Why Injectors?

Cars use four-stroke internal combustion engines that burn fuel to create motion. Fuel is introduced into the engine through the use of carburettors and injectors. The 4 Cycles of the internal combustion engine are:

  • Intake stroke – Suck
  • Compression stroke – Squeeze
  • Power Stroke – Bang
  • Exhaust Stroke – Blow

The engine sucks in the air using the pistons, compresses the air, introduces fuel and spark, which then creates little explosions inside the engine. These explosions (gas expansion) pushes down the piston and the resultant gases are then blown out in the Exhaust stroke. For more information on how engines work, click “here

For this post, we want to discuss how fuel is introduced into the cylinder to enable combustion.


Early car engines, as far back as the Ford Model T, used carburetors to introduce fuel into the combustion chamber.

Well, how do they work?

Carburetors work using the Venturi principle. The Venturi Principle/Effect states that when air flows through a restriction of a tube (narrowing), the air speed increases, and air pressure decreases. This drop in air pressure draws fuel up and out of the carburetor bowl through a series of precisely machined jets and passages, and into the cylinder head. 

It uses atmospheric pressure to create a pressure differential which then causes fuel, which is been supplied through an engine-driven fuel pump into a bowl in the carburetor, to be sucked up from the bowl and sprayed into the air being sucked into the engine. 

A carburetor
A carburetor


Injectors, however, are a more efficient measured and direct means of introducing fuel into the combustion chamber. They get their name from their look and their job. They look like injection syringes and inject fuel into the combustion chambers with an ability to open and close many times per second.

Fuel injector
Fuel injector

Injectors need many complicated systems to function. Some of them are;

  1. Airflow sensor
  2. Electronically controlled fuel pump
  3. Electronic Control Unit (ECU) or brain box
  4. Injectors
  5. Throttle (accelerator) position sensor
  6. Lambda sensor on the Exhaust

… to name a few

How injectors work.

On turning the key, the electronic control unit powers the fuel pump up. The fuel pump sends fuel through the fuel lines to the fuel rails which carry the injectors. 

Once the accelerator is depressed, a butterfly valve opens in the intake manifold, and it opens as wide as the accelerator is depressed, the throttle position sensor feeds the throttle position information to the ECU. At the same time, the butterfly valve which opens allows more air to rush into the engine, the airflow sensor then measures the rate at which air flows past it into the engine and feeds that information to the ECU too.


The ECU then uses this available information to calculate the volume of air available and Intern gauges the necessary amount of fuel which will be needed for either power or efficiency depending on the throttle position, and then sends electricity of a specific amperage, voltage and frequency to the injector sot that it opens and sprays a specific volume of fuel as a mist into the combustion chamber.

An injector at work
An injector at work

And it repeats the cycle at least seven hundred times per minute.


  • Emissions and fuel economy. Fuel injection, because it can be more precisely controlled, results in more efficient use of fuel, reduced fuel consumption and fewer emissions, which is the main reason it began to replace the carburetor in the 1970s.
  • Power and performance. Again, because fuel injection and modern electronic controls are more accurate, fuel delivery can be tuned to match driver demand. Carburetors are precise, but not accurate, in that they cannot account for changes in air or fuel temperature or atmospheric pressure.
  • Cost and complexity. Being purely mechanical devices, carburetors have it hands down over fuel injection with regard to cost and complexity. With a can of carburetor cleaner, simple hand tools and maybe a couple of spare parts, you can rebuild a carburetor on your porch or at a rest stop. Whereas with fuel injection, even with years of training and experience and a few thousand dollars in diagnostics gear, you will still need a tow truck to get you off the road should your system burn out on you. Most small engines, such as those on motorcycles, lawn mowers and snow-blowers, are still equipped with carburetors, simply because they are not emissions-regulated, inexpensive, simple and reliable.


For more info, click here

In the next post, we will talk about how to detect if your injector is bad and what you should do about it.


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