The purpose of the fuel system is to supply the right amount of fuel for the engine to burn, and control the speed of the engine.

Two types of  fuel systems have been used in gasoline engines in modern times:

...We will look at both of them; but first, how does the fuel get to the engine?

Parts of the fuel supply system:

1. Gas Tank - The gas tank is usually made of two pieces of stamped steel, spot welded together to form a container, or it can be made of plastic. It may have baffles inside to keep gas from sloshing around. It will have a fuel pickup located at the deepest part, a filler neck, a vent to allow air back in when the fuel is removed, and it will have a gauge to give the driver information on fuel level. Many cars have the fuel pump located in the gas tank. This keeps a normally noisy electric fuel pump quiet.
                       The gas tank can be located in many different locations. Safety should enter into the design of the location of the gas tank, but safety often takes a back seat to profit. All manufacturers have been guilty of poor gas tank locations at one time or another. Examples are the Ford Pinto, and the Chevrolet  & GMC C/K Pickup truck from 1973 - 1988. Both were notorious for having their gas tanks explode in a relatively minor accident. The safest place for a gas tank is somewhere within the perimeter of the four wheels. Gas tanks could be made crash proof ( fuel cells used in race cars are designed not to explode ) but again , profits often win over safety.

2. Fuel Lines and vent lines - Galvanized steel fuel lines carry the fuel from the tank to the engine. Manufacturers try to minimize the amount of rubber line used, because rubber deteriorates over time, but some rubber lines must be used to allow the engine to flex on its' mounts. From 1972 model year on, the gas tank and carburetor float bowl have been vented into a charcoal canister under the hood, so a vent line runs forward from the tank, to the canister. Most fuel injection systems vent the excess fuel supplied by the pump, but not used by the engine, back into the tank by a fuel return line.

3. Fuel Pump - Two types of fuel pumps are used in modern cars:
           Mechanical Pump - driven by the engine itself. Usually by an eccentric (cam) on the camshaft. The cam pushes down on a rocker arm which pulls up on a pull rod compressing a spring, pulling up the diaphragm, and filling the pump chamber with fuel.  When the cam turns, and releases the rocker, the spring is able to push down on the diaphragm, pumping the fuel to the carburetor. Pump pressure is regulated by the tension of the spring. 
           Electric Pump - can be mounted anywhere in the fuel line, but in modern cars is usually submerged in the gas tank. This keeps them cool, and quiet.
   Be aware that an electric fuel pump will supply a certain amount of pressure, and are different between an engine using a carburetor, and one using fuel injection.
   Pumps for fuel injection run fuel pressures of from 30 to 75 PSI, whereas on for a carburetor will only supply from 3 to 7 PSI. They are not interchangeable.
                   Electric pumps can be solenoid types which work similar to a mechanical pump, but instead of a cam moving the diaphragm, an electric solenoid does the work; or they can be an impeller type which uses an electric motor to drive a little impeller wheel. Fuel pressure is regulated by a pressure regulator located on the end of the fuel supply rail on the engine.

Carburetors

What does the Carburetor Do?

2 Jobs:

1. Mixes air and fuel together in the correct proportion under all conditions.

2. Regulates the speed of the engine.

    The carburetor must mix the air and fuel together in the correct proportions under all conditions, and those conditions change depending upon whether the engine is cold or hot, idling or at high RPM, accelerating, decelerating, or staying the same speed.
To deal with those changing conditions, the carb has different "circuits".

    Air fuel ratio will need to be anywhere from 8:1 to 15:1 by weight. We are not used to thinking about air as having weight, so if it was described by volume one gallon of fuel would need approximately 15,000 gallons of air to burn it.

    To help us understand how a carb works, we are going to "build" one.
We'll start with a tube or pipe for the air to go through, called an "air horn". We'll mount it on an intake manifold sp one carb will feed all our cylinders, otherwise, we'd have to use a separate carb for each cylinder. 

    We'll put a mounting flange on the bottom, and where the manifold meets the head, and put gaskets in between the flanges, so that the only way air can get into the cylinder, is through the air horn.

    Install a small reservoir called a float bowl (6),off the side of the air horn (1), to hold the gas before it goes into the air horn. We will maintain the level of fuel in the float bowl with a float which rides up and down on the gas (7).

