All You Need to Know about Fuel Injection Systems in SI Engines


Do you know the fact that fuel injection systems have gained attention over carburetors for Spark-Ignition engines? Yes, the fuel injection system is better in many ways and has replaced carburetors in almost every premium motorcycle. Read ahead to know the function of the fuel injection system for SI engines and different types of fuel injection systems.




Function of Fuel Inject System

The fuel injection system in SI engine controls the ratio of air and fuel in the mixture and delivers it at the right time into the cylinder. Before the fuel injection system, carburetors were used. Carburetor depends on the vacuum created in the engine during the suction stroke. In comparison, a fuel injection system is electronically controlled to inject fuel at high pressure. Since an electronically controlled fuel injection system delivers at high pressure, it gives increased power by better atomisation and lowers emissions. A fuel injection system is easy to maintain and provides clean combustion.


Types of Fuel Injection System

We have two types of fuel injection systems based on the number of injectors. An injector is nothing but a small device used to spray fuel as a tiny mist rather than a jet. The two types of injection systems are,

  • Single Point Fuel Injection System and
  • Multi-Point Fuel Injection System


Types of Fuel Injection System

Single Point Fuel Injection System

Single point fuel injection system resembles a carburetor setup. It has one or two injectors placed at the throttle body, which is a passage through which the air intake of the vehicle is controlled. Hence this single point fuel injection system is also called throttle body injection. The injector is controlled by ECU( Electric Control Unit). 

The fuel in the fuel tank is pressurized by means of a fuel pump and sent to the injector. The injector receives the signal from ECU and then injects fuel into the throttle valve. The fuel mixes with air, and the air-fuel mixture enters the cylinder through the inlet manifold. This is how a single point injection system works. But in a single point injection system, the injector is centralized. This resulted in the uneven distribution of air-fuel mixture to each cylinder. To overcome this issue, a multi-point fuel injection system was invented.



Multi-Point Fuel Injection System(MPFI)

This multi-fuel injection system is almost similar to the CRDI system(Common Rail Direct Injection). As the name says, fuel is injected through multiple points. Each cylinder is provided with a separate injector and is connected with a common rail. In this, the injectors are placed near the intake manifold and hence called a port injector system.

The pressurized fuel is sent to the common rail and is then distributed to individual injectors. After receiving the signals from the ECU the fuel is sprayed and it is mixed with the air. Then the air-fuel mixture enters the cylinder. This is how a multi-point injection system works. 


There are two types of multi-point fuel injection systems. If ECU uses manifold pressure, engine speed and air density as the parameters to control the injector, then it is known as D-MPFI system(D stands for Druck, which means pressure). If ECU uses air flow rate and engine speed as the main parameters to control the injector then it is known as L-MPFI system(L stands for Luft, which means air).


There is one more classification of the fuel injection system. Based on the location of the injector there are two types of fuel injection systems. They are,

  • Indirect Injection System and
  • Direct Injection System

The fuel is sprayed near the inlet manifold or inlet port in an indirect injection system. In a direct injection system, the fuel is sprayed directly into the cylinder resembling the diesel engine’s fuel injection system. Spraying fuel directly into the cylinder increases fuel efficiency and reduces emissions. When fuel is sprayed directly into the cylinder, it is sprayed at a high pressure of 2900 psi.


The SI engine fuel injection system determines the ratio of air-fuel mixture which is a significant factor for the efficiency of the engine or mileage. The optimum ratio is 14.7:1 for gasoline engines(14.7 parts of air and 1 part of fuel), which is called the stoichiometric ratio. If the mixture is too lean or too high, then the performance of the engine would be affected. Thus SI engine fuel injection system which is controlled by ECU plays a significant role in engine performance. You can learn more about engines from Skill-Lync’s course.



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