Types of Engines

The two primary categories of engines are the spark ignition (Otto-cycle) and the compression ignition (Diesel-cycle). In the spark ignition engine, the fuel is metered and introduced into the engine by either a carburetor or an injection system, Figure 5.5. When a carburetor is used, the fuel is metered by jets and orifices in the carburetor and added to the air as it flows through the venturi of the carburetor. Mixing occurs as the fuel-air mixture moves through the intake system and into the cylinder. A spark plug ignites the fuel-air mixture at the proper time.

Fuel injection systems improve the fuel efficiency of a gasoline engine because the multiple sensors and computer can better match the fuel needs of the engine. Historically the additional cost of the system over a carburetor fuel system has limited its use to larger, multicylinder engines, but rising fuel costs and emission standards have been the impetus for increasing the use of fuel injection systems on smaller engines. An engine that uses injection does not have a carburetor; the fuel is usually injected into the air stream at the throttle body or intake port. In some engines, a single injector is located at the throttle body (throttle body injection) that replaces the carburetor. This single injector meters the fuel for all of the cylinders. In the port injection system, an injector is positioned just outside the intake valve (port) leading into each cylinder. The injector in conjunction with the injector pump meters and delivers the required amount of fuel to the cylinder. The fuel and air are mixed as they flow into the cylinder. Engineers are continuing to work on direct inject for gasoline engines and the technology is starting to be used in automobile engines.

In a compression ignition engine (Diesel) the fuel is injected directly into the combustion chamber, Figure 5.6. The injection is timed to occur just before TDC on the compression stroke. Combustion occurs almost instantaneously because compression causes the air temperature within the combustion chamber to rise to 1,000°F and above. Thus, there is no need for a spark plug to ignite the fuel. The injector pump controls the amount of fuel delivered to each cylinder.

FIGURE 5.5. Throttle body and port injection.
Ergonomics

Piston

FIGURE 5.6. Parts of diesel engine.

Piston

FIGURE 5.6. Parts of diesel engine.

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