Lab Manual | constructional details, working principles and operation of the fuel supply systems

Aim

To study and prepare report on the constructional details, working principles and operation of the following fuel supply systems

a) Carburetor

b) Diesel fuel injection system

c) Gasoline fuel injection system

Theory

Labeled diagram, construction details, working principle and operation of the fuel supply systems

Functions of a carburetor

The main functions which a carburetor is required to perform are

1 To keep a small reserve of fuel at a constant head

2 To vaporize the fuel to prepare a homogeneous air fuel mixture

3 To supply correct amount or the air fuel mixture at the correct strength under all conditions of load and speed of the engine

Simple carburetor

To understand the principle on which a carburetor works, consider a simple carburetor as shown in fig. The main parts are a float chamber, fuel jet, venturi, nozzle and a throttle valve. The float in the float chamber is made of deep drawn brass sheet and is kept hollow for lightness. Such floats have a tendency to leak along the joint seams. Due to this reason floats are now made of nylon plastic or expanded synthetic rubber. The needle valve attached to the float lever serves to close or open the fuel inlet to the float chamber depending upon the requirements. The needle valve consists of a cylindrical stem with a conical tip made of steel or else a solid steel stem with a rubber seat tip. Alternatively, there may be a three piece valve with a rubber seat tip and a spring – loaded ball in the body of the stem. The later two types maintain the liquid – tight seal during vibrations, which is not possible with ordinary single piece needle valve. When the fuel level falls below a definite predetermined value, the float also falls along with fuel level, thus opening the passage for the fuel supply. The fuel starts flowing in and the float rises gradually till the fuel level reaches the desired value. At this time, the float needle closes the fuel inlet passage. Thus a constant head of fuel is maintained in the float chamber. This constant level of fuel is slightly below the nozzle outlet, so that the fuel may not drop all the time from the nozzle, even when the engine is not working. This provision also prevents the fuel from spilling out when the car is tilted on account of hilly or highly cambered road. In practice fuel level in the float chamber is maintained about 5mm below the nozzle outlet. A small vent in the float chamber keeps the pressure inside atmospheric. In modern practice it is preferred to vent the float chamber to the air intake of the carburetor. Such an arrangement prevents dust particles from mixing with the petrol through the vent as there is always an air cleaner on the intake side of the carburetor which filters the incoming air.

Fuel Injection System

The function of a fuel injection system is to inject proper quantity of fuel into the engine cylinders at the correct time and at a predetermined rate. The fuel injection systems may be broadly classified into the solid injection system and the air injection system. In the solid injection system, only the liquid fuel is injected, whereas in the injection system liquid fuel is injected along with compressed air. The air injection system is less reliable, less efficient and requires an air compressor for supplying air at 7 Mpa or higher pressures (which consumes upto 10% of the power output of the engine) due to which reasons it has become obsolete. The solid injection system will, therefore, be discussed here in detail.

Two types of solid injection systems are in use:

1. common rail fuel injection system

2. individual pump fuel injection system

Layout of a common rail fuel injection system is shown in fig. This type of fuel supply system is used in the Detroit diesel engine, commonly known as Jimmy diesel. In this a single injection pump with injector, called as unit injector is employed on each cylinder. The unit injectors are operated by rocker arms and springs similar to the engine valves. A linkage connects the control racks of all the unit injectors, so that fuel injection in all the cylinders may be equal and simultaneously controlled.

The fuel is taken from the fuel tank by the feed pump and is supplied at low pressure through a filter, to all the unit injectors. This avoids the high pressure fuel lines necessary in the individual pump system. Any excess fuel from the relief valve is returned to the fuel tank.

Individual pump fuel injection system using in line injection pump is shown in fig. Fuel is drawn from the fuel tank by means of a fuel feed pump which is operated from the injection pump camshaft. Generally, the plunger type or the diaphragm type of fuel feed pumps are employed in automobiles. The pump is provided with hand priming lever so that the diesel oil can be forced into the system and the air bled out without turning the engine. The fuel is then passed through a filter and thence to the fuel injection pump. Without the filter or with a poor quality filter, abrasive matter would reach the fuel injection pump and injectors, resulting in poor starting, irregular idling and deterioration in performance due to decreased fuel delivery from the injection pump. The abrasive matter would also cause faulty spraying and leakage in the injectors thus resulting in increased fuel consumption and heavy exhaust smoke.

Reference Books

1) Automotive Mechanics – Crouse / Anglin

2) Automobile Engineering – Dr Kirpal Singh

Viva Questions

1 Name of the various fuel supply system?

2 Which one is most accurate fuel supply system for petrol cars?

3 Where the fuel pump is usually located in the car?

4 Why the carburetor float is made hollow?

5 What is the function of check valve is the carburetor?

6 What is the function of venturi in the carburetor?

7 Name any Indian vehicle using solex carburetor?

8 What is the purpose of petrol injection?

9 What is the common rail fuel injection system?

10 What are the various types of petrol injection system?

11 What is the function of a fuel feed pump/

What is the function of fuel injector