ENGINE OPERATING CYCLES | ENGINE COMPONENTS

1.3 ENGINE OPERATING CYCLES

Most of this is about reciprocating engines, where each piston moves

back and forth in a cylinder and transmits power from the high-pressure and

temperature burned gases inside the cylinder through the piston and the

connecting rod and crank mechanism to the drive shaft as shown in Fig. 1.1.

The rotation of the crank produces a cyclical piston motion. The piston

comes to rest at the top-center (TC) crank position and bottom-center (BC)

crank position when the cylinder volume is a minimum or maximum,

respectively. d The minimum cylinder volume is called the clearance volume

Vc. The volume swept out by the piston, the difference between the maximum

or total volume Vt and the clearance volume, is called the displaced or swept

volume Vd. The ratio of maximum volume to minimum volume is the

compression ratio rc. Values of rc are 8 to 12 for SI engines and typically in

the ranges of 14 to 22 for CI engines.

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Basic geometry of the reciprocating internal combustion engine.

Figure 1.1 Basic geometry of the reciprocating internal combustion engine.

V c , Vd, and Vt indicate clearance, displaced, and total cylinder volumes.

The majority of reciprocating engines operate on what is known as the

four-stroke cycle. Each cylinder requires four strokes of its piston—two

revolutions of the crankshaft—to complete the sequence of events that

produces one power stroke. Both SI and CI engines use this cycle that

comprises

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The four-stroke operating cycle

Figure 1.2 The four-stroke operating cycle

1. An intake stroke, which starts with the piston at TC and ends with

the piston at BC, which draws fresh air or fuel-air mixture into the

cylinder. To increase the mass inducted, the inlet valve opens shortly

before the stroke starts and closes after it ends.

2. A compression stroke, which starts with the piston at BC and ends at

TC, when the mixture inside the cylinder is compressed to a small

fraction of its initial volume. Toward the end of the compression

stroke, combustion is initiated and the cylinder pressure rises more

rapidly.

3. A power stroke, or expansion stroke, which starts with the piston at

TC and ends at BC as the high-temperature, high-pressure gases push

the piston down and force the crank to rotate. About five times as

much work is done on the piston during the power stroke as the piston

had to do during compression. As the piston approaches BC, the

exhaust valve opens to initiate the exhaust process and drop the

cylinder pressure to close to the exhaust system pressure.

4. An exhaust stroke, where, as the piston moves from BC to TC, the

remaining burned gases exit the cylinder: first, because the cylinder

pressure may be significantly higher than the exhaust pressure; then as

these gases are swept out by the piston as it moves toward TC. As the

piston approaches TC the inlet valve opens and just after TC the

exhaust valve closes. The cycle then starts again.

Though often called the Otto cycle after its inventor, Nicolaus Otto, who

built the first engine operating on these principles in 1876, the more

descriptive four-stroke nomenclature is preferred.

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Simplest types of two-stroke engine designs.

The four-stroke cycle requires, for each engine cylinder, two crankshaft

revolutions for each power stroke. To obtain a higher power output from a

given engine size, and a simpler valve design, the two-stroke cycle was

developed. The two-stroke cycle is applicable to both SI and CI engines.

Figure 1.3 shows one of the simplest types of two-stroke engine designs.

Ports in the cylinder liner, opened and closed by the piston motion, control

the exhaust flow out of the cylinder and the fresh charge flow into the

cylinder, while the piston is close to BC. The two strokes are:

Figure 1.3 The two-stroke operating cycle. A crankcase-scavenged engine

is shown.1. A compression stroke, which starts with the closing of the fresh

charge transfer ports and then the exhaust ports, and compresses the

cylinder contents as the piston moves up the cylinder, and also draws

fresh charge into the crankcase through the inlet Reed valve. As the

piston approaches TC, combustion is initiated.

2. A power or expansion stroke, similar to that in the four-stroke cycle

until the piston approaches BC, when first the exhaust ports and then

the transfer ports are uncovered ( Fig. 1.3). Most of the burnt gases

exit the cylinder in an exhaust blowdown process. When the transfer

ports are uncovered, the fresh charge that has been compressed in the

crankcase flows into the cylinder. The piston and the ports are

generally shaped to deflect the incoming charge from flowing directly

into the exhaust ports, and to achieve effective scavenging of the

residual in-cylinder burned gases by this fresh charge.

Each engine cycle with one power stroke is completed in one crankshaft

revolution. However, it is difficult to fill completely the displaced volume

with fresh charge, and some of the fresh mixture flows directly out of the

cylinder during the scavenging process. e The example shown is a crossscavenged

design; other approaches use loop-scavenging or uniflow gas

exchange processes

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1.4 ENGINE COMPONENTS

Cutaway drawings of a four-stroke spark-ignition (SI) engine and a diesel

(CI) engine are shown in Figs. 1.4 and 1.5, respectively. The SI engine is a

four-cylinder in-line automobile engine. The major components are labeled.

The diesel is a six-cylinder in-line heavy-duty truck engine. The function of

the major components of these engines and their construction materials will

now be reviewed.

Figure 1.4 Cutaway drawing of 2.2-liter displacement four-cylinder sparkignition

engine. Bore 87.5 mm, stroke 92 mm,

compression ratio 8.9.

Figure 1.5 Cross-section drawing of a four-stroke cycle 6.7-liter in-line

six-cylinder turbocharged diesel engine. Bore 107 mm, stroke 124 mm,

compression ratio 17.3, maximum torque 1200 N · m at 1600 rev/min,

maximum power 285 kW at 2800 rev/min. ( Courtesy Cummins Engines.)

