Types of Fuel Injection System

The combustion engine in our cars would not function without fuel. Supplying this essential fuel into the combustion chambers are the fuel injection systems. Many attributes of your car are highly influenced by the type of fuel injection system its engine has, mileage, ride quality, engine life, etc. A fuel injection system is a mechatronic circuit that is the combination of mechanical and electronic circuits to supply fuel of ideal volume into the engine.

Since the fuel injection system is such a vital component, engineers around the globe have evolved it to its most efficient and effective version. In this day and age, while there are various types of fuel injection systems available, the broad categorization brings them down to four main types: Single point fuel injection, Multi-point fuel injection, Sequential fuel injection, and Direct fuel injection. Let’s go through these categories one by one and understand their functionality.

Single-Point Fuel injection or Throttle-Body injection

A single-point injection system features a common fuel injector for all the cylinders in the combustion chamber of the engine. This is the oldest and simplest form of the fuel injection system. In place of the carburettor, the single-point injection uses one or two fuel injector nozzles in the throttle body which is why it’s called Throttle body injection (TBI) as well. 

The fuel is sprayed in all cylinders at the same time, however, as opposed to the carburettor jet, it comes from an injector and can be monitored by an electronic control unit (ECU). While it has an edge over the carburettor, it has a slight disadvantage that since it uses a single injector it disturbs the performance of an engine at high RPMs and causes rough ride quality as the required fuel supply isn’t fulfilled. Moreover, a small fraction of fuel gets condensed outside the intake manifold of the cylinders and this causes fuel wastage.

Multi-Point Fuel injection or Port injection

In multi-point fuel injection (MPFI) technology, every cylinder in the combustion chamber of the engine is given an injector at the front of their inlet valves (outside the intake port), which is why it’s also called ‘Port Injection’.

Every injector sprays fuel at the same time and each cylinder gets a more precise volume of fuel, with less possibility of fuel condensation outside the intake manifold. While MPFI has the advantage of lower fuel wastage as compared to TBI, since the fuel is sprayed at the same time in all the cylinders, it doesn’t get properly synced with the rotation of all the pistons. This results in the fuel idling in the engine and port for as long as 150 milliseconds. All the same, from the performance point of view, the MPFIs function much better as compared to TBIs.

Sequential Fuel injection

The sequential fuel injection system eliminates the only disadvantage of MPFI and is the most widely used fuel injection system today. In a sequential fuel injection system, the fuel injectors function with respect to the cylinders they are connected to. Every injector injects the fuel only when the intake valve of the cylinder opens. It remains idle for the rest of the steps. An ECU monitors the movement of cylinders and triggers the injectors only when required. The sequential fuel injection is the most effective and efficient of all the fuel injections systems currently available in the automotive industry.

Direct injection

In direct fuel injection, the system is focused on placing the injector inside the cylinder to directly inject the fuel, bypassing the intake valve or manifold. While this type of fuel injection system is usually seen in diesel engines, it has a significant space in petrol engines as well, where it’s known as GDI (gasoline direct injection). 

In the previously mentioned systems, the fuel is sprayed on the intake and there is always a potential of fuel condensation to some extent. However, in the DI system, all the fuel is directly injected into the cylinder which produces maximum fuel economy and this is its greatest advantage. In diesel engines, direct fuel injection has been used since the 1920s, whereas, in petrol engines, it’s been used since around World War II. Automakers have also found that GDI engines are relatively more powerful and quite convenient for better CNG fuel efficiency.

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