Fuel injection System

Introduction

Automobile engineers have been using carburettors for a very long period of time as a fuel-supplying mechanism in SI engines. They had been quite useful and effective for a considerable duration of time. They were very basic and simple devices working purely on mechanical principles. Even today, you can find carburettors on most of the two-wheelers. Way back in the 1990s, automobile engineers started using fuel injectors in petrol engines. Before this, they were only used in Diesel engines. This change improved the performance and efficiency of SI engines considerably. This led to a significant increase in mileage and performance of the engines. It also opened a way to introduce different sensors and electronic control on engines. So let us understand the working of Fuel injectors in detail.

Also Read: Chassis Of Motorbikes

The electronic fuel injection system 

The electronic fuel injection system has four main components:

1) Fuel injector

2) ECU

3) Fuel Pump
4) Pressure release valve

1. Fuel Injector and the ECU

Fuel injectors are small electronically or mechanically controlled valves which can open or close within a fraction of seconds. These valves are used to supply fuel to the engines. The opening and closing time of fuel injectors can be controlled using the Engine control module and hence the amount of fuel supply to the engines can be regulated. The cross-sectional view of an electronic fuel injector can be seen below

2. Fuel Pump

Fuel injection system also consists of a fuel supply pump, fitted below the fuel tank.  This pump maintains the supply of fuel in the fuel circuit at a constant rate. The fuel enters the injector from the top and is collected in a cavity around the plunger. The Engine control unit (ECU), also called the engine control module is an electronic control circuit which controls the amount of fuel supplied through Fuel Injector. 

3.Pressure Release Valve (PRV)

The pump keeps on pumping the fuel uniformly in the fuel circuit, irrespective of whether the supply is needed or not. The opening and closing of the injector nozzle depends upon the riding requirements and is non-uniform. The difference in the rates of fuel pumping and spraying through nozzles can lead to an abrupt increase in the pressure inside the fuel pipe and the injector. This may damage them or may lead to the leakage of fuel from the joints. To prevent all this, a Pressure Regulating Valve or PRV is provided in the fuel supply circuit. The PRV releases the pressure inside the fuel circuit whenever it exceeds the optimum values. Hence maintaining the fuel supply at constant pressure preventing any damage to fuel pipe or the injector. 

Fuel Injectors

The main components of a fuel injector are: 

1) Plunger

2) Return Spring and

3) Solenoid

The fuel pump keeps on supplying the fuel to the fuel injector. This fuel remains stored in the cavity around the plunger. The tip of the plunger is of needle shape. It sits on the orifice of the spray tip and blocks it completely. Till the plunger sits on the spray tip, fuel remains completely blocked inside the injector.
The solenoid acts as an electromagnet. It can be magnetized or demagnetized as per the commands given by the ECU.

Picture Representing The Magnetic Field Of A Solenoid

When the fuel supply is required, the ECU sends current supply to the solenoid and it gets magnetized. The magnetized solenoid now pulls up the plunger and the pressurised fuel in the injector is sprayed out.  The amount of fuel to be injected is controlled by varying the time for which the spray nozzle tip is open. The spray nozzle sprays very fine and minute fuel droplets, which can mix up very easily with the air and create a uniform mixture. This helps in uniform combustion of the fuel leading to improved efficiency and performance of the engine. 

Fuel Injectors Vs Carburettors

Carburettors had been popular for a very long time. But, now the fuel injectors are replacing them at a fast pace. The craving for high performance and strict emission norms is causing the fuel injectors to outperform the carburettors. Let's discuss the advantages of fuel injectors over carburettors one by one:

1. Fuel Injectors are controlled electronically through ECU while Carburetors are purely mechanical devices.

2. Fuel Injectors can produce stoichiometric Air-Fuel ratio more precisely than the Carburetors. This improves mileage and also reduces the level of pollutants in the exhaust gases.

3. Fuel injectors response quickly and can vary the Air-Fuel ratio depending upon the requirements.

4. The fuel injectors eliminate the problem of cold starting as they are programmed to provide rich Air-Fuel mixture at the time of starting. This is a big problem in carbureted engines and we need to pull the choke for starting the vehicle in cold conditions.

Port Vs Direct Injection System

The fuel injection systems can be classified as port or direct injection, based on the location of the fuel injector. Both these systems have their own pros and cons making them useful for different purposes. Let's discuss them one by one:

Port Injection  System

The port injection system has their fuel injectors located in the intake manifold. They inject the fuel at a lower pressure in the air flowing inside the intake manifold. This allows the fuel to mix thoroughly with the air and generate a fine Air-Fuel mixture. The fuel also cools the incoming air and makes it denser. This allows more air to be sucked in, which can burn more fuel. The fuel also helps in cleaning of the valve seats, ensuring better air-fuel supply for a longer period of time. Also, they are simple in design and need a smaller pump.

Direct  injection system 

Direct Injection System have their fuel injector located in their cylinder heads. These injectors spray the fuel directly inside the combustion chamber. This helps in cooling of cylinder walls and reduce the chances of knocking. This allows the engine to be operated on a comparatively higher air-fuel ratio,  producing a better efficiency and fuel economy. The injectors in direct injection system work on very high pressure, as they need to spray the fuel inside the already pressurised combustion chamber. The high pressure helps in better atomisation of fuel and spreads it all across the engine cylinder. This further helps in improving the combustion of fuel.