30079_59.pdf

CHAPTER 59

INTERNAL COMBUSTION ENGINES

Ronald Douglas Matthews

General Motors Foundation Combustion Sciences

and Automotive Research Laboratories

The University of Texas at Austin

Austin, Texas

59.1

TYPES AND PRINCIPLES OF

OPERATION

59.3.1 Experimental

Measurements

1801

1814

59.1.1 Spark Ignition Engines

1 802

59.3.2 Theoretical

Considerations

and Modeling

59.1.2 Compression Ignition

(Diesel) Engines

1808

1816

59.3.3 Engine Comparisons

1820

59.2 FUELSANDKNOCK

1808

59.2.1 Knock in Spark Ignition

Engines

59.4 EMISSIONSANDFUEL

ECONOMY REGULATIONS

1808

1822

59.2.2 Knock in the Diesel

Engine

59.4.1 Light-Duty Vehicles

1822

1810

59.4.2 Heavy-Duty Vehicles

1825

59.2.3 Characteristics of Fuels

1810

59.4.3 Nonhighway Heavy-Duty

Standards

1826

59.3 PERFORMANCEAND

EFFICIENCY

1814

SYMBOLS

1826

An internal combustion engine is a device that operates on an open thermodynamic cycle and is used

to convert the chemical energy of a fuel to rotational mechanical energy. This rotational mechanical

energy is most often used directly to provide motive power through an appropriate drive train, such

as for an automotive application. The rotational mechanical energy may also be used directly to drive

a propeller for marine or aircraft applications. Alternatively, the internal combustion engine may be

coupled to a generator to provide electric power or may be coupled to hydraulic pump or a gas

compressor. It may be noted that the favorable power-to-weight ratio of the internal combustion

engine makes it ideally suited to mobile applications and therefore most internal combustion engines

are manufactured for the motor vehicle, rail, marine, and aircraft industries. The high power-to-weight

ratio of the internal combustion engine is also responsible for its use in other applications where a

lightweight power source is needed, such as for chain saws and lawn mowers.

This chapter is devoted to discussion of the internal combustion engine, including types, principles

of operation, fuels, theory, performance, efficiency, and emissions.

59.1 TYPES AND PRINCIPLES OF OPERATION

This chapter discusses internal combustion engines that have an intermittent combustion process. Gas

turbines, which are internal combustion engines that incorporate a continuous combustion system,

are discussed in a separate chapter.

Internal combustion (IC) engines may be most generally classified by the method used to initiate

combustion as either spark ignition (SI) or compression ignition (CI or diesel) engines. Another

general classification scheme involves whether the rotational mechanical energy is obtained via re-

ciprocating piston motion, as is more common, or directly via the rotational motion of a rotor in a

rotary (Wankel) engine (see Fig. 59.1). The physical principles of a rotary engine are equivalent to

those of a piston engine if the geometric considerations are properly accounted for, so that the

following discussion will focus on the piston engine and the rotary engine will be discussed only

briefly. All of these IC engines include five general processes:

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Fig. 59.1 IC engine configurations: (a) inline 4; (b) V6; (c) rotary (Wankel); (d) horizontal, flat, or

opposed cylinder; (e) opposed piston; (/) radial.

1. An intake process, during which air or a fuel-air mixture is inducted into the combustion

chamber

2. A compression process, during which the air or fuel-air mixture is compressed to higher

temperature, pressure, and density

3. A combustion process, during which the chemical energy of the fuel is converted to thermal

energy of the products of combustion

4. An expansion process, during which a portion of the thermal energy of the working fluid is

converted to mechanical energy

5. An exhaust process, during which most of the products of combustion are expelled from the

combustion chamber

The mechanics of how these five general processes are incorporated in an engine may be used to

more specifically classify different types of internal combustion engines.

59.1.1 Spark Ignition Engines

In SI engines, the combustion process is initiated by a precisely timed discharge of a spark across

an electrode gap in the combustion chamber. Before ignition, the combustible mixture may be either

homogeneous (i.e., the fuel-air mixture ratio may be approximately uniform throughout the com-

bustion chamber) or stratified (i.e., the fuel-air mixture ratio may be more fuel-lean in some regions

of the combustion chamber than in other portions). In all SI engines, except the direct injection

stratified charge (DISC) SI engine, the power output is controlled by controlling the air flow rate

(and thus the volumetric efficiency) through the engine and the fuel-air ratio is approximately con-

stant (and approximately stoichiometric) for almost all operating conditions. The power output of the

DISC engine is controlled by varying the fuel flow rate, and thus the fuel-air ratio is variable while

the volumetric efficiency is approximately constant. The fuel and air are premixed before entering

the combustion chamber in all SI engines except the direct injection SI engine. These various cate-

gories of SI engines are discussed below.

Homogeneous Charge Sl Engines

In the homogeneous charge SI engine, a mixture of fuel and air is inducted during the intake process.

