Fuel system of diesel engine consists of the given components: 1. Fuel tank 2. Fuel Lift Pump or Feed Pump 3. Fuel Filter 4. Fuel Injection Pump 5. High Pressure Pipe 6. Fuel Injector (Injection Nozzles).
Fuel is drawn from the fuel tank by the Feed pump and forced to the Injection pump through Fuel filter. The Injection pump supplies high pressure fuel to injection nozzles through delivery valves and high pressure pipes. Fuel is injected into the Combustion chamber through Injection nozzles. The fuel that leaks out from the injection nozzles passes out through leakage pipe and returns to the fuel tank through the over flow pipe.
Overflow valve installed at the top of the filter, keeps the feed pressure under specified limit. If the feed pressure exceeds the specified limit, the overflow valve opens and then the excessive fuel returns to fuel tank through overflow pipe.
Component # 1. Fuel Tank:
It is a storage tank of suitable size and shape, usually made of mild steel sheet. Atmospheric pressure is maintained in the tank with the help of a pin hole on the cap. Usually a wire gauge strainer is provided under the cap to prevent foreign particles. Usually a drain plug is provided at the bottom for flow of fuel.
Component # 2. Fuel Lift Pump (Feed Pump or Transfer Pump):
Fuel lift pump may be:
(i) Plunger type,
(ii) Diaphragm type
i. Plunger Type:
It may be single acting or double acting pump usually mounted on the side of the Injection pump housing. It is driven by Injection pump camshaft.
It transfers fuel from the fuel tank to the inlet gallery of the Injection pump through fuel filter. The fuel pressure at the feed pump must be in the range of 1.5 to 2.5 kg/cm .
It delivers adequate amount of fuel to the injection pump.
The pump consists of – (i) Body, (ii) Piston, (iii) Inlet valve, (iv) Pressure valve.
The valves are tightly pressed against their seats by springs.
The piston is free to slide in the bore. The fuel contained in the space below the piston is forced to flow through filter to the injection pump. At the same time, downward movement of the piston creates a depression in the space above the piston, which causes the fuel to be drawn in the transfer pump from the fuel tank through inlet valve and filter. When starting the engine and venting any possible air, the priming pump provides manual pump operation also.
ii. Diaphragm Type Feed Pump:
It consists of:
(i) Main body
(ii) Diaphragm
(iii) Spring
(iv) Rocker arm
(v) Inlet and outlet valve
(vi) Sediment bowl
Suction is created in the main chamber of the pump by pulling down the diaphragm. The fuel is sucked through the inlet valve. During this period, outlet check valve remains closed. When there is no pressure of the cam on the rocker arm of the pump, it returns its original position due to the tension of the return spring.
Upper movement of the diaphragm creates a pushing effect on the fuel, which has entered the chamber during its downward movement. Under the pressure of the fuel, outlet valve opens and the fuel is pumped through outlet chamber. Thus the fuel flows from the tank to fuel injection pump.
Preliminary Filter (Sediment Bowl Assembly):
This filter is mostly fitted on the fuel lift pump. It prevents dust and foreign materials from reaching inside the fuel line. It usually consists of a glass cap with gasket. Heavy dust particles settle at the bottom and clean fuel flows ahead.
Component # 3. Fuel Filter:
Mostly two stage filters are used in diesel engines:
i. Primary filter removes coarse material, water and dust.
ii. Secondary filter removes fine dust particles.
Filters are made of different materials like:
(а) Metal element
(b) Felt and cloth element
(c) Paper disc
(d) Metal element consists of porous metal filter elements.
(e) Felt and cloth element is a cloth of special weaving, which facilitates easy filtration.
(f) Paper disc type filtering element is enclosed in a metal case, which can be replaced easily after being clogged.
Component # 4. Fuel Injection Pump:
It is a high pressure pump, which supplies fuel to the injectors according to the firing order of the engine. It is used to create pressure varying from 120 kg/cm2 to 300 kg/cm2. It supplies motored quantity of fuel to each cylinder at appropriate time.
Fuel Injection pump is usually of two types:
I. Multi element pump
II. Distributor (Rotary) type pump
I. Multi Element Pump:
Each fuel pump unit consists of the following components:
(a) Plunger and barrel
(b) Camshaft
(c) Delivery valve
(a) Plunger and Barrel:
Each pump element consists of – 1. Plunger and 2. Barrel.
