FAA Powerplant Written Practice Test

When should the combustion process be completed if the fuel/air ratio and ignition timing are correct?

1. Before TDC at the beginning of the power stroke

2. At TDC

3. Just before TDC

4. Just after TDC at the beginning of the power stroke

The correct answer is: Just after TDC at the beginning of the power stroke

The combustion process in an engine should be completed just after top dead center (TDC) at the beginning of the power stroke to ensure maximum efficiency and power output. When the ignition timing is correctly set, the fuel-air mixture ignites at the appropriate moment, allowing the pressure generated from combustion to push the piston down effectively. Ignition must occur early enough that the combustion process is already well underway when the piston reaches TDC, but not so early that the peak pressure occurs before the piston has descended. If combustion finishes just after TDC, it means that the expansion of gases, or the power produced, fully utilizes the downward motion of the piston. This timing maximizes the work done on the crankshaft, ensuring the engine operates effectively and produces optimal power. If combustion completed before TDC, the effective force on the piston would decline due to the piston’s upward motion while combustion was still occurring, which would reduce efficiency. Similarly, if combustion finished too early (i.e., excessively before TDC), the piston might be traveling downward before the pressure build-up has a chance to perform useful work. Therefore, finishing combustion just after TDC aligns perfectly with the design of internal combustion engines for optimal performance.