TABLE OF CONTENTS | Authors Note | Even though carburettor fuel systems are now very rare, they are still used on some specialist vehicles. For this reason, and because it serves as a good introduction to fuel systems, I decided to include this section. |
All vehicle fuel systems consist of the carburettor or fuel injectors, the fuel tank, the fuel pump, and the fuel filter, together with connecting pipes. An engine works by the massive expansion of an ignited fuel air mixture acting on a piston. The job of the fuel system is to produce this mixture at just the right ratio to run the engine under all operating conditions. There are three main ways this is achieved:
petrol is mixed with air in a carburettor;
petrol is injected into the manifold, throttle body cylinder or to mix with the air;
diesel is injected under very high pressure directly into the air already in the engine combustion chamber.
This section will only examine the carburettor systems; diesel and injection comes under engine management later.
Figure 7.7 shows a simple fixed choke carburettor operating under various conditions. The float and needle valve assembly ensures a constant level of petrol in the float chamber. The venturi causes an increase in air speed and hence a drop in pressure in the area of the outlet. The main jet regulates how much fuel can be forced into this intake air stream by the higher pressure now apparent in the float chamber. The...
