How Fuel Injection Systems Work

A detailed explanation of how modern fuel injection systems work and their advantages that help keep fuel mileage and performance under control.

Fuel injection is the biggest advancement to the automotive fuel delivery system since the invention of the automobile, itself. Better fuel mileage, cleaner smelling exhaust, and when was the last time you had to pump the gas pedal 30 times in the middle of winter to get the car started?

Fuel injection also made the Remote Starter possible.

This article will cover both basic systems, though true understanding only comes with time and experience.

There are two different fuel delivery systems, Throttle Body Injection (TBI) and Multi-Port Injection (MPI).

There are variations of these depending on the manufacturer called Computerized Fuel Injection, Port Fuel Injection, Central Fuel Injection and so on, but they are all the same concept as TBI and MPI.

The easiest and first setup to come about was the TBI system in the 1980s.

For over 90 years engines had fuel delivered via a carburetor (carb) where air was drawn in through a small hole - creating a low-pressure center -, which drew fuel from a bowl within the carb. From there the air and fuel mixture was drawn into the individual chambers by vacuum and burned by the spark plugs.

Today, the carb has been replaced by a similar looking 'Throttle Body', which contains one or more injectors.

Instead of relying on a low pressure center to move fuel from the bowl of a carb the fuel pump pressurizes the fuel and forces it out as needed.

This way, the computer can control the amount of fuel needed at any given time.

Underneath the Throttle body the fuel and air move to the individual chambers via the intake tunnels just as in a carbureted engine and burned.

There isn't a lot of science or math to this set up, which makes it easier to install and cheaper - with less parts - overall. There are still drawbacks, however, as you'll see explained with MPI.

The second delivery system manages fuel flow and efficiency much better than TBI.

Instead of having a carb or TBI unit placed at the central inlet of air, there are several injectors (one per cylinder) placed just at the entrance of the combustion chamber.

Air flows into the engine at the throttle plate and travels down the intake manifold toward the chamber. Just as the air reaches the opening into the chamber an atomized burst of fuel is added to it.

From there, both the fuel and air enter the chamber and are burned to produce power.



The MPI injection system is much more efficient and creates more power this way because there is no loss of fuel to the chamber, meaning the fuel can't come into contact with the walls of the intake tunnels. This can create a pool of liquid gasoline that doesn't make it to the burning process and thins out the air/fuel mixture.

Plus, with more injectors and less travel between each nozzle and the chamber the computer can calculate a better amount to add to each individual cylinder. The computer doesn't have to predict how much fuel will be needed and how much will be lost to the intake walls.

But, how does the computer know when and how much fuel to add?

There is a complex system of sensors on today's vehicles, most of which are dedicated to fuel delivery and spark (burn) timing.

All of these sensors "talk" to the computer and to each other to "decide" when fuel or spark is to be delivered.

The Crankshaft Position (CKP) sensor monitors a specially tabbed ring on the crankshaft that tells the computer where it is in the firing sequence. "Is the crank at number one?"

The Camshaft Position (CAM) sensor monitors the camshaft (which rotates at one half the speed of the crankshaft) to let the engine know that the mechanical parts are in proper working order and at which stroke the engine is at.

"If the crank is at number one, is this the compression stroke (where fuel would be added) or the exhaust stroke? (Where fuel would be a bad thing)."

The Throttle Position (TPS) sensor measures the angle of the throttle plate, or "How far is your foot pushing the pedal?"

Engine Coolant Temp (ECT) sensors monitor how warm the engine is at any given point. Colder engines require more fuel than warmer ones.

Intake Air Temperature (IAT) sensor let the computer know how warm the air coming into the engine is. The warmer the air, the thinner it becomes. Therefore, it'll need less fuel mixed with it.

Mass Air Flow (MAF) and Manifold Absolute Pressure (MAP) sensors measure the volume of air, or 'how much', is coming into the engine. This is how the computer knows what altitude the car is in relation to Sea Level because, just like with temperature, the higher the altitude the thinner the air.

Knock sensors tell the computer if the spark plugs are firing at the right time. If a spark plug is late, the fuel and air can ignite itself in an uncontrolled manner and do severe damage to the engine.

Finally, Oxygen (O2, or HO2S) sensors are mounted in the exhaust pipes to monitor the burned mixture. Depending on make, model and year there can be between 1 and 4 sensors in the exhaust.

What they are looking for is oxygen content left over after the engine is done with it. If the content is low, the sensors tell the computer to add less fuel. If there is a lot of oxygen then the engine is running 'lean' and the computer adds more fuel.

All of these sensors combine their knowledge. The computer looks this all over to determine the best burn mixture for power and how to keep more fuel in the tank instead of wasting it out of the exhaust. It also determines the best way to keep the engine from being damaged due to a lean mix, which can burn up a piston and keeping the mix from being to rich and destroying a very expensive catalytic converter that cleans up the toxic fumes created from the engine.

When the 'Check Engine' light comes on the computer is telling the driver that something isn't right. Either a sensor has stopped working, or the information isn't right and could be creating damage.

'Check Engine' lights aren't used for practicing with electrical tape.

© High Speed Ventures 2011