Introduction to Spark ignition four stroke engine

This is about spark ignition engine

1. M.A.N. Induwara Munasinghe BSc Eng (Hons)(Moratuwa), MEng (Canada), AMIE (SL), AMIMechE (UK) 4 Stroke Spark Ignition Engine ME3103: Automotive Engineering I Department of Mechanical Engineering, Faculty of Engineering, General Sir John Kotelawala Defense University

2. Department of Mechanical Engineering GENERAL SIR JOHN KOTELAWALA DEFENCE UNIVERSITY Automotive Engineering I Department of Mechanical Engineering 4 Stroke SI Engine

3. Four Stroke Petrol Engine Exhaust Valve Spark Plug Exhaust Gas Path Piston Connecting Rod Crank Shaft Crank Case Cylinder Inlet Gas Path Inlet Valve Valve Springs Cam Rocker Arms Basic Parts o f t he Four Stroke Petrol Engine

4. Four-stroke Spark-ignition Engine Suction or intake stroke Compression stroke Expansion or power stroke and Exhaust stroke

5. Working principle of a Four Stroke SI Engine 1. Suction or Intake Stroke : Suction stroke starts when the piston is at the top dead center and about to move downwards. The inlet valve is open at this time and the exhaust valve is closed. Due to the suction created by the motion of the piston towards the bottom dead center, the charge consisting of fuel-air mixture is drawn into the cylinder. When the piston reaches the bottom dead center the suction stroke ends and the inlet valve closes.

6. Working principle of a Four Stroke SI Engine 2. Compression Stroke : The charge taken into the cylinder during the suction stroke is compressed by the return stroke of the piston. During this stroke both inlet and exhaust valves are in closed position. The mixture that fills the entire cylinder volume is now compressed into the clearance volume.

7. Working principle of a Four Stroke SI Engine 3. Power Stroke : At the end of the compression stroke the mixture is ignited with the help of a spark plug located on the cylinder head. In ideal engines it is assumed that burning takes place instantaneously when the piston is at the TDC and hence the burning process can be approximated as heat addition at constant volume.

8. Working principle of a Four Stroke SI Engine 3. Power Stroke : During the burning process the chemical energy of the fuel is converted into heat energy producing a temperature rise of about 2000 °C. The pressure at the end of the combustion process is considerably increased due to the heat release from the fuel. This sudden rise of pressure pushes the piston with a great force, and rotates the crankshaft. The crankshaft, in turn, drives the machine connected to it.

9. Working principle of a Four Stroke SI Engine 4. Exhaust Stroke : At the end of the expansion stroke the exhaust valve opens and the inlet valve remains closed. The pressure falls to atmospheric level a part of the burnt gases escape. The piston starts moving from the BDC to TDC and sweeps the burnt gases out from the cylinder almost at atmospheric pressure. The exhaust valve closes when the piston reaches T.D.C. at the end of the exhaust stroke and some residual gases trapped in the clearance volume remain in the cylinder.

10. Working principle of a Four Stroke SI Engine 4. Exhaust Stroke : Residual gases mix with the fresh charge coming in during the following cycle, forming its working fluid. Each cylinder of a four stroke engine completes the above four operations in two engine revolutions, one revolution of the crankshaft occurs during the suction and compression strokes and the second revolution during the power and exhaust strokes. Thus for one complete cycle there’s only one power stroke while the crankshaft turns by two revolutions.

11. Ideal Valve Timing Diagram

12. Modified Otto Cycle

13. Isothermal [T] Isentropic [S] Isobaric [P] Isochoric [V] P V Polytropic V P

14. Inlet valve is open Time period Both valves are close Time period Exhaust valve is open Time period Both valves are open Time period

15. is the regulation of the positions in the cycle at which the valves are set to open and close. The inlet valve opens 5° before TDC for slow speed engine and at 00 TDC for high speed eñine ; and closes 20 — 25° after BDC for slow speed engine and 40-50° after BDC for high speed engine. The exhaust valve opens 35° before BDC for slow speed engine and 45° before BDC for high speed engine ; and closes 10° after TDC for slow speed engine and 10° after TDC for high speed engine. The ignition valve opens 40° before TDC for slow speed and 30° before TDC for high speed engines.

16. Modified OTTO Cycle Inlet valve is open before the piston reaches TDC on the exhaust stroke. The fuel / air mixture is ignited before the piston reaches TDC on the compression stroke. Inlet valve is close after the BDC on the compression stroke. Exhaust valve is open before the piston reaches BDC on the power stroke. Exhaust valve is close after the TDC on the intake stroke

17. Valve lead - when inlet valve is open before the piston reaches TDC, when exhaust valve is open before the piston reaches BDC Terms Used In Modified Otto Cycle Valve lag - when inlet valve is close after the piston passes BDC, when exhaust valve is close after the piston passes TDC Valve overlap - This is the time where both valves are open in the same time

18. Why Inlet valve would open before the piston reaches TDC on the exhaust stroke ? - The lower pressure of the cylinder, caused by the momentum of the out going exhaust gasses will assist the flow of new gas in to the cylinder - Incoming gas will help to scavenge the exhaust gasses remaining in the cylinder head (in the valve overlap time period) - It will fully open by the time the piston has begun to move down the cylinder.

19. Why Inlet valve would close after the piston reaches BDC (lagged) on the suction stroke?

20. The Gas Pressure Curve

21. Cylinder Pressure Graph

22. Engine Firing Order S/No Type Firing Order 1 4 - Cylinder Inline 1-3-4-2 or 1-2-4-3 2 6 - Cylinder Inline 1-5-3-6-2-4 3 8 - Cylinder V-Type 1R-4L-2R-3L-4R-1L-3R-2L 4 12 - Cylinder V-Type 1L-2R-5L-4R-3L-1R-6L-5R-2L-3R-4L-6R 5 4 - Cylinder Opposed 1-3-2-4 or 1-4-2-3 6 6 - Cylinder Opposed 1-4-5-2-3-6 7 8 - Cylinder Opposed 1-5-8-3-2-6-7-4 8 9 - Cylinder Radial 1-3-5-7-9-2-4-6-8 9 14 - Cylinder Radial 1-10-5-14-9-4-13-8-3-12-7-2-11-6 10 18 - Cylinder Radial 1-12-5-16-9-2-13-6-17-10-3-14-7-18-11-4-15-8

23. Exercise:

24. Ineffective Crank Angles

25. Internal Combustion Engine Theory and Performance

26. What is Boyle’s Law? The volume of any dry gas varies inversely with the absolute pressure sustained by it, the temperature remaining constant. PV = Constant

27. What is Charle’s Law? The volume of a gas is directly proportional to its absolute temperature, assuming the quantity of gas and pressure remain constant V/T = Constant

28. Conversion of Heat Energy to Mechanical Energy When a mixture of gasoline and air is ignited, the combustion process increases the kin e tic energy of the molecules in the ga s ses When the gas is confined it results in increased pressure (potential energy) The conversion of the potential energy in fuel to the kin e tic enegry of the engine’s motion is controlled by certain laws of physics.

29. Theory of Engine Operation The mixture of fuel and air burns when it is ignited and produces heat The heat is absorbed by the gases in the cylinder , and they tend to expand The increase in pressure acting on the piston head forces it to move and transmit the motion to the crankshaft through connecting rod.