What factors reduce the efficiency of a heat engine from its ideal value?

The efficiency (η) of a heat engine is defined as,
η = 1 – T₂/T₁
Here T₁ is the temperature of source and T₂ is the temperature of sink.
The efficiency of an engine depends upon the temperatures between which it operates and is independent of the nature of working substance. The efficiency of an engine will be 100 % (η = 1) if the temperature of sink (T₂) is zero. Since absolute zero is not attainable, even an ideal engine cannot have efficiency as 1. This signifies that, it is impossible to extract heat from a single body and convert the whole of it into work.
The loss of energy due to the friction, viscosity and insufficient combustion will reduce the efficiency of a heat engine from its ideal value.