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SECTION II DIeSeL eNGINe FUNDaMeNtaLS
Engine Terminology
When you are learning the fundamentals about anything, including engines, it is helpful to start with common vocabulary and terminology. Understanding the following terms and concepts will help you understand this and the following chapters. ■ Bore: The diameter of a cylinder FIGURE 6-4A . This measurement is used as a reference point for measuring cylinder wear. ■ Stroke: The distance traveled by the piston from the top to the bottom of the cylinder FIGURE 6-4B . ■ Stroke ratio: The ratio between a cylinder’s bore and stroke, also referred to as squareness. This figure can be found using the following formula: Bore/Stroke = Stroke Ratio Engine application characteristics, or what types of work an engine is best suited for, are determined by the bore/stroke relationship FIGURE 6-5 . Breathing characteristics and valve timing characteristics of engines also change due to the shape of the areas where gas is exchanged between the intake and exhaust valves and the time the pistons take to move through the top and bottom of their strokes. ■ Square engine: An engine that has equal, or nearly equal, bore and stroke dimensions. Square engines are commonly used in spark-ignition (SI) passenger cars. They are a reasonable compromise between highrevving, over-square performance engines and undersquare, high-torque diesel or truck engines. ■ Under-square engine: An engine with a stroke dimension that is longer than the bore dimension, also known as a long-stroke engine. These engines are used to
A
Bore Stroke
■
■
Over Square
Under Square
Bore = 1 Stroke
Bore > 1 Stroke
Bore < 1 Stroke
perform heavy hauling and vocational work, such as diesel or SI engines used in pick-up trucks. Under-square engines have a stroke ratio more useful for operating at low speed and developing peak torque, the engine speed where cylinder pressures are highest, at lower engine speeds. In under-square engines the crankshaft throw, which is the distance between the centerline of the crankshaft main bearing journals and the crankpin, is longer. This naturally produces more torque, a measurement of rotational force transmitted from the crankshaft to the flywheel, from the same cylinder pressure as a square or over-square engine. The downside of an under-square engine is that the long stroke length increases friction and mid-stroke piston speed. More cylinder side thrust from the piston and greater crankshaft stress takes place in under-square engines. Under-square engines need wider crankcase dimensions to accommodate longer throws on the crankshaft compared to over-square engines, which are not as large perpendicularly to the crankshaft (they have a smaller vertical height). Over-square engine: An engine with a stroke that is shorter than the bore diameter. Over-square engines are best suited to high-speed operation. Stroke ratios in over-square engines are better adapted to use in high-speed applications where engines need to develop the highest torque at high speed. Over-square engines, also called short-stroke engines, are commonly used by cars with SI systems. The short stroke allows the piston to change direction quickly at top dead center (TDC) and bottom dead center (BDC), which allows higher revolutions per minute (rpm) without excessive piston speed. Cylinder displacement: The volume displaced by the piston as it moves from TDC to BDC FIGURE 6-6 . Cylinder displacement is also called swept volume. Cylinder displacement can be found using the following formula: Cylinder Displacement = π × Radius2 × Stroke
Bottom Dead Center
A. Bore. B. Stroke.
Square
FIGURE 6-5 The bore/stroke relationship predicts the best application for an engine.
B
FIGURE 6-4
= Squareness
■
Total cylinder displacement: The volume of all of the cylinders in an engine FIGURE 6-7 . Total cylinder
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