Fuel Supply to SI Engines

Fuel Supply to SI Engines (Carburetion):

Introduction:

The process of preparing air-fuel mixture in SI engine outside the engine cylinder is known as Carburetion. The device used for this purpose is known as Carburetor.

In petrol engines, the air and fuel is mixed outside the engine and partly evaporated mixture is supplied to the engine. The fuels such as petrol, benzol and alcohol used in SI engine vaporizes easily if injected in the flow of air, therefore, the engine suction is sufficient to create the air flow and fuel injected easily evaporates. The oil fuels which are used in CI engines do not vaporize easily. Therefore, a separate injection system is used. These systems will be discussed in the next blog.

The vaporization process of the fuel in the current of air depends mainly upon, the physical properties of fuel, the temperature of incoming air in the intake manifold, the pressure difference causing the flow of fuel in the air, design of intake manifold and the time available for evaporation.

The simple arrangement of the mixture supply to SI engine is shown in the figure below:

During the suction, the air is sucked as vacuum is created inside the engine cylinder. The fuel is injected in the air from the carburetor and a mixture is supplied to the engine cylinder.

It is desirable to have a complete vaporized mixture in the engine cylinder but some of the large droplets may reach the cylinder in the form of liquid and they are mixed and vaporized during the compression stroke.

The time available for atomization, mixing and vaporization is so small, (0.02 second when engine is running at 3000 RPM) the design of the system becomes more difficult. The temperature is one of the factors which accelerate vaporization but this would reduce the power output due to reduction in mass flow.

The design of carburetor is difficult and complicated as the requirements by the engine for A:F ratio vary from 1:1 to 15:1 under different operating conditions. Therefore, the design of different components and devices incorporated in the carburetor to fulfill all the above mentioned requirements will be discussed in subsequent articles.

Fuel Supply system to Automobile and Fuel Pumps:

The general arrangement of fuel supply system to SI engine used in automobile is shown in figure below:

Generally on four wheeler automobile, the fuel tank is located away from the engine to avoid vapor lock and fire hazard. The fuel is pumped with the help of a petrol pump from the storage tank to the carburetor as the tank level is always below the level of the float chamber of the carburetor, the pressure developed should be just sufficient to overcome the frictional losses passing through the filter and pipe line carrying the petrol from the tank to the carburetor float chamber.

Types of Petrol pump:

There are two types of petrol pumps commonly used in practice as Mechanical pump and Electrical pump.

a.      Mechanical Petrol Pump:

It is located near the engine as it is operated by the engine itself. It is mounted on the side of the crank case and operated by an eccentric on the cam shaft as shown in the figure below:

This consists of a chamber divided into two compartments as shown in the figure. The top portion contains a filter and sediment boul and has two spring loaded valves to control the flow of petrol. The lower portion contains a spring which regulated the pressure of the petrol supply and an operating link and rocker arm driven by the cam shaft. The diaphragm fitted as shown in the figure is alternately pulled down by the link and then pushed up by the spring.

The pump lever is made of two parts in such a way that it only pulls the diaphragm down and then it is taken up by the spring as the lever action goes out.

As the eccentric (mounted on the cam shaft) pushes the lever towards the right, the rocker arm pushes the push rod and the diaphragm down creating a vacuum inside the down chamber of the pump. This vacuum is sufficient to open the inlet valve and sucks the petrol. As the eccentric goes out of the action, the push rod and the diaphragm is pushed up by the spring action and pressurizes the fuel taken in. This pressure (1.1 to 1.3 bar) is sufficient to open the delivery valve and supply the petrol to the carburetor. As this is done, the lever comes into the operation. Once more to pull the diaphragm down and suck the petrol again and operation is repeated.

The filter is covered by a glass boul as shown in the figure so that the accumulated sediment can be seen and boul can be removed easily for cleaning purposes.

The double lever arrangement is essentially used because, if it were in single piece, the lever would operate the diaphragm and pump the petrol continuously to the carburetor even the carburetor float is full. This would cause the float chamber to overflow. Double lever arrangement overcomes this difficulty.

The mechanical pump is highly reliable but it operates only when the engine is running. A hand lever, used for priming the pump, operated in the same manner as the pump lever pulls the diaphragm down when operated, drawing the petrol in.

The mechanical pump is always subjected to heat as located near the engine and can cause vapor lock although insulated.

The diaphragm may become porous after a long service and in such case it must be changed immediately to prevent the petrol leaking into the engine and mixing with the lubricating oil.

b.      Electric Pump:

The electric pump works on the same principle as mechanical pump except that the diaphragm is operated by a solenoid instead of by a cam shaft.

The arrangement of the pump is shown in the figure below:

It consists of an electromagnetic coil which is connected to the battery through a contact points. These contact points are separated by a pull rod which is also connected to the diaphragm by a diaphragm spring.

The advantages of electric pump over mechanical are:

1.      It can be started without starting the engine. It starts as soon as the ignition circuit is switched on.

2.      It is not affected by the engine heat and totally free from vapor lock.

More details about the Fuel Supply to SI Engines will be discussed in next coming blog. Air fuel mixture and mixture requirements will be discussed.