Home Uncategorized Basic Studies About Fuel System Of Diesel Engine

Basic Studies About Fuel System Of Diesel Engine


The process of combustion in the compression ignition engine is different from that of a spark engine. The period between the injection and ignition of fuel is define as delay period (ignition delay).

This lie between 0.001 to 0.002 second according to speed of the engine. Therefore the whole performance of the engine is totally dependent on the delay period. Less the delay period , better the diesel engine performance.

Important Topics

  1. Function requirements of a fuel injection system
  2. Fuel injection system
  3. Components of a fuel injection system
  4. Fuel injector consist of the following components
  5. Nozzle and types of nozzle
  6. Fuel filter
  7. Micro fuel injection system for multi cylinder engine  

Function requirements of a fuel injection system of diesel engine

Proper running and good performance of the engine , the following requirement :-

  • The fuel should be introduce into the combustion chamber at right period of cycle.
  • The amount of fuel inject per cycle should be meter accurately.
  • The rate of injection  should be such that the desire heat release pattern is achieve during combustion.
  • The injection fuel should be break into very fine droplets.
  • The fuel should be spray in such a manner that if result in mixing of fuel and air.
  • The beginning and end of the injection should be sharp.
  • Equal quantity of fuel should be supply to all cylinders in case of multi cylinder engine.
  • The injection timing should change according to the load required and energy speed.

Fuel injection system

Two type of fuel injection system are as follows :-

  • Air injection
  • Solid injection

Air injection

The air is first compressed to very high pressure. The compress air is then inject into the engine cylinder and it carries the fuels with itself. The rate of fuel admission can be control by varying the pressure of air to be inject.

This method was originally used in the stationary marine engines, but it is obsolete now.

The meter quantity of fuel is pump into the fuel valve with the help of the fuel pump drive cam shaft. This fuel valve is open with the help of  mechanical linkage operate by the cam shaft which controls the timing of fuel injection. This fuel valve is also connect to a high pressure air line which supplies air.

When the fuel valve is open , the blast air sweep the fuel along with it and a well atomize fuel spray is supply to the combustion chamber.

Advantages of air injection

  • It provides better atomization and distribution of fuel.
  • Heavy and viscous fuel can also be inject.
  • The fuel pump is require to developed only a small pressure.

Disadvantages of air injection

  • The system is complicate and expensive due to requirement of multi compressor.
  • The power produced by engine decrease.
  • The fuel valve sealing requires considerable skill.
  • The system becomes quit dangerous due to the presence of high pressure air.

Solid injection

The fuel under high pressure is directly inject into the combustion chamber. It burn due to the heat of compression of the air. The  system requires a fuel pump to deliver the fuel at high pressure (300 bar). This system used for all types of small and big diesel engines. This system is also define as airless injection. Airless injection system is much superior to air injection as far as the fuel economy is concerned.

Solid injection can be classified into following ways :-

  • Individual pump and nozzle system
  • Common rail system
  • Distribution system

Individual pump :- In this system , each cylinder is provide with a separate pump and injection.

Common rail system :- A high pressure fuel pump delivers fuel to a fuel header or an accumulator, whose pressure is keep constant with the help a pressure regulating valve.

Distribution system :- This high pressure pump is use for metering , timing and compressing the fuel and then fuel is deliver to the common rotating distribution.

Components of a fuel injection system

Fuel injection system of diesel engine consist of following components :-

  • Tank
  • Fuel feed pump
  • Injection pump
  • Injector
  • Fuel filter

Fuel tank :- The fuel tank is use to store the fuel. It may be place at the higher levels (stationary engines) or at lower level (in case of automobile engine).

Fuel feed pump :- The purpose of the fuel feed pump is use to supply the fuel from the fuel tank to the injection pump.

Fuel injection pump :- The purpose of the fuel injection pump is to meter the correct quantity of fuel and deliver at the correct time to the engine cylinder according to the varying load and speed requirements.

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Type of fuel injection pump :-

  • Distribution type fuel injection pump
  • Plunger type fuel injection pump

Fuel Injector :- A nozzle mount on the combustion chamber which supply the fuel to the engine cylinder in the form of a fine spray is define as fuel injector.