The plunger has helical groove. The rotation of the plunger changes the position of its helical groove with respect to the barrel port. The fuel enters the barrel through these ports. Control rod of the pump governs the movement of plunger. This control rod is connected to the governor by a linkage. Locking screw controls the length of stroke of the plunger. Calibration is necessary in such cases.
Working of Plunger:
When the plunger is at BDC, the barrel is occupied by the fuel which has entered through the barrel ports either under the force of gravity or from a fuel pump. As the plunger rises, the fuel is pushed back through the barrel ports until they are closed by the top of the plunger position. The effective stroke of the plunger begins at this point. Further upward movement forces the fuel through the delivery valve, causing it to lift up from its seat. The fuel then enters the pipe, connecting the pump to injector. A corresponding amount of fuel leaves it at the other end and enters the cylinder through the injector.
This action continues until the plunger reaches a position when the lower edge of the control helix uncovers the barrel ports allowing the fuel to pass out via the vertical slot in the plunger.
By rotating the plunger, the effective stroke can be shortened or lengthened depending upon which part of the helix is in line with the barrel port, through which the fuel escapes at the end of stroke. Thus the quantity of fuel, injected into the cylinder can be varied.
(b) Camshaft:
The camshaft of a fuel pump is timed with the crankshaft and operates the intake and exhaust valves. It also operates the plunger at the correct sequence according to the firing order of the engine.
The fuel from the fuel tank passes through filters and flows to Fuel Injection pump and from the pump, the plunger lifts the fuel to the injector of a particular cylinder.
(c) Delivery Valve:
This acts as a one way valve or non-return valve. It seals off the delivery pipe from the barrel during the intake stroke until next delivery stroke. It is fitted in the F.I. pump between the pump element and the delivery pipe or high pressure line. It consists of a delivery valve and a valve body in one unit. Another important function of delivery valve is to relieve (unload) the pressure in the delivery pipe.
II. Distributor (Rotary) Type Fuel Injection Pump:
In this type of pump, one plunger and one barrel assembly deliver fuel not to one cylinder but to several cylinders. The plunger not only reciprocates, but rotates also in a close fitting barrel. This helps in distributing fuel to a number of cylinders in turn. In the upper part of the barrel, there are inlet ports through which fuel enters the barrel.
There are delivery passes connecting the barrel bore with the inclined passages drilled in the head. Through this passage, fuel is delivered to delivery valve holder. From these the fuel goes to the appropriate injectors through high pressure pipes. The barrel is sealed on the outside by rubber rings.
Important Characteristics of Diesel Fuel:
i. Proper Viscosity:
If fuel viscosity is higher than proper viscosity, droplet of the fuel, injected into the combustion chamber becomes comparatively large and then the fuel is hard to burn. On the contrary, lower viscosity fuel may not provide proper lubrication of plungers and nozzles of injection pump, resulting in quick wear of parts. So fuel of proper viscosity should be chosen.
ii. Impurities and compound elements in diesel fuel should be low in percentage. Impurities and compound elements like (i) Water, (ii) Ash, (iii) Sulfur, and (iv) Carbon residues should be low is percentage. Increase of percentage of the impurities and compound elements would decrease engine output, and cause quick wear of fuel injection system components and engine components.
Precaution in Handling Fuel System:
i. Fuel:
(a) High quality fuel is necessary to the fuel injection pump.
Since the pump elements and nozzles are super-high precision parts to have a clearance of about 0.001 mm, any kind of dirt and water will damage the pump elements and nozzles.
(b) Only filtered fuel should be supplied to the fuel tank.
ii. Fuel Tank Cleaning:
Water and deposits should be drained from the fuel tank at least every six months. Strainer of feed pump and other strainers should be cleaned frequently.
iii. Air Venting for Fuel System:
When air has entered the fuel lines or the suction chamber of the injection pump, Venting should be done properly Air is removed by the priming pump through the bleeding holes of the injection pump.
iv. Lubricant:
Recommended lubricant should only be used. Oil level should be between upper line and lower line of oil level gauge. Oil should be changed at the specified time.
Component # 5. High Pressure Pipe:
Resistance to fuel flow in high pressure pipe and its length have influences on fuel injection quantity, so only specified high pressure pipe should be used. High pressure pipe should never be bent sharply. The radius of the bend should not be less than 50 mm. The pipe should never be bent by applying heat.
Delivery Valve:
Delivery valve holder should be tightened with specified torque. Excessive tightening will cause cracks on pump house.
Nozzle Holder:
Nozzle holder retaining nut should be tightened with specified torque. Over tightening will make control rack resistance heavy.
