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The Fuel System

    Just as your body needs food, your engine needs fuel. But how does it get from the gas tank to the cylinders?

Last time we talked about what an engine is. This week, we'll talk about how it burns the fuel and converts it into motion. In order to have combustion you need three things; Air, Fuel and Heat. In an engine there are three systems that do this. The Air Induction System, The Fuel System and the Ignition System. Since all manufacturers do things in a different way, we'll look at these systems in general.

First, getting the fuel to the engine. The fuel is drawn from the fuel tank by either a mechanical or electric fuel pump. All cars with Electronic fuel injection have an electric fuel pump. Cars with carburetors, generally, have a mechanical fuel pump. Electric fuel pumps are mounted inside the fuel tank as part of the fuel-sending unit. Mechanical fuel pumps are mounted on the engine block and are driven by the camshaft.

The fuel pump sends the fuel through steel lines to the fuel filter. The fuel filter removes any particles of dirt or metal that may be in the fuel. Some fuel filters will also remove moisture from the fuel as well. From the filter it goes either to the carburetor or the fuel injectors. In the case of the carburetor, the fuel is mixed with air to form the optimum fuel mixture for the current conditions.

The ideal fuel mixture is 14.7:1. That is to say 14.7 parts of air to 1 part of fuel. This is very difficult for a carburetor to do because it has no idea how much air is going into the engine. In order to do it properly, you need a four-gas emission analyzer to see what % of carbon monoxide (CO) and how many parts per million (PPM) of hydrocarbons (HC) is being emitted by the engine. By turning the adjuster screws on the carburetor and watching the emissions, you can come pretty close. In some states there are specific levels of CO and HC that must be met or the vehicle is not considered street legal. From the carburetor the fuel mixture travels through the intake manifold and is delivered to each cylinder.

In a fuel-injected engine, fuel is introduced into the engine by the fuel injector which is computer controlled. I'll explain more about that later. There are two, basic, types of fuel injection. Port fuel injection (PFI) and throttle body injection (TBI). In TBI there is one or two injectors mounted in a housing similar to a carburetor and the fuel is constantly injected into the air stream. In PFI there is one injector for each cylinder mounted in the intake manifold so that it is pointing towards the intake valve. By opening and closing, the injectors can, very precisely, meter the correct amount of fuel to the engine as determined by the Powertrain Control Module (PCM).

Lastly, there is a fuel pressure regulator that controls the pressure of the fuel in the lines. It has a vacuum line connecting it to the intake manifold to measure manifold vacuum. As engine RPM increases, manifold vacuum decreases causing the fuel pressure to increase.

Okay, so now you have an understanding of how the fuel gets to the engine. Next time we'll discuss the other two systems involved and see how they interrelate with each other. Until then, happy motoring!!!

Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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