GB573714A - Improvements in and relating to two-stroke cycle internal combustion engines - Google Patents

Abstract

573,714. Two-stroke engines. ARMSTRONG WHITWORTH & CO. (ENGINEERS), Ltd., SIR W. G., MANSFIELD, W. P., and WHITE, J. Feb. 1, 1944, No. 1862. [Class 7 (ii)] [Also in Group XXVI] In a two-stroke internal-combustion engine wherein the exhaust gases are arranged to drive a turbine a, the duct or ducts b between the engine and turbine are constructed so that following each exhaust action the pressure distribution in each duct comprises a lowpressure zone adjacent the cylinder, to facilitate charging, and a high-pressure zone adjacent the turbine to drive the latter. In the form shown, the turbine drives a charging blower c, but in some cases the turbine may drive an auxiliary machine or may return its output to the main engine. It may also be arranged that a pressure wave reflected back along the exhaust duct may cause a rise in pressure adjacent the cylinder towards the end of the charging period so that some of the charge which has passed through the cylinder is compressed and returned to the cylinder as a supercharge. The total quantity of fresh charge per cylinder should be sufficient to charge the cylinder to the required pressure and also to pass an excess charge, which is of the order of .15-.6 cylinder volume at N.T.P., into the exhaust passage f. The area of the exhaust duct and the rate of discharge of the exhaust gases are arranged to cause high pressure gases to be delivered to the turbine for a substantial portion of the period between successive exhaust actions. Preferably, there is a 'reduction in area at the turbine inlet d so that some of the exhaust wave energy is reflected back to the cylinder and the duct length is such that the return wave reaches the exhaust orifices towards the end of the charging period. A valve may be located at d to control the amount of energy passing through the turbine in relation to that which is reflected. Preferably, when the return wave action is used to effect supercharging, the exhaust openings close later than the inlet openings, and the supercharging occurs after the inlet openings have closed and low-pressure charging completed. In one example, an exhaust overlap of 10 to 20 degrees may be used, the return wave reaching the exhaust openings at or slightly before the beginning of the overlap period. If the exhaust return action is not used, the exhaust openings close before the inlet openings. The timing diagram for a four-cylinder engine is shown in Fig. 3 and a similar diagram is given for a three-cylinder engine. Calculation of the length of exhaust duct is based on a mean exhaust wave velocity of 450-600 metres per second, depending principally on engine speed and exhaust area. The cross-sectional area of the duct b should be preferably 20-50 per cent greater than the exhaust area open before the inlet orifices open. In multi-cylinder engines, there may be separate exhaust ducts for each cylinder, each connected separately to the turbine or, as shown, there may be a single exhaust manifold e provided the exhaust from one cylinder does not raise the pressure in the exhaust branch of another cylinder at a time when low pressure is desired therein. Preferably the combined volume of exhaust passages f, g, Fig. 2, should exceed the volume of fresh charge to be returned to the cylinder. In some cases, a progressive reflection action may be obtained by gradually decreasing the area of the duct b. Gearing, controlled by a clutch, may be arranged between the engine and turbine so that the engine may drive the compressor to facilitate starting.