DIFFERENT TYPES OF FOUR-STROKE SI ENGINES

1.7 DIFFERENT TYPES OF FOUR-STROKE SI ENGINES

A variety of SI engines are used in practice, depending on the application.

Small SI engines are used in many applications: in the home (e.g., lawn

mowers, chain saws), in portable power generation, as outboard motorboat

engines, and in motorcycles. These are often single-cylinder engines

producing a few kW of power. In the above applications, light weight, small

bulk, and low cost in relation to the power generated are the most important

characteristics; fuel consumption, engine vibration, and engine durability are

less important. A single-cylinder engine gives only one power stroke per

crank revolution (two-stroke cycle) or two revolutions (four-stroke cycle).

Hence, the torque pulses are widely spaced, and engine vibration and

smoothness are significant problems.

Multi Cylinder engines are invariably used in automotive practice. As

rated power increases, the advantages of smaller cylinders in regard to size,

weight, power density, improved engine balance, and smoothness point

toward increasing the number of cylinders per engine: see Sec. 1.5 .

Multi Cylinder SI engines (in the power range 25 to 400 kW) are used in cars,

light trucks, vans, light-duty commercial vehicles, and in stationary

applications to produce mechanical and electrical power. Many of these

markets are shared with diesel (CI) engines. Low engine emissions and high

operating efficiency are important, especially in these transportation

applications. Precise control of fuel and airflow is critical to achieving these

objectives. What used to be the dominant fuel metering device—the

carburetor—has been superceded by electrically controlled fuel injection

into each intake port. Now, injection of the gasoline directly into each engine

the cylinder is coming into production ( Fig. 1.7). Each of these technology steps

improves control of the amount of fuel entering each cylinder per cycle, and

thus the dynamic response of the engine to changes in load (engine output)

and speed.

The work transfer per cycle to each piston depends on the amount of fuel

burned per cylinder per cycle, which depends on the amount of fresh air

inducted each cycle. Variable valve control over the engine’s speed range

can be used to increase the mass of air inducted into each cylinder in four stroke

SI engines (especially at low and high speed), and thus increase the

wide-open-throttle torque and power. Engine output from a given

displacement engine can be increased by boosting—increasing the density of

the air supplied to the engine intake by compressing atmospheric air. Thus,

compressing the air prior to entry into the cylinder with a supercharger or a

turbocharger increases the output from a given displacement engine.

Examples of various types of SI engines in practical use follow to provide

the context for reviewing critical engine processes, a primary objective of

this text.

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1.7.1 Spark-Ignition Engines with Port Fuel Injection

Spark-ignition (SI) engines have traditionally been operated with a premixed

fuel vapor/air mixture inside the cylinder, prepared by feeding liquid

gasoline into the engine intake. Carburetors were used to meter the fuel flow

in proportion to the airflow. This technology has largely been replaced by

port fuel injection (see Fig. 1.8) where an injector in each cylinder’s intake

port or manifold injects a pulsed fuel spray toward the intake valve, once per

cycle. The hot valve surface and warm intake port (once the engine has

warmed-up) promote rapid evaporation of the liquid fuel, and the airflow

through the port(s) past the intake valve(s) and into the cylinder, coupled

with the in-cylinder flow and mixing with the hot residual gas, produces a

nearly homogeneous mixture by the time combustion starts. Here, we show

some examples of SI engines with this method of mixture preparation.

Figure 1.10 shows a small single-cylinder air-cooled SI engine with a

displaced volume of 149 cm3 and power output of 2.8 kW (3.9 hp). The

objective of such simple construction SI engines is to produce modest power

levels at low cost. A primary benefit of air-cooled, as compared to the

water-cooled, engines are lower engine weight. The fins on the cylinder block

and head are necessary to increase the external heat-transfer surface area to

achieve the required heat rejection. In small engines, such as in Fig. 1.10,

natural convection promotes adequate airflow around the outside of the

engine. In larger engines, an air blower provides forced air convection over

the block. The blower is driven off the driveshaft.

