EXPERIMENT NO. 2
AIM: To study the fuel supply system of a petrol engine
Model used : Fiat Padmini
Introduction : The basic fuel supply system in an automobile with petrol engine consists of a fuel tank,
fuel lines, fuel pump, fuel filter, air cleaner, carburetor, inlet manifold and supply & return pipelines.
The basic requirement of the fuel supply system is to supply a combustible mixture of air
and fuel to the engine. The fuel system must change the proportions of air and fuel for different
operating conditions. Eg : During idling, starting a rich mixture is needed.
Main parts of a fuel system are :
1.
2.
3.
4.
5.
Fuel tank
Fuel pump
Fuel filter
Air cleaner
Carburettor
All parts are explained one by one in forthcoming pages
Fuel tank : The fuel tank is normally located at the rear of the vehicle. It is usually made of sheet metal
(aluminium alloy). It is divided into interconnected components by means of baffle plates. This
management reduces surging of fuel on account of sudden braking or cornering. The top cover of the
fuel fitter tube is vented to atmosphere so that the pressure inside the tank is always atmospheric, as
well to allow the vaporized fuel escape from the fuel tank. The fuel pipe lines take the fuel from fuel
tank to the carburetor through various other components.
Fuel pump:-The fuel pump is used to deliver fuel from tank to carburetor. There are two types of fuel
pump, mechanical and electrical pump. Mechanical pump is operated by an eccentric on the engine cam
shaft. It is usually diaphragm type of pump. As the camshaft rotates the eccentric rocks the rocker arm
back and forth. The inner end of rocker arm is linked to a flexible diaphragm. The eccentric operates the
rocker arm which in conjuction with the return spring pushes the diaphragm up and down. Downward
movement of the diaphragm causes vacuum in the chamber which causes the inlet valve to open and
the fuel then goes through the strainer to the chamber. The next upward movement of the diaphragm
causes the inlet valve to close while the outlet valve opens and the fuel goes out to the carburetor float
chamber.
Fuel filter:-Fuel filter is commonly used for filtering the petrol. The dust, water molecules etc. are
separated out of the fuel. A very simple and effective device used is the camel’s leather, which is first
moistened with petrol which will allow only the petrol to pass through it and the water will be
intercepted.
Air cleaner:-The fuel system mixes air and fuel to produce a combustible mixture. As hundreds of cubic
meters of air per hour are used by the engine of an automobile. It is important that this air should be
very clean. Impurities like dust in the air causes rapid wear of the engine, particularly of the cylinders
pistons, rings, valves and guides. The air cleaner contains a ring if filled material (fine mesh metal screen
ribbons, pleated paper, cellulose fibers or polyurethane). The filter element Is corrugated to increase
the surface area exposed to the incoming air so that the resistance offered by the air cleaner is reduced
to minimum sides of the elements are sealed against dust particles when air passes through the
element. It provides a fine maze that traps most of the airborne particles. Now days, thermostatic
controlled type of air cleaners are being used. Some air cleaner are of oil bath type.
Carburetor:-Carburation is the mixing of the gasoline fuel with air to get a combustible mixture. The
function of carburetor is to supply a combustible mixture of varying degrees of richness to suit engine
operating conditions. It also helps in keeping a small reserve of fuel at a constant head. Proper
vaporization and atomization of fuel is carried out by the carburetor. In a simple carburetor, the air is
taken in through the venturi, where the nozzle is placed. Through the nozzle the fuel comes out as the
pressure difference is created b/w venturi and flow chamber. Thus incoming air brought about the
vaporization and atomization of fuel.
Solex carburetor :- it is a downdraught type of carburetor which is famous for ease of starting, good
performance, and reliability. The unique feature of this carburetor is the bi-starter or progressive starter
for cold starting. It has various provisions for idling, starting, cursing as well as for acceleration. Various
circuits of the solex carburetor are:a)
b)
c)
d)
Cold starting
Idling or low speed operation
Normal running
Acceleration
Cold stating & warming:- unique feature of solex carburetor I the provision of a bi-starter or a
progressive starter. The starter valve is in the form of a flat disc with holes of different sizes. These holes
connect the petrol jet and starter jet sides to the passage which opens into the air horn just below the
throttle valve depending upon the position of the starter lever either bigger or smaller holes come
opposite the passage. Initially for starting, richer mixture is required and after the engine starts the
richness requirement decreases. So in the start position bigger holes are the connecting holes. The
throttle valve being in the clo0sed position the whole of the engine suction is applied to starting
passage, sucking petrol from jet and air from starter air je. The jets and passages are so shaped that the
mixture supplied to the carburetor is rich enough for starting. After the engine has started, the started
lever is brought to the second position, in between the initial and final positions. Now the smaller holes
in the valve completes the circuit, thus reducing the amount of petrol. Also in this position, the throttle
valve is partly open, so that the petrol is also coming out from the mainjet. The reduced mixture supply
from the starter system in this situation is however sufficient to keep the engine running, till it reaches
the normal running temperature when the starter is brought to “off” position.
Idling or low speed operation :- during idling or low speed operation a considerably rich mixture Is
required. An idle port controlled by idle adjusting screw is provided in the engine side of throttle valve.
As the throttle is almost closed, the engine suction is applied at the pilot petrol jet, which supplies
petrol. The jet itself draws the petrol from the main jet circuit. The air is drawn in from the pilot air jet.
The petrol & air mixture in the idling passage & the mixture comes out of the idle part. To ensure the
smooth transfer from idle and slow speed circuit to the main jet circuit without the occurrence of flat
spot, slow running openings are provided on the venturi side of throttle valve. As the throttle is opened
wide, the suction at idle port decreases. But suction is also applied at the slow speed openings which
more than affsets the loss of suction at the idle port and thus flat is provided.
Normal running :-the solex carburetor has a conventional float in a float chamber. For normal running
the fuel is provided by the main jet and the air by the choke tube or venturi the fuel from the main jet
passes into the well of air bleed emulsion system. The correct balance of air and fuel is automatically
ensured by air entering through and being calibrated by the air correction jet. The metered fuel is
discharged through nozzle in the middle of venturi.
Acceleration :-during acceleration, again a very rich mixture is needed. To avoid the “flat spot” during
acceleration a separate membrane pump is provided which delivers spark of extra fuel for acceleration.
Pump lever is connected to the accelerator pedal so that when the same is pressed, the lever moves
towards left, thus pressing the membrane towards left & forcing petrol in to main jet circuit. When the
pedal is lest free, the lever moves the membrane back towards right creating vaccum towards left which
opens the ball valve provided, thus admits the fuel in the chamber into the pump itself