DIFFERENT TYPES OF DIESEL ENGINES | TWO-STROKE CYCLE ENGINE OPERATION

1.9 DIFFERENT TYPES OF DIESEL ENGINES

A large number of diesel engine configurations and designs are in common

use. The largest marine and stationary power-generating diesels are two stroke

cycle engines and are discussed in the next section, 1.10. Small- and

medium-size diesel engines use the four-stroke cycle. Because air capacity is

an important constraint on the amount of fuel that can be burned in the diesel

engine, and therefore on power, turbocharging is used extensively. Larger

engines are almost always turbocharged. Small low-cost diesels are not

usually turbocharged. The details of the engine design also vary significantly

over the diesel size range. In particular, different combustion chamber

geometries and fuel-injection characteristics are required to deal effectively

with a major diesel engine design problem: achieving sufficiently rapid fuel air

mixing rates to complete the fuel-burning process in the short time

available. Smaller engines run at higher maximum speeds. Thus, a wide

variety of inlet port geometries, cylinder head and piston cavity or bowl

shapes, and fuel-injection patterns are used to achieve the airflows and fuel

flows needed to accomplish fast enough combustion over the diesel size

range.

Figure 1.31 shows a diesel engine typical of the heavy-duty truck

application. The design shown is a six-cylinder in-line engine. The drawing

indicates that diesel engines are generally substantially more rugged and

heavier than SI engines because stress levels are higher due to the

significantly higher pressure levels of the diesel cycle. The engine shown has

a displacement of 12.9 liters, a compression ratio of 16.5, and is

turbocharged. This type of diesel is called a direct-injection diesel since the

fuel is injected directly into a combustion chamber above the piston crown.

The combustion chamber shown is a “bowl-in-piston” design, which puts

most of the clearance volume into a compact cavity in the piston crown. With

this type of diesel engine, it is often necessary to use a swirling airflow

rotating about the cylinder axis, which is created by suitable design of the

inlet port and valve, to achieve fast enough fuel-air mixing and fuel-burning

rates. The fuel injector, shown on the cylinder axis in the drawing, has a

multihole nozzle. It typically has four to six holes in this application. The fuel

jets move out radially from the nozzle holes close to the center of the piston

bowl into the (swirling) airflow. Figure 1.31 Six-cylinder 12.9-liter turbocharged direct-injection truck diesel engine: transverse and longitudinal sections. Bore 135 mm, stroke 150

mm, compression ratio 16:1, governed speed 2100 rev/min, maximum output

294 kW, maximum torque 1667 N · m at 1300 rev/min.

Figure 1.32 shows a 2.2-liter four-cylinder high-speed direct-injection

(HSDI) diesel is typical of those used in automobiles. It has four valves per

cylinder, with the fuel injector and bowl-in-piston combustion chamber

centered on each cylinder axis. These types of engines are highly boosted to

give high torque at low- to mid-engine speeds per unit displaced cylinder

volume. The compressor exit air, cooled in a heat exchanger, enters the

cylinder via intake ports that generate swirl about the cylinder axis. During

the compression stroke, the flow of this swirling air into the reentrant bowlin-

piston significantly enhances the swirl, thereby achieving high rates of

mixing of air with the injected fuel sprays close to the top center. Maximum rated

speeds (4500 to 5000 rev/min) are lower than maximum speeds of gasoline

SI engines due to fuel-air mixing rate limitations. The compression ratio of

these HSDI engines is 18 to 20:1, somewhat above the value that gives

maximum engine efficiency. This is done to obtain higher air compression

temperatures to enable cold engines starting at low cranking speeds. An

electrically heated ceramic glow plug is usually inserted through the cylinder

head in these size engines to assist the cold starting process.

Figure 1.32 Small 2.2-liter four-cylinder high-speed direct-injection

automobile diesel engine: maximum power 93 kW; maximum torque 285 N ·

m at 1750 rev/min; maximum boost 0.9 bar (gauge); compression ratio 18.5.

The smallest diesels operate at higher engine speeds than do larger

engines: hence the time available for burning the fuel is less and the fuel injection

and combustion systems must achieve faster fuel-air mixing rates.

Figure 1.32 shows how this rapid combustion is realized in HSDI diesels.

