This analysis determines the influence of engine design and operating variables upon the effectiveness of the exhaust process. This effectiveness is measured by the calculated pressure ratio (cylinder pressure, divided by exhaust-manifold pressure) near the end of the exhaust stroke. The variables that have an imporant effect upon this pressure ratio are the gas-velocity parameter (which is a ratio including the engine speed, piston displacement, exhaust-valve area, exhaust-valve flow coefficient at maximum valve lift, etc.) and the effective exhaust-valve closing angle. The variables that have an unimportant effect are the pressure ratio at the effective exhaust-valve opening angle (which, however, does become important for highly supercharged engines), the effective exhaust-valve opening angle, the engine compression ratio, the ratio of crank throw to connecting-rod length, and the ratio of specific heats. The maximum value recommended for the gas-velocity parameter is 0.325. From this value the minimum exhaust-valve area can be calculated directly for any given engine speed, displacement volume, etc. For large values of the gas-velocity parameter the exhaust valve should be closed as late as possible. For most current engine-operating conditions the correct exhaust-valve area does not depend upon the intake-valve area or upon the ratio of intake- to exhaust-manifold pressure (except for highly supercharged engines).

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