Ch7 Engines Combustion

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    CH A P TER 7

    INTE RNAL-COMBUSTION E NGINES

    Internal-combustion engines are used exten-

    sively in the Navy. They serve as propulsion units

    i n a va r i e ty o f sh ips and boa t s . In t e rna l -

    combustion engines are also used as prime movers

    (drive units) for auxiliary machinery. Because they

    have pistons that employ a back-and-forth

    motion, gasoline and diesel engines are also

    classified as reciprocating engines.

    This chapter provides you with the general

    construction features and operating principles of

    various types of internal-combustion engines.

    After reading this chapter, you will have a basic

    understanding of the components that make up

    an internal-combustion engine and how these

    components work together to develop power.

    RECIPROCATING ENGINES

    The internal-combustion engines (diesel and

    gasoline) are machines that convert heat energy

    into mechanical energy. The transformation of

    heat energy to mechanical energy by the engine

    is based on a fundamental law of physics. Gas

    will expand when heat is applied. The law also

    s ta tes tha t when a gas i s compressed, the

    temperature of the gas will increase. If the gas is

    confined with no outlet for expansion, the

    pressure of the gas will be increased when heat

    is applied. In the internal-combustion engine, the

    burning of a fuel within a closed cylinder results

    in an expans ion of gases. The pressu re creat ed on

    top of a piston by the expanding gases causes it

    to move.The back-and-forth motion of the pistons in

    an engine is known a s reciprocat ing motion. This

    reciprocating motion (straight-line motion) must

    be changed to rotary motion (turning motion) to

    perform a useful function, such as propelling a

    boat or ship through the water or driving a

    genera tor to provide electr icity. A cran ksh aft an d

    a connecting rod change this reciprocating

    motion to rotary motion (fig. 7-1).

    Figure 7-1.Cy li n d e r , p i st o n , c o n n e c t i n g r o d , a n d c r a n k -s h a f t f o r o n e c y l in d e r o f a n e n g i n e .

    All internal-combustion engines are basically

    th e same. They all rely on th ree th ingsair, fuel,

    and ignition.

    Fuel contains potential energy for operating

    the engine; air contains the oxygen necessary for

    combustion; and ignition starts combustion. All

    are fundamental, and an engine will not operate

    without all of them. Any discussion of engines

    must be based on these three factors and the steps

    and mechanisms involved in delivering them to

    the combustion chamber at the proper time.

    GASOLINE VERSUS

    DIESEL ENGINES

    There are two main differences between

    gasoline and diesel engines. They are the methods

    of getting the fuel into the cylinders and of

    igniting the fuel-air mixtures. In the gasoline

    engine, the air and gasoline are mixed together

    outside the combustion chamber. The mixture

    7-1

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    Figu re 7-2.Co mp a r i s o n o f e v e n t s i n d i e s e l a n d g a s o l in e f ou r -c y c le e n g i n e s .

    then passes through the intake manifold, where The diesel engine uses neither spark plugs nor

    it star ts to vaporize. Then th e mixtur e enters th e a carbur etor. On t he inta ke str oke, only fresh a ir

    cylinder through the intake valve. Here it is is drawn into the cylinder t hrough th e intake valve

    completely vap orized by th e h eat of compr ession and manifold. On the compression stroke, the air

    as the piston moves upward on the compression is compressed and the temperature in the cylinder

    s t roke. When the pis ton i s near the top of it s r i ses to a point between 700 F and 1200 F. At

    stroke (top dead center or TDC), the mixture is the proper time, the diesel fuel is injected into the

    ignited by a spark from the spark plug. cylinder by a fuel injection system . When th e fuel

    7-2

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    enters the cylinder, it ignites. Figure 7-2 shows the

    comparison of the four strokes of four-cycle diesel

    and gasoline engines.

    The speed of a diesel or gasoline engine is

    controlled by the amount of fuel-air mixture that

    is burned in the cylinders. The primary difference

    is the method in which th e fuel and air ent er the

    combustion chamber. In a diesel engine, the fuelis injected directly into the combustion chamber,

    where it mixes with air. In a gasoline engine, the

    fuel and a ir are mixed in the inta ke man ifold and

    then drawn into the combustion chamber.

