In this article we will discuss about:- 1. Principle of Diesel Engine 2. Special Features of Diesel Engine 3. Operation 4. Combustion Process.
Principle of Diesel Engine:
In such engines, only air is drawn into the cylinder instead of mixture of air and fuel as in carburetor type engine. Compression of the air during the compression stroke makes the charge very hot. Towards the end of the stroke, diesel fuel is injected into the cylinder in atomised form which ignites in the cylinder, causing rise in pressure which forces the piston down. No electrical ignition system is provided for sparking.
The basic elements of diesel engine are same as spark ignition engine but the method of fuel introduction and ignition are different to a great extent. The engine has high compression ratio hence the air in the cylinder attains very high temperature and pressure at the end of the compression stroke. At the end of the compression stroke, the fuel is sprayed into the cylinder through atomisers (injectors).
The cylinder contains air at high pressure and temperature; hence the fuel begins to burn as soon as the fuel reaches the cylinder in atomised form. Such engines are called compression ignition engines because the ignition of fuel takes place due to heat of compression. Diesel engine is equipped with fuel injection pump and injectors. The injectors protrude into the combustion space of the engine.
Special Features of Diesel Engine:
i. Engine has high compression ratio ranging from 14:1 to 22:1.
ii. During compression stroke, the engine attains high pressure ranging from 30 to 45 kg/cm2 and high temperature of about 500°C.
iii. At the end of the compression stroke, fuel is injected into the cylinder through injectors (atomisers) at a very high pressure ranging from 120 to 200 kg/cm2.
iv. Ignition takes place due to heat of compression only.
v. There is no external spark in diesel engine.
vi. Diesel engine has better slogging or lugging ability i.e. it maintains higher torque for a longer duration of time at a lower speed.
Operation of Diesel Engine:
For operation of diesel engine, the piston is placed inside the cylinder and it is attached to the crankshaft through the connecting rod. The piston is moved up and down in the cylinder. This up and down motion of the piston is changed into rotary motion of the crankshaft by the connecting rod. Fly wheel is attached to the rear end of the crankshaft. This makes the shaft revolve uniformly when the engine is running.
The cylinder is tightly closed at the top by cylinder head which houses inlet and exhaust valves. Inlet valve admits air into the cylinder and exhaust valve allows the burnt gases to go out of the engine. The valves are held closed by valve spring and are made to open by means of rocker arms, which are operated by camshaft through valve lifters and push rods.
The camshaft and fuel injection pump shaft are driven by the crankshaft through gears. The fuel supplied by fuel injection pump is injected into the cylinder through fuel injector. Fuel is ignited by heat of compression and piston is forced back by the expanding gases. Thus cycle is repeated.
Combustion Process in Diesel Engine:
In Diesel engine, only air is compressed in the cylinder by the piston. Then fuel is injected into the cylinder approximately. 15° before top dead centre (TDC) and continues to approximately 10° after TDC (Fig. 3.7). Due to fuel being in highly atomised form, it is self-ignited by heat of the compressed air. That’s why compression ratio of diesel engine is required to be high (14 to 22), and the compressed air temperature to be over 500°C.
The combustion takes place in the following four stages:
(a) Delay Period Before Ignition (Fig. 3.7):
During this period, some fuel has been admitted but has not yet been ignited. At the completion of Delay period i.e. at the point B, ignition starts with rapid pressure rise.
(b) Period of Rapid Pressure Rise and Heat Transmission (Fig. 3.7 during B-C):
Once ignition has been started and a flame established, much heat is transmitted to all fuel particles, resulting in rapid combustion and pressure rise in the combustion chamber.
(c) Period of Direct Burning of Later Injected Fuel (Fig. 3.7 during C-D):
At this stage, the temperature and pressure in the combustion chamber are so high that later injected fuel burn as soon as they enter.
(d) Period of After Burn (Fig. 3.7 during D-E):
At point D, fuel injection stops and fuel which has failed to burn till this period, burn during the period D-E.
If during the “Delay period”, excessive fuel has been injected into the combustion chamber, the fuel will burn explosively, during the period B-C. Thus excessive pressure is created in the combustion chamber, which results in engine “Knocking”.
Fuel Injection Pump:
The fuel injection pump is the most important unit of diesel engine. It consists of barrel and a plunger. A cam is there to raise the plunger and the plunger returns back by the tension of a spring. The plunger fits so tightly that it seals off without a gasket at very high pressure. On the downward stroke of the plunger, the fuel enters from the suction chamber, and fills the barrel.
When the plunger goes upward, the fuel is compressed until the top edge of the cylindrical part of the plunger covers the portion, leading to the suction chamber. From there the fuel is forced out through the delivery valve and delivered to the pipe going to injector nozzle. The amount of fuel is adjusted to suit the requirement by means of a governor which changes the angular position of the plunger in the barrel.
Injector or Atomiser:
The atomiser or the injector receives the fuel from the fuel injection pump and sprays the fuel into the combustion chamber. The needle spindle of the atomiser is spring loaded at the top with a certain pressure. When the pressure exerted by the fuel overcomes the spring load, the needle valve is lifted from its seat and the oil is injected through the orifices.