Petrol Engine

           Petrol engine is an internal combustion engine with the spark ignition so it is also known spark ignition that means S I Engine. Fuel used in petrol engine is petrol and similar volatile fuel. The petrol engine works on principle of constant volume cycle. In most petrol engine the fuel and air are usually premixed before compression. It is a reciprocating piston engine, in which a number of piston move down and up in cylinder.                                                                                          

 A mixture of petrol and air is introduced into the space above the piston and ignited. The gases produced force the piston down, generating power.The engine operating cycle is repeated every four strokes (upward or downward movement) of the piston, this being known as four stroke cycle as shown in figure1.4. 

                                            

                         Figure1.4 4-Stroke S I engine (petrol engine)

Four stroke SI engine:-                                                                                                                    Conventional SI engine operates using four “strokes”, with either an up or down movement of each piston.

These strokes are named:    Intake, Compression, power and exhaust.
                      One complete cycle in the 4-strokr/cycle SI engine requires two complete rotation of the crankshaft. As the crankshaft rotates, the piston moves up and down in the cylinder.In the two complete revolution of the crankshaft that make up one cycle. 

Intake Strokes:-

                 Intake stroke of the combustion cycle is when the piston travels down the cylinder with the intake port open. During this stroke, inlet valve open and exhaust valve closed, the pressure in the cylinder will be atmosphere.As the piston moves from top to bottom, the volume in the cylinder increases, while simultaneously pressure decreases.
               This creates a pressure difference between the atmosphere and inside of cylinder. Due to this pressure difference the petrol and air mixture will enter into the cylinder through fuel injection system or carburetor.In a carbureted engine, during the intake stroke of the piston a vacuum is created in the inlet manifold. With a multi cylinder engine the vacuum is almost constant. Electronic fuel injection system spray fuel at high pressure either directly into the combustion chamber or into the intake port of the cylinder during the intake stroke. Using fuel injection enables improved control over the air-fuel mixture and reduces the power required to draw fuel from the jets. The diagram below shows a typical electronic fuel injection system.

      Figure 1.5(a) Intake stroke

Compression stroke:-

                              The compression stroke is the upward movement of the piston in the cylinder with the valves closed following the intake stroke. During this stroke both the inlet valve and exhaust valve are closed, the piston moves from bottom to top as shown in figure 1.5(b).This upward motion compress the fuel/ petrol air mixture inside the combustion chamber raising the pressure.   

Near the end of this stroke, the petrol and air mixture is ignited by the electric spark given out by the spark plug. The difference between the initial volume of the cylinder and the final volume at the top of the compression stroke is known as the compression ratio. The fuel/petrol –air mixture and compression ratio is critical to avoid pre-ignition which is the abnormal ignition of fuel in the combustion chamber before the combustion stroke

                                             Figure1.5 (b) Compression stroke

Power stroke:-

                    The power stroke is the downward movement of the piston in the cylinder with the valves closed. During this stroke both the inlet valve and outlet valve are closed, the piston moves from top to bottom.                                                        

The high pressure and high temperature burnt gases force the piston to perform this stroke, called power stroke. This stroke is also known as combustion stroke or working stroke. The engine produces power during this stroke.

As the piston moves from top to bottom, the pressure of hot gases decreases and volume increases.

Near the end of this stroke, the exhaust valves open which will release the burnt gases to the atmosphere. Figure1.5(c) shows the power stroke.

                                               Figure1.5(c) Power stroke

Exhaust stroke:-

               Exhaust gases are pushed out of the cylinder by the upward motion of the piston following the ignition stroke. During this stroke, the outlet valve is open and inlet valve is closed. The piston moves from bottom to top and during this motion piston pushes the exhaust gases out of the cylinder at constant pressure.

Typically exhaust gases contain oxygen, nitrogen, water vapor, carbon dioxide, carbon monoxide, hydrogen, nitrous oxides, particulates and unburned hydrocarbons.  

Figure1.5 (d) shows the Exhaust stroke. At the one movement the inlet valve open and the new cycle starts

                                           Figure 1.5(d) Exhaust stroke

Two stroke SI Engine:-

                       The engine cycle is completed after just two strokes (up or down) of the piston, which is differentiates it from the more common four-stroke cycle.

Some power mowers and lightweight motorcycle use two-stroke petrol engines, which are cheaper and simpler than four-strokes. In a typical two-stroke motorcycle engine, fuel mixture is drawn into the crankcase as the piston moves up on its first stroke to compress the mixture above it. Then the compressed mixture is ignited, and hot gases are produced, which drive the piston down on its second stroke.

 As it moves down, it uncovers an opening (port) that allows the fresh fuel mixture in the crankcase to flow into the combustion space above the piston.

At the same time, the exhaust gases leave through another port. In two stroke SI engine only two strokes is used by combing the two strokes in half cycle.

These strokes are named: intake/compression, power/exhaust.
Figure 1.6(a) show the Two Stroke S.I Engine

                             Figure 1.6(a) Two Stroke S.I Engine

Intake/compression stroke:-

           The fuel-air mixture has entered the cylinder, and the piston begins to move up i.e. Bottom to Top. This compresses the charge in the cylinder and draws a vacuum in the crankcase, pulling in more air, fuel, and oil from the carburetor.    The compressed charge is ignited by the spark plug, and the cycle begins.The figure 1.6(b) shows the Intake or Compression stroke .
                                             

  Figure 1.6(b)  Copression Stroke

Power/exhaust stroke:-

                                This stroke occurs immediately after the ignition of the charge. The piston is forced down i.e. movement from Top Dead Centre to Bottom Dead Centre.                                                                                                                                        After a certain point, the top of the piston passes the exhaust port, and most of the pressurized exhaust gases escape. As the piston continuous down, it compress the air/fuel/oil mixture in the crankcase. Once the top of the piston passes the transfer port, the compressed charge enters the cylinder from the crankcase and any remaining exhaust is forced out. The figure 1.6(c) shows the Power or exhust stroke .
                                     

                                   Figure 1.6(c)  Power or exhust stroke .

Diesel Engine