Petrol Engine vs Diesel Engine | Physics

Although different engines may differ in their construction Technology but they are based on the principle of a Carnot cycle. A typical 4 strokes petrol engine also undergoes for successive processes in each cycle.  

(a)

1. Intake Stroke:

The cycle starts on the intake stroke in which piston moves outward and petrol air mixture is drawn through an inlet valve into the cylinder from the carburetor at atmospheric pressure.

2. Compression Stroke:

On the compression stroke, the inlet valve is closed and the mixture is compressed adiabatically by inward movement of the piston.

3. Power Stroke:

On the power stroke, a spark fires the mixture causing a rapid increase in pressure and temperature the burning mixture expands adiabatically and forces the piston to move outward this is the stroke which delivers powers to crank shaft to drive the flywheels.

4. Exhaust Stroke:

On the exhaust stroke, the outlet valve opens. The residual gases are expelled and piston moves inward.

The cycle then begins again. Most motor bikes have one-cylinder engine but cars usually have 4 cylinders on the same crankshaft as shown in the figure b.

(b)

The cylinders are timed to fire turn by turn in succession for a smooth running of the car. The actual efficiency of properly tuned engine is usually not more than 25% to 30% because of friction and other heat losses.

Diesel engine:

No spark plug is needed in the diesel engine. Diesel is sprayed into the cylinder add maximum compression. Because air is at high temperature immediately after compression, the fuel mixture ignites that’s on contact with the air in the cylinder and pushes the piston outward. the efficiency of diesel engine is about 35% to 40%.

 

APPROXIMATE EFFICIENCIES OF VARIOUS DEVICES

DEVICE                                                                           EFFICIENCY %

1. Electric generator                                                             70 - 99

2. Electric motor                                                                   50 - 93

3. Dry cell battery                                                                    90

4. Domestic gas furnace                                                       70 - 85

5. Storage battery                                                                   72

6. Hydrogen-oxygen fuel cell                                                  60

7. Liquid fuel rocket                                                                 47

8. Steam turbine                                                                    35 - 46

9. Fossil-fuel power plant                                                      30 - 40

10. Nuclear power plant                                                        30 -35

11. Nuclear reactor                                                                  39

12. Aircraft gas turbine engine                                                36

13. Solid-state laser                                                                30

14. Internal combustion gasoline engine                               20 - 30

15. Gallium arsenide solar cell                                                >20

16. Fluorescent lamp                                                                20

17. Silicon solar cell                                                              12 - 16

18. Steam locomotive                                                                8

19. Incandescent bulb                                                               5

20. Watt’s steam engine                                                            1