indirect injection system

Understanding an indirect injection system

Indirect injection refers to fuel injection in an internal combustion engine where the combustion chamber is not directly injected with fuel. Indirect injection petrol engines, in which the fuel is delivered using an injector before the intake valve, have mostly lost use in favor of direct injection.

However, some manufacturers, including Toyota, Volkswagen, and Ford, have created a “dual injection” system that combines port (indirect) injectors with direct injectors to combine the advantages of both fuel injection systems. Greater power and fuel efficiency may result from direct injection since the fuel could be precisely injected into the combustion chamber below high pressure.

In internal combustion engines where fuel must be an injection into a combustion chamber, various methods have been employed by manufacturers to accomplish the process including direct and indirect injection.

An indirect injection in an internal combustion engine is when fuel is not directly injected into the combustion chamber. It’s common among auto enthusiasts that an indirect injection is equipped on only diesel engines, that’s the truth but it’s now employed on gasoline engines.

In gasoline engines with an indirect injection system, there is a fuel injector that delivers fuel at some point before the intake valve. Whereas, in diesel engines, fuel is delivered into a chamber of the combustion chamber, known as the chamber. This will be further explained.

indirect injection system

Today you’ll get to know the definition, functions, classification, components, and workings of an indirect injection system. you’ll also get to know the advantages and disadvantages of indirect injection in gasoline (petrol) and diesel engines.


What is an indirect injection system?

Just as earlier mentioned, indirect fuel injection is a fuel injection process in internal combustion engines, in which fuel is not directly injected into the chamber. The invention helps to achieve higher engine speeds in diesel engines. it’s generally used in light-duty diesel engines, found in the earlier generation passenger cars, sedans, and multi-utility vehicles.

In an indirect injection system, injectors do not spray fuel directly over the piston, but instead, spray the fuel into a pre-combustion chamber located inside the cylinder head. Some auto manufacturers now install a dual-injection system that combines direct and indirect injectors to obtain the benefits of both injection systems.

A direct injection system allows the fuel to be precisely metered into the combustion chamber under high pressure. this helps to obtain greater power and fuel efficiency. Limitations with this system are that it generally leads to particulate matter and also the fuel no longer contacts the intake valves. This will accumulate the amount of carbon over time.

Because of this, an indirect injection system will play a perfect role as it keeps spraying fuel on the intake valves. It will assist in removing the accumulated carbon on the intake valves and in low-load conditions.

Functions of indirect injection

Below are the functions of indirect injection in diesel engines:

  • Indirect injection serves the purpose of increasing the engine speed as the power output is increased.
  • The addition of the per-chamber increases heat loss to the cooling system.
  • Indirect injection allows air to move faster to mix with the fuel in the system.

Gasoline engines

Having deposits from the crankcase ventilation system cleaned off the intake valves is a benefit of indirect injection petrol engines over direct injection petrol engines. Because the fuel and air are combined more evenly in indirect injection engines than in direct injection engines, they also tend to produce less particulate matter.

Characteristic of an indirect injection system

Below are the characteristics of an indirect combustion chamber:

  • Swirl chamber – a swirl chamber allows about 50% of fuel to enter into it during a compression stroke. It’s a spherical chamber located in the cylinder head.
  • Pre-combustion chamber – during the compression stroke, air from the main cylinder moves to the pre-combustion chamber to begin the combustion. This chamber is located at the cylinder head and is connected to the engine cylinder by some tiny holes.
  • Air cell chamber – this is a cylindrical chamber with a hole in one end. It’s mounted more or less coaxially to the fuel injection; it’s done so that thermal contact can be minimized with the mass of the head.

Components of an indirect injection system

Components of an indirect injection in an automotive diesel engine include the top of the head, piston, injection holes, cylinder head, injection pump, head close-up, combustion chambers, and glow plugs. All this will be mentioned and explained in the working principle section.

Diagram of an indirect injection system:

diagram of direct and indirect injection system

Working principle

The working of an indirect injection is quite simple and easy to understand. In the system, fuel is sprayed into a separate chamber, known as a pre-combustion chamber. Unlike the conventional internal combustion diesel engines that atomize fuel directly over the piston.

This pre-combustion chamber is located inside the cylinder head, creating a swirling action in the compressed air. With this, the mixing of fuel with air is achieved uniformly. in this system, the air is allowed to move fast while mixing the fuel and air.

During the compression stroke, the air from the engine cylinder will first enter the pre-combustion chamber. As it compresses, its temperature rises, making the air becomes extremely hot. At this point, an injector injects fuel into the pre-combustion chamber. For this reason, combustion begins and then the entire cylinder will be spread.

Watch the video to learn more about how an indirect injection system works:

Advantages and disadvantages of an indirect injection system


Below are the benefits of an indirect injection system in diesel engines:

  • Possibility of producing smaller size diesel engines suitable for cars.
  • Higher engine speeds can be achieved than direct injection diesel engines
  • Because of low injection pressure, the injectors are cheaper to produce.
  • Lower stresses are created on its internal components. This is making It possible to make the basic common platform for petrol and diesel versions of the same engine to save cost.
  • Injection direction is less important.
  • Much simpler to design and manufacture.
  • Higher engine speed is reached due to the continuous burning in the chamber.

Read more: Understanding the fuel injection system in automobile engines


Despite the advantages of indirect injection, some limitations still occur. below are the disadvantages of an indirect injection system in diesel engines:

  • Indirect injection systems require glow plugs for a cold engine start on diesel engines.
  • Fuel efficiency is lower than with direct injection because of the large exposed areas in the system causing heat loss. And also, pressure loss due to air motion through the throats.
  • Since heat and pressure of combustion are applied to one point on the piston as it exists in the precombustion chamber. This makes such engines to be less suited to high power outputs than direct injection diesel engines.

Read more: Working principles, advantages, and disadvantages of diesel engine

Related Article:

In conclusion, an indirect injection system is a type of fuel injection process in an internal combustion engine with good benefits. Today we have explained its working, functions, and components. the advantages and disadvantages of an indirect injection were also discussed. I hope you enjoyed the reading, if so, kindly comment, share, and recommend this site to other technical students. Thanks!


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