Historically, in the smallest sizes diesel engines, this can be accomplished by

using an indirect-injection or prechamber type of diesel. Fuel is injected into

an auxiliary combustion chamber that is separated from the main combustion

chamber above the piston by a passageway or nozzle. During the latter stages

of the compression process, air is forced through this nozzle from the

cylinder into the prechamber at high velocity. Fuel is injected into the highly

turbulent and often rapidly swirling flow in this prechamber, and very high

fuel-air mixing rates are achieved. Combustion starts in the prechamber, and

the resulting pressure rise in the prechamber forces burning gases, fuel, and

air into the main chamber. Since this outflow is also extremely vigorous,

rapid mixing then occurs in the main chamber as the burning jet exiting the

prechamber mixes with the remaining air and combustion is completed. A

glow plug is also shown in the auxiliary chamber; this plug is electrically

heated during cold engine start-up to raise the temperature of the air charge

and the fuel sufficiently to achieve autoignition. The compression ratio of this

the engine is high—around 20. Indirect-injection diesel engines require higher

compression ratios than direct-injection engines to start adequately when

cold.

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1.10 TWO-STROKE CYCLE ENGINE OPERATION

The two-stroke engine is used at the small-size and very large-size ends of

the engine market. In small sizes, the two-stroke cycle SI engine is cheap,

compact and light, simple, and robust. This is the basis of its market appeal

in mopeds, scooters, motorcycles, and snowmobiles, in portable devices

such as chain-saws and brushcutters, in agricultural and construction devices

such as lawn mowers, disc saws, and snow blowers, in the outboard marine

engine arena, and in light and in remotely piloted aircraft. The very large

diesel engines used in marine and power-generation applications are also

two-stroke cycle engines. These large internal combustion engines are the

most efficient and cost effective prime movers currently available. The two stroke

diesel has also been used in the locomotive and in parts of the truck

market. 29,30 The passenger-car and truck engine markets are now, however,

dominated by four-stroke cycle engines. Key operating features of the two-stroke cycle

are its power stroke every crankshaft revolution and its scavenging of the burned gases from the engine cylinder with fresh charge. Doubling the number of power strokes per unit

time relative to the four-stroke cycle increases the power output per unit

displaced volume. It does not, however, increase by a factor of two. The

outputs of two-stroke engines range from only 20% to 60% above those of

equivalent-size four-stroke units. This lower increase in practice is a result

of the poorer than ideal charging efficiency, that is, incomplete filling of the

cylinder volume with fresh air due to incomplete scavenging of the residual

burned gases. Doubling the number of power strokes per unit time also

halves the intervals between combustion-generated pressure impulses. This

results in a smoother crankshaft torque versus time profile.

The two-stroke cycle’s process of scavenging the burned gases from the

engine cylinder with fresh charge—its gas exchange process—has several

consequences. First, charging losses are inevitable. Under higher load

conditions, in a typical small two-stroke engine, some 20% or more of the

fresh charge that enters the cylinder is lost due to short-circuiting to the

exhaust. When the fuel is mixed with air prior to cylinder entry, this process

results in very high hydrocarbon emissions and poor fuel consumption

compared with the four-stroke cycle engine. However, as both exhaust and

charging occur around BC, the exhaust and intake ports can be situated near

the bottom end of the cylinder and can be covered and uncovered by a long skirt

piston (see Fig. 1.33). This simple geometric two-stroke cycle

configuration obviates the need for valves and their actuating gear. It also

substantially simplifies the engine structure and the production process, and

significantly reduces engine cost.

Figure 1.33 Cylinder-pressure versus cylinder-volume trace for a two stroke

cycle engine cylinder. Exhaust and transfer or scavenge ports are

uncovered by the piston as shown. In the two-stroke engine cycle, the compression, combustion, and expansion processes are similar to the equivalent four-stroke cycle

processes; it is the intake and exhaust processes that are different ( Fig.