Traditionally, the fuel was mixed with the air in the venturi section of a carburetor. More recently,

as more precise control of the fuel-air ratio became desirable, throttle body fuel injection took the

place of carburetors for most automotive applications. Even more recently, intake port fuel injection

has almost entirely replaced throttle body injection. The five processes mentioned above may be

combined in the homogeneous charge SI engine to produce an engine that operates on either a 4-

stroke cycle or on a 2-stroke cycle.

4-Stroke Homogeneous Charge SI Engines. In the more common 4-stroke cycle (see Fig. 59.2),

the first stroke is the movement of the piston from top dead center (TDC—the closest approach of

the piston to the cylinder head, yielding the minimum combustion chamber volume) to bottom dead

center (BDC—when the piston is farthest from the cylinder head, yielding the maximum combustion

chamber volume), during which the intake valve is open and the fresh fuel-air charge is inducted

into the combustion chamber. The second stroke is the compression process, during which the intake

and exhaust valves are both in the closed position and the piston moves from BDC back to TDC.

The compression process is followed by combustion of the fuel-air mixture. Combustion is a rapid

hydrocarbon oxidation process (not an explosion) of finite duration. Because the combustion process

requires a finite, though very short, period of time, the spark is timed to initiate combustion slightly

before the piston reaches TDC to allow the maximum pressure to occur slightly after TDC (peak

pressure should, optimally, occur after TDC to provide a torque arm for the force caused by the high

cylinder pressure). The combustion process is essentially complete shortly after the piston has receded

away from TDC. However, for the purposes of a simple analysis and because combustion is very

rapid, to aid explanation it may be approximated as being instantaneous and occurring while the

piston is motionless at TDC. The third stroke is the expansion process or power stroke, during which

the piston returns to BDC. The fourth stroke is the exhaust process, during which the exhaust valve

is open and the piston proceeds from BDC to TDC and expels the products of combustion. The

exhaust process for a 4-stroke engine is actually composed of two parts, the first of which is blow-

down. When the exhaust valve opens, the cylinder pressure is much higher than the pressure in the

exhaust manifold and this large pressure difference forces much of the exhaust out during what is

called "blowdown" while the piston is almost motionless. Most of the remaining products of com-

bustion are forced out during the exhaust stroke, but an "exhaust residual" is always left in the

combustion chamber and mixes with the fresh charge that is inducted during the subsequent intake

stroke. Once the piston reaches TDC, the intake valve opens and the exhaust valve closes and the

cycle repeats, starting with a new intake stroke.

This explanation of the 4-stroke SI engine processes implied that the valves open or close in-

stantaneously when the piston is either at TDC or BDC, when in fact the valves open and close

relatively slowly. To afford the maximum open area at the appropriate time in each process, the

exhaust valve opens before BDC during expansion, the intake valve closes after BDC during the

compression stroke, and both the intake and exhaust valves are open during the valve overlap period

since the intake valve opens before TDC during the exhaust stroke while the exhaust valve closes

after TDC during the intake stroke. Considerations of valve timing are not necessary for this simple

explanation of the 4-stroke cycle but do have significant effects on performance and efficiency.

Similarly, spark timing will not be discussed in detail but does have significant effects on performance,

fuel economy, and emissions.

The rotary (Wankel) engine is sometimes perceived to operate on the 2-stroke cycle because it

shares several features with 2-stroke SI engines: a complete thermodynamic cycle within a single

revolution of the output shaft (which is called an eccentric shaft rather than a crank shaft) and lack

of intake and exhaust valves and associated valve train. However, unlike a 2-stroke, the rotary has a

true exhaust "stroke" and a true intake "stroke" and operates quite well without boosting the pressure

of the fresh charge above that of the exhaust manifold. That is, the rotary operates on the 4-stroke

cycle.

2-Stroke Homogeneous Charge SI Engines. Alternatively, these five processes may be incor-

porated into a homogeneous charge SI engine that requires only two strokes per cycle (see Fig. 59.3).

All commercially available 2-stroke SI engines are of the homogeneous charge type. That is, any

nonuniformity of the fuel-air ratio within the combustion chamber is unintentional in current 2-stroke

SI engines. The 2-stroke SI engine does not have valves, but rather has intake "transfer" and exhaust

ports that are normally located across from each other near the position of the crown of the piston

when the piston is at BDC. When the piston moves toward TDC, it covers the ports and the com-

pression process begins. As previously discussed, for the ideal SI cycle, combustion may be perceived

to occur instantaneously while the piston is motionless at TDC. The expansion process then occurs

as the high pressure resulting from combustion pushes the piston back toward BDC. As the piston

approaches BDC, the exhaust port is generally uncovered first, followed shortly thereafter by uncov-

ering of the intake transfer port. The high pressure in the combustion chamber relative to that of the

(a) (b) (c) (d) fe)

Fig. 59.2 Schematic of processes for 4-stroke Sl piston and rotary engines (for 4-stroke Cl, replace spark plug with fuel injector): (a) intake, (b) compression, (c) spark

ignition and combustion (for Cl, fuel injection, and autoignition), (d) expansion or power stroke, (e) exhaust.

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