Fuel System

The fuel system, which includes in reality, emission controls, feeds your gasoline engine or diesel engine the fuel that it needs to run. (There are also natural gas, hydrogen cell, and a few other experimental fuel systems. They are so rare and few at this point that our discussion excludes them. They well may be players in the near future.) There are many parts to the entire fuel system, with some vehicles having more than others. The failure of ANY part in the fuel system usually means that the engine will not run. If it is running when the part fail, the engine will probably shut down. A typical fuel system consists of these parts:

Fuel - A gasoline engine runs on gasoline, or mixtures that are primarily gasoline. There are various octane ratings, various additives for one reason or another, and various methods of refinement, but the fuel is for all practical purposes, gasoline. Temperature, humidity, moisture saturation, various other impurities and the octane value of the gasoline, all affect the combustion quality of the fuel. The higher the octane number (see our Octane Rating Calculator), the better the knocking resistance of the fuel. Diesel engines run on diesel fuel, either pure kerosene or a derivative of kerosine. It also comes in various refined products and grades for commercial use, industrial applications, farm and agricultural use, aviation, and automotive applications. See Diesel and Gasoline Engines for more detail on the differences in the engine types.

Fuel delivery - There are four delivery systems that I know of, carburetors, fuel injection, superchargers and turbochargers, the latter two being used in combination with one of the first two.

Fuel tank - A storage and "waiting to be used" reservoir tank for your vehicle's fuel. The fuel tank or tanks connect to the fuel cap, which is what you open to put fuel in the vehicle. The part that is between the tank and the cap is called the filler neck or filler tube. Just inside the cap of modern generation vehicles is a limiting opening to prevent gas station leaded gas pump handles from fitting into the unleaded vehicle's filler tube. Some fuel tanks and fuel systems are pressurized; most are not. Some tanks have a fuel expansion valve, some contain the fuel pump, some have a filter screen in them, all have fuel gauge sending units in them and all have a fuel pickup tube connecting to the fuel line. Some fuel tanks have a small drain valve in the lowest collecting point to drain water and contaminants, such as water, from the tank.

Fuel gauge - This is an indicator, either a meter or digital display, showing the quantity of fuel in the tank. There are also gauges to show the flow rate, fuel pressure and fuel temperature on some systems. There is a sending unit for quantity in the fuel tank that based on a float, sends an electrical value to the gauge, indicating a value from empty to full.

Fuel pump - On newer vehicles the fuel pump is often installed in the fuel tank to pump fuel into the fuel line. Older vehicles have the fuel pump attached to the engine itself or attached to the fuel line between the fuel delivery device and the fuel tank. Fuel pumps can be electrical, mechanical or in a very few cases, vacuum driven. If the pump is physically attached to the engine, it is probably mechanical or vacuum, or a combination of both. Otherwise it is probably electric and is powered by your vehicle's battery and electrical system. The fuel pump moves fuel from the tank to the delivery system; in the process it has to deliver enough fuel to supply the requirements of an engine under all normal operating conditions, maintain the proper pressure in the line between the delivery device and the pump to prevent the fuel from boiling, and to prevent fuel system vapor lock. Not all systems deliver a constant SUPPLY or RATE of fuel; some however require it.

Fuel lines - Rubber, metal or neoprene tubing or hose that carries fuel from one part of the fuel system to another. As a normal rule, they are made of rolled steel or, sometimes, of drawn copper on new vehicles. Steel tubing, when used for any type fuel lines, is generally rust protected by being copper or zinc plated. Fuel lines are placed as far away from exhaust pipes, mufflers, and manifolds as possible, so that excessive heat will not cause vapor lock. They are attached to the frame, the engine, and other units in such a way that the effect of vibration is minimized, and so that they are free of contact with sharp edges which might cause abrasive wear. In areas where there is a lot of real or potential movement, as between the vehicle`s frame and rubber shock mounted engine, short lengths of fuel resistant flexible tubing are used.

Fuel filter - Virtually all systems contain one or more fuel filters. Some delivery devices, certainly fuel injectors, are more critical than others requiring the use of extremely clean fuel. Clean fuel is one major factor to engine life and performance. Fuel contaminants, including water, cause rough running engines or complete failure. Fuel injectors have tiny openings which clog easily from dirty fuel; properly filtering the fuel is the only way to prevent this clogging process. Carburetors also have small openings for fuel, called jets. While they are sensitive to contaminants also, they are not as sensitive as injectors. Filters can be anywhere in the system but there is usually at least one immediately (prior to) before fuel enters the delivery device. They can be metallic composition (often bronze or brass), made of paper, woven fibers or composite material. Most contaminants come from the ingestion of fuel into the tank from your fuel station.

Throttle valve - All delivery systems MUST have a throttle valve or similarly purposed device connected to both the accelerator inside the vehicle and the point where fuel enters the engine. This controls the quantity of fuel allowed into the engine. For example, when you depress the accelerator with your foot, the valve opens proportionately to your foot pressure, allowing more or less fuel into the engine, thus controlling your speed potential.

Carburetor - With a carburetor as the fuel delivery device, often termed a normally aspirated engine, the fuel and air is mixed in the carburetor before it enters the intake manifold. The intake manifold then delivers the pre-mixed air/gas fuel mixture to the cylinders, all of them, during the intake stroke of each cylinder of the engine, through the cylinder's intake valve or valves. The mixture is then drawn in to the cylinder and both the intake and the exhaust valves subsequently close as part of the cycle. The opening and closing of the valves, lifting and depressing of the pistons within the cylinders, drawing in and exhausting of fuel and gases is controlled in a series of mechanical movements through the camshaft, which is geared directly or indirectly to the crankshaft. That is also synchronized with the time that a spark plug ignites the mixture. The whole process is generally called the "engine timing".

