achieved. By

repeated activation of the fast-acting solenoid valve within the injector,

multiple injection pulses in each injection cycle can be realized. It is often

advantageous to use a short pilot injection before the main injection, to

initiate combustion with a small amount of fuel, and reduce engine noise.

Multiple pulse main injections can help control emissions. Both solenoid controlled

injectors and piezo-actuated injectors are used. Figure 1.30 shows

a piezoelectric injector that provides more precise control of injection pulses

and improves fuel atomization within the cylinder.

Figure 1.29 Common rail accumulator fuel-injection system on a fourcylinder

automobile diesel.25 (1) Air-mass flow meter; (2) Electronic control

unit; (3) High-pressure pump; (4) High-pressure accumulator (rail); (5)

Injectors; (6) Crankshaft speed sensor; (7) Coolant temperature sensor; (8)

Fuel filter; (9) Accelerator-pedal sensor. ( Courtesy Robert Bosch GmbH and SAE.)

Figure 1.30 Bosch piezoelectric fuel injector that provides more precise

fuel injection control on opening and closing, and finer atomization.27

(Courtesy Robert Bosch GmbH and SAE.)

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