Knowledge Of Automotive Fuel Systems

Basic Automotive Fuel Systems

To understand the basics of the automotive fuel systems used today is pretty simple. Understanding these fuel systems will definitely increase your confidence in sales and more importantly the confidence of your customers. Most vehicles in use today are powered by gasoline or diesel fuel, although alternative fuel vehicles powered by E8 5, natural gas, propane, and combinations of fuels are common. Bi-fuel vehicles have two separate fuel systems, where either one can power the vehicle. One fuel system is designed to run on gasoline and the other is designed for CNG (Compressed Natural Gas) or LPG (propane). There are dedicated CNG and LPG powered vehicles, with LPG being the most common. Hybrid vehicles use a combination of conventional fuel and electric power. They can operate on either conventional fuel with a small four-stroke piston engine or with an onboard electric motor and battery system. In most cases the gasoline engine is the primary power source and the electric motor is used as an assist, particularly at take off and higher speeds. This allows a smaller, more fuel efficient engine to be used. When the vehicle comes to a stop the engine may shut down completely, and when the throttle pedal is depressed the electric motor will provide the initial power with the gas engine starting as the vehicle begins to move. Battery charging is provided by the gas engine and through regenerative braking. Vehicles built today use some form of fuel injection system. The most common major components of a gasoline or diesel fuel system are the intake manifold, throttle body (except diesel), air filter, fuel injectors, PCM/ECM, wiring harnesses, airflow meter, fuel filter, fuel pump, fuel tank, fuel lines and pressure regulator.

Intake Manifold

The intake manifold comes in many shapes, depending on the application. It is basically a system of passageways or runners made of reinforced plastic or aluminum to convey air or the air/fuel mixture to the engine. The manifold is bolted to the head(s). An air filter removes dirt and dust from the intake air, to avoid damage to engine parts. Most vehicles use a port injected system where fuel is sprayed at the intake valve. In GDI (Gasoline Direct Injection) and diesel engines, fuel is injected under high pressure directly into the cylinder. In all of these applications only air is conveyed through the intake manifold, which allows engineers to tune the length of the manifold runners to optimize torque and/or horsepower at various engine loads.

Fuel Injectors

Fuel injectors are electronic solenoids that open to allow the fuel, which is under pressure from the fuel pump, to be sprayed into the manifold. In a port injection system, fuel is supplied to the injectors through a fuel rail, which is connected to the fuel pump. The fuel rail also contains a fuel pressure regulator that may adjust fuel pressure mechanically or be controlled by the PCM/ECM. The pump pressurizes fuel in the tank and supplies it to the fuel rail at a steady rate. Since the vehicle will use this fuel at varying rates depending on engine load, the regulator or some other PCM/ECM driven device varies the fuel supply to meet the demand. Dirt in the fuel can block the fuel lines and injector passages, as well as wear components due to the abrasive action of the dirt particles.

Fuel Injection Control & Types Of Fuel Injection

Electronic fuel injection uses a computer called a PCM (Powertrain Control Module) or ECM (Engine Control Module), to calculate how much fuel should be provided for efficient operation and low emissions. The computer uses data from various sensors, such as the Throttle Position (TP) sensor, Manifold Absolute Pressure (MAP) sensor, Mass Air Flow (MAF) sensor, Vehicle Speed Sensor (VSS), Engine Coolant Temperature (ECT) sensor, Intake Air Temperature (IAT) sensor and Oxygen (H02s) sensor. Not all of these sensors are always used. The type of sensor used may vary and additional sensors may also be used. There are three common types of gasoline fuel injection: Throttle Body Injection (TBI), port injection and GDI. TBI places one or more injectors in a carburetor like body, which also contains a throttle plate. Port injection has individual injectors in each intake port aimed at the intake valve. GDI positions the fuel injector inside the combustion chamber and allows for improved fuel delivery and timing. Some port injection systems fire injectors several at a time but most all current systems use Sequential Fuel Injection (SFI), which times the fuel injection to the engine firing order.

Fuel Filters, Fuel Pumps & Fuel Tanks

Fuel filters are an important part of every fuel system. They will vary in size and shape; some are mounted under the hood with the engine, while others are placed somewhere between the gas tank and the engine, or in the fuel tank. The common arrangement for a fuel injected vehicle is an in-tank fllter or screen that is not serviced as normal maintenance and an inline filter that is serviced as part of the maintenance schedule. Fuel must be moved from the fuel tank to the engine. This is done by means of a fuel pump, usually mounted inside the fuel tank. Some in-tank units are combination fuel pumps and fuel sending units for the vehicle’s fuel gauge. Many vehicles have an inertia switch that shuts off the fuel pump if the vehicle is involved in a collision. OBD II vehicles use pressure sensors and vapor control systems called Evaporative systems to capture fuel vapors during normal operation of the vehicle and draw them into the engine to be burned. The PCM/ECM monitors fuel tank pressure to determine the presence of system leaks. If a fuel cap is left off it can cause the MIL (Malfunction Indicator Light) to illuminate until the cap is replaced and, in some cases, the related DTC (Diagnostic Trouble Code) is cleared. Fuel lines carry fuel from the fuel tank to the fuel injection system. They are usually made from metal tubing or flexible hose. The hoses and tubes have to be specially designed to be used with fuel.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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