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Petrol Vs Diesel

Petrol Vs Diesel Engine

Engines are the powerhouse of vehicles and systems. It provides the energy to perform certain operations as desired in the machines. Here particularly we are talking about the engines used in the vehicles that we see on the road. Petrol and Diesel Engines have their specific applications and they both follow different cycles for providing power.

Petrol Engines are also called SI Engines, i.e. Spark Ignition Engines. Petrol Engines are based on the Otto cycle. The name is given after the inventor of the cycle. The ideal Otto cycle has 4 processes namely:

  • Reversible Adiabatic Compression

  • Reversible Constant Volume Heat Addition

  • Reversible Adiabatic Expansion

  • Reversible Constant Volume Heat Rejection

These 4 processes represent the 4 strokes in the petrol engine which are namely:

  • Compression Stroke

  • Combustion Stroke

  • Power Stroke

  • Exhaust Stroke

The Cycle is represented like this:

Fig. 1 : p -V & T- S Diagram of Otto Cycle

The area between the boundaries of the curves in the net work produced by the ideal cycle. But as told earlier this is an ideal cycle the process given here are not possible to replicate easily. Some irreversibility’s are involved in these processes and cycles. Hence the net-work output is also reduced because of the loss of entropy. The analysis using the cycles is done on both P-V and T-S plots. The graph of the real otto cycle is as in fig 2.

Fig. 2 : Real Otto Cycle Plot

Petrol Engines use spark plugs to create the required amount of temperature, to ignite the fuel because the firing temperature of petrol is very high around 280 degrees C. Spark Plug creates a spark for a small instance and that helps to create the power stroke. The thing to note here is that in SI engines the Air-fuel mixture is sent into the engine. The appropriate mixing of the air and petrol happens in the carburetor. The compression ratio of the petrol engine lies in the range of 9-11.

Diesel Engines are also called compression ignition engines. This cycle is based on Diesel Cycle. The name Diesel is given by its inventor Rudolf Diesel. It has also 4 processes in the ideal cycle which are:

  • Reversible Adiabatic Compression

  • Reversible Constant Pressure Heat Addition

  • Reversible Adiabatic Expansion

  • Reversible Constant Volume Heat Rejection

These also correspond to the 4 Stroke of the Engine which I have explained earlier.

The Cycle of the Diesel Engine can be represented as in fig. 3.

Fig. 4: Comparison of Actual & Otto Cycles (closed cycle)

Diesel Engine uses fuel injector in the power stroke as the ignition here happens with the help of compressed air. The compressed air temperature is very high when the fuel is injected with the help of an atomizer. This helps the system to perform proper combustion. The compression ratio lies in the range of 16 – 22.

This cycle is used for heavy vehicles or where a higher compression ratio is required.

Let’s compare the efficiencies of the diesel and the petrol engine. Using P-V and T-S plots

Fig. 5: Otto Vs Diesel Cycle Comparison

The plot here shows the comparison of efficiencies for the same compression ratios. The efficiency is clearly dependent on the fact that which loop has more area. From visual representation, we can confirm that the Otto cycle is having more area, which indirectly means more generation of power output. Hence when the compression ratios are the same otto cycle is more efficient than diesel cycles.

Fig. 6: Otto and Diesel Cycle for Maximum Pressure

The above fig. 6 shows the comparison of both Otto and diesel cycles for maximum pressure. Here again, we have to look for the maximum area of the loop. From our visual understanding, we can say that the diesel cycle is more efficient as it can produce more power than the otto cycle for the same heat input.

Thus, we conclude that the Diesel cycle is more efficient when the max pressure is the same, on the other hand, the otto cycle is more efficient when the compression ratio is the same.


  • For heavy vehicles, Diesel Engine is Preferred over Petrol Engine.

  • The compression ratio for Diesel Engines is more than for Petrol engines.

  • The spark plug is used to ignite the fuel in the Petrol Engine, whereas a Fuel injector is used in the case of a Diesel Engine.

  • Air Fuel mixture is used in Petrol Engines, whereas Air is directly used in Diesel Engines

  • For same Compression ratio efficiency is more in Petrol Engines

  • For the same maximum pressure, the Diesel Engine is more efficient.