Internal Combustion Engine

Topics: Internal combustion engine, Combustion, Diesel engine Pages: 25 (6874 words) Published: June 19, 2013
Chapter 11 Internal Combustion Engines
11.1 Introduction Internal combustion engines differ from external combustion engines in that the energy released from the burning of fuel occurs inside the engine rather than in a separate combustion chamber. Examples of external combustion engines are gas and steam turbines. The gas turbine power plant utilizes products of combustion from a separate combustor as the working fluid. These gases are used to drive the gas turbine and produce useful power. The steam power plant utilizes a separate boiler for burning fuels and creating hot gases which convert water to steam. The steam drives the steam turbine to produce useful power. On the other hand, internal combustion engines usually burn gasoline or diesel fuel inside the engine itself. If they use gasoline, they are called spark-ignition engines, since the spark from a spark plug ignites a mixture of air and gasoline trapped in the cylinder of the engine. The spark ignition (SI) engine operates ideally on the Otto cycle. The diesel engine, also called the combustion ignition (CI) engine, burns diesel fuel which is ignited as it is injected into the cylinder filled with very hot compressed air. Although there are some rotary internal combustion engines, internal combustion engines are usually reciprocating engines. Spark ignition engines usually use gasoline mixed with air, and these form the products of combustion upon being ignited. The high-pressure gases formed during combustion of the fuel and air provide impetus to the mobile pistons which reciprocate in cylinders. The pistons are connected to a rotating shaft, the crankshaft, by means of a connecting rod, which is connected at one end to the wrist pin located in the interior of the piston and at the other end to the crank pin of the crankshaft. As the crankshaft rotates through 360 degrees, the piston moves from the top of the cylin-

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Copyright n 1999 by Marcel Dekker, Inc. All Rights Reserved.

der (assuming a vertical cylinder axis) to the bottom and back to the top; thus, the piston makes two full strokes per revolution of the crankshaft. Since the engine cycle comprises four strokes of the piston: the intake stroke, the compression stroke, the expansion stroke and the exhaust stroke, the complete cycle for a fourstroke engine requires two revolutions of the crankshaft.

Exhaust

(a)

(b)

(c)

(d)

Figure 11.1 Four-stroke Cycle

Figure 11.1 (a) shows the piston moving down during the intake stroke. Note that the valve on the left is open and is admitting air to the cylinder as the piston moves down. Figure 11.1 (b) shows that the valve on the left as well as that on the right closed as the piston moves up while the piston compresses the fuel-air mixture previously admitted. When the piston approaches top dead center, a process of combustion is initiated by a spark created in a spark plug located in the center of the cylinder head. The effect of combustion is to heat the trapped gas and thus to raise its pressure; its chemical constitution is modified somewhat as well, e.g., the carbon in the fuel unites with the oxygen in the air to form carbon dioxide gas, and the hydrogen combines with the oxygen of the air to form water vapor. At this point Figure TM

Copyright n 1999 by Marcel Dekker, Inc. All Rights Reserved.

11.1 (c) applies, and the pressurized piston is forced down as the hot gases expand. At a crank angle of about 50° before bottom dead center the exhaust valve on the right side opens, and the gas in the cylinder blows out through the valve by virtue of the pressure excess of the gas in the cylinder. After bottom dead center is passed, the upward moving piston sweeps the cylinder almost clear of the gases formed in the combustion process; this is the exhaust stroke indicated in Figure 11.1 (d). The sweeping process is not complete, because a small volume of burned gas, the residual gas, exists in the cylinder when it is at top dead...

References: Heywood, J.B. (1988). Internal Combustion Engine Fundamentals. New York: McGraw-Hill.
Moran, M.J. and Shapiro, H.N. (1992). Fundamentals of Engineering Thermodynamics. New York: Wiley. Ohta, T. (1994). Energy Technology: Sources, Systems and Frontier Conversion. Oxford: Elsevier.
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