Four-Stroke Engines

Originally supercharging was employed only in four-stroke engines. A diagram shows a four-stroke engine with gas turbine supercharging. The exhaust gases flow from the cylinder along pipe 1 and enter the casing of gas turbine 2

where they expand in nozzles 7 and acquire a high velocity

used in blades 4 to drive disk 3. The turbine disk is fitted onto shaft 5 whose other end carries impeller б of a centrifugal pump. The air leaving the vanes of the impeller enters diverging ducts 9 where its velocity drops, while its

pressure grows. After this, from scroll housing 8 of the pump

the air is delivered to the engine cylinder through a pipe.

Exhaust turbocharged, single acting, four-stroke cycle marine engines are in successful operation which deliver as much as 200 % more power than atmospheric induction engines of the same speed and dimensions. Almost all marine four-stroke engines at present in service operate on the pulse system.

To ensure adequate scavenge of the cylinders in an engine with a supercharger the intake and exhaust valves of four-stroke engines remain open for some time simultaneously (overlap). A valve overlap of approximately 140° is normal. In a typical valve timing arrangement the air inlet valve opens at 80° before top dead-centre and closes at 40° after bottom dead-centre and the exhaust valve opens at 50° before bottom dead-centre and closes at 60° after top dead-centre.

Optimum values of power output and specific fuel consumption can be achieved only by effective utilisation of the high-energy engine exhaust pulses. The engine exhaust system should be so designed that it is impossible for gases from one cylinder to contaminate the charge-air in another cylinder, either by blowing back through the exhaust valve or by interfering with the charge of gases from the cylinder. During the period of overlap the exhaust pressure must be less than the air-charging pressure. This ensures effective scavenging of the cylinder for elimination of all residual exhaust gas and for cooling purposes. It has been found, in practice, that if the period between discharge of successive cylinders into a common manifold is less than about 240°, then interference will take place between the scavenging of one cylinder and the exhaust of the next one. This means that engines with more than three cylinders must either be fitted with more than one turboblower, or the turboblower must have multi-inlet casings which have separate passages right up to the turbine nozzles (pulse system). Either method can lead to inefficient turbine operation.

For constant pressure operation, all cylinders exhaust into a common receiver (manifold) which tends to maintain an almost constant pressure. The advantage of this system lies in the ease with which engines may be adapted for pressure-charging, because a complicated multi-pipe exhaust arrangement is not needed, and also little change in valve timing is required. It also leads to much higher turbine efficiencies. An additional advantage is that the lack of restrictions, within reasonable limits, on exhaust-pipe length permits greater flexibility in positioning the turboblower relative to the engine. The main disadvantage of the constant-pressure system is the poor performance obtained at part-load conditions. In addition, owing to the relatively large exhaust manifold, the system is insensitive to changes in engine operating conditions; and the resultant delay in turboblower acceleration, or deceleration, results in poor combustion during transition periods.

Explanatory notes a valve overlap перекрытие клапанов

blowing back заброс выхлопных газов назад в цилиндр через вы-

хлопной клапан interfering создание помех