76 Diesel Technology Copyright Goodheart-Willcox Co., Inc. timing is very precise and will vary between engine models. The following timing points are examples only. Fuel injection normally begins about 15° BTDC on the compression stroke and ends about 1.5° ATDC on the power stroke. The power stroke continues until the exhaust valve opens, usually around 90° to 95° ATDC. The exhaust portion of the power stroke continues until the intake ports are opened. Intake ports normally open around 60° BBDC on the power stroke and remain open around 60° ABDC on the compression stroke. This means the intake ports are open for about 120° of crankshaft rotation during the entire combus- tion cycle. The exhaust valves are timed to close at about 65° ABDC on the compression stroke. Exhaust valves are usually open for about 155° to 160° of the entire combustion cycle. Figure 4-32 illustrates some basic differences between typical four-cycle and two-cycle engines. During 720° of crankshaft rotation, a two-stroke engine completes two cycles, while a four-stroke engine completes just one. Two- cycle power strokes last approximately 90° of crankshaft rotation. In contrast, a single power stroke in a four-cycle engine lasts approximately 180° of rotation. Cylinder Number and Configuration Diesel engines can be classified by the number of cylin- ders in the engine. Single cylinder engines are often used for portable power plants or for running irrigation systems. For commercial on- and off-highway vehicles, four-, six-, and eight-cylinder engines are popular. For industrial uses such as power generation, locomotives, and marine propulsion, twelve-, sixteen-, twenty-, and twenty-four cylinder engines are used. The arrangement or configuration of cylinders is also used to distinguish diesel engines from one another. Inline Engines The most popular engine configuration is the vertical inline design, with six cylinders being the standard for most trucking and mobile equipment applications, Figure 4-33. Inline engines are less expensive to overhaul than V-type engines, and their components and accessory items tend to be more accessible and easy to service. Inline engines are Figure 4-31. Duration of piston strokes and valve positions in a two-stroke engine. (Detroit Diesel Corp.)