Quantcast Mean Effective Pressure

ENGINEMAN  1  &  C Table  5-1.—Factors  That  Influence  Engine  Performance 1. Fuel characteristics 2.  Compression  ratio 3. Engine operating conditions Combustion chamber design Valve  arrangements Size of valves Manifold arrangements Hot spots (presence/absence) Location of spark plugs Number of spark plugs 4. Pressure and temperature of air in the engine cylinder at start of compression 5.  Pressure/temperature  of  the  charge  in  the  engine cylinder at the start of compression factors are the mean effective pressure, the length of stroke, the cylinder bore, and the number of revolutions  per  minute  (piston  speed)  of  the engine. The latter, piston speed, is limited by the frictional heat and by the inertia of the moving parts. Mean Effective Pressure The  mean  effective  pressure  (MEP)  is  the average pressure exerted on the piston during each power stroke, and is determined from a formula or by means of a planimeter. There are two kinds of  mep:  indicated  mean  effective  pressure  (imep), which is developed in the cylinder and can be measured;  and  brake  mean  effective  pressure (bmep),   which   is   computed   from   the   brake horsepower (bhp) delivered by the engine. Diesel  Engines X X X X X X X X Gasoline  Engines X X X X X X X X X X Length of Stroke The distance a piston travels between top and bottom dead centers (TDC, BDC) is known as the length of stroke. This distance is one of the fac- tors that determines the piston speed. In some modern  diesel  engines,  piston  speeds  may  reach about 1600 feet per minute (fpm). Cylinder Bore Bore is used to identify the diameter of the cylinder. The cylinder bore must be known in order to compute the area of the piston crown upon   which   the   pressure   acts   to   create   the driving force. This pressure is calculated and ex- pressed for an area of one square inch as pounds per square inch (psi). 5-2


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