TAILIEUCHUNG - Troubleshooting and Repair of Diesel Engines P2

To get around this restriction, many diesels incorporate an air pump in the form of an exhaust-driven turbocharger or a mechanical supercharger. Forced induction can double power outputs without violating the smoke limit. And, as far as turbochargers are concerned, the supercharge effect is free. That is, the energy that drives the turbo would otherwise be wasted out the exhaust pipe as heat and exhaust-gas velocity. The absence of an air restriction and an ignition system that operates as a function of engine architecture can wrest control of the engine from the operator. All that’s needed is for significant amounts. | Ignition and combustion 13 To get around this restriction many diesels incorporate an air pump in the form of an exhaust-driven turbocharger or a mechanical supercharger. Forced induction can double power outputs without violating the smoke limit. And as far as turbochargers are concerned the supercharge effect is free. That is the energy that drives the turbo would otherwise be wasted out the exhaust pipe as heat and exhaust-gas velocity. The absence of an air restriction and an ignition system that operates as a function of engine architecture can wrest control of the engine from the operator. All that s needed is for significant amounts of crankcase oil to find its way into the combustion chambers. Oil might be drawn into the chambers past worn piston rings or from a failed turbocharger seal. Some industrial engines have an air trip on the intake manifold for this contingency but many do not. A runaway engine generally accelerates itself to perdition because few operators have the presence of mind to engage the air trip or stuff a rag into the intake. Ignition and combustion SI engines are fired by an electrical spark timed to occur just before the piston reaches the top of the compression stroke. Because the full charge of fuel and air is present combustion proceeds rapidly in the form of a controlled explosion. The rise in cylinder pressure occurs during the span of a few crankshaft degrees. Thus the cylinder volume above the piston undergoes little change between ignition and peak pressure. Engineers exaggerating a bit describe SI engines as constant volume engines Fig. 2-4 . Compared to SI the onset of diesel ignition is a leisurely process Fig. 2-4 . Some time is required for the fuel spray to vaporize and more time is required for the spray to reach ignition temperature. Fuel continues to be injected during the delay period. Once ignited the accumulated fuel burns rapidly with correspondingly rapid increases in cylinder temperature and pressure. The .

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