Fuel Supply System in Diesel Engine (With Diagrams)

Fuel Supply System in Diesel Engine!

Introduction to Fuel Supply System for CI Engines:

The fuel supply system of a diesel engine can be called as the heart of the engine, since the engine performance directly depends upon the proper functioning of this system—which must supply, meter, inject and atomize the fuel.

Fuel injection systems are manufactured with great accuracy, hence they are costlier.

Fuel will flow either because of gravity or fuel feed pump, which is provided to supply fuel through the filter to the injection pump. Which pumps the fuel to the injectors which are provided in the cylinder heads.

The fuel injection systems are of 2 types:

1. Air Injection System:

In this case fuel is injected under the pressure of air. For supplying high pressure air multistage air compressors are required, which are very much costly and hence this system is not in use.

2. Solid Injection System:

In this case diesel fuel is directly injected by fuel pump (Bosch Pump).

Further these are of 3 types of solid injection systems:

A. Individual Pump System:

As shown fuel will flow from the storage tank to filters to low pressure pumps. This low pressure pump pumps the fuel to 4 separate metering and pressure pumps.

These separate metering and pressure pumps will pump the fuel to individual injectors which are provided in the cylinder heads. These are used in large slow speed engines.

B. Distributor System:

Fuel will flow from storage tank to low pressure pump through filters, then to metering and pressure pumps. This metering and pressure pump pumps the fuels to distributor unit which distributes and sends required quantity of fuel to each injectors/each cylinders. Used in small and medium size engines.

C. Common Rail System:

In this case fuel flows from storage tank to low pressure pump through filters. Low pressure pump, pumps the fuel to high pressure pump, which pumps the fuel to high pressure pump, which pumps the fuel to common rail. Thus high pressure fuel is collected in common rail and from here through the metering devices required quantity of fuel goes to injectors/cylinders. Generally Cummins and multi-cylinder engines use this system.

Fuel Pump and Injector:

Rack (1) is connected to the accelerator pedal or governor, which moves in and out when the accelerator pedal is operated.

Rack is in contact with the Quadrant gear (2) (a part of gear), which has cylindrical bottom part (Skirting cylinder). This skirting cylinder has cross slot. In this cross slot, cross bottom part of the plunger (3) is held. As the rack moves in and out—quadrant gear rotates—in turn plunger with helical groove moves in the cylinder (4).

The cylinder has inlet and overflow ports. This fuel pump and injector operates under primed conditions. Valve (5) is resting on the valve seat by the spring (6). Injector and pump are connected by the delivery pipe (7).

In the injector (8) is the nozzle body, (9) is the nozzle valve and (10) is valve cap nut, (11) is the spindle-held in position by spring (12).

 It is to be noted that the plunger has up and down reciprocating motion—which is obtained by the camshaft below it and has rotary motion because of rack. When the rack moves in and out depending upon the power requirement. Quadrant gear moves—in turn plunger rotates—plunger has helical groove—so height of groove with respect to ports varies—so amount of fuel to be injected will vary.

During the upward motion of plunger— once the ports are closed—valve is lifted from its seat because of fuel pressure and fuel flows through the delivery pipe—through the fuel passage (13) to nozzle valve (9). Because of fuel pressure—nozzle valve (9) is lifted against the compression of spring (12) and fuel is injected till the edge of helical groove comes in contact with port—when pressures of fuel is released and injection stops.

Types of Fuel Injectors:

1. Air Blast Injectors – These are used in air injection systems. Nowadays air injection systems are not in use as they require multistage compressors. And hence these injectors are no more used.

2. Mechanically Operated Injectors – These injectors are operated by a mechanism similar to that used to operate. IC engine valves i.e., it uses camshaft, push rods, rocker arms, etc. Cam operates the plunger.

3. Automatic Fuel Injector – All automobile CI engines use these Automatic Fuel Injectors. They comprise needle valve, which is lifted up by fuel pressure. This fuel pressure is created by fuel pump.

Types of Nozzles:

Following types of nozzles are normally used with diesel engines:

1. Single hole type

2. Multiple hole type

3. Pintle type

1. Single Hole Type:

At the centre of nozzle body a hole of 0.2 mm dia is provided.

Spray cone is ∠ is @ 15°.

Used in open combustion chambers.

High pressure is required to get same velocity. No good mixing with air. It has a tendency to dribble.

2. Multiple Hole Type:

Proper mixing with air 4 to 18 holes. Size of the holes will be 0.35 to 1.5 mm.

3. Pintle Type:

To avoid weak injection and dribbling, the spindle is provided with projection called Pintle. It protrudes through the mouth of nozzle body. It may be cylindrical or conical in shape. Dribbling is avoided.

Used in precombustion chambers, air cells, swirl chambers.

4. Pintaux:

Good for cold starting. It is a development of pinter nozzle. It has an auxiliary hole in the nozzle body. It results in good cold starting.

Disadvantage:

Side hole may be choked-better filter needed.

Electronic Fuel Injection (EFI) System:

Electronics is introduced in automobiles in 1965. About 30—40% of cost of vehicles is for electronic items. Max power and best economy are attained by using electronics and computers in automobiles.

EFI systems was various sensors to sense various parameters like temperature, pressure of gases, position of throttle valve, air flow rate etc.

Sensors feed this data to Electronic Control Unit (ECU)—which is basically a computer. This ECU—processes the data and operates injectors and other devices to have maximum power, with best economy, and low emissions.

Multipoint Fuel Injection System (MPFI):

Multipoint Fuel Injection system is used to supply air-fuel mixture of proper strength and in required quantity to each cylinder of a multi-cylinder engine, at all engine speeds loads.

MPFI—Systems Functions under 2-Basic Arrangements:

1. Port Injection:

In this case injector is placed in the intake manifold, near the inlet valve. The injector sprays petrol into air flowing through the intake manifold. Homogenous air-fuel mixture produced enters cylinder. Note that each cylinder has a separate injector placed in its intake manifold.

Advantages:

1. Uniform fuel distribution

2. Increase in power output

3. More precise control of air-fuel ratio.

2. Throttle Body Injection:

In this case injector is provided at a single point in the throttle body. Throttle valve controls the amount of air entering intake manifold.

Quantity of Fuel and Size of Nozzle Orifice: