ENGINEMAN 1 & CTable 5-1.—Factors That Influence Engine Performance1. Fuel characteristics2. Compression ratio3. Engine operating conditionsCombustion chamber designValve arrangementsSize of valvesManifold arrangementsHot spots (presence/absence)Location of spark plugsNumber of spark plugs4. Pressure and temperature of air in the engine cylinderat start of compression5. Pressure/temperature of the charge in the enginecylinder at the start of compressionfactors are the mean effective pressure, the lengthof stroke, the cylinder bore, and the number ofrevolutions per minute (piston speed) of theengine. The latter, piston speed, is limited by thefrictional heat and by the inertia of the movingparts.Mean Effective PressureThe mean effective pressure (MEP) is theaverage pressure exerted on the piston during eachpower stroke, and is determined from a formulaor by means of a planimeter. There are two kindsof mep: indicated mean effective pressure (imep),which is developed in the cylinder and can bemeasured; and brake mean effective pressure(bmep), which is computed from the brakehorsepower (bhp) delivered by the engine.Diesel EnginesXXXXXXXXGasoline EnginesXXXXXXXXXXLength of StrokeThe distance a piston travels between top andbottom dead centers (TDC, BDC) is known as thelength of stroke. This distance is one of the fac-tors that determines the piston speed. In somemodern diesel engines, piston speeds may reachabout 1600 feet per minute (fpm).Cylinder BoreBore is used to identify the diameter of thecylinder. The cylinder bore must be known inorder to compute the area of the piston crownupon which the pressure acts to create thedriving force. This pressure is calculated and ex-pressed for an area of one square inch as poundsper square inch (psi).5-2
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