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Got Gas?-Part 2-Fuel Injection Basics

© Garth R. Smith

In November we looked at that relic of 20th century transportation-the carburetor. Since nearly all cars built since the mid 80's have done away with the carb, let's look at what replaced it. There are several forms of fuel injection in use today, but they all work basically the same way. The fuel pump sends fuel through the fuel lines to the injectors which deliver it to the combustion cylinders.

One of the earliest forms of fuel injection was Throttle Body Injection or TBI. With this injection system the fuel pump, normally located inside the fuel tank, pumps fuel at high pressure, normally around 30 pounds per inch, through the fuel lines. Somewhere along the lines the fuel passes through a filter which filter out any small sediment particles out of the fuel. From the filter the gas continues on to the throttle body. The throttle body sits atop the intake manifold and looks quite similar to a carburetor. Unlike a carb, however, a throtlle body does not have choke flaps on top and has one or two small fuel injectors mounted on top. These injectors spray a small amount of fuel into the throtlle body where it is mixed with air and sent to the cylinders through the intake manifold.

The injectors spray a measured amount of fuel based on computer commands. The vehicles electronic control module, or ECM, is the cars brain. It determines the amount of fuel to be delivered based on information it receives from various sensors such as the throttle position sensor (TPS) which measures how much pressure you are putting on the accelerator. It also gets reading from the oxygen sensors (O2) which measure the amount of gas being burned in the exhaust and adjusts the fuel amount used to lessen emissions, the air temp sensor which measures the outside air temp. Many ECM's also must receive a reading on oil pressure before they allow the fuel pump to operate, this helps to prevent engine damage due to running an engine with no oil pressure.

After the injectors deliver the metered amount of fuel, any gas left over is recirculated to the fuel tank. This differs from old carburetors in that fuel was simply dumped into the engine and any left over was burned in the exhaust by the catalytic converter. Not very efficient is it? Because a tank of fuel may be recirculated several times before it is all used it is very important to replace the fuel filter about every 25,000 miles to ensure that it does not impede fuel flow.

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