Trouble Shooting in Fuel Injection System Preliminary Check:
Before trouble shooting, check the following:
(a) Check all fuel lines for leakage and damages.
(b) Check injection timing.
(c) Inspect nozzle spray. Loosen each fitting between delivery valve holder and high pressure pipe, and then check each valve holder for fuel leakage with the pump stopped. If leakage is found, the delivery valve is faulty.
(d) Check feed pump. Loosening inlet fitting to the pump housing, operate the priming pump. Fuel should overflow from fuel hose.
(e) Check if the control rack smoothly moves. Remove the control rack cover, push the control rack into the pump housing and release it. The control rack should back smoothly.
(f) Check pump element chamber for rust and damage.
(g) Check lubricant for viscosity and quantity with the oil level gauge.
Trouble Shooting:
Component # 6. Fuel Injector:
It is the component which delivers finely atomised fuel under high pressure to the combustion chamber of the engine. Modern tractor engines use fuel injectors which have multiple holes. Main parts of injector are: nozzle body and needle valve. The nozzle body and needle valve are fabricated from alloy steel. The needle valve is pressed against a conical seat in the nozzle body by a spring. The injection pressure is adjusted by adjusting the screw.
In operation, fuel from the fuel injection pump enters the nozzle body through high pressure pipe. When the fuel pressure becomes so high that it exceeds the set spring pressure, the needle valve lifts off its seat. The fuel is forced out of the nozzle spray holes into the combustion chamber. The injector is installed in a brass tube or sleeve which is fitted in the cylinder head and is held in position by a special clamp.
The nozzles of injector are of three types:
1. Pintle type
2. Throttle type
3. Hole type
1. Pintle Type:
It has got a nozzle tip projection, which is little thinner than the nozzle tip. This prevents the nozzle tip from being clogged with carbon.
2. Throttle Type:
It is a nozzle with specially shaped nozzle tip projection. It causes economy of fuel.
3. Hole Type:
It does not have a nozzle tip. The valve seat is directly connected to a conically shaped outlet.
Fuel Injection System:
Diesel fuel is injected in diesel engine through injectors with the help of fuel injection pump. The system using injectors, fuel injection pump, fuel filter, and fuel lines is called fuel injection system.
The main functions of fuel injection system are:
1. To measure the correct amount of fuel required by engine speed and load.
2. To maintain correct timing for beginning and end of injection.
3. To inject the fuel into the combustion space against high compression pressure.
4. To atomise the fuel for quick ignition.
Process of fuel injection in diesel engine is of two types:
(a) Air injection
(b) Solid injection
(a) Air Injection:
In this process, the engine uses compressed air to force the fuel into the cylinder. It is a bulky system and hence it is not considered very suitable for vehicles and tractors. It is mostly used on heavy duty stationary engines.
(b) Solid Injection:
A high pressure pump is used for forcing the fuel into the combustion chamber.
The common methods are:
(i) Common rail system
(ii) Individual pump system
(iii) Distributory system
(i) Common Rail System:
This system utilizes one pump only. Fuel is sucked from the fuel tank and is supplied to the combustion chamber. One injector is fitted for each cylinder. The pump delivers the fuel under high pressure to the common rail from where it is supplied to every injector.
(ii) Individual Pump System:
There is a separate pump for each cylinder. The fuel tank supplies fuel to different pumps from where it goes to different cylinders.
(iii) Distributory System:
This system consists of a distributory unit along with the pump. The pump supplies fuel to the distributory unit from where the fuel separately.
Combustion Chamber:
A combustion chamber is a space inside the engine, where the combustion of fuel takes place. In diesel engine, the fuel is atomised, vapourised and burnt inside combustion chamber, whereas in spark ignition engine, atomisation of fuel takes place in the carburetor and vapourisation occurs in carburetor as well as the inlet manifold.
Combustion Chamber is classified as:
1. Direct injection chamber.
2. Indirect injection chamber.
1. Direct Injection Chamber:
Fuel is injected directly in the compressed air of the cylinder. The entire fuel is not burnt quickly. Only a part of the fuel comes in contact with the heated air of the engine which gets ignited immediately. Starting of engine is easy is this case.
2. Indirect Injection Chamber:
It may be of two types:
(i) Pre-Combustion chamber.
(ii) Air cell chamber.
