When does energy used to compress the air get lost in relation to the exhaust valve opening?

The correct answer is TDC (Top Dead Center). This is where the piston reaches its highest point in the cylinder, having completed the compression stroke before the power stroke begins.

During the compression stroke, as the piston moves up towards TDC, energy is utilized to compress the air within the cylinder. If the exhaust valve is not sufficiently closed during this critical phase, some of that energy can escape as the air begins to exit the cylinder prematurely. This results in a loss of efficiency, as the engine cannot extract all the potential work from the compressed air-fuel mixture during the combustion phase.

Thus, it's crucial that the timing of the exhaust valve opening is carefully calculated to ensure that it occurs after TDC during the expansion (power) stroke rather than before or at TDC. This timing allows the engine to maximize the energy derived from the combustion process, improving overall efficiency and performance.