Fuel Supply System

“The fuel system of an internal combustion Engine is intended to produce a combustible mixture, composed of the fuel stored in the fuel tank and atmospheric air, and then deliver both to the cylinders.”

Fuel Injection for SI Engine

In a S.I engine, a measured quantity of petrol or gas is already mixed with air in a calculated proportion before being sucked by engine. Then this combustible charge having right quantity of fuel-air is ignited at the appropriate time at the end of compression stroke in the engine with the help of a spark plug.

Depending on the method of mixing petrol or gas in the air in right proportion as per the requirement of engine, the fuel supply system of S.I engine is conventionally is of two types.

1. Carburetion

In the carburetion method, fuel stored in the fuel tank is supplied to carburetor by means of a simple diaphragm pump through a fuel filter. The job of pump is only to supply fuel from fuel tank to float chamber of carburetor. Sometimes when the fuel tank is above the carburetor like in 2-wheeler engine, fuel will come by gravity and fuel pump is not needed.

The design & working of a simple carburetor fitted in the suction line or Inlet manifold of engine is very simple. As shown in figure there is a venturi in the flow passage of air being sucked by engine. A jet is situated in the venturi and connected to the float chamber of carburetor, where fuel is stored at atmospheric pressure. The float keeps the fuel at a constant level in the float chamber. While passing through the venturi, pressure of air reduces and a pressure difference is created across the fuel jet connecting float chamber with venturi. Due to this pressure difference fuel is continuously supplied to air flowing through venturi. As jet is of very small inner diameter, fuel i.e. petrol is sprayed in the flowing air and due to its high volatility, it vaporizes and forms a combustible mixture of fuel vapour plus air. The fuel air ratio is automatically controlled by the speed of air through the venturi which eventually depends on speed/rpm of engine. The size of venturi and jet are designed on the basis of desired fuel air ratio. In an actual carburetor, some additional systems are there to satisfy the demands of engine under varying conditions like cold starting, engine idling, requirement of additional power at high speed & load etc.

2.  Electronic fuel injection (MPFI system)

In this system an electrically driven fuel pump draws fuel from fuel tank and supplies it to a common header or tube. A pressure regulator fitted at the end maintains a constant pressure of fuel approx. 3 bars in the header. The header is connected to different branches of inlet manifold through fuel injectors. For each cylinder of engine there is separate fuel injector which injects fuel in the corresponding air passage of that cylinder. Due to this the system is called multi-point fuel injection (MPFI) system. The fuel injectors are precision built solenoid valves having single or multiple orifices. Due to constant pressure of fuel maintained in the common header, the quantity of fuel injected depends only on the time period for which the solenoid valve type fuel injectors are kept in open position.

An on-board ECU (Electronic control unit) i.e. microprocessor controls the quantity of fuel injected to each cylinder individually and also the ignition timing of each cylinder. The data input to the ECU comes from a number of sensors located all over the engine. These sensors collect the following data continuously.

1. Ambient temperature

2. Inlet manifold vacuum or Air Velocity

3. Exhaust gases temperature

4. Exhaust O2 content

5. Throttle position

6. Engine r.p.m.

7. Crankshaft & position

8. Engine coolant temperature

Based on programmed interpretation or processing of this data, ECU calculates the amount of fuel needed to maintain stoichiometry i.e. air/fuel ratio of 14.7:1 and converts it into required pulse width i.e. time period for which it keeps the solenoid injector energized. ECU also gives command to spark ignition system. In this way ECU ensures overall satisfactory performance of the engine from start to shut down including emission control by sending right quantity and quality of fuel air mixture to each cylinder of engine at right time based on requirement of engine and also ignites it at right time.

Fuel Injection for CI Engine

In C.I engine, the air is taken in during the suction stroke & compressed to a high pressure (28 to 70 bar) & high temperature (5200 to 7200 C) according to the compression ratio used (12:1 to 20:1). The high temperature of air at the end of stroke is sufficient to ignite the fuel.  Fuel is injected into the cylinder at the end of compression stroke, the pressure of fuel injected lie between 100 to 200 bar. During the process of injection fuel is broken into very fine droplets. The droplets vaporized taking the heat from hot air & from a combustible mixture & start burning.

Function of Fuel Injection System

The main function of fuel injection system are:-

1. Filter the fuel.
2. Meter or measure the correct quantity of fuel to be injected.
3. Time the fuel injection.
4. Control the rate of fuel injection.
5. Atomize or break up the fuel to fine particles.
6. Properly distribute the fuel in the combustion chamber.

Types of  CI Fuel Injection System

The diesel injection systems used nowadays are of two types:

(1) Mechanical Injection System

1. Air Injection System

2. Airless or Solid  Injection System

(a) Common-rail injection system

(b) Individual Pump Injection System

(c) Distribution System

(2) Electronic Injection System

1.  Mechanical injection system

This system was universally being used in Diesel Engines until the introduction of new fuel injection technology like CRDI etc. But still due to the reason of more initial cost involved in adopting newer technology, it is being used in Diesel Engines of different sizes. In this type, there are two basic components as one pressurizing unit (High Pressure Pump) and other atomizing unit (High pressure nozzle or injector). It's generally two types namely Air & Airless fuel injection system.

1. Air Injection :- In this method of fuel injection the air is compressed in the compressor to a very high pressure & then injected through the fuel nozzle into the engine cylinder. This method is generally use in the large stationary and marine engines.

Advantages of Air Injection System:- 

1. Its provide better atomization & distribution of fuel. 

2. Inferior fuel can be used. 

3. As the combustion is more complete, the Break mean effective pressure (b.m.e.p) is higher than without other type of injection system. 

Disadvantages of Air Injection System:- 

1. Its required a pressure multi-stage compression. 

2. A separate mechanical linkage are required to time the operation of fuel valve. 

2. Airless or Solid Injection :- In this method of fuel injection the fuel is directly injected into the combustion chamber without primary atomization. Its also termed as Mechanical Injection. 

The main components of airless injection system are:- 

1. Fuel Tank. 

2. Fuel Feed Pump. 

3. Fuel Filter. 

4. Injection Pump. 

5. Governor 

6. Fuel Piping & Fuel Injector. 

There are 3 types of airless fuel injection system.

(a) Common-rail injection system

(b) Individual Pump Injection System

(c) Distribution System

(a) Common-rail injection system

It consists of the common header for all the fuel injector nozzles. The fuel will be supplied to the header by a High-Pressure pump. The high pressure in the header will pass the fuel to each nozzle at each cylinder. At the correct time, a rocker and rod mechanism will let the valve open for the fuel to enter into the cylinder through the nozzles.

(b) Individual Pump Injection System

In this system there is a separate pump and separate injector for each cylinder of engine. The pump creates high pressure of fuel and also meters and times the injection of fuel through injector. It is a plunger type pump driven by engine power itself. 

(c) Distribution System

In this Distributed system, there will be a pump which will take care of all the three operation Pressurizing, metering and timing on injection. The fuel will be sent to the rotating distributor to inject the fuel into each cylinder. The uniform distribution of the fuel can be achieved due to the one metering device. There is one more advantage is that the cost of the injection system is also less.

(2) Electronic injection system (with CRDI technology)

In this system of diesel injection, a common rail diesel injection (CRDI) technology is used. It is more or less same as Common Rail System of Mechanical Injection but the difference comes in the control over metering & timing of injectors which is done by an on-board computer system or electronic control unit. Thus it resembles with the operation of MPFI system of S.I engine. The basic system is same; there is a high pressure fuel pump which maintains high pressure in a common rail (steel tube) through high pressure regulator. But here the pressure maintained is very high of the order of 2000 bar as compared to 3-5 bars in MPFI system. The fuel injectors are very special either solenoid type or piezzo electric type which control the fuel injection from common rail to each cylinder very precisely.

The opening time, pulse width etc of fuel injectors can be electrically controlled by the E.C.U. Here is the main advantage of system that the fuel can be injected in more than one pulse in a very controlled manner unlike only one pulse or one injection per cycle in the mechanical system. Here a pilot injection is done before the main injection for fast burning and less ignition delay of the fuel. It reduces the noise level very much and also ensures complete burning of fuel, high efficiency, low emission and good cold start. This new technology has considerably removed the demerits of diesel engines like high noise level, high pollution, difficulty in starting etc and improved fuel efficiency a lot.