Final answer:
True, the exhaust valve in a four-stroke gasoline engine is opened before the piston reaches bottom dead center during the power stroke to efficiently expel exhaust gases using the residual pressure from the combustion process.
Step-by-step explanation:
The query relates to the functioning of exhaust valves in an internal combustion engine, specifically a four-stroke gasoline engine. The assertion that the exhaust valve is opened before the piston reaches the bottom dead center (BDC) on the power stroke is indeed true. In this phase, there is residual pressure within the cylinder from the combustion process, which facilitates the expulsion of exhaust gases when the exhaust valve opens.
During the exhaust stroke, the engine utilizes the remaining pressure to push out the exhaust gases. This phase begins before the BDC of the power stroke to ensure an efficient expulsion of the gases, which is crucial for the engine's performance and readiness for the next intake cycle. This method of operation is designed to enhance the efficiency of the engine by reducing the amount of work required to expel the exhaust gases and to prepare the combustion chamber for the next cycle.