(i) Pre-Combustion Chamber:
There is small chamber above the cylinder of the engine, which may be spherical or cylindrical in shape. Fuel is injected directly in this small chamber. At the time of fuel injection, the air of the chamber is disturbed by the upward movement of the piston.
As the combustion takes place, very high pressure is produced inside the small chamber which forces the unburnt fuel with high velocity into the main chamber. Starting of the engine is not very easy due to relatively low temperature of the cylinder.
(ii) Air Cell Chamber:
It is a space provided in the piston or cylinder to trap air during the compression stroke. Later air blows out into the combustion chamber. There is a spherical cavity in the cylinder head, piston head or the cylinder wall. Fuel is injected in the main chamber and combustion of fuel takes place inside the cylinder. When the piston moves downward, the air cell discharges air in the form of spray inside the cylinder which results in rapid burning of the fuel.
Turbocharger:
It is a turbo-compressor driven by the exhaust gases of the engine to supply air under pressure to the cylinders of the engine.
Turbocharger is useful because the power output of a diesel engine can be increased by supplying compressed air to the engine cylinders. If more air is delivered to the cylinders the fuel charge can also be increased and will release more energy.
The turbocharger consists of a centrifugal compressor with impellers and a gas turbine unit. The compressor impeller and the turbine wheel are rigidly fixed on a common shaft. Compressor impeller draws air from the atmosphere and delivers it to the intake manifold and from there it goes to the engine cylinders thus improving the volumetric efficiency of the engine.
Fuel Filter:
It is a device to remove dirt from fuel oil. Solid particles and dust in diesel fuel are very harmful for giving a fine degree of filtration. Fuel injection equipment in diesel engines are extremely sensitive to dirt and solid particles present in fuel. A filter is used to remove the dirt and solid particles from the fuel to ensure trouble free fuel supply.
It consists of a hollow cylindrical element contained in a shell, an annular space being left between the shell and the element. The filtering element consists of metal gauge in conjunction with various media such as packed fibres, woven cloth, felt, paper etc. These filters are replaced at certain intervals, specified by the manufacturer.
Usually there are two filters in diesel engine:
1. Primary filter and
2. Secondary filter
The primary filter removes water and coarse particle of dirt from the fuel. The secondary filter removes fine sediments from the fuel. Usually the primary filter is placed between the tank and the transfer pump.
Air Cleaner:
It is a device which filters and removes dust, moisture and other foreign matter from the air before if reaches the engine cylinder.
Air cleaner is usually of two types:
1. Dry type air cleaner and
2. Oil bath type air cleaner
1. Dry type air cleaner:
The filtering element in this case is a type of felt. The air passes through the element. The element has got larger surface area so the air speed becomes relatively low and consequently particle or dirt in the air is deposited on or stopped by its surface.
2. Oil bath type air cleaner:
In this type of air cleaner, the incoming air impinges upon the surface of the oil, kept in a container in the lower part of the casing. The foreign particles of the air are trapped in the oil and then the air passes through a wire element before reaching the inlet manifold of the engine. The wire element also arrests the remaining dirt particles of the air.
Process to Remove Airlock in the Diesel Engine:
Very frequently there is airlock in diesel engines. It is very important to remove airlock from’ the fuel system for satisfactory running of the engine.
The following sequence should be followed for this purpose:
1. Open the fuel cock and fill the pre-cleaner bowl with fuel.
2. Loosen the bleeding screws of the fuel filters and pump manually till diesel oil flows out without bubbles from both filters. Tighten the screw of the filters one by one.
3. Loosen the bleeding screws of the fuel injection pump and pump manually until bubble free diesel oil flows around these screws tighten the screws.
4. Loosen the capnuts on the injectors one by one and bleed the fuel till the bubble free fuel comes out from the capnuts. Tighten the capnuts one by one.
Precautions in Handling Diesel Fuels in Diesel Engine:
1. Fuel injection pump and injectors are very precise components of diesel engine. Dust is very harmful for fuel injection equipments. Hence extreme care should be taken to use perfectly clean fuel. 90% of the troubles of diesel engines can be removed if absolutely clean fuel is used in the engine.
2. It is very dangerous to touch the spray of the diesel oil. The atomised fuel may enter the flesh of the body and poison the blood.
3. Disassembly and assembly of nozzles should be done in fresh fuel with extreme care.
4. Fuel filters should be cleaned at regular interval and they should be changed at specified time.
5. Fuel injection pump, injectors and governors should be opened and repaired by authorised technicians only and not by others.
6. While filling the diesel tank, a strainer should always be used.