Diesel Operational Terminology

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Before a detailed operation of a diesel engine can be explained, several terms must be defined.

Bore   and   Stroke

Bore and stroke are terms used to define the size of an engine.  As previously stated, bore refers to the diameter of the engine's cylinder, and stroke refers to the distance the piston travels from the top of the cylinder to the bottom.  The highest point of travel by the piston is called top dead center (TDC), and the lowest point of travel is called bottom dead center (BDC).  There are 180o of travel between TDC and BDC, or one stroke.

Engine   Displacement
Engine displacement is one of the terms used to compare one engine to another. Displacement refers 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 displacement of an engine, the volume of one cylinder must be determined (volume of a cylinder =(πr2)h  where h = the stroke).  The volume of one cylinder is multiplied by the number of cylinders to obtain the total engine displacement.

Degree   of   Crankshaft   Rotation
All events that occur in an engine are related to the location of the piston.  Because the piston is connected to the crankshaft, any location of the piston corresponds directly to a specific number of degrees of crankshaft rotation.  Location of the crank can then be stated as XX degrees before or XX degrees after top or bottom dead center.

Firing   Order
Firing order refers to the order in which each of the cylinders in a multicylinder engine fires (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 the number 3 cylinder fires, and so on.  Engines are designed so that the power strokes are as uniform as possible, that is, as the crankshaft rotates a certain number of degrees, one of the cylinders will go through a power stroke.  This reduces vibration and allows the power generated by the engine to be applied to the load in a smoother fashion than if they were all to fire at once or in odd multiples.