Question

combustion engine.

Discuss the thermodynamics of the Otto cycle for a combustion engine.

Why does the efficiency depend on the compression ratio?

What is the problem with that? Why is the Otto cycle better than a steam engine? Or, what is the main innovation

Answer 1

Thermodynamic Cycles used in internal combustion gasoline powered engines. The first such engine was built by the German engineer Otto in 1861. The Otto cycle consists of four strokes of the piston for each explosion.

1. Intake stroke. A mixture of air and gasoline is drawn into the cylinder.
2. Compression stroke. The fuel-air mixture compressed, causing a rise in temperature.
3. Explosion. The mixture is ignited by spark plugs with the piston staying nearly fixed, leading to a rise in pressure and temperature.
4. Power Stroke. The hot gas is then allowed to do mechanical work by pushing the piston back.
5. Valve exhaust. An outlet valve is opened and some exhaust escapes with the piston remaining fixed.
6. Exhaust stroke. The piston forces the remaining exhaust to escape. The outlet valve is then closed, the inlet valve opened, and the cycle repeats.

The Otto cycle's efficiency is 1 - r^(1-k), where r is the compression ratio and
k is the specific-heat ratio (always > 1), so efficiency improves when r is large.
However, the possibility of "knocking" limits the compression ratio in a
spark-ignition engine.