Combustion efficiency is a measure of how efficiently a device consumes fuel. Ideally, it would be measured at 100%, meaning that the fuel was completely consumed. In practice, this level of efficiency is impossible to achieve, but it's possible to come close. The lower the combustion efficiency, the less efficient the device is, making it expensive to run, wasteful of fuel, and harmful for the environment.
Several parameters are considered when measuring combustion efficiency. One is the composition of the fuel, for the purpose of determining how much potential energy is present. Another is the amount of oxygen entering the device, with higher oxygen levels tending to promote more complete combustion. The temperature and composition of the gases being vented from the device is also important; certain types of flue gases indicate that combustion is not complete, and high temperatures indicate that heat is being wasted by being vented off instead of utilized.
One of the classic examples of combustion inefficiency is a woodburning fireplace, which often has an efficiency rate of less than 10%. This means that the fireplace is not fully utilizing the energy in the wood, generating lots of byproducts of incomplete combustion like soot and harmful gases, and losing heat up the chimney rather than heating a room. Conversely, specialized types of gas heaters can have efficiency rates of over 90% when they are operating properly.
Combustion efficiency is an important concern for a number of reasons. For people paying to fuel devices which burn fuel for energy or heat, the more efficiently a device uses fuel, the less it costs to run, which is good. For people concerned about the environment, more efficient combustion is beneficial because it reduces pollution; this also reduces wear and tear on equipment caused by accumulation of particulates and harmful flue gases.
Ideally, combustion efficiency would reach a stoichiometric stage, a perfectly balanced reaction which consumes 100% of the fuel. This would require highly controlled conditions which are simply not available in the real world. However, it is possible to monitor a device to determine its efficiency during different operating periods, for the purpose of making adjustments which can increase efficiency. This monitoring can be critical on the industrial level, when fuel waste can become quite costly, but people can also benefit from monitoring combustion efficiency in their own homes, as they can save on fuel costs substantially just by making sure that devices are efficient.