Ideal cycle for spark-ignition engines; efficiency depends on compression ratio.
Process: isentropic compression, constant-volume heat addition, isentropic expansion, constant-volume heat rejection. η=1-1/r^{γ-1}, with compression ratio r (8–11 in petrol engines). Higher r gives higher efficiency but knock limits r.
r=10 and γ=1.4 gives 60% ideal efficiency.
The Otto cycle is the ideal model for petrol engines, so it shows how compression ratio controls efficiency.
Ideal cycle for spark-ignition engines; efficiency depends on compression ratio.
The Otto cycle is the ideal model for petrol engines, so it shows how compression ratio controls efficiency.
The compression ratio. A higher ratio gives higher efficiency, but it is limited by knock in real petrol engines.
Real combustion is not instantaneous, heat is lost to the walls, and friction and pumping losses reduce the output.
Typical applications include estimating the effect of raising the compression ratio of a petrol engine, comparing the ideal efficiency of spark-ignition engines with real measurements and teaching the basic stroke sequence of a four-stroke engine.
Study "Irreversibility and lost work" first (topic 29 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
r=V_max/V_min. Mean effective pressure. Real efficiency about 30–35%.
r=10 and γ=1.4 gives 60% ideal efficiency.
The next topic is "Diesel cycle", which builds directly on this one.
It is classed as Beginner in Thermodynamics & Fluids (topic 30 of 105). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.
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