Automobile Fuel System Complete Overview

Any combustible substance, which liberates (releases) heat energy on combustion, is called fuel. Fuel is in the form of liquid gas energy, which is being converted into heat energy by the process of combustion and ignition, in automotive engine. The power and performance of an automotive engine depends upon the type of fuel used. The fuel supply system takes liquid fuel from the fuel tank and mixes with air to form the combustible mixture for onward feeding to cylinder.


(a) Fuel Tank. The fuel tank is a reservoir for liquid gasoline and it is made of galvanized mild steel sheet. It is fastened under the chassis by straps, nut and bolt with rubber packing in between. Petrol connection is taken from bottom, sometimes a fine wire gauge strainer is fitted to filter the course suspended impurities. Drain plug is fitted at the bottom of the tank. Sometimes plug portion of the tank is dished, so that condensed water can settle there. In rainy season this condensed water must be drained at regular intervals. The fuel tank has two or three compartments, inter connected with each other. These compartments are made so that fuel does not strike with walls of the tank, when vehicle is moving on rough road. Fuel tank has a neck made on its top, where tank cap is placed. On the neck a small air bleed pipe is also welded to kill the vacuum created by consumed fuel. In some of the tank air vent hole is drilled in the cap itself.

(b) Fuel Pump. The function of fuel pump is to pump, fuel from the tank and feed it to carburetor under pressure. These pumps can be operated either mechanically or electrically.


Purpose of fuel gauge is to indicate the fuel contents available in the fuel tank of the vehicle. Fuel gauges are various types either operated mechanically or electrically to register the quantity of gasoline in the tank. It is fitted on the instrument panel, in the mechanical operated design, a float operates the needle through a simple geared mechanism and a needle in the tank indicates fuel level. Some of the MT vehicles are fitted with electrically operated fuel gauge. This gauge is in two parts i.e tank unit fitted in the fuel tank and gauge unit fitted on dash board. Both are connected with single wire and operated by ignition switch.


It is series of passages leading from the carburetor to intake ports of the engine. It is designed to aid in even distribution of fuel air mixture to all the cylinders. It is fixed to cylinder head with studs or bolts with packing placed at each opening to ensure airtight joint. The design of the induction manifold is very important. The bore of manifold should be the same as that of the inlet valve port. When the mixture flows around a gradual bend, there is little loss of fuel volumetric efficiency compared with a sharp bend. When a curved shape is employed, the mean radius of the curves should be as great as possible because the fuel particles tends to stick to the curve due to centrifugal force, causing improper vaporization and weak mixture entering the cylinder.

Induction Manifold


As the name implies it cleans the air going to the cylinder. We know that air contains lots of minute particles of dust and dirt not visible to naked eye. If these particles are not removed, they will enter the cylinder and work as emery paste, when mixed with the lubricating oil. This will wear the parts faster. The air cleaner is fitted on top of carburetor or connected to the throat of the carburetor. Besides cleaning the air, it also reduces the hissing sound (carburetor roar) produced by fast moving of air to intake pipe. In petrol vehicle it does not allow the flame to come out due to popping back. To eliminate the carburetor roar silencing chambers are formed in various type of air cleaner. The induction sound waves generated in the induction manifold passes back to the air cleaner through carburetor. These sounds are being absorbed by annular passage and subdivisions made in the air cleaner. These subdivisions being of different sizes absorb sound waves of different lengths. To suit the various requirement of the vehicle there are mainly three types of air cleaner.

(a) Wet Type. These are most commonly used in all automobiles. These are also called oil bath type. In this air enters from the top and made to deflect over the oil in the bowl. When the dusty air strikes the surface of the oil, the dust particles stick to oil and later on settle in the base of the pan. When the incoming air strikes to the oil, certain amount of oil also tries to go along with air rushing to the manifold. To prevent this wire gauge element is fitted on top of it. For maintenance the air cleaner should always be kept clean, the oil in the bowl should be replaced at each servicing and wire gauge filter should be clean. While fitting the bowl with oil ensure to fill the oil up to level marked on the bowl.

Air Cleaner

(b) Dry Type. It is usually known as paper type of air cleaner. It is mostly used on smaller vehicle. Filter paper is used in corrugated form to give more cleaning area. The paper element can be cleaned by blowing compressed air in it at each service and when found very dirty, these are replaced with new one.

