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INTERNAL COMBUSTION ENGINES

By Nancy Sims,2014-11-25 10:35
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INTERNAL COMBUSTION ENGINES

    INTERNAL COMBUSTION ENGINES

    By: Dennis Ballou and Barbara McCord

     This paper presents the essential design features of the spark ignition (SI), piston-cylinder engine. 1: Referring to Figure 1

     S - Stroke

     s - Distance between crank and

     wrist pin axes

     - Crank angle

     B - Bore

     r Connecting rod

     a Crankshaft offset

     V Clearance volume c

     V Displacement volume d

     TDC Top dead center

     BDC Bottom dead center

     Figure 1: Piston and Cylinder Geometry of

     Reciprocating Engine

    Definitions

Top Dead Center (TDC) - Maximum travel of piston toward cylinder head. The cylinder volume at TDC

    is called the clearance volume.

    Bottom Dead Center (BDC) - Minimum travel of piston toward crankshaft.

Bore (B) - Cylinder diameter (piston diameter = cylinder diameter - clearance).

Stroke (S) - Distance between TDC and BDC.

Displacement (V) - volume of cylinder between TDC and BDC. d

     1 Reference 1., p. 36.

     1

     2 The Four-Stroke, Air Standard Ideal Otto Cycle

     3 Referring to Figure 2:

First stroke, Process 6-1 (Induction).

    The piston travels from TDC to BDC with the intake valve open and the exhaust valve closed (some valve overlap occurs near the ends of strokes to accommodate the finite time required for valve operation). The temperature of the incoming air is increased 25-35 ~C over the surrounding air as the air passes through the hot intake manifold.

     Second Stroke, Process 1-2 (Compression).

     At BDC the intake valve closes. The piston travels to TDC compressing the cylinder contents at constant entropy. Just before TDC, the spark plug fires initiating combustion.

Combustion, Process 2-3.

    This process is modeled at constant volume even though combustion requires a finite time in a real engine (cylinder is moving). Peak cycle temperature and pressure occur at state 3.

    Third Stroke, Process 3-4 (Expansion or power stroke).

    With all valves closed, the piston travels from TDC to BDC. The process is modeled at constant entropy.

Exhaust Blowdown, Process 4-5.

    Near the end of the power stroke, the exhaust valve is opened. The resulting pressure differential forces cylinder gases out dropping the pressure to that of the exhaust manifold. The process is modeled at constant volume.

    Figure 2: Ideal air-Standard Otto Cycle

    Fourth Stroke, Process 5-6.

    With the exhaust valve open, the piston travels from BDC to TDC expelling most of the remaining exhaust gases.

Note: The four strokes require two complete

    revolutions of the crankshaft.

     2 Id., pp. 25 and 72. 3 Id., p. 73.

     2

    Thermodynamic Analysis

     Process 6-1. wPvv,?;;61016

Process 1-2. q0 wuu,?;;121212

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