MULTICYLINDER ENGINES | SPARK-IGNITION ENGINE OPERATION

1.5 MULTICYLINDER ENGINES

Small engines are used in many applications: for example, lawn mowers,

chain saws, in portable power generation, as outboard motorboat engines,

and motorcycles. These are often single-cylinder engines. In the above

applications, simplicity and low cost in relation to the power generated are

the most important characteristics; fuel consumption, engine vibration, high

power to weight or volume ratio, and engine durability are usually less

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

revolution (two-stroke cycle) or two revolutions (four-stroke cycle). Hence,

the individual cycle torque pulses are widely spaced, and engine vibration

and smoothness are significant issues.

Multi Cylinder configurations are invariably used in practice in all but the

smallest engines. As rated power increases, the advantages of smaller

cylinders in regard to bulk size, weight, improved engine performance, and

engine balance and smoothness all point toward increasing the number of

cylinders so the engine’s total displaced volume is spread out amongst

several smaller cylinders. The increased frequency of power strokes with

smaller and increasing number of cylinders produces more frequent and

smaller torque pulses, and thus smoother output. The forces in each

components are smaller, so structural design requirements are reduced.

Multi Cylinder engines can also achieve a much better state of balance than

single-cylinder engines. A force must be applied to each piston to accelerate

it during the first half of its travel from BC or TC. The piston then exerts a

force on the crankshaft as it decelerates during the second part of the stroke.

It is desirable to cancel these inertia forces through the choice of number and

arrangement of cylinders to achieve a primary balance. Note, however, that

the motion of the piston is more rapid during the upper half of its stroke than

during the lower half (a consequence of the connecting rod and crank

mechanism evident from Fig. 1.1; see also Sec. 2.2 ). The resulting inequality

in acceleration and deceleration of pairs of pistons (one moving up and one

moving down) produces corresponding differences in inertia forces

generated. Certain combinations of cylinder number and arrangement balance

out these secondary inertia force effects.

For a given engine displaced volume, the larger the number of cylinders,

the higher the engine’s maximum power. The reciprocating speed of an

The engine's pistons are limited by the airflow into each cylinder. Once the flow

through the intake valve becomes sonic—reaches the speed of sound—higher

piston speeds do not increase airflow. For a given engine displacement,

increasing the number of cylinders, and thus reducing their size, raises the

crankshaft rotational speed at which this sonic airflow limit is reached. Since

engine power is proportional to the engine’s rotational (crankshaft) speed,

maximum performance is improved.

Other operational issues are affected by cylinder size. The relative

importance of heat losses from the in-cylinder gases depends on the relative

importance of the combustion chamber surface area to its volume. The SI

engine compression-ratio limiting phenomenon called knock is adversely

affected by the flame travel distance (spark plug gap to farthest combustion

chamber wall).

Common four-stroke multi cylinder configurations are shown in Fig. 1.6. 13

These multi cylinder configurations normally use equal crankshaft rotation

firing intervals between cylinders. In in-line engines, the cylinders are

arranged in a single plane. Three-, four-, five-, and six-cylinder in-line

configurations are used. Four-cylinder in-line engines are the most common

arrangement for automobile engines from 1.2 to about 2.5-liter displacement.

An example of this in-line arrangement is shown in Fig. 1.4. It is compact—

an important consideration for small passenger cars. It provides two torque

pulses per revolution of the crankshaft, and primary inertia forces (though not

secondary forces) are balanced. Six-cylinder in-line diesel engines are

commonly used in the truck market with up to 12-liter displacement.

Figure 1.6 Multicylinder engine configurations: (1) In-line engine; (2) Vengine;

(3) Radial engine; (4) Opposed-cylinder engine; (5) U-engine; (6)

Opposed-piston engine.13 (Courtesy Robert Bosch GmbH and SAE.)

The vee (V) arrangement, with two banks of cylinders set at an angle to

each other, provides a compact engine block and is used extensively for

larger displacement automotive engines. Vee six, eight, ten, and twelve

configurations are used. In a V-6 engine, the six cylinders are arranged in two

banks of three, usually with a 60° angle between their axis. Six cylinders are

normally used in gasoline SI engines in the 2.4- to 3.6-liter displacement

range. Six-cylinder engines provide smoother operation with three torque

pulses per revolution. The in-line arrangement is fully balanced. However, it

gives rise to crankshaft torsional vibration, and also makes even distribution

of air to each cylinder is more difficult. The V-6 arrangement is more compact

than an in-line 6, and provides primary balance of the reciprocating

components. With the V-engine, however, a rocking moment is imposed on

the crankshaft due to the secondary inertia forces, which results in the engine

being less well balanced than the in-line version. The V-8 arrangement, in

sizes between 3.2 and 6 or more liters, is commonly used to provide

compact, smooth, low-vibration, larger-displacement, SI engines, as are V-

10 and V-12 designs.