(c) Pre Cleaner Type. The pre cleaners are used for all engines, which works in dusty condition such as heavy earth moving machine and tractor. Pre- cleaner is small air cleaner fitted before the main air cleaner. Most of the pre-cleaners are cyclone type. In this type the deflector plates are fitted on intake side, the air is made to pass through deflector plate, which deflect the incoming air at certain angle and give it a cyclonic motion resulting in setting heavier dust particles in the pan. The dust particles are removed from pan at regular intervals. In some of the pre cleaner glass bowl collector is fitted to collect the dust.

Paper and Oil Bath type Air Cleaner


The most commonly used gasoline filter is the ordinary fine mesh gauge type, which will intercept most of the foreign matter but cannot be relied upon to stop water or the finest dirt from reaching the carburetor. Chamois leather is a very satisfactory material for filtering gasoline which might have been contaminated. It possess the property that if it is first moistened with gasoline it will pass gasoline but not water. In some of the vehicle a strainer is placed in a glass bowl which requires cleaning at each service. Nowadays mostly sealed unit type filters are used. They are not cleanable type hence complete assembly is to be replaced, when found dirty. Depending upon the fuel system in use, filters may be fitted in any of the following positions:

(a) In the filler neck of the fuel tank.

(b) At the base of the fuel tank on inlet of fuel pipe.

(c) Between the fuel tank and the fuel pump.

(d) Incorporated in the fuel pump.

(e) In the carburettor where the main feed pipe is attached


The requirement of fuel feed system is to supply the stored fuel to the carburetor under certain pressure. For this there are mainly three types of fuel feed system used on motor vehicles. These are:

(a) Gravity feed system.

(b) Mechanical pump feed system.

(c) Electrical or Electronic pump feed system.


The tank is mounted at higher level than the carburetor and fuel flows by force of gravity. It is not commonly used because of its uncertainty to supply the fuel while climbing a hill and the fire hazard owing to the closeness of the tank to engine. This system mainly employed on motorcycle, scooter and moped etc.

Gravity Feed System


AC mechanical pumps are now extensively used in fuel system where it is required to draw fuel from a low level tank and supply to the carburetor at pressure of approximately 2.5 pounds/Sq. inch. The pump is secured to the engine and its rocker arm is operated either directly or by an eccentric of camshaft, or by a pushrod interposed between the rocker arm and eccentric cam. The pump body is made of zinc alloy.


Rotation of the eccentric lift the rocker arm, thus causing the link to pull the diaphragm downwards and compress the diaphragm spring. This action causes a drop in pressure in the pressure chamber. As the high point of the cam passes the rocker arm, the light diaphragm return spring pushes the diaphragm upwards. This action closes the inlet valve and opens the out let valve, and the fuel in the pump chamber is forced out via a pipe line to the carburetor float chamber. Thus there is one inlet and one delivery stroke of the pump for each complete revolution of the eccentric.

Idling: When the fuel in the carburetor reaches the predetermined level a needle valve closes which prevents the further entry of more fuel. Continued delivery of fuel by the pump causing a back pressure which pushes the diaphragm down against its spring which allows the rocker arm to “idle” without working the diaphragm. This continues until the carburetor require more fuel i.e. the float needle is opened which causes the pressure reduction in the pump chamber, thus allowing the diaphragm spring to force the diaphragm upward again and normal pumping action is resumed.

Mechanical (AC) Fuel Pump


In some of the vehicles instead of mechanical fuel pump electrical fuel pump is fitted which is connected through ignition key and works on 12 volts DC power supply provided by vehicle battery. This pump is very compact in design so it can be fitted anywhere in the fuel line in between fuel tank and carburetor.

Electrical or Electronic Fuel Pump


There are following advantages of electrical pump:

(a) Light in Weight and compact in design.

(b) Universal fitment.

(c) Eliminate vapour lock.

(d) Easy in cold starting.

(e) Low Power requirement.

(f) Water tight and vibration resistance.


(a) Clear the pump and its surroundings.

(b) Disconnect the inlet and outlet unions.

(c) Block the inlet pump line so that no fuel will leak.

(d) Loosen the two mounting nuts and remove the pump without damaging gasket.

(e) Refitting is carried out in reverse of removal order.


(a) Park the car and choke the wheel.

(b) Drain the fuel from fuel tank and replace the drain plug.

(c) Remove the fuel pipe connection from the fuel tank.

(d) Disconnect the fuel gauge electrical connection.

(e) Loosen the fuel tank mounting bolts from both sides of the luggage booth.

(f) Loosen the rubber hose, jubilee clip from the fuel tank neck and disconnect the rubber hose. Take out the tank from the luggage booth.

(g) Remove the fuel pipelines from the inlet of the AC fuel pump as well as outlet of pump.

(h) Loosen the inlet pipe from carburetor and remove the pipe.


(a) Pour some petrol in the fuel tank.

(b) Take assistance to hold the fuel tank from one side and hold it from the other side. Lift the tank up and down as well as to the sides, so that whatever the dust or dirt is in the tank should get mixed with the fuel.

(c) After that put tank upside down suddenly in the container. When the fuel is coming out through the fuel tank neck, the dirt and dust will also come out along with it. Likewise carry out this procedure two to three times. The fuel tank will be thoroughly flushed. Blow out the fuel pipeline with the compressed air. Fit the tank and fuel pipeline in reverse order. Fill up the fuel into the tank and checkup for any leakage.


It is a unit, which takes fuel in liquid form from AC fuel pump, converts it into vapour form and mixing it with air in correct proportion for continuous operation of gasoline engine, in different road and load condition. The main functions of the carburetor satisfy the engine’s varying requirement, the mixture strength of approximately the following ratio, by weight are required.

(a) Cold Weather Starting. A very rich mixture of four parts of air to one part of gasoline.

(b) Starting and Slow Running. A rich mixture of ten parts of air to one part of gasoline.

(c) Cruising. A weak mixture of 16.5 parts of air to one part of gasoline and sometime 20:1.

(d) High Speed. A rich mixture of twelve parts of air to one part of gasoline.


(a) Purpose of carburetor.

(i) To supply rich mixture when engine is cold at the time of starting (Choke Circuit).

(ii) To correct the supply of fuel after attaining normal working temperature (Slow/ High Speed Circuit).

(iii) To supply correct quantity of fuel at idle speed (Idle/Low Speed Circuit).

(iv) To supply enough fuel to suit the road and load condition (High / Econostat Circuit).

(v) Response to sudden acceleration by supplying rich mixture (Accelerating pump circuit).

(vi) Atomize the fuel with air at all load condition in proper ratio.

(b) Slow Running Adjustment of Solex carburetor

(i) Rectify all other defects before starting the adjustment i.e. tappet adjustment and ignition timing.

(ii) Fully screw in the Volume Control screw and screw it out 1½ to 1¾ turn.

(iii) Start the engine and run till it attains normal working temp.

(iv) Adjust the throttle control screw to obtain 600 rpm approximately.

(v) Screw in or out the Volume control screw till maximum rpm is achieved without hunting.

(vi) Repeat this operation (iv & v) until assure that there is no increase in rpm by screwing in or out the Volume control screw, this is the symptoms of correct slow running adjustment.


(a) Clear the carburetor and its surroundings.

(b) Remove the air cleaner assembly.

(c) Disconnect the inlet fuel pipe connection.

(d) Disconnect the accelerator and choke linkage.

(e) Disconnect the solenoid connection and vacuum advance pipe connection.

(f) Loosen the two mounting nuts and remove the carburetor assembly.

(g) Refitting is carried out in reverse of removal order.


(a) Multi Point Fuel Injection (MPFI) is the one of the latest fuel feeding systems in automobile engines. The main advantage of this system is reduced fuel consumption and low exhaust emission.

(b) The fuel injection System consists of an electronic injector mounted in the intake manifold of the engine, which in response to an electric signal from the computer injects a proper amount of fuel into the intake air passage.

(c) Unlike the conventional carburetor system, where the fuel quantity is drawn into the intake manifold by the amount of negative pressure created by the suction of the engine, this system employs a number of sensors and a computer. The sensors send signals to the computer about the exact state in which the engine is operating and the computer based on a prerecorded program calculates and operates the injection so that the exact quantity of fuel is injected into the system as per its requirements.

(d) Most, multi-point fuel injection systems are intermittent. This means that the injectors are not energized at the same time. There are different types of intermitted injection. The injector can be turned on and off in pair or in a specific sequence. Most, but not all, Multi-point systems are ported. Ported injection means that the fuel is sprayed directly in to an intake runner. This eliminates the problems associated with intake manifolds. This contrasts to fuel being sprayed in the intake manifold as in the single point injection system.


In this system the delivery of the fuel to the injectors are controlled by the electronic fuel injection (EFI). This system is divided in to three sub group. They are.

(a) Air Induction. The incoming air is regulated by the throttle valve. It is located in the throttle body. The throttle valve is connected by linkage to the accelerator pedal. As the accelerator pedal is depressed, the valve is opened, allowing air to enter the intake.

(b) Fuel Injection. The fuel from the tank is carried under pressure to the fuel injectors. This is done by an electric fuel pump located in or near the fuel tank. The excess fuel is returned to the fuel tank. A relay for the electric fuel pump is used to complete the circuit to the fuel pump. This cuts off the current to the fuel pump in the event of an accident.

