Diesel Engine FundamentalsDOE-HDBK-1018/1-93DIESEL ENGINESOperationalTerminologyBefore a detailed operation of a diesel engine can be explained, several terms must be defined.Boreand StrokeBore and stroke are terms used to define the size of an engine. As previously stated, borerefers to the diameter of the engine's cylinder, and stroke refers to the distance the pistontravels from the top of the cylinder to the bottom. The highest point of travel by thepiston is called top dead center (TDC), and the lowest point of travel is called bottomdead center(BDC). There are 180oof travel between TDC and BDC, or one stroke.EngineDisplacementEngine displacement is one of the terms used to compare one engine to another.Displacementrefers to the total volume displaced by all the pistons during one stroke.The displacement is usually given in cubic inches or liters. To calculate the displacementof an engine, the volume of one cylinder must be determined (volume of a cylinder =(pr2)h where h = the stroke). The volume of one cylinder is multiplied by the numberof cylinders to obtain the total engine displacement.Degreeof CrankshaftRotationAll events that occur in an engine are related to the location of the piston. Because thepiston is connected to the crankshaft, any location of the piston corresponds directly toa specific number of degrees of crankshaft rotation. Location of the crank can then be stated as XX degrees before or XX degrees after topor bottom dead center.FiringOrderFiring orderrefers to the order in which each of the cylinders in a multicylinder enginefires (power stroke). For example, a four cylinder engine's firing order could be 1-4-3-2.This means that the number 1 cylinder fires, then the number 4 cylinder fires, then thenumber 3 cylinder fires, and so on. Engines are designed so that the power strokes areas uniform as possible, that is, as the crankshaft rotates a certain number of degrees, oneof the cylinders will go through a power stroke. This reduces vibration and allows thepower generated by the engine to be applied to the load in a smoother fashion than if theywere all to fire at once or in odd multiples.Rev. 0ME-01Page 17
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