How is engine efficiency typically expressed?

Engine efficiency is typically expressed as a percentage because it represents the ratio of useful work output to the total energy input. This provides a clear and standardized way to assess how effectively an engine converts fuel into mechanical energy. For example, if an engine uses 100 units of fuel energy and produces 30 units of useful work, its efficiency would be calculated as 30%. Expressing efficiency as a percentage makes it easy to compare the performance of different engines under various operating conditions. The percentage format allows for a straightforward understanding of how much of the fuel energy is actually being utilized for work versus being wasted in the form of heat or other losses.