The engine cylinders are contained in the engine block.

The block has

traditionally been made of gray cast iron because of its good wear resistance

and low cost, but is often now made of aluminum. Passages for the cooling

water is cast into the block. Heavy-duty and truck engines often use

removable cylinder sleeves pressed into the block that can be replaced when

worn. These are called wet liners or dry liners depending on whether the

sleeve is in direct contact with the cooling water. Aluminum is used in

automotive SI engine blocks to reduce engine weight. Iron cylinder liners

may be inserted at the casting stage, or later on in the machining and

assembly process. The crankcase is often integral with the cylinder block.

The crankshaft has traditionally been a steel forging; nodular cast iron

crankshafts are also accepted practice in automotive engines. The crankshaft is supported in main bearings. The number of crankshaft bearings depends

largely on the engine’s loading and maximum speed. The maximum number of

main bearings is one more than the number of cylinders; there may be less.

The crank has eccentric portions (crank throws); the connecting rod big-end

bearings attach to the crank pin on each throw. Both main and connecting rod

bearings use steel-backed precision inserts with bronze, babbitt, or aluminum

as the bearing materials. The crankcase is sealed at the bottom with a

pressed-steel or cast aluminum oil pan, which acts as an oil reservoir for the

lubricating system.

Pistons are made of aluminum in smaller engines or cast iron in larger

slower-speed engines. The piston both seals the cylinder and transmits the

combustion-generated gas pressure to the crank pin via the connecting rod.

The connecting rod, usually a steel or alloy forging (though sometimes

aluminum), is fastened to the piston by means of a steel piston pin through the

rod upper end. The piston pin is usually hollow to reduce its weight.

The oscillating motion of the connecting rod exerts an oscillating force on

the cylinder walls via the piston skirt (the region below the piston rings). The

a piston skirt is usually shaped to provide appropriate thrust surfaces. The

piston is fitted with rings that ride in grooves cut in the piston head to seal

against gas leakage and control oil flow. The upper ring is the compression

ring that is forced outward against the cylinder wall and downward onto the

groove face. The lower rings scrape the surplus oil from the cylinder wall to

reduce exposure to the hot burned gases, and return it to the crankcase. The

crankcase must be ventilated to remove gases that blow by the piston rings,

to prevent pressure buildup. The crankcase gases are recycled to the engine

intake.

The cylinder head (or heads in V engines) seals off the cylinders and is

made of aluminum or cast iron. It must be strong and rigid to distribute the

gas forces acting on the head as uniformly as possible through the engine

block. The cylinder head contains the spark plug (for an SI engine) or fuel

injector (for a CI or direct-injection engine), and, in overhead valve engines,

parts of the valve mechanism.

The valves shown in Fig. 1.4 are poppet valves, the valve type normally

used in four-stroke engines. The engine shown has one intake and one exhaust

valve: most modern engines have four valves per cylinder (two intake and

two exhaust valves), or three valves (two intake and one exhaust). Valves are

made from forged alloy steel; the cooling of the exhaust valve, which

operates at up to about 700°C, may be enhanced by using a hollow stem

partially filled with sodium, which through evaporation and condensation

carries heat from the hot valve head to the cooler stem. Most modern SI

engines have overhead valve locations (sometimes called valve-in-head or Ahead

configurations) as shown in Fig. 1.4. This geometry leads to a compact

combustion chamber with minimum heat losses and flame travel time, and

improves the breathing capacity. Older geometries such as the L head where

valves are to one side of the cylinder are now only used in small low-cost

engines.

The valve stem moves in a valve guide, which can be an integral part of

the cylinder head (or engine block for L-head engines), or may be a separate

unit pressed into the head (or block). The valve seats may be cut in the head

or block metal (if cast iron) or hard steel inserts may be pressed into the

head or block. A valve spring, attached to the valve stem with a spring

washer and split keeper, holds the valve closed. A valve rotator turns the

valves a few degrees on opening to wipe the valve seat, avoid local hot

spots, and prevent deposits building up in the valve guide.

A camshaft made of cast iron or forged steel with one cam per valve (or

pair of valves in four valves per cylinder engines) is used to open and close

the valves. The cam surfaces are hardened to obtain adequate life. In fourstroke

cycle engines, camshafts turn at one-half the crankshaft speed.

Mechanical or hydraulic lifters or tappets slide in the block and ride or roll

on the cam. Depending on valve and camshaft location, additional members

are required to transmit the tappet motion to the valve stem; for example, in

in-head valve engines with the camshaft at the side, a pushrod and rocker

arms are used. A trend in high-speed automotive engines is to mount the

camshaft over the head with the cams acting either directly or through a

pivoted follower on the valve. Also, variable control of valve opening and

closing as a function of engine operating conditions, in its simplest form

using a camshaft phasing device, is replacing fixed valve timing engine

designs. Camshafts are gear, belt, or chain driven from the crankshaft.

An intake manifold (aluminum, cast iron, or plastic) and an exhaust

manifold (generally of cast iron) complete the SI engine assembly. Other

engine components specific to SI engines—fuel injectors, ignition systems—

are described in more detail in the remaining sections in this chapter.

The four-stroke cycle diesel engine shown in Fig. 1.5 is an in-line six cylinder

design commonly used for large trucks. The engine is turbocharged

to increase the amount of air that enters the cylinder each cycle. The

turbocharger consists of a centrifugal compressor (which compresses the air

prior to entry into the cylinder on the same shaft as the exhaust-gas-driven

turbine that powers the compressor). In diesel engines, the fuel injectors are

mounted in the cylinder head. Diesel fuel-injection systems are discussed in

more detail in

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