thermodynamics explained

Otto and Diesel cycles explained

Gas cycles, no phase change. Otto burns at constant V, Diesel at constant P. η = 1 − 1/r^γ−1; diesel wins with a higher r

The engine under most car hoods runs on air alone, not on water that changes phase, and it heats that air by burning fuel inside the cylinder. Otto and Diesel share the same four strokes and differ in only one thing: whether the fuel burns at fixed volume or at fixed pressure.

The air is pressed without heat crossing, heat is added at fixed volume when the spark fires, the gas expands and delivers work, and heat is rejected again at fixed volume. In a real 4-stroke engine only the expanding part delivers force, and the inertia stored in the wheel pushes the piston through the other parts.

The ideal Otto efficiency hangs only on the compression ratio, η = 1 − 1r^γ−1. Pushing the same air into a smaller part of the cylinder raises the efficiency, and as the ratio grows the extra gain shrinks, so at a very high ratio the benefit flattens.

The two cycles split where the fuel burns. Otto is a spark at fixed volume, a vertical jump on P-V, and Diesel burns at fixed pressure while the piston moves and fuel is sprayed, a horizontal step on P-V. The spark burns the premixed gas in a flash, so the piston has no time to move and the volume stays nearly the same, while Diesel burns the fuel little by little and the pressure stays nearly steady as the piston descends.

A gasoline engine presses fuel and air together, so knock caps the compression ratio. A diesel presses air alone and sprays fuel only at the top, so it runs at a much higher ratio and reaches a higher efficiency. That is why trucks, ships, and heavy machines, where fuel economy matters, mostly use diesel, while the lighter, faster gasoline engine suits passenger cars.

Both are gas cycles with no phase change, they split only on whether the burn is at fixed volume or fixed pressure, and the compression ratio is the master knob. Next comes the job of running a cycle backward to lift heat uphill, which is refrigeration. A real engine does not use only one of the two; it burns in a blend, and that blend is called the dual cycle.

In PracticeOtto and Diesel are internal-combustion cycles that use air and do not change phase. They share four parts (adiabatic compression → combustion → adiabatic expansion → constant-volume exhaust), but Otto adds heat at fixed volume with a spark and Diesel adds heat at fixed pressure by compression. The ideal Otto efficiency η = 1 − 1r^γ−1 is fixed by the compression ratio alone and rises as that ratio grows. Gasoline presses the fuel too, so knock caps the ratio, while a diesel presses air alone and can run a much higher ratio, so it is more efficient in practice. At the same ratio a diesel sits a little below Otto because of the stretch where fuel is sprayed, but the higher ratio reverses that gap. The following D3 turns these cycles backward into refrigerators and heat pumps, spending work to lift heat from cold to hot.

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