The radial engine configuration, with cylinders arranged in one or more

radial planes, as shown, was common in larger piston-driven aircraft

engines. Opposed cylinder engine designs are occasionally used. As Fig. 1.6

indicates, the motion of pairs of pistons with this design is fully balanced.

The U-cylinder configuration, where the pistons move in the same direction,

and the opposed-piston configuration have been used in special purpose two stroke

engine concepts.

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1.6 SPARK-IGNITION ENGINE OPERATION

In SI engines, the fuel must be vaporized and well mixed with the air

inducted into the cylinder, prior to combustion. Historically, the fuel flow

was metered with a carburetor or single-point fuel-injection system ( Fig.

1.7a) upstream of the throttle, which controls the airflow. This approach has

been superseded by intake-port fuel injection ( Fig. 1.7 b) where a pulsed

liquid fuel spray is directed toward the intake valve. Injection of gasoline

directly into each cylinder ( Fig. 1.7 c) is now in large-scale production.

Moving the point of fuel injection closer to the cylinder enables better

dynamic response during engine transients.

Figure 1.7 Different fuel-injection approaches for gasoline spark-ignition

engines. ( a) Single-point injection; ( b) Multipoint port injection; ( c) Direct in-cylinder injection. (1) Fuel supply; (2) Air supply; (3) Throttle valve; (4)

Intake manifold; (5) Injectors; (6) Engine. 13 ( Courtesy Robert Bosch GmbH

and SAE.)

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Figure 1.8 shows the layout of a modern SI engine management system.

The airflow, fuel flow, exhaust gas characteristics, and engine operating state

are all monitored and controlled as shown to provide the desired engine

performance with good combustion characteristics, high efficiency, and low

exhaust air pollutant emissions. The ratio of mass flow of air to mass flow of

fuel must be held approximately constant at about 15 to ensure reliable

combustion and facilitate exhaust emissions control. The appropriate fuel

flow is determined for the engine airflow in the following manner. The

airflow into the intake system is measured with an air mass-flow meter. A

the throttle valve or plate, which can be opened or closed, controls the airflow.

The appropriate amount of fuel required per cylinder per cycle to generate

The desired engine output is then determined by the engine control unit. In

naturally-aspirated engines, the intake airflow is reduced by throttling to

below atmospheric pressure by reducing the flow area when the power

required (at any engine speed) is below the maximum, which is obtained

when the throttle is wide open.

Figure 1.8 Schematic of modern port-injected engine management system

(Bosch ME-Motronic system). (1) Carbon fuel-vapor absorbing canister; (2)

Hot-film air-mass meter with integrated temperature sensor; (3) Throttle

device; (4) Canister-purge valve; (5) Intake-manifold pressure sensor; (6)

Fuel-distribution pipe; (7) Injector; (8) Actuators and sensors for variable

valve timing; (9) Ignition coil with attached spark plug; (10) Camshaft phase

sensor; (11) Lambda oxygen sensor upstream of primary catalytic converter;

(12) Engine control unit; (13) Exhaust-gas recirculation valve; (14) Speed

sensor; (15) Knock sensor; (16) Engine-temperature sensor; (17) Primary

three-way catalytic converter; (18) Lambda oxygen sensor downstream of

primary catalytic converter: (19) CAN interface; (20) Fault lamp; (21)

Diagnosis interface; (22) Interface to immobilizer control unit; (23)

Accelerator-pedal module with pedal-travel sensor; (24) Fuel tank; (25) Intank

unit with electric fuel pump, fuel filter, and fuel-pressure regulator; (26)

Main three-way catalytic converter. 13 ( Courtesy Robert Bosch GmbH and

SAE.)

The sequence of events that take place inside the engine cylinder is

illustrated in Fig. 1.9. Several variables are plotted against crank angle

through the entire four-stroke SI engine cycle. Crank angle is a useful

independent variable because the various engine processes occupy almost

constant crank angle intervals over a wide range of engine-operating

conditions. The figure shows the valve opening and closing angles, and

volume relationship, for a typical fixed valve-timing automotive SI engine.

