More Than You Ever Wanted to Know About Mechanical Engineering: Thermodynamic Efficiency
We’re starting to take a look at some of the basic tools for analysis of heat, energy, and thermodynamic systems. One concept that will come up over and over for us is efficiency.
It’s a pretty familiar concept. Efficiency is how much you get out for what you put in. Or, more formally, the ratio of your desired output to your required input. In equations, we’ll write it as η.
Say, for example, you have a motor drawing 60W from an electric outlet and doing 52W of shaft work. In this case, the motor is about 86% efficient.
Suppose, instead of drawing electrical power from the mains, your motor is supplied by a generator. The generator will have an efficiency of its own, which is the electrical power it outputs over the mechanical work that goes into it. Say it takes 100W to spin the generator and it can produce 80W of electricity from that shaft work for an efficiency of 80%.
If we want to know the overall efficiency of the system that comprises both the motor and the generator, we just multiply their individual efficiencies together.
Note that the overall efficiency of a system will always be lower than the individual efficiencies of its components.