FAA Powerplant Written Practice Test

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

1. The energy produced increases

2. The energy produced remains the same

3. The energy produced decreases

4. The mixture becomes more efficient

The correct answer is: The energy produced decreases

When the fuel/air ratio shifts from 15:1 to 16:1, the mixture becomes more fuel-rich, meaning there is less air relative to the amount of fuel being burned. In a combustion process, the stoichiometric fuel/air ratio represents the optimal balance for complete combustion, where all fuel is burned with the appropriate amount of oxygen. At a ratio of 15:1, the combustion process is close to optimal, allowing for efficient burning of the fuel, maximizing the amount of energy extracted from the reaction. However, when you increase the ratio to 16:1, indicating more fuel than what is ideal for complete combustion, the additional fuel does not find enough oxygen to burn effectively. This incomplete combustion results in the production of unburned fuel, which means that not all of the chemical energy contained in the fuel is converted into useful work. Consequently, the overall energy produced decreases because surplus fuel does not contribute to the energy output, leading to a less efficient combustion process. In summary, when the fuel/air ratio becomes more fuel-rich beyond the optimal stoichiometric point, the energy produced from combustion decreases due to incomplete burning of the fuel.