LO 9-4 Identify and describe the characteristics of timing in a diesel engine
1. Valve Timing in a Diesel Engine
1.1. Valve Overlap
LO 9-04: Identify and describe the characteristics of timing in a diesel engine. |
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Valve Overlap Valve overlap is the duration, measured in degrees of crankshaft rotation, at which both the intake and the exhaust valves are open FIGURE 9-24. Overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke. Because fuel does not enter the cylinders with air during the intake stroke, diesels can use large amounts of valve overlap and not waste fuel or generate hydrocarbon (HC) emissions. A large valve overlap helps charge the cylinders with more air because the intake valve is opened before TDC at the end of the exhaust stroke. Pressurized intake air pushes out exhaust gases to improve scavenging, and it helps cool the exhaust valve as air crosses over from the intake port and across the open exhaust valve. Larger displacement, high horsepower engines hauling heavy loads commonly use a greater amount of overlap. These engines will have a bore-to-stroke ratio where the stroke length is much longer than the bore diameter, meaning it has an undersquare bore-stroke ratio. MD diesels with higher maximum engine speeds with more mixed use will approach a square bore-stroke ratio. A MD diesel will always have a much longer overlap period than any gasoline engine. In fact, some spark ignition racing engines will use camshafts or valve timing operation patterned after diesels. Large overlap helps an engine to breathe better, delivering more torque as engine speed and load increase. The downside of large overlap is that it also produces soot loading of the intake manifold and more intake noise, as exhaust pulses can travel from open intake and exhaust valves into the intake manifold at low speed. Small valve overlap delivers higher torque at lower engine speeds, but the power will drop off as the engine speed and load increase. |
FIGURE 09-24: Large valve overlap is characteristic of turbocharged diesel engines due to their stroke length and need for a larger volume of air intake. It produces superior breathing characteristics at higher revolutions per minute and heavy load conditions. |
| Technicians Tip | ||
| Soot loading of an intake manifold is common in diesel engines even without an EGR system. During low-speed, low-load operation, some exhaust gases will enter into the intake manifold when both the intake and the exhaust valves are open. EGR and closed crankcase ventilation systems can also contribute to soot loading and sludge formation in the intake manifold. Soot can safely accumulate in the intake manifold, which is a relatively harmless condition unless buildup is excessive. A soot-restricted air intake flow can generate NOx-related fault codes. |
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