Figure 1.10 Cutaway drawing of single-cylinder air-cooled spark-ignition

engine. Displacement 149 cm 3, bore 65 mm, stroke 45 mm, compression

ratio 9.2, maximum power 2.8 kW at 3000 rev/min. ( Courtesy Kohler Co.)

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Figure 1.11 shows a turbocharged automobile engine that incorporates

many of the features now used to improve engine performance and efficiency.

The in-line arrangement with four cylinders provides a compact block, and

when turbocharged, increases the power per unit engine displaced volume

significantly. This engine features all-aluminum construction, four valves per

cylinder, dual overhead camshafts, friction reducing roller finger followers

in the valve train, variable phasing on each cam to control the relative

phasing of intake and exhaust valves, and piston-cooling oil jets to control

piston temperatures in this high-performance engine. Other performance

enhancing features now being designed into such automobile engines are

cylinder cut out (or displacement on demand) where, for example, in a V-8

engine, at the lighter loads, the valves in half the cylinders are deactivated so

only four cylinders provide torque. This reduces the pumping work over the

exhaust and intake strokes and thereby improves engine fuel consumption.

Figure 1.11 Cutaway drawing of General Motors four-cylinder

turbocharged DI Ecotec gasoline spark-ignition engine. Displacement 2.0

liters, bore 86 mm, stroke 86 mm, compression ratio 9.2, maximum power

187 kW (250 hp) at 5300 rev/min, maximum torque 353 N · m (260 lb · ft) at

2000 rev/min. 14 ( Courtesy General Motors Corporation.)

Variable valve control improves engine performance, efficiency, and

emissions. The simpler systems used vary the relative

phasing of the intake and exhaust valve opening and closing by rotating the

camshafts relative to the crankshaft. Valve lift profiles and open duration

remain fixed. This is the approach used in the engine shown in Fig. 1.11.

(The cam-phasing system is apparent in the upper center of the engine drawing.)

More sophisticated approaches vary valve timing, lift profile, and open

duration (e.g., BMW’s Valvetronic system 15). This technology can eliminate

the need for throttle valves by accurately controlling the cylinder charging

process by intake valve control

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1.7.2 SI Engines for Hybrid Electric Vehicles

The use of internal combustion engines in automotive hybrid propulsion

systems is prompting additional SI engine developments. In such a hybrid

system, an internal combustion engine, a generator, battery, and electric

motors are combined. Figure 1.12 shows three categories of hybrid systems: a

parallel hybrid, a series hybrid, and a power split hybrid. In the parallel

approach, the engine can drive the wheels directly, the battery can drive via

the electric motor, or both can be combined to drive the wheels to obtain a

high overall propulsion system efficiency at all loads and speeds.

Figure 1.12 Diagrams of parallel, series, and power split hybrid electric

vehicle propulsion systems.

In the series approach, the electric motor drives the vehicle’s wheels. The

engine can drive through the generator and motor, or recharge the battery via

the generator. Since the vehicle is propelled solely by electrical energy, the

the engine is not coupled to the wheels. Thus its operating conditions are not

dependent on the vehicle’s operation so it can be operated in its higher

efficiency modes. In the power-split system, a planetary gear set is used to

transmit power from the engine. This arrangement allows both parallel and

series-type operation to be combined. Power from the engine can flow

directly to the wheels via the ring of the planetary gear system. Engine power

can also flow through the generator, producing electrical power that can

drive the wheels through the electric motor.

These hybrid propulsion systems provide increased vehicle drive

efficiency relative to direct internal combustion engine drive for three basic

reasons. First, regenerative braking—applying a braking torque by

connecting the generator to the vehicle’s wheels is then used to recharge the

battery—converts a substantial fraction of the vehicle’s kinetic energy as the

vehicle slows down to store electrical energy. Second, when the engine is

being used, it can operate much of the time at a higher efficiency than would

be the case with a stand-alone engine vehicle propulsion system. Third, the

battery electric drive mode allows the engine to be shut down when the

vehicle is decelerating or idling.