Fuel injector consist of the following components

The high pressure fuel coming out of fuel pump enters the injector. The nozzle valve is lift up due to high pressure fuel entering at the bottom of the valve and the fuel is sprayed into the combustion chamber in finely atomized droplets.

The adjusting screw helps to adjust the tension in the spring and hence the valve opening pressure.

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Nozzle and types of nozzle

Nozzle is that part of an injector through which the liquid fuel is spray into the combustion chamber.

The main requirements of an injector nozzle are :-

  • To inject the fuel at a sufficiently high pressure in the engine cylinder in the form of very fine droplets.
  • Penetration should be such that the fuel should not impinge on cylinder walls, otherwise, this may result in poor starting.
  • Fuel supply and cut off should be rapid and there should be no dribbling.

Types of injection nozzle are :

  • Single hole nozzle :- Its consist of a single hole bored centrally through the nozzle body and closed by the needle valve.
  • Multi hole nozzle :- In consist of a number of holes bored in the tip of the nozzle. This type of nozzle find in extensive use in automobile engine.
  • Pintle nozzle :- The stem of nozzle valve is extend to form a pin or pintle which extend through the mouth of the nozzle.
  • Circumferential nozzle :- The injection fuel particles tend to project in the form of plane with wide angle cone.
  • Pintaux nozzle :- When an auxiliary hole is provided at the nose of a pintle nozzle , it is define as pintaux nozzle.

Fuel filter

A filter which cleans the fuel by removing the vapour , dirt particles or air bubbles is define as fuel filters.

Types of fuel filter

  • Preliminary fuel filter
  • Secondary fuel filter

Preliminary fuel filter :- It consists of a bowl, a perforated tube a filter element. This filter is mounted on the suction side of the fuel feed pump.

Secondary fuel filter :- The direction of fuel flow ensures that all the fuel passes through the filter. The fuel passes through the secondary fuel filter before it is delivered to the fuel injection pump.

Micro fuel injection system for multi cylinder engine  in diesel engine 

The fuel pump is mounted in the fuel injection pump. The strainer bowl is clamped at the bottom and hand primer is screwed on the top of the body. The fuel feed pump is operated by cam shaft.

This pump lifts the fuel from the fuel tank which is located at the lower level of injection pump. This fuel is then supplied to the fuel injection pump through the primary or preliminary and secondary filters.

The fuel injection pump supplies the fuel under a very high pressure to the fuel injector. The leak off fuel from the injector is supplied back to the fuel tank.

The filter units are provided with air vent screw and drain plug. An over flow valve is also incorporated in the fuel filter unit to permit extra fuel to flow back to fuel tank.

The cam shaft of pump unit is driven from engine. All the pumps are controlled by the same control rack.


Q1.When an auxiliary hole is provided at the nose of a pintle nozzle , it is define as pintaux nozzle.

Ans. Air injection
Solid injection

Q2. Write the components of fuel injection system for diesel engine.

Ans . 1) Fuel tank,
2) Fuel injection pump
3) Fuel filter
4) Fuel feed pump
5)Fuel injector

Q3. What is nozzle in case C.I engine ?

Ans. Nozzle is that part of an injector through which liquid fuel is sprayed into the combustion chamber.

Q4. List various types of nozzles.

Ans. a). Single hole nozzle
         b). Multi hole nozzle
        c). Pintle nozzle
        d). Circumferential orifice nozzle
        e). Pintaux nozzle

Q5. Describe the working of fuel feed pump with neat sketch.

Ans. The purpose of the fuel feed pump is to supply the fuel from the fuel tank to the injection pump.
Working of fuel feed pump :- A schematic diagram of fuel feed pump. It is spring loaded plunger type fuel feed pump. The plunger is actuated through a push rod by a cam mounted on the cam shaft.

At the minimum lift position of the cam spring force on the plunger creates a suction which causes the fuel to flow from the main tank into the pump when the cam is turned to its  maximum lift position, the plunger is lifted upward. At the same time when the inlet valve is closed and the fuel is forced through the outlet valve.



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