Diesel Engine Fundamentals
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 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 =
(pr2)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 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.