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Figure 1.13 shows an SI engine designed specifically for this application.

The engine employs a modified version of the four-stroke cycle called the

Atkinson cycle, where the volume ratio used for expansion is higher than the

volume ratio for compression. The engine shown has a displaced volume of

1.5 liters, a geometric (TC to BC) compression/expansion ratio of 13:1, and

uses variable valve timing with late intake valve closing during compression

and late exhaust valve opening during expansion to achieve a higher effective

expansion than compression. This increases engine efficiency. Maximum

engine speed is held to 5000 rev/min to minimize the pumping penalties of

this Atkinson cycle approach

Figure 1.13 Four-cylinder Toyota spark-ignition engine designed for a

hybrid electric automobile propulsion system.16 This 1.5-liter (bore = 75

mm, stroke = 84.7 mm), four valves per cylinder, variable valve timing

engine uses the Atkinson cycle with a geometric compression/expansion ratio

of 13:1. Maximum power is 57 kW (76 hp) at 5000 rev/min. Valve timings

are: intake opening 18 to –15° BTC, closing 72 to 105° ABC; exhaust

opening 34° BBC, closing 2° ATC.

An alternative to this hybrid electric vehicle (HEV) system, which overall

is powered solely by a fuel such as gasoline, is the plug-in hybrid (PHEV)

system. Here a larger battery, with some 10 to 30 mile (15 to 50 km) all

electric driving range rather than the electric range of a few miles of the HEV

system, is used that can be recharged from the electrical grid. Thus the PHEV

can be driven with electricity or with a hydrocarbon fuel similarly to an

HEV. There is an important but different role for SI engines (and potentially

diesels) to play as a key component of these more efficient hybrid systems:

the engine preserves the driving flexibility that vehicles require, as the

electrification of propulsion systems continues to evolve.

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1.7.3 Boosted SI Engines

The work transfer to each piston per cycle that can be obtained from a given

a displacement engine determines the amount of torque the engine can deliver.

This work transfer depends on the amount of fuel that can be burned in each

cylinder each cycle. This depends on the amount of fresh air that is inducted

into each cylinder each cycle. Increasing the air density prior to its entry into

each cylinder thus increases the maximum torque that an engine of a given

displacement can deliver. This can be done with a supercharger, a

compressor mechanically driven by the engine. More often it is done with a

turbocharger, a compressor-turbine combination, which uses the energy

available in the engine exhaust stream to provide via the turbine the power

required to compress the intake air.

Figure 1.14 shows a cutaway drawing of a turbocharged automobile SI

engine, which illustrates how the turbocharger connects with the engine’s

cylinders. The airflow passes through an air filter (1) into a centrifugal

compressor (2) where the radially outward flowing air is compressed by the

rotating varies. Next the air flows through an intercooler (3) to reduce the

compressed air temperature (further increasing its density), through the intake

manifold (4) into the intake port where the fuel is injected, past the intake

valve (5), and into the cylinder (6). When the exhaust valve (7) opens, the hot

and higher-than-atmospheric pressure exhaust gas flows through the valve

and exhaust manifold (8) into the turbine (9). The exhaust gas is directed

radially inward and circumferentially at high velocity by vanes (nozzles)

onto the turbine wheel’s blades were some of the exhaust gas energy is

extracted as work or power.

The turbine drives the compressor. A wastegate

(valve) just upstream of the turbine bypasses some of the exhaust gas flow

when necessary to prevent the boost pressure becoming too high. The

wastegate linkage (11) is controlled by a boost pressure regulator (12).

Figure 1.15 shows a cutaway drawing of a small automotive turbocharger.

The arrangement of the compressor and turbine rotors connected via the

the central shaft and of the turbine and compressor flow passages are evident.

Figure 1.14 Drawing of turbocharger system connected to four-cylinder

automobile spark-ignition engine. See text for details. ( Courtesy Regie

Nationale des Usines.)

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Figure 1.15 Cutaway view of small automotive SI engine turbocharger. (

Courtesy Nissan Motor Co. Ltd.)

