What is the Otto cycle? exploring Otto cycle efficiency

What is the Otto cycle?

Otto cycle is a gas power cycle and the Power cycle refers to systems that generate power using a working fluid that remains as gas throughout the entire process. This cycle is called so because it was invented by “Otto”. This cycle is also known as the constant volume cycle. Now you came to know what is the Otto cycle now let’s discuss more about this Otto cycle

Petrol, gas and many types of oil engines work on this cycle. It is the standard of comparison for internal combustion engines.

Assumptions Made in the Otto Cycle

• Compression and expansion processes are adiabatic and reversible.
• During the constant volume heat addition process, combustion is complete with 100% combustion efficiency.
• Valves open and close at the piston’s top and bottom dead centre position inside the cylinder.
• Suction and exhaust take place at constant pressure.

PV and TS diagram of the Otto cycle

TS diagram of Otto cycle

Point 1 represents that cylinder is full of air with volume V1, pressure p1, and absolute temperature T1.

• Line 1-2 represents the adiabatic compression of air
• Line 2-3 shows the heat supply to the air at constant volume.
• Line 3-4 represents the adiabatic expansion of the air
• Ling 4-1 shows the rejection of heat by air at a constant volume

How does an Otto cycle work?

In the air cycle analysis, certain processes are neglected specifically, the induction and exhaust processes (represented by line 0-1 and 1-0 on the diagram) are assumed to have no net effect on the system, as any work done during these processes would be equal and opposite, and therefore cancel each other.

Starting from the bottom of the outer dead centre (point 1 on the p-V and T-s diagrams), the air is then compressed by the piston through a reversible adiabatic process as it moves inward until it reaches the top or inner dead centre position (process 1-2). Throughout this compression process, the entropy of the air remains constant.

Next, heat is added at constant volume (process 2-3), which corresponds to the instantaneous burning of fuel in an actual engine through an electric spark. This causes the state of the air to change from point 2 to point 3.

Following this, the air undergoes a reversible adiabatic expansion (process 3-4), as the piston moves outward from the top dead centre to the bottom dead centre.

At the end of this expansion process, the heat is rejected by the gases at constant volume (process 4-1), completing the cycle.

In an actual engine, this final step involves the instantaneous opening of the exhaust valve at point 4, which allows the pressure to drop to atmospheric pressure and initiates the exhaust stroke (process 1-0) as the piston moves back to the bottom dead centre.

Otto Cycle Efficiency

Thermal efficiency in terms of T3 and T4

From this equation, we can conclude that the efficiency of the Otto cycle is lower than the Carnot efficiency (1- T1/T3) since heat is rejected at a temperature T4 which is higher than the minimum temperature T1 of the cycle

How can the efficiency of the Otto cycle be enhanced?

It is evident from the equations that the efficiency of the Otto cycle is not affected by the amount of heat supplied but is determined solely by the compression ratio “r” and the ratio of specific heats “𝜸,” as depicted in the figure. As the compression ratio “r” and “𝜸” increase, the efficiency also increases.

However, it is observed that raising the compression ratio to excessively high levels only results in a slight improvement in efficiency, as shown by the flattening of the curve, and may not be a worthwhile endeavor.

Using monoatomic gases such as helium(𝜸= 1.66) or argon(𝜸= 1.97) instead of air could make the Otto cycle more efficient. However, since the cycle is an open system, we have to use atmospheric air as the working fluid.

When the value of 𝜸 is 1.4, we call the efficiency the “cold air standard efficiency” and when the value of 𝜸 is 1.3, we call it the “hot air standard efficiency”. During most of the cycle, the air’s temperature is very high, around 1000 K to 2000 K, and its 𝜸 value is around 1.3.

If you want to compare the Otto cycle with the diesel cycle to know which one is better then click/tap here

References

• Thermal Engineering by R.K Rajput
• Internal Combustion Engine by M.L Mathur, R.P Sharma

https://en.wikipedia.org/wiki/Otto_cycle