cylinder during the complete operating cycle. This pressure  variation can be represented by the kind of graph shown in  Figure 1-39 . Notice that the cylinder pressure rises on the  compression stroke, reaches a peak after top center, and  decreases as the piston moves down on the power stroke.  Since the cylinder pressure varies during the operating cycle,  an average pressure (line AB) is computed. This average  pressure, if applied steadily during the time of the power  stroke, would do the same amount of work as the varying  pressure during the same period. This average pressure is  known as indicated mean effective pressure and is included  in the indicated horsepower calculation with other engine  specifications. If the characteristics and the indicated mean  effective pressure of an engine are known, it is possible to  calculate the indicated horsepower rating. manifold pressure, and the magnitude of both affects the  pressure of the fuel charge just before the instant of ignition.  If the pressure at this time becomes too high, pre-ignition  or detonation occur and produce overheating. Pre-ignition  is when the fuel air charge starts to burn before the spark  plug fires. Detonation occurs when the fuel air charge  is ignited by the spark plug, but instead of burning at a  controlled rate, it explodes causing cylinder temperatures  and pressures to spike very quickly. If this condition exists  for very long, the engine can be damaged or destroyed. One of the reasons for using engines with high compression  ratios is to obtain long-range fuel economy, to convert more  heat energy into useful work than is done in engines of low  compression ratio. Since more heat of the charge is converted  into useful work, less heat is absorbed by the cylinder walls.  This factor promotes cooler engine operation, which in turn  increases the thermal efficiency. Here again, a compromise  is needed between the demand for fuel economy and the  demand for maximum horsepower without detonation.  Some manufacturers of high compression engines suppress  detonation at high manifold pressures by using high octane  fuel and limiting maximum manifold pressure. The indicated horsepower for a four-stroke cycle engine can  be calculated from the following formula, in which the letter  symbols in the numerator are arranged to spell the word  “PLANK” to assist in memorizing the formula: The indicated horsepower produced by an engine is the  horsepower calculated from the indicated mean effective  pressure and the other factors which affect the power output  of an engine. Indicated horsepower is the power developed  in the combustion chambers without reference to friction  losses within the engine. This horsepower is calculated as  a function of the actual cylinder pressure recorded during  engine operation.  To facilitate the indicated horsepower calculations, a  mechanical indicating device, such as is attached to the  engine cylinder, captures the actual pressure existing in the  In the formula above, the area of the piston multiplied by the  indicated mean effective pressure gives the force acting on the  piston in pounds. This force multiplied by the length of the  Figure 1-39. Compression ratio. 1-31 Top center BC Bottom center TC BC Indicated mean effective pressure (IMEP) Spark Pressure in cylinder Peak pressure Compression Power Exhaust Intake A D C B E P I Indicated Horsepower Where: P = Indicated mean effective pressure, in psi L = Length of the stroke, in feet or in fractions of a foot A = Area of the piston head or cross-sectional area of the  cylinder, in square inches N = Number of power strokes per minute:   rpm 2 K = Number of cylinders Indicated horsepower =  PLANK 33,000