CI Engine Fuel System
CI engines use diesel fuel and fuel injection system. The fuel system in a diesel engine is intended to supply fuel to the cylinder, atomise and partly distribute it in the combustion chamber. Early CI engines used air pressure to deliver the fuel into the cylinder, but nowadays liquid fuel, supplied by a reciprocating plunger pump, is delivered through small bore steel pipelines to the injector, which breaks up the fuel to the required degree and deliver it to combustion region in the chamber.
The fuel supply system including modern jerk pump system is presented in the chapter. The diesel engine speed is controlled by changing the area of the passage at the intake of the fuel injection pump. Therefore governors, which control the intake fuel to the cylinder, are dealt in the chapter at appropriate sections. The injection of fuel and various injectors are also given equal importance in the discussion.
In a normal engine, both induction of charge and exhaust of combustion take place at atmospheric pressure. In a supercharged engine exhaust pressure is atmospheric but the suction pressure is raised above atmospheric pressure which increases the pressure limit of the cylinder and hence its brake mean effective pressure (bmep) is also increased. A diesel engine is inherently suited for supercharging, therefore, is presented in the chapter.
CI Engine Fuel Supply System
The main components of the fuel system are fuel tank, fuel pipes, coarse filter, fuel-feed pump, fine filter, high-pressure pump and injector. Figure 10.1 presents a fuel system for diesel engines.
The high-pressure pump is the main and the most complicated component of the fuel system. It meters out the fuel matching with the engine duty, supplies it to the injector and ensures the required injection pressure. The fuel is pumped at a very high pressure through an injector with a nozzle from which it emerges as a fine spray into the cylinder at the end of compression stroke.
It is very important to correctly filter fuel oil before it reaches the injection pump even if the correct grade and type of fuel oil is used. Both coarse filters and fine filters are used for filtration of the fuel oil. The fuel feed pump transfers fuel from the tank to the high-pressure pump (injection pump).
Generally an in-line pump or a distributor pump is fitted with the engine. The pump does not function correctly in the presence of air in the system, and therefore venting screws or valves
are provided at various locations for bleeding out of air. This is particularly necessary if the fuel supply line is distributed or is running out of fuel. A priming lever, generally fitted to the feed pump, is used to operate the pump manually to bleed out of entrapped air in the system.
Fig. 10.1. CI engine fuel system.
Lubrication of main working parts of the system is usually carried out by the fuel oil itself. A predetermined leakage of oil past the injector needle is directed to waste or returned to the fuel tank.
The classification of fuel system for diesel engine is based on the design of its principal elements, the high-pressure pump and the injector. Diesel motor vehicles generally employ two types of fuel systems.
(i) High pressure pump and injector as separate units
(ii) High-pressure pump and injector as a single unit.