    The float controls a needle (4) and seat (3) which shuts off the gas when float level is reached. A vent (2) allows atmospheric pressure to act on the fuel in the float bowl so we don't get a vacuum created and starve the engine for gas. The float can be made of hollow brass, or foam.

    Somehow, we have to get the gas from the float bowl into the air horn, this is done with a main discharge tube.

    Notice how the one end of the main discharge tube is in the bottom of the float bowl, and the other is in the air horn, with the nozzle just above float level.

    We also need the amount of gas to increase with the amount of air. 

Bernoille's principle states that: 
            "when the speed of an air stream increases, its pressure decreases".

    What this means is that if you force a stream of air to speed up, a vacuum is created in it. This is the same principle that allows airplanes to fly. The top of the wing is a curved surface, while the bottom is flat. As the wing moves through the air, the air moving over the wing speeds up, while the air moving under the wing stays the same speed. This creates a vacuum over the wing (lift). When the lift is greater than the weight of the aircraft, it will fly.
    In a carburetor, we put a smaller version of an airplane's wing around the main discharge tube, just above float level. We call this restriction, a "venturi". The faster the air moves through the venturi, the more vacuum is created in it. 

    There is a pressure differential between the float bowl and the venturi, and so the fuel flows out the main discharge tube and into the air stream. The fuel is drawn up from float level, and out into the air stream where it forms tiny droplets and vapourizes. The faster the air flows, the more fuel comes out. If a smaller venturi is added inside the main venturi, venturi vacuum is increased.

    A venturi is simply a restriction in the air horn that forces the air to speed up when it goes through it. The faster the air goes through, the more fuel comes out. This is called "venturi vacuum" and is greatest at fastest air flow. 

    An "air bleed" causes the fuel to break up into smaller droplets to vapourize better. the smaller the droplets we can break the gas into, the faster it will vapourize. 

    A "main jet " is added to the bottom end of the main discharge tube to limit the amount of fuel. The main jet is just a screw in brass plug with a hole drilled in it. The larger the hole is, the more gas will pass through it, so the richer the air - fuel ratio will be. If you want a leaner air - fuel ratio, put in a smaller main jet.

    We need to control engine speed. 

    This is done by restricting the flow of air into the engine by putting a shaft through the body of the carb, and placing a disc on the shaft. This is the "throttle plate". When it is placed across the air horn, it restricts the air flow and therefore, engine speed. When the plate is moved parallel to the air flow, there is no restriction, and therefore, the engine speeds up.

    Any restriction in the air flow creates a vacuum in behind the restriction. Remember the venturi; at high airflow, the venturi was a restriction, and therefore venturi vacuum was created in the venturi. At low airflow the venturi was not a restriction, the air passed through it easily. At closed throttle, the throttle plate restricts the amount of air getting into the engine, and therefore a vacuum is created downstream from the throttle plate. This is called "manifold vacuum". Manifold vacuum is greatest at closed throttle, and is least at wide open throttle. Manifold vacuum and venturi vacuum work opposite each other. When manifold vacuum is high, venturi vacuum is low, and vice versa.

    Our carburetor would now work. If we took our drawing down to the metalwork shop, built our carb, and bolted it on an engine, the engine would run. It wouldn't start very well, if at all,....it wouldn't idle,.......it wouldn't run at low speed,....... it wouldn't produce much power,........it wouldn't accelerate......, but if all we wanted to do was drive down the road at 40 MPH it would run! Obviously our carb still needs some work. We will go through it and solve its drivability problems one by one.

First Problem: 

       - the engine is difficult to start especially when it is cold, and doesn't run well until it is warm.

Why?

        - an engine runs on fuel vapour. Liquids don't burn. The problem with gasoline is that it is a liquid. Before we can get it to burn, we must mix it with air. Gasoline vapourizes very well at high temperatures, but not at low temperatures. 

Solution :    The Choke System

        Because only a 50% of the fuel vapourizes, we must add twice as much to make sure there is enough to burn. 

        A butterfly valve is added above the venturi, so that when the choke valve is shut, engine vacuum acts on the main discharge tube to draw a huge amount of air into the air stream

        Engine cranking speed is from 150 to 300 RPM, so at that speed, the engine's fuel requirement is low. The main circuit is capable of supplying enough gas to allow the engine to run at 2500RPM, so it can easily supply enough fuel for a rich mixture on start. When the engine is cranking over cold, the choke plate should be shut tight, and a huge amount of fuel enters the air stream.