    Mechanically, the diesel engine is similar to

    the gasoline engine. The intake, compression,

    power, and exhaust strokes occur in the same

    order. The arrangement of the pistons, connecting

    rods, crankshaft, and engine valves are also the

    same.

    DEVELOPMENT OF POWER

    The power of an internal-combustion engine

    comes from the burning of a mixture of fuel and

    air in a small, enclosed space. When this mixture

    burns, it expands greatly. The push or pressure

    created is used to move the piston. The piston then

    rotates the crankshaft. The rotating crankshaft

    is then used to perform the desired work.

    Since the same actions occur in all cylinders

    of an engine, we will discuss only one cylinder and

    its related pa rts. The four major pa rts consist of

    a cylinder, piston, cra nk sha ft, and conn ecting rod

    (fig. 7-1).

    First we must have a cylinder that is closed

    at one end. The cylinder is sta tionar y within the

    engine block.

    Inside this cylinder is the piston (a movable

    metal plug) tha t fits snu gly into the cylinder but

    can still slide up and down easily. Movement of

    the piston is caused by the burning fuel-air

    mixture in the cylinder.

    You have already learned that the back-and-

    forth movement of the piston is called recipro-

    cating motion, which must be changed to rotary

    motion. This change is accomplished by a throwon the crankshaft and a connecting rod that

    connects the piston and the crank throw.

    The number of piston strokes occurring

    during any one series of operations (cycles) is

    limited to either two or four, depending on the

    design of the engine.

    When the piston of the engine slides down-

    ward because of the pressure of the expanding

    gases in the cylinder, the upper end of the

    Figure 7-3 . P i s t o n s t r o k e .

    connecting rod moves downward with the piston

    in a straight line. The lower end of the connecting

    rod moves down and in a circular motion at the

    same time. This moves the crank throw and, in

    turn , ro ta tes the shaf t . This ro ta t ion i s the

    desired result. So remember, the crankshaft and

    connecting rod combination is a mechanism for

    the purpose of changing back-and-forth (recipro-

    cating) motion to circular (rotary) motion.

    BASIC ENGINE STROKES

    Each movement of the piston from top to

    bottom or from bottom to top is called a stroke.

    The piston takes two strokes (an upstroke and

    a downst roke) as the crankshaf t makes one

    complete revolution. When the piston is at the topof a stroke (fig. 7-3, view A), it is said to be at

    top dead center (TDC). When th e piston is a t t he

    bottom of a stroke (fig. 7-3, view B), it is said

    to be at bottom dead center (BDC).

    In the basic engine you have studied so far,

    we have n ot consider ed provisions for getting t he

    fuel-air mixtu re into the cylinder or burn ed gases

    out of th e cylinder. Ther e ar e two openin gs in th e

    enclosed end of a cylinder. One of the openings,

    7-3

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    Figu re 7-4.F o u r -s t r o k e d i e s e l e n g i n e .

    7-4

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    or por t s , pe rm i t s an in t ake o f a i r , o r an

    in t ake o f a m ix tu re o f fue l and a i r , i n to

    the combustion area of the cylinder ( intake

    valve). The other opening, or port , permits

    the burned gases to escape from the cylinder

    (exhaust valve). The two ports have valves

    in them. These valves close off ei ther one

    or the other of the ports , or both of them,during various stages of engine operation. The

    cam shaf t ( a sha f t w i th a num ber o f cam

    lobes along its length) opens the valves and

    ho lds them open fo r shor t pe r iods dur ing

    the piston stroke. The camshaft is driven by

    the crankshaf t through t iming gears or by

    a t im ing cha in . O n a four - s t roke eng ine ,

    t h e c a m s h a f t t u r n s a t o n e - h a l f c r a n k s h a f t

    speed. This permits each valve to open and

    close once for every two turns of the crank-

    shaft.

    The following sections give a simplifiedexp lana t ion o f t he ac t ion tha t t akes p l ace

    within the engine cylinder. For the purpose of

    explanation, we will show the action of a four-

    stroke diesel engine. This type of engine is

    referred to as a four-stroke engine because it

    requ ires four complete pist on str okes to complete

    one cycle. These str okes ar e known as the intak e

    stroke, the compression stroke, the power stroke