The power output of the lawnmower engine is not directly proportional to the number of RPMs, as the simple formula Equation (6.1) suggests. It actually reaches a peak at about N = 2500 RPM and declines for higher RPM. This behavior can be approximated by the formula: η_Mech = exp(-1.26 × 10^-7 × N²).¹⁶ Prepare a graph of power in kW versus RPM from 0 to 8000 in increments of 500; comment on the shape of the graph.

Refer to a 3.5 horsepower lawn mower engine at a nominal speed of 2.50 × 10³ RPM. A typical lawn mower engine, and the inside of its cylinder head, is shown below. Note: the depression in the cylinder head is the clearance for the engine’s valves. If such an engine is available to you in class, you can disassemble it and measure the bore, stroke, and clearance volume, CV, of the engine.

Equation (6.1)

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Valve clearances Piston Valves Fully assembled engine Partially disassembled engine