(i) Regulator. The fuel pressure regulator keeps the fuel pressure at the injector’s constant under all different driving conditions.

(ii) Injector. The fuel injector is an electromechanical device that sprays and atomizes the fuel. The injector is operated electrically to supply the metered fuel as per the requirements. The injection nozzle spray opening area is constant and fuel injection pressure is also constant. Therefore the injection quantity is determined by the amount of time the solenoid coil is energized.

(iii) Controlling of Fuel injectors. The Multi-point fuel injection system consists of sensors which detect the engine conditions. The engine-ECU (Engine Control Unit) which controls the system based on the signals from the sensors and the actuators which operate under the control of the engine ECU. The engine-ECU carries out activities such as fuel injection control, idle speed control and ignition timing control.

Lay out of MPFI


(a) Crankshaft Position Sensor. This determines when the injector is energized. Most crankshaft position sensors double as an engine speed sensor. The higher the RPM, the more times the injector(s) is/ are energized.

(b) Manifold Absolute Pressure (MAP) Sensor. This senses the engine load. The greater the load, (low vacuum), the longer the injector is energized.

(c) Throttle Position Sensor. This determines the angle of the throttle blades. The greater the angle the longer the fuel injector is energized.

(d) Coolant Temperature Sensor. This senses the temperature of the engine. Air-cooled engine needs the injector to be energized longer. Other systems provide a cold start valve to make the fuel mixture rich in cold engine.

(e) Manifold Air Temperature Sensor. This senses the temp. of the incoming air. Cold air is denser. To compensate for the denser air the injector ON time must be increased.

(f) Oxygen Sensor. This senses the amount of oxygen in the exhaust. When there is an excess of oxygen in the exhaust (lean mixture) and ON time of the injector is increased. When there is a lack of oxygen (rich mixture), in the exhaust, the “ON” time is reduced. When the computer uses the signal from the oxygen sensor, it is referred to as closed loop. When the exhaust gases are below 6000 F, the oxygen sensor does not provide the microprocessor with a signal. The microprocessor operates from pre-determined values. This is referred to as open loop. Also, when the throttle is wide open the signal from sensor is ignored, regardless of the temperature.


Actuators are the devices which are controlled by the ECU. There are 04 actuators used in MPFI System mentioned as follows.

(a) Fuel Injector. It is mounted on the intake port of the cylinder head. As per the direction of ECU, the injectors inject correct amount of fuel into the intake port.

(b) Canister Purge Valve. The fuel produced in the fuel tank is collected in the canister assembly. ECU directs the canister control valve to open the passage and to draw stored fumes into the air cleaner. During engine running condition, this collected is drawn and passes along with the air into the engine to burn. Therefore, the fuel economy is obtained and pollution of atmosphere is reduced.

(c) Fuel Pump. Relay As soon as the ignition switch is put to “ON” position, ECU energises the fuel pump relay and the fuel pump starts to pump the fuel from the tank.

(d) Ignition Coil Assembly. There are two ignition coil i.e. Amp-I and Amp-II. Amp-I is connected to No 1 and 4 cylinder spark plug and Amp-II is connected to No 2 and 3 cylinder spark plug. Amp-I ignition is directly mounted on No4 cylinder spark plug and No 1 sparking plug is connected by HT lead. Similarly Amp-II is dire4ctly mounted on No 2 spark plug and No3 spark plug is connected by HT lead. So at a time 1& 4 and 2&3 spark plugs will be fired in pair.


The microprocessor Assembly of the MPFI system receives the information from the various sensors. The sensors are nothing but a measuring device. The measurements are then converted into electrical signals. The signal is then compared by the microprocessor to the reference voltage. This determines the amount of time that an actuator is energized. An actuator is a device controlled by microprocessor. The fuel injector is one of the actuators that the microprocessor controls. The longer the fuel injector is energized, the richer the fuel mixture becomes. The injector pulse width is the length of ON time of injectors.


(a) Correct air and fuel ratio at all conditions in all speeds and reduced fuel consumption.

(b) Uniform air/fuel mixture distribution to all cylinders and improved performance.

(c) Easy correction of air/fuel ratio and a high level of fuel controllability and quick response.

(d) Much less occurrence of icing, vapour lock and percolation.

(e) Automatic altitude and temperature compensation.

(f) Improved cold starting and warming up behaviour and low exhaust emissions.

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Author: Aliva Tripathy

Taking out time from a housewife life and contributing to AxiBook is a passion for me. I love doing this and gets mind filled with huge satisfaction with thoughtful feedbacks from you all. Do love caring for others and love sharing knowledge more than this.

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