Increasing the intake air density, through boosting, increases the mass of

air trapped within the cylinder, the mass of fuel burned, and thus the torque a

a given size engine can protrude. Torque increases of more than a factor of two

can be realized. Turbocharging of SI engines is made difficult by the SI

engine’s knock constraint.

The onset of knock (the rapid spontaneous ignition

of a fraction of the in-cylinder fuel-air mixture) during the latter part of

combustion) depends on the maximum mixture temperature and pressure

reached inside the engine cylinder, and boosting raises both these variables.

Special measures such as reducing the compression ratio, higher octane—

better knock resisting—fuels have to be used to control knock. Direct fuel

injection into the cylinder (see following section), with its charge-cooling

effect, eases this problem.

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A different type of boosted SI engine is large natural-gas

fueled engines. These are used in electric power generation, propulsion, and marine

applications. An example is shown in Fig. 1.16. It uses an encapsulated spark

plug with orifices to improve ignition.

Figure 1.16 Large natural-gas-fueled boosted SI engine used in electric

power generation. Bore 170 mm, stroke 190 mm, displaced volume per

cylinder 4.3 liters, compression ratio 12:1, power (eight cylinders) 965 kW

at 1500 rev/min. ( Courtesy Caterpillar, Inc.)

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1.7.4 Direct-Injection SI Engines

Since the 1920s, attempts have been made to develop internal combustion

engines that combine the best features of the SI engine and the diesel. By

injecting the gasoline fuel directly into each cylinder of the engine, better

control of the fuel’s behavior can be achieved, improving the engine’s

dynamic performance, permitting use of higher compression ratios, and

reducing the losses resulting from throttling the airflow in the standard port injected

SI engine. Diesels are more efficient because they operate close to

the optimum compression ratio (14 to 18), operate fuel lean (with excess

air), and control engine output by varying the fuel flow rate while leaving the

airflow unthrottled. Historically, direct-injection SI engines have often been

called stratified-charge engines since to realize all these benefits, the

mixing process between the evaporating fuel jet and the air in the cylinder

must produce a “stratified” or nonuniform fuel-air mixture, with an easily

ignitable composition at the spark plug at the time of ignition, and with

excess air surrounding the fuel-containing spray.

Over the years, many different types of stratified-charge engine have been

proposed; some are now being used in practice. 17 The operating principles

of three of these early designs are shown in Fig. 1.17. The combustion

chambers are bowl-in-piston designs, and a high degree of air swirl (rotation

about the cylinder axis) is created during intake and enhanced in the piston

bowl during compression to achieve rapid fuel-air mixing. With the Texaco

18 and MAN 19 systems ( Figs. 1.17a and b), fuel is injected into the cylinder

in tangentially into the bowl during the latter stages of compression. A long duration

spark discharge ignites the fuel-air jet as it passes the spark plug;

the flame spreads downstream, and consumes the fuel-air mixture. Figure

1.17 c shows the Ford PROCO system20 with its centrally located injector

and hollow cone spray injected earlier in the compression stroke to get more

complete fuel vapor/air mixing, so that high air utilization could be achieved

to obtain high outputs.

Figure 1.17 Three historical stratified-charge engines that were developed

for production: (a) Texaco Controlled Combustion System (TCCS);18 (b)

M.A.N.-FM Combustion System;19 (c) Ford PROCO Combustion System

Modern direct-injection SI engines are often divided in so-called sprayguided,

wall-guided, and air-guided categories: see Fig. 1.18. This

classification is based on the primary mechanism used to control the

development of the fuel spray. In practice, mixture stratification is achieved

through a combination of these mechanisms. The Texaco TCCS system in Fig.

