Patent GB-573715-A (GB)
Title: Improvements in and relating to two-stroke cycle internal combustion engines
IPC: F02B25/00
Filed: 1944-02-01
Abstract: 573,715. Two-stroke engines. ARMSTRONG WHITWORTH & CO. (ENGINEERS), Ltd., SIR W. G., MANSFIELD, W. P., and WHITE, J. Feb. 1, 1944, No. 1864. [Class 7 (ii)] In a two-stroke cycle internal-combustion engine, the charge is more than that required for charging or supercharging the cylinders, the excess charge passing into the exhaust duct which is constructed so that after each exhaust event a low pressure exists adjacent the cylinder during most of the charging period to facilitate scavenging and charging, and an obstruction is provided in the exhaust duct from which the reflection of exhaust energy causes a raised pressure near the exhaust orifices towards the end of the charging period, whereby further escape of fresh charge to exhaust is prevented, or some of the fresh charge which has passed into the duct is compressed and returned to the cylinder to form a supercharge. In some cases, a blower connected to the inlet duct a, Fig. 1, may provide the low-pressure scavenging and charging. The exhaust orifices preferably close after the inlet orifices so that the return action occurs after low-pressure charging is completed. An exhaust overlap of 10-20 degrees may be used for moderate speed engines. If overlap is not used, the exhaust and inlet orifices should preferably be at opposite ends of the cylinder. Calculation of the required length of the exhaust duct should be based on a mean exhaust wave velocity of 450/600 metres per second. The optimum length will be inversely proportional to engine speed, and means may be used for altering the effective length of the exhaust duct. The exhaust duct end may be partially closed by a fitting d, Fig. 3, to give the required reflection, and in some cases the size of the escape opening e may be altered by valve means. The duct may taper, as shown, so that its diameter increases towards the pipe e. In some cases, the reflection may be caused by gradually decreasing the duct area instead of providing a plane reflecting surface. In an alternative exhaust duct, Fig. 4, a closed portion f of the duct is separated from an open end pipe g leading to an expansion chamber h. The reflection action of the open pipe reaches the cylinder before the reflection from the closed pipe f, the former action assisting the charging of the cylinder before the latter action which returns charge to the cylinder to effect supercharging. In another modification, the outer end of the duct may be closed by a cyclically-operated valve which is closed to permit the required reflecting action and then opened to permit escape of the gases before the next exhaust event. One form of valve is shown in Fig. 6 and consists of a rotary watercooled disc i provided with orifices l to give the required open and closed periods. With multicylinder engines, separate exhaust ducts may be used for each cylinder, or, alternatively, a manifold c, Fig. 1, may be used provided the charging periods are so related to the number of cylinders that exhaust from one cylinder does not raise the pressure in the exhaust branch of a preceding cylinder at a time when low pressure is required in such branch. The disc valves may in some cases be in a plane parallel to the crank-shaft axis. If the supercharging action is not required, the exhaust gases may be byepassed so that they do not pass through the disc valve. The excess charge passed through the cylinders should be of the order of 0.15 to 0.6 cylinder volume at N.T.P. Specification 573,714 is referred to.