Introduction

The fuel system handles two main tasks necessary for diesel engine operation: (1) it provides storage space for the diesel fuel, (2) it delivers the correct quantities of filtered diesel fuel to the cylinders. It is also responsible for admitting an unrestricted supply of filtered air to the engine. The second function-delivering fuel to the cylinders-is handled by a high-pressure fuel injection system that is totally unlike the fuel injection systems used on contemporary spark-ignition engines

Because the diesel engine relies on a high compression ratio (23:1) to generate heat that will ignite vaporized diesel fuel, the maximum possible quantity of air must be admitted to each cylinder on every air intake stroke. So the diesel engine has no throttle to restrict airflow as on spark-ignition engines; the intake ports are always "wide open." In the case of the optional turbo-diesel engine, a turbocharger is used to pump air into the engine under pressure. The diesel engine's power output-hence its rpm-is determined solely by the quantity of fuel injected into the superheated air inside the combustion pre-chambers. At idle, the air-to-fuel ratio may be greater than 100:1; even at top speed, the mixture never approaches the comparatively rich stoichiometric air-to-fuel ratio of approximately 14.5:1 that is common to spark-ignition engines. This lean operation is largely responsible for the diesel engine's excellent fuel economy. (A stoichiometric ratio is one in which all fuel and oxygen are consumed in combustion; a diesel always has oxygen left over.)

The diesel engine's fuel injection pump draws fuel through a water separator and filter from the tank. This fuel is then put under very high pressure. The fuel distributor portion of the pump selects (according to the engine's firing order) the cylinder to which the fuel will go. Finally, the pressurized fuel enters the combustion pre-chamber of the correct cylinder via an injector. Because diesel fuel is not a particularly volatile liquid, thorough vaporization is necessary to obtain efficient burning. Each injector is designed both to restrict the admission of fuel until a pressure of about 125 bar (1786 psi) has been attained and also to spray the fuel into the pre-chamber with a uniform pattern of highly vaporized fuel.