1.17 a and the PROCO system in Fig. 1.17 c are examples of the former. The

MAN system, Fig. 1.17 b, is primarily wall guided (with air swirl also

playing an important role). The Texaco system, Fig. 1.17 a, is also air

guided, with high air swirl generated during intake and augmented by the

bowl-in-piston combustion chamber during compression. Many systems with

significantly different geometric details are now being developed and

employed in production: 17 see Sec. 7.7.2 . Generally, spray-guided

approaches require a closer spacing between the injector and spark plug

electrode location, as shown in

Fig. 1.18, to limit the dispersion of the fuel

spray and provide substantial mixture stratification. Wider spacing allows

more time for fuel-air mixing, produces a more uniform composition spray,

but then requires a specific combination of charge motion and wall guiding to

achieve the desired spray behavior, and combustion, and emissions

characteristics Figure 1.18 Illustrations of spray-guided, wall-guided, and air-guided

direct-injection SI combustion systems.

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Figure 1.19 shows a production example of a direct-injected (DI) wallguided

system. This Mitsubishi gasoline DI engine used a spherically shaped

cavity in the piston crown and an upright intake port to generate a reverse

tumbling airflow in the cylinder during intake, to “guide” the developing

spray toward the spark plug in the center of the cylinder head. Figure 1.20

shows a Toyota direct-injection engine design that uses a fan-shaped fuel

spray directed into a shell-shaped bowl in the piston crown to provide rapid

air-fuel mixing and fuel vaporization, and by the in-cylinder flow set-up by

the straight intake port, to guide the spray so it reaches the spark plug

location at the appropriate point in the cycle.

To achieve high engine outputs,

both these concepts transition from late injection (i.e., injection during the

latter half of the compression stroke) when stratified operation is desired, to

early injection (injection during the intake stroke) when essentially complete

mixing of the injected fuel with all the air in the cylinder is required. This

latter mode is called homogeneous-charge operation, as distinct from

stratified operation. Figure 1.20 Toyota gasoline direct-injection SI engine design uses a wideangle fan-shaped fine-atomization fuel spray injected into a shell-shaped

piston cavity to achieve a stratified mixture with late injection, and

homogeneous mixture with early (during intake stroke) injection.22 This

concept is also used in Toyota’s homogeneous-charge direct-injection

engines. Figure 1.19 Mitsubishi gasoline direct-injection SI engine design. It uses a

wide spacing between injector and spark plug; the spray is guided by the

hemispherical piston cavity, and the reverse tumble produced by the upright

intake port. In this 1.83-liter, four-cylinder, 12:1 compression ratio engine,

the bore is 81 mm and the stroke is 89 mm. The fuel system uses an

electromagnetic-controlled high pressure (5 MPa) swirl injector. 21

Homogeneous-charge operation at all engine loads and speeds is a viable

direct-injection SI engine approach, and is used in production engines. While

the efficiency benefit of stratified operation with excess air (which the diesel

enjoys) is lost, the in-cylinder charge cooling due to liquid fuel vaporization

that increases the amount of air inducted and reduces the propensity of the

engine to knock, and the more accurate control of fuel flow during engine

transients, are retained. Homogeneous direct-injection engine concepts thus

can increase compression ratio and efficiency, increase maximum power, and

benefit from the effective emission-control technology that has been

developed for port-injected SI engines.

Most production designs of direct-injection engines have used the fourstroke

cycle. Direct-injection is, however, especially helpful in controlling

fuel carry through in two-stroke cycle engines. Direct fuel is

especially attractive with turbocharged engines to increase their power

density. The charge cooling, which evaporation of the in-cylinder fuel spray

produces, reduces the engine’s propensity to knock.

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1.7.5 Prechamber SI Engines

An alternative to these open-chamber SI engines described above is a

prechamber engine concept, which has been mass produced. It uses a small

prechamber fed during intake with an auxiliary fuel system to obtain an

easily ignitable mixture around the spark plug. This concept, first proposed

by Ricardo in the 1920s and extensively developed in the Soviet Union and

Japan, is often called a jet-ignition or torch-ignition stratified-charge

engine. Its operating principles are illustrated in.

