Scavenging is only applicable to 2-stroke engines, and is the process of clearing from the cylinder any remaining products of combustion from the previous cycle. Air, at a low pressure, is introduced into the cylinder through scavenge ports which are opened shortly after the opening of the exhaust valve. The prior opening of the exhaust valve allows the exhaust gases to expand out of the cylinder, reducing the pressure in the cylinder to well below that of the scavenge air.
Were that not case, the air would not be able to ‘clean out’ the cylinder arid recharge it with fresh air for the next cycle. The effectiveness with which the air clears the cylinder is called its ‘scavenge efficiency’ and is a comparison between the fresh air and the total of fresh air and any gases still remaining in the cylinder. Thus, in the case of 100% scavenge efficiency, all the gases have been cleared out and a completely new charge of fresh air is in the cylinder.
All major engine builders now adopt a process of scavenging known as ‘uniflow’ which, as the name implies, operates by directing the air through the cylinder in one direction only. Such system is easily capable of achieving 100% scavenge efficiency and is absolutely essential in modern long stroke engines. The main alternative to this system is ‘loop’ scavenge where the air/exhaust flow through the cylinder takes the form of a loop. This process, though it simplifies the design of the cylinder head ect., cannot get much above 95% efficient, and is even less efficient on long stroke engines. Loop scavenge, achieved by having both exhaust and scavenge ports in the liner, the opening and closing of which regulated by the piston, has been superseded by the uniflow process where only scavenge ports are used in the liner. In uniflow the ports are located at the lower end of the liner, uniformly around the circumference. It is quite common to have these ports ‘angled’ tangentially to the liner in such a way that the air passing through them develops a swirling characteristic that not only helps to clean out the liner, but also aids combustion. This is because the slow moving air coming from ‘behind’ the injector, carries the products of combustion away from the tip and simultaneously provides it with fresh air throughout the injection period.
Scavenging can be achieved providing there is an adequate flow of air into the cylinders from the ‘scavenge spaces’. Increasing the pressure of this air flow not only helps with scavenging but, more importantly, increases the density of the charge air remaining in the cylinder. This process of increasing the air pressure and thereby the air density is called ‘supercharging’, and can be achieved by increasing the speed of the pumps/pistons etc. which are supplying the scavenge air.
Any mechanical drive taken from the engine to drive these pumps/pistons absorbs energy (power) from the engine. The main problem with such a process is the lack of response of such pumps to changes in load. When an engine is exposed to an increase in load, head wind or current for example, the engine will tend to slow down, causing a reduction in speed of the pumps supplying the air. Thus, just when the main engine needs more air to bum the extra fuel injected to match the increase in load, the supply could decrease. The result is incomplete combustion, black smoke and all the other undesirable side effects of reduced combustion efficiency.
Turbochargers on the other hand, driven by the exhaust gases from the cylinder, respond directly to changes in load. As the fuel injection rate is altered to suit the load, so the mass (and therefore the energy within it) of the exhaust gas generated changes proportionately. This self-regulating ability of the turbocharger has improved the power outputs and versatility of the diesel engine.
Turbochargers are really superchargers driven by heat from the exhaust gas that may otherwise be wasted or lost. Thus not only is it possible to increase the charge air pressure, and through that combustion efficiency, but, by recovering heat from the exhaust gas, the overall thermal efficiency of the plant is also improved.
The effect of supercharging is ‘to increase the air density in the cylinder, allowing a proportionate increase in the fuel injected and thereby give a corresponding increase in power output’.
To accept these increases in power, and the corresponding increase in peak pressures, (though the increase in peak pressures is modified as far as possible by reducing the compression ratio) certain modifications need to be made to the engine.
To absorb the increased firing pressures and mep, the strength of the combustion chamber has to be improved, not simply by increased wall thickness as that would only reduce heat transfer, but by improved design. Piston crown design has advanced over recent years, one of the latest being the honey-comb design that combines effective cooling with great strength. Similarly, bore cooling has come into its own to allow the wall thickness to be maintained whilst still providing adequate and effective cooling.
The firing forces are carried through the piston down to the crosshead bearing and ultimately to the main bearings and transverse girders. It has become necessary therefore to improve the load carrying ability of these components, a process that is repeated each time a new model or modified engine comes on to the market. The crossheads in particular have undergone many refinements over the years, as have the transverse girders that are at present solid forged to provide adequate support and strength.