High-Pressure Components

Light automotive diesel engine components are exposed to much higher operating temperatures, pressures, and forces than gasoline engines of similar size. Also, diesel engine parts are usually heavier and built more ruggedly than those of similar output gasoline engines. The high compression ratio heats the air in the combustion chamber to a temperature high enough to ignite the fuel when it is injected. These high injection pressures are needed to overcome the compression and combustion pressures in the combustion chamber and break up the fuel into atomized particles.

As previously described, because of these high pressures, the injector pump and the injector are made from highly polished, accurately sized components in order to limit loss of pressure in the system.

The injector pump can be an in-line type Figure 54-2, or it can be a rotary type Figure 54-3. Both types are driven by the engine. During operation, the quantity of air taken in on the intake stroke is not determined by the driver’s control of the throttle. However, the driver does control how much fuel is delivered to the engine.

A characteristic of all diesel engines is that at a fixed fuel setting, the amount of fuel delivered to the engine will increase as engine speed and pump speed increase. This is called the rising characteristic of the fuel injection system, and unless it is controlled, overspeeding of the engine will result. To achieve this control, all diesel engines use a governor to manage how much fuel is delivered from the injection pump, to the injectors, and into the engine.

FIGURE 54-2
A typical in-line fuel injection pump.
FIGURE 54-3
A Bosch VE distributor fuel injection pump.

Governors can be operated mechanically, pneumatically, hydraulically, or electronically. They can also be used in combination with one another. Usually, today’s governors are electronically controlled.

Diesel engines need assistance to make cold starting easier. Most diesel fuel injection systems inject extra fuel when starting to ensure sufficient fuel will vaporize and burn in the combustion chamber. To make cold starting easier, some engines use preheaters, also called glow plugs. They heat only the air; it is important to understand that they do not ignite the fuel during the combustion process, but simply heat the intake air so that during compression the air in the cylinder is preheated to a point where normal ignition/combustion can take place during the engine cycle.