To maintain high mixture flows at high engine speeds (and thus high power

outputs) the inlet valve, which opens before TC, closes substantially after

BC. During intake, the inducted fuel and air mix in the cylinder with the

residual burned gases remaining from the previous cycle. After the intake

valve closes, the cylinder contents are compressed to above atmospheric

pressure and temperature as the cylinder volume is reduced. Some heat

transfer between the in-cylinder gases and the piston, cylinder head, and

cylinder walls occur—first a heating of the gases, then a cooling, but the effect

on unburned gas properties is modest.

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Figure 1.9 Sequence of events in four-stroke spark-ignition engine operating

cycle. Cylinder pressure p (solid line, firing cycle; dashed line,

motored cycle), cylinder volume V/V max, and mass fraction burned xb are

plotted against crank angle.

Between about 10 and 40 crank angle degrees before TC, an electrical

discharge across the spark plug starts the combustion process. Before the

desired ignition point, the ignition driver switches a current to the primary

circuit of the ignition coil. At the ignition point, the primary winding is

interrupted, generating in the secondary ignition coil winding that is

connected to the spark plug, a high voltage across the plug electrodes as the

magnetic field collapses. This switching is done electronically. A flame

develops from the spark discharge, propagates through the mixture of air,

fuel, and residual gas in the cylinder, and extinguishes at the combustion

chamber walls. The duration of this burning process varies with engine

design and operation, but is typically 40 to 60 crank angle degrees, as shown

in Fig. 1.9. As fuel-air mixture burns in the flame, the cylinder pressure

(solid line in Fig. 1.9) rises above the level due to compression alone

(dashed line). This latter curve—called the motored cylinder pressure—is

the pressure trace obtained from a motorized or non firing engine. f Note that

due to differences in the flow pattern and mixture composition between

cylinders and within each cylinder, cycle-by-cycle, the development of each

the combustion process differs somewhat. As a result, the shape of the pressure

versus crank angle curve in each cylinder, and cycle-by-cycle, is not exactly

the same.

There is an optimum spark timing which, for a given mass of fuel, air, and

residual inside the cylinder, gives maximum torque. More advanced (earlier)

timing or retarded (later) timing than this optimum gives lower output. Called

maximum brake-torque (MBT) timing, g this optimum timing is an empirical

compromise between starting combustion too early in the compression stroke

(when the work transfer is to the cylinder gases) and completing combustion

too late in the expansion stroke (and so lowering peak expansion stroke

pressures).

About two-thirds of the way through the expansion stroke, the exhaust

the valve starts to open. The cylinder pressure is significantly higher than the

exhaust manifold pressure and a blowdown process occurs. The burned gases

flow through the valve into the exhaust port and manifold until the cylinder

pressure and exhaust pressure equilibrate. The duration of this process

depends on the pressure level in the cylinder. The piston then displaces most

of the remaining burned gases from the cylinder into the manifold during the

exhaust stroke. The exhaust valve opens before the end of the expansion

stroke to ensure that the blowdown process does not last too far into the

exhaust stroke when the piston travels upwards. The actual timing is a

compromise that balances reduced work transfer to the piston before BC

against reduced work transfer to the cylinder contents after BC.

The exhaust valve remains open until just after TC; the intake opens just

before TC. The valves are opened and closed slowly to avoid noise and

excessive cam wear. To ensure the valves are fully open when piston

velocities are at their highest, the valve open periods usually overlap

somewhat. If the intake flow is throttled to below exhaust manifold pressure,

then backflow of burned gases from the cylinder into the intake manifold

occurs when the intake valve is first opened. During the valve overlap

period, backflow of burned gas from the exhaust port into the cylinder

occurs.

With variable valve control, the trade-offs that fixed valve timing requires

can be relaxed. The simplest approach varies intake and exhaust valve timing

by rotating the camshafts to change their phasing relative to the crankshaft.

More complex systems vary valve lift as well as varying the valve opening

and closing angles. Variable valve control is attractive because it improves

maximum engine power (at high speed) and maximum torque at lower

speeds, and can improve part-load engine efficiency. It can also be used to

control the mass of burned residual gas, and fresh air, trapped in the engine

cylinder.

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