The performance of large aero-engines is limited by the weight of the charge that can be inspired at the high speeds and by the tendency to knock, which is due chiefly to the hot exhaust valve. The performance can be considerably increased when a valve control system is employed, which offers small resistances and large passages for the gas flow. Besides this, the walls of the combustion chamber must be kept at a low and uniform temperature, in order to ensure freedom from knocking even with high compression.

As tests show that the knock resistance and the piston pressure in engines with rotary valves are sensibly greater than in the same engines with poppet valves, measurement results are used in order to ascertain the possible limiting values obtainable for the compression ratio, the mean piston pressure and the specific fuel consumption of large aero-engines.