Efficiency

As you can see here, an ideal engine cycle is made out to be a closed process, in this ideal cycle, engine efficiency is made out to be 100%. However we know thats not the case with engines today.
The math is involved, but basically an Otto Cycle Engine is dependent upon the compression ratio in the cylinders. Its the ratio of the expanded volume to the compressed volume.
According to sources, the average efficiencies of Otto Cycle based engines is around 20%.

Why Such Lousy Efficiency?

First, in a car engine, parts of the Otto cycle do not proceed without intense heat exchange, thus lowering the change in temperature and lowering the efficiency.
Second, the gas/air mixture that we use today isn't an 'idea' gas to use for the best compression ratio due to the fact that lots of energy is lost due to internal excitation.
These two major components, combined along with friction, all together adds up to a huge efficiency loss.