Theoretical Power

In the previous chapter, theoretical power was defined as the calculated horsepower based on the bore, cylinder pressure, and engine speed. Figure 5.8 shows a typical cylinder pressure curve for a naturally aspirated engine. During the intake stroke the pressure is below zero, atmospheric. The pressure starts to rise as the compression stroke occurs and jumps rapidly at combustion. During the power stroke and exhaust stroke, the cylinder pressure gradually drops back to zero. The average pressure for the complete cycle is used to calculate engine power.

The theoretical or indicated engine horsepower can be determined using the mean effective cylinder pressure (MEP). The indicated or theoretical power is the power produced in the engine by combustion of the fuel. It does not account for the power lost to friction and other losses. The MEP can be determined from the graph, but these calculations are beyond the math level for this text. The equation for calculating indicated engine power in units of horsepower for 4 cycle engines is:

The equation is changed for 2 cycle engines:

where P = Mean effective pressure (psi); S = Stroke (ft); A = Cylinder area (in2); N = Engine speed (rpm); n = Number of cylinders. Note: the dimension for stroke in this equation is feet to cancel out the units of feet in the 33,000 constant.


0 45 90 135 130 225 270 31 5 360 405 450 495 540 585 630 675 720

Crankshaft rotation

FIGURE 5.8. Cylinder pressure example.

Problem: Determine the indicated horsepower for a four cycle single cylinder engine, that has a mean effective pressure of 152.0 psi. The bore is 2.75 in, the stroke is 2.65 in, and the engine speed is 3,000 rpm.

Solution: The engine power for 4 cycle engines can be calculated using the following equation:

152.0 — x i 2.65 in x -j^— ) x I n x i —^—j I x 3,000 rpm

33,000 x 2


66,000 P

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