Question 1

What is the Otto cycle?

Accepted Answer

The Otto cycle is the idealised thermodynamic cycle that describes a spark-ignition (petrol) engine. It consists of two adiabatic processes (compression and expansion) and two constant-volume processes (heat addition during combustion and heat rejection during exhaust).

Question 2

What are the four strokes of a petrol engine?

Accepted Answer

Intake (fuel-air mixture drawn in), compression (piston squeezes the mixture adiabatically), power/expansion (combustion pushes the piston down), and exhaust (burnt gases pushed out). In the idealised Otto cycle these map to adiabatic compression, isochoric heat addition, adiabatic expansion, and isochoric heat rejection.

Question 3

What is the efficiency of the Otto cycle?

Accepted Answer

The ideal thermal efficiency is eta = 1 - 1/r^(gamma-1) = 1 - r^(1-gamma), where r is the compression ratio and gamma is the heat-capacity ratio (about 1.4 for air). It depends only on the compression ratio, not on the amount of heat added.

Question 4

Why does higher compression ratio improve efficiency?

Accepted Answer

A higher compression ratio means the gas is expanded over a larger volume range, extracting more work per unit of heat. Since eta = 1 - 1/r^(gamma-1), increasing r always raises efficiency, which is why high-performance engines use high compression ratios.

Question 5

What limits the compression ratio in real engines?

Accepted Answer

Knock (auto-ignition of the fuel-air mixture before the spark) limits compression ratio in petrol engines. Higher-octane fuels resist knock and allow higher ratios. Typical petrol engines use 8:1 to 12:1; diesel engines, which compress only air, reach 14:1 to 22:1.

Question 6

What is an adiabatic process?

Accepted Answer

An adiabatic process happens with no heat exchange with the surroundings. For an ideal gas it follows PV^gamma = constant. The fast compression and expansion strokes are modelled as adiabatic because they occur too quickly for significant heat transfer.

Question 7

What does the area inside the PV diagram represent?

Accepted Answer

The enclosed area of the PV loop equals the net work done by the engine per cycle. A larger loop means more work output. Efficiency is that net work divided by the heat added during combustion.

Question 8

What is gamma (the heat-capacity ratio)?

Accepted Answer

Gamma = Cp/Cv is the ratio of specific heats at constant pressure and constant volume. For diatomic gases like air it is about 1.4. It governs how steeply pressure changes during adiabatic compression and expansion.

Question 9

How is the Otto cycle different from the Diesel cycle?

Accepted Answer

In the Otto cycle heat is added at constant volume (spark ignition). In the Diesel cycle heat is added at constant pressure (compression ignition). Diesel engines use higher compression ratios and are generally more efficient for that reason.

Question 10

Why is real engine efficiency lower than the ideal value?

Accepted Answer

Real engines lose energy to friction, heat transfer through cylinder walls, incomplete combustion, pumping losses, and finite-time processes that are not truly adiabatic or isochoric. Real petrol engines reach 25 to 35 percent, below the ideal Otto value.

🚗 Otto Cycle — How a Petrol Engine Works

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