        This mixture is the richest mixture of all, and is around 8 : 1. In the drawing above, the choke shaft is centered on the butterfly valve. This is, in fact, incorrect.  This would mean that that choke plate would not open  when the engine starts, it would remain shut, and the engine would starve for air. The shaft is really not on center, but off towards one side, so the choke plate opens slightly when the engine starts. this allows a leaner air - fuel ratio of around 10:1 when the engine starts.

        Because of the extremely rich mixture on choke, it is very easy to "flood" the engine. This is when there is so much liquid fuel in the combustion chamber that there is not enough air to support the combustion and the engine won't run. A choke "unloader" opens the choke plate when the throttle is depressed all the way. This allows the maximum amount of air through the engine to clear out all the extra gas, and allow the engine to start. 

        Choke Operation:

1.         Manual - a cable runs to the choke so when the driver pulls on a knob on the dash, the choke plate shuts. The worst thing about a manual choke, is that the driver forgets to turn it off.

2.         Automatic - A bi-metallic or thermostatic coil, which is sensitive to heat closes the choke plate when the engine is cold. When the coil heats up, it allows the off - center shaft, and a vacuum device called a "vacuum break", to open the choke. As the engine heats up, the choke plate automatically opens the choke up, leaning out the mixture. The bi - metallic coil has to be heated somehow. It can be heated by heat from the exhaust pipe, electrically, or by water from the cooling system.

        Fast Idle

        Because of the poor vapourization of fuel, poor fitting engine parts, thick oil, and extra friction inside the engine when cold, the engine would stall at idle if it was left the same as it was when hot. A "fast idle cam" opens the throttle slightly when the choke is on to prevent stalling.

        Heat Riser Valve

        When the engine is cold, a "Heat Riser Valve" forces hot exhaust gas to circulate under the carb, and around the intake manifold to warm it up, and help the fuel vapourize better. 

        Thermostatic Air Cleaner

        From 1968 on, most cars have used an air cleaner that draws hot air from a stove, or cover, around the exhaust manifold, to help vapourize the fuel better when the engine is cold. When the engine heats up, a thermostatic switch opens the cold air intake for better gas mileage.

Fuel Injection

Virtually all cars sold in North America have use electronic, computer controlled fuel injection to supply the fuel to the engine. Fuel injection gives better fuel economy, with more power and lower exhaust emissions, and better drivability. The computer has transformed the fuel system.

Fuel Supply

Electric fuel pumps can be located anywhere in the fuel system, but since any pump pushes better than it sucks, it makes more sense to locate it as close to the source of the fuel as possible. Most manufacturers locate the fuel pump in the fuel tank, below the level of the gas in the tank. Fuel pumps located in the tank are usually impeller type pumps whereas pumps located in the fuel lines are usually bellows type pumps. Because the pump will continue to run with the engine shut off, a relay controls the pump to make sure that doesn't happen. A roll over switch shuts the pump off in event of an accident. Details on its location, and procedure for re-setting it, will be in the service manual.

A motor drives a little vaned wheel like a water wheel called an impeller type pump. Liquid keeps the pump cool, so its important you don't let the fuel level get too low, or the pump burns out. The pump itself can be worth as much as $400.00, and two or three hours of labour to drop the tank and replace the pump.

Fuel is forced through a folded paper filter located somewhere between the pump and the fuel rail. Steel fuel lines and braided hoses route the fuel under the car body to the fuel rail on the engine. Fuel pressure is considerably higher when the car is equipped with fuel injection, so the lines and filter must be able to withstand more pressure. The fuel rail feeds fuel to the individual fuel injectors in a multi port system. A Shrader valve (like a tire valve) allows the technician to check fuel pressure in the system. A fuel pressure regulator maintains fuel pressure to the injectors at a constant value even though manifold vacuum and voltage to the pump are constantly changing. Fuel pressure in a carbureted system can be as from 1 to 7 psi. Fuel pressure in a fuel injected gasoline engine can be from about 35 to 75 psi. Fuel pressure in a diesel engine can be more than 600psi!

 The Fuel Injection System

Diesel Engines inject the fuel directly into the combustion chamber. How do they work?