Fig. 1.21, which shows a

three-valve carbureted version of the concept. 23 A separate carburetor and

intake manifold feed a fuel-rich mixture (which contains fuel beyond the

amount that can be burned with the available air) through a separate small

intake valve into the prechamber that contains the spark plug. At the same

time, a lean mixture (which contains excess air beyond that required to burn

the fuel completely) is fed to the main combustion chamber through the

normal intake manifold. During intake, the rich prechamber flow fully purges

the prechamber volume. After intake valve closing, lean mixture from the

main chamber is compressed into the prechamber bringing the mixture at the

spark plug to an easily ignitable, slightly rich composition. After combustion

starts in the prechamber, rich burning mixture issues as a jet (or series of

jets) through one or more orifices into the main chamber, entraining and

igniting the lean main chamber charge. This engine is really a jet-ignition

concept whose primary function is to extend the operating limit of

conventionally ignited SI engines to mixtures leaner than could normally be

burned. This approach has been used in large natural gas engines to provide

rapid initiation of the combustion process. Figure 1.21 Schematic of three-valve

torch-ignition stratified-charge spark-ignition engine.

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1.7.6 Rotary Engines

The reciprocating engine geometry discussed so far dominates the practical

world of internal combustion engines. However, motivated by the fact that

engine power is delivered through a rotating drive shaft, over the years many

rotary engine designs have been proposed. 5 One of these, the Wankel rotary

engine 6, 7 has and continues to be used in limited production. Its attractive

features are its compactness and higher engine speed (which result in high

power/weight and power/volume ratios), and its inherent balance and

smoothness. These benefits are, however, offset by its higher heat transfer,

and the engine’s gas sealing and leakage problems.

Figure 1.22 shows the major mechanical parts of a simple single-rotor

Wankel engine and illustrates its geometry and operation. There are two

rotating parts: the triangular-shaped rotor and the output shaft with its integral

eccentric. The rotor revolves directly on the eccentric. The rotor has an

internal timing gear, which meshes with the fixed timing gear on one side

housing to maintain the correct phase relationship between the rotor and

eccentric shaft rotations. Thus the rotor rotates and orbits around the shaft

axis. Breathing is through ports in the center housing (and sometimes the side

housings). The combustion chamber lies between the center housing and rotor

surface and is sealed by seals at each apex of the rotor and around the

perimeters of the rotor sides. Figure 1.22 also shows how the Wankel rotary

geometry operates with the four-stroke cycle. The figure shows the induction,

compression, power, and exhaust processes of the four-stroke cycle for the

chamber defined by rotor surface AB. The remaining two chambers defined

by the other rotor surfaces undergo exactly the same sequence. As the rotor

makes one complete rotation, during which the eccentric shaft rotates through

three revolutions, each chamber produces one power “stroke.” Three power

pulses occur, therefore, for each rotor revolution; thus for each eccentric

(output) shaft revolution, there is one torque pulse.

Figure 1.23 shows a

cutaway drawing of an intake-port injected two-rotor automobile Wankel

engine. The two rotors are out of phase to provide a greater number of torque

pulses per shaft revolution. Note the combustion chamber cut out in each

rotor face, and the rotor apex and side seals. Two spark plugs per firing

chamber are often used to obtain a faster combustion process. Large area

side intake and exhaust ports are used to increase airflow, and improve

burned gas outflow. Figure 1.22 (a) Major components of the Wankel rotary engine.

( b)Induction, compression, power, and exhaust processes of the four-stroke

cycle for the chamber defined by rotor surface AB. ( From Mobil Technical

Bulletin, Rotary Engines, © Mobil Oil Corporation, 1971)Figure 1.23

Mazda 1.3-liter RENESIS two-rotor Wankel rotary engine.

Compression ratio 10:1, 15-mm eccentricity (offset between eccentric shaft

axis and rotor centerlines), 105-mm generating radius (distance between

rotor centerline and apex), trochaic (rotor) chamber width is 80 mm, giving

654 cm 3 displaced volume for each rotor chamber (1308 cm 3 total).

Produces 184 kW at 8200 rev/min and 216 N · m of torque at 5500 rev/min.

24 ( Courtesy Mazda Motor Co.)

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