Fuel injection - Many newer cars have a fuel injection fuel delivery device, a system that injects the air and fuel, though not always pre-mixed outside of the cylinder, into each engine cylinder. Fuel injection is a alternative method of fuel delivery, eliminating the need for a carburetor. Fuel injection systems often produce better fuel economy, better performance, or both, and make the engine more responsive to changes in the environment. Modern fuel injection systems are electronic, controlled by one or more computers. They are far more sophisticated the carburetors or early forms of fuel injection, utilizing at a minimum, the vehicle's on board computer to control the flow of fuel electronically. See the Diesel and Gasoline Engines information page for subtle differences in fuel and delivery. Older fuel injection systems have a manifold, similar to a carbureted intake manifold, to put pre-mixed fuel into gas engine cylinders, all of them, during the intake stroke of each cylinder of the engine, through the cylinder's intake valve or valves. The mixture is then drawn in to the cylinder and both the intake and the exhaust valves subsequently close as part of the cycle. The opening and closing of the valves, lifting and depressing of the pistons within the cylinders, drawing in and exhausting of fuel and gases is controlled in a series of mechanical movements through the camshaft, which is geared directly or indirectly to the crankshaft. That is also synchronized with the time that a spark plug ignites the mixture. The whole process is generally called the "engine timing". Electronic Fuel Injection systems (EFI), generally pump air and atomized gas into the cylinder directly. All other processes are the same. Injectors on diesel systems inject only fuel into already injected and compressed air within the cylinder. All other processes are more or less the same.

Intercooler, air chiller or air cooler - This is a device that reduces the temperature of air entering the air delivery device. Cooler air has more molecules than warm air and can therefore mix with more fuel, producing a more intense combustion.

Turbocharger - Turbochargers, as are superchargers, are based on forced air induction into the fuel delivery or air delivery system, depending on the particular type of system. They are often called turbos for short. Following the principle that the more properly mixed fuel and air inside the cylinder at the time is compressed and ignited, the more powerful the explosion and subsequent thrust of the piston, eventually getting more power to the power wheels on the road. Though there are many different manufacturers and designs, a generic turbo has only two main parts. One is the compressor, connected to the vehicle's fuel and air delivery devices, and the other is a turbine, connected to the vehicle's exhaust system. The engine exhaust is directed through the turbo's turbine, turning the turbine wheel, the central part of any turbine. The turbine wheel is directly joined to the compressor wheel by a connecting rod. When the turbine wheel spins from exhaust gases' movement, it also spins the compressor wheel, propelling volumes of outside introduced but filtered air within the compressor, and forces it into the air delivery device of your engine. The effect of turbos is measured in pounds per square inch (PSI) of air boost. Though there is no "standard for boost", most turbos produce a boost range of between 3 and 10 PSI. More fuel is also added into the fuel delivery device in proportional amounts to the air, either by computer control or other less efficient methods of control. The result is a more powerful combustion, producing more power. Typically, turbos are used to produce more power; in such applications they are not fuel economical. However they can be used in high altitude applications (very thin air and low oxygen content) just to get an adequate amount of air for normal combustion. They perform best and most efficiently at mid-range to high engine RPM. It is possible to use a low end (low RPM) turbo in conjunction with the standard turbo. Forced air systems, if used on gasoline engines, generally require higher octane gas. (There may be a few other advantages and certainly some drawbacks in such use, but they are beyond the scope of this information.)

Supercharger - Superchargers, frequently just called blowers, are similar to turbochargers in overall operation with the exception of NOT using the vehicle's exhaust system to power the compressor. They use a parasitic action to the engine to compress the intake air, most often, a pulley and belt to turn the compressor. In reality, turbochargers are superchargers which are driven by engine exhaust, though they are not generally used in terminology interchangably. There are at least three types of superchargers in current automotive use, perhaps more. Since they are connected directly to the engine for compressor operation, the faster the engine speed, the faster the supercharger speed. Multiple units CAN BE used on one engine. The result is the same as a turbocharger. Forced air systems, if used on gasoline engines, generally require higher octane gas. (There may be a few other advantages and certainly some drawbacks in such use, but they are beyond the scope of this information.)

Ram induction - Ram air effect, known as ram induction, is also a forced air effect. This was first used in the early 60s. It uses the force of the vehicle moving into an air supply to capture and force air, pressured only by the speed of the vehicle, to do more or less the same thing as a supercharger or turbocharger. While the boost is usually only 1.5 to perhaps 3 PSI, it is non-parasitic on the engine. Control of fuel introduction is difficult to control and the only real application is for high speed racing, NASCAR or Indy style, or in aviation applications. It has not been used in any production vehicles in recent years.

Emission controls - Devices such as oxygen, NOx and temperature sensors, air pumps, catalytic converters, particulate filters, thermal management devices, fuel and air computers, and evaporative recyclers fall into the controls group. Virtually NONE of these can be serviced without the use of a computer (On Board Diagnostics (OBD and OBD II)) to diagnose problems or set the devices.

Emission valves - A positive crankcase vent valve (PCV) recycles most unburned gases by sending them back into the combustion chambers of the engine, where they are safely and reasonably efficiently, burned. Generically termed a breather, PCV is a system directing ambient atmospheric pressure to the engine's crankcase. The atmospheric pressure then pushes any blowby gases to a lower pressure area within. The air that is directed into the crankcase MUST first be filtered. Otherwise, the dust and sand particles, particularly in our environment, will destroy the internal engine parts though friction, wear and tear. When there is too much blowby, it is routed back through the crankcase breather element to the engine's carburetor or injector throttle body with the incoming fresh air to be re-introduced and burned in the cylinders. The breather also helps to extend the useful life of the engine's regular air filter due to the fact blowby contains oil vapor from the crankcase.