1.33). The sequence of events in a port-scavenged two-stroke engine is

illustrated in

Fig. 1.34. In such engines, both exhaust and the scavenging (or

transfer) ports are at the same end of the cylinder and are uncovered as the

piston approaches BC ( Fig. 1.34). After the exhaust ports open, the cylinder

pressure falls rapidly as burned gases flow out of the cylinder into the

exhaust system, in a blowdown process as shown. The scavenging or transfer

ports then open, and once the cylinder pressure p falls below the scavenging

pressure pi, fresh charge flows into the cylinder. Burned gases, displaced by

this fresh charge, continue to flow out of the exhaust port (along with some of

the fresh charge). Once the ports close as the piston starts the compression

stroke, compression, fuel-injection and fuel-air mixing in direct-injection

engines, combustion, and expansion processes proceed as in the equivalent

four-stroke engine cycle. 29, 30 (Fresh charge is fuel vapor and air in engines

where fuel and air are “premixed” before entry into the cylinder: the fresh

charge is air if direct fuel injection is used.) Figure 1.34 Sequence of events during expansion, gas exchange, and

compression processes in a loop-scavenged two-stroke cycle compression ignition

engine. Cylinder volume/clearance volume V/Vc, cylinder pressure

p, exhaust port open area Ae, and intake port open area Ai are plotted against

crank angle.

Figure 1.35 shows how this two-stroke cycle is realized in a small

gasoline engine.

This is an experimental single-cylinder crankcase scavenged

engine, with a pneumatic direct fuel injection system to avoid high

hydrocarbon emissions and achieve good fuel consumption. Air flows into

the crankcase, through Reed valves, as the piston travels up the cylinder. This

air is compressed as the piston travels down. Air flows into the cylinder

once the transfer or scavenging ports, which connect the crankcase to the

cylinder, are uncovered. Fuel is injected after the transfer ports have been

closed off. This engine employs a novel combustion approach—often called

activated radical combustion. By retaining the appropriate amount of burned

residual gas within the cylinder by restricting the flow out of the exhaust

ports with a throttle valve, the in-cylinder unburned gas temperature at the

the end of compression can be raised and controlled. This in-cylinder mixture

can then be made to autoignition at the appropriate point in the cycle—at the

end of compression, just before TC. This spontaneous autoignition of the

complete well-mixed fuel vapor, air, and burned residual in-cylinder charge

is an alternative combustion process to the standard spark-ignited or diesel

combustion processes. It is often called homogeneous charge compression

ignition (HCCI) or controlled autoignition (CAI). This HCCI combustion

the concept is also being developed for four-stroke cycle engines. At high

outputs, when effective scavenging of the burned gases becomes important,

this two-stroke cycle engine reverts to the normal spark-ignition engine flame

propagation process.

Figure 1.35 Honda experimental two-stroke cycle loop-scavenged engine

with pneumatic direct in-cylinder fuel injection, employing an activated

radical combustion process—controlled autoignition of the in-cylinder fuel vapor,

air, burned residual mixture just before top center. 402 cm 3

displacement, bore and stroke 80 mm, trapped compression ratio 6:1.

Generates 33 kW at 6900 rev/min

Diesels, in the very large size engines, used for marine propulsion and

electrical power generation, also operate using the two-stroke cycle. Figure

1.36 shows such a two-stroke cycle marine engine, available with 4 to 12

cylinders, with a maximum bore of 0.6 to 0.9 m and stroke of 2 to 3 m, which

operates at speeds of about 100 rev/min. These engines are normally of the

crosshead type shown to reduce piston side forces on the cylinder. The gas exchange process is initiated by opening the exhaust valve in the cylinder

head, followed by the piston uncovering the transfer ports at the lower end of

the cylinder liner. The expanding exhaust gases leave the cylinder via the

exhaust valve and manifold, and pass through the turbocharger turbine.

Compressed air enters the cylinder via the transfer ports, continuing the

scavenging process; the air is supplied from the turbocharger compressor and

intercooler. At part load, electrically driven blowers cut in to compress the

scavenge air. Because these large engines operate at low speed, the motion

induced by the injected fuel jets is sufficient to mix the fuel with air and burn

it in the time available. Thus, a simple open combustion chamber shape can

be used, which achieves efficient combustion even with the low-quality

heavy fuels used with these types of engines. The pistons are water cooled in

these very large engines. The splash or jetted oil piston cooling used in

small- and medium-size diesels is not adequate. Figure 1.36 Cross-section of an

IHI-Sulzer uniflow-scavenged large two stroke

cycle turbocharged diesel engine, developing 1590 kW per cylinder at

127 rev/min. Stroke to bore ratio about 2:5.

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