FAA Powerplant Written Practice Test

What happens to the energy produced by combustion when the stoichiometric mixture changes from 15:1 to 16:1?

1. The energy produced increases

2. The energy produced remains the same

3. The energy produced by combustion decreases

4. The energy produced becomes volatile

The correct answer is: The energy produced by combustion decreases

When the air-fuel mixture changes from a stoichiometric ratio of 15:1 to 16:1, the energy produced by combustion decreases. In a combustion process, a stoichiometric mixture refers to the ideal ratio of air to fuel that allows for complete combustion, maximizing energy extraction. As the air-fuel ratio increases beyond the stoichiometric point, it indicates a leaner mixture. A leaner fuel mixture means there is more air relative to fuel, which can result in a reduction of the combustion efficiency. This means that not all fuel is burned effectively, leading to less overall energy being released during the combustion process. Furthermore, in a leaner mixture, the temperature of the combustion products is often lower, which can also contribute to a decrease in the thermal energy available for conversion into mechanical work. Therefore, when moving from a 15:1 to a 16:1 air-fuel ratio, the efficiency and energy output of the combustion process diminish, confirming that the energy produced by combustion decreases.