# What is the efficency of internal combustion engine?

1. Sep 24, 2014

### Aayam

just how effecient is the internal combustion engine. please tell me(any/all you know/ or can find online), efficency of an average engine, best achieved ever effecincey(ideal scenario), and the maximum theoritical efficency.. i am talking about energy put in vs the amout that is useable.(by this i mean exclude heat lost, etc, just give me the energy that is outputted that a veihcle can use.), im looking for just the idea of engine, car, plane, etc doesnt matter. thanks :)

2. Sep 25, 2014

### gezibash

It depends, however to answer your question as directly as possible I will assume you mean a car engine, spark-ignited and one that uses air as an oxidizer. Typical compression ratios (air is compressed in the engine prior to explosion) range from about 7-10, so with air the average theoretical efficiency (Otto Cycle thermal efficiency) becomes,
$$\eta_{\text{th}} \approx 0.573$$
If you want to compute the thermal efficiency using your own compression ratio, use this formula. It applies to the Otto Cycle,
$$\eta_{\text{th}} = 1 - \frac{1}{r^{k-1}}$$
where $r$ stands for compression ratio and $k$ is the specific heat ratio of the working fluid. For air, it's about $k=1.4$.