LO 9-4 Identify and describe the characteristics of timing in a diesel engine

1. Valve Timing in a Diesel Engine

Valve Timing in a Diesel Engine

Before examining valve train mechanisms, it’s helpful to understand the unique valve timing characteristics of diesel engines because it influences valve train construction and operation. Two features of diesel engine operation that influence valve timing are as follows:

• Lower maximum engine speeds
• Stroke length

Lower engine speeds mean that larger, heavier valve train components are not as likely to be affected by inertia forces when valve train mechanisms change direction to either open or close the valves. Stroke length is longer in diesel engines to produce higher compression ratios. Because stroke length is established by the length of the crankshaft throw, longer throws in a diesel cause the piston speed to slow down at the top and bottom of each stroke relative to crankshaft speed. When changing direction while moving through bottom dead center (BDC) and top dead center (TDC) and back again, piston movement relative to crankshaft rotation is very limited. However, during mid-stroke, piston speeds are fastest in a diesel. Minimal piston movement at the top and bottom of each stroke allows the valves to remain open longer at the beginning and end of each stroke. The addition of a turbocharger to pressurize the intake systems allows more time to charge the cylinder with more air at the beginning of the intake stroke and allows exhaust gases to leave the cylinder.