Types of scavenging Loop or cross scavenge Uniflow. • Cross flow • The main difference between the two types is that uniflow requires an exhaust valve or piston to operate. Loop or cross flow relies on the piston to open and close exhaust ports After ignition of the fuel the piston travels down the liner uncovering firstly the exhaust ports. The exhaust gas at a pressure above atmospheric is expelled. This is often referred to as blowdown and its effect can be seen on the power card for all the types of scavenging as a rapid drop in cylinder pressure towards the end of the cycle. Loop • The method of loop scavenging is similar to the cross flow except the exhaust and scavenge ports may be found on the same side • Some 2 stroke engines do not have exhaust valves; As well as scavenge ports in the cylinder liner, they are fitted with exhaust ports located just above the scavenge ports. As the piston uncovers the exhaust ports on the power stroke, the exhaust gas starts to leave the cylinder. When the scavenge ports are uncovered, scavenge air loops around the cylinder and pushes the remaining exhaust gas out of the cylinder. This type of engine is known as a loop scavenged engine. Note that the piston skirt is much longer than that for a uniflow scavenged engine. This is because the skirt has to seal the scavenge and exhaust ports when the piston is at TDC. • Although simpler in construction with less moving parts, these engines are not as efficient or as powerful as uniflow scavenged engines. The scavenging of the cylinder is not 100%, and thus less fuel can be burnt per stroke. • All modern large 2 stroke crosshead engines now being built are of the uniflow scavenged type. Uniflow UNIFLOW • 2 stroke engines with an exhaust valve mounted in the cylinder head are known as uniflow scavenged engines. This is because the flow of scavenging air is in one (uni) direction. MAN B&W MC series uniflow scavenged engine The scavenge air enters through the scavenge ports in the lower part of the cylinder liner, the exhaust gas is expelled through the centrally mounted exhaust valve in the cylinder cover. The scavenge ports are angled to generating a rotational movement of the rising column of air. Air is forced out of the cylinder by the rising piston leading to low flow resistance, the effect is often compared to squeezing the contents out of a tube. Disadvantages of the loop/crossflow method of scavenging The greatest disadvantage of this system, and the one that has led to the abandonment of its usage where once it was widespread is its inefficiency in clearing the cylinder of all combustion products. Following the exhaust blowdown the scavenge ports are opened. The period available for scavenging is limited to the recovering of the exhaust ports and is only at its most effective until the closing of the scavenge ports. Therefore, high air velocities are designed in, the air entering through steeply angled shaped ports. The possibility exists for the scavenge air to shortcut directly to the exhaust ports, a situation which worsens with blockage of the scavenge ports due to carbon build up. Due to the inefficiencies above there is a high volume of scavenge air requirement with this design. This has led to the complicated underpiston effect designs to augment the turboblower output with some engine power being absorbed dropping cycle efficiency. A problem with having fixed ports is that difficulty is encountered with port timing. On the piston down stroke the exhaust port is opened followed by the scavenge port to make effective use of blowdown. However, the same timing for closing the ports means that the effective compression stroke is reduced. To try to remedy this differing means of closing the exhaust ports before the piston covered the ports was tried, One such method was by engine driven rotating valves which opened and closed the exhaust ports. All the designs increased complexity and often proved unreliable due to the arduous conditions they had to operate in. To prevent exhaust gas entering the cylinder under the piston as the piston moves up to TDC, extended piston skirts are fitted. This adds to the reciprocating mass and increases load on the crosshead bearing. The small amount of side thrust not absorbed by the crosshead is spread over the larger of the skirt reducing loading and wear on the liner, however, problems of increased lubrication requirements for the increased surface area largely negate any advantage. The requirement for both the exhaust and scavenge ports being fitted into the liner makes for a more complicated design with increased liner lubrication difficulties especially in way of the exhaust ports. This region suffering not only the washing away effect of the gas flow but also contamination from combustion products and increased temperature. Cylinder lubricating oil volume demand is therefore higher with this design. In an attempt to improve the scavenge efficiency shaped pistons have been used which produce a combustion chamber shape not the best for efficient combustion. Asymmetrical piston designs can also lead to excessive thermal loading and complicated strengthening and cooling designs. One advantage of the Loop method of scavenging is that it does away with the requirement for an exhaust valve or opposed piston. This means that all the extra running gear associated with this can be omitted. That means, simpler cylinder cover design, simpler and less stressed camshaft and camshaft drive train. Where an exhaust valve is fitted in the cover, there is increase thermal stressing especially in way of the valve where higher temperatures are encountered. Additional advantages of the Uniflow method The fitting of an exhaust valve does give a major advantage in that the timing of opening and closing of the valve can be altered which is used to its fullest with modern designs with 'Variable exhaust timing' control fitted. This means that the effect of the scavenge air inertia entering the cylinder, and optimising the closing of the valve to increase the effective compression (which starts when the exhaust valve is closed) can all be taken into account for the varying loads and engine speeds. The increased scavenging efficiency with this type of scavenging creates greater scope for increased economy and so all modern designs are based on this design. The opposed piston design once in favour due to its inherent dynamic load balancing has now largely disappeared due to its increased mechanical complexity. There is a minimum air demand as the ingressing air pushes the combustion products ahead of it with little requirement for scavenging by dilution. As the air flow is symmetrical rising up the liner the thermal influencing on the liner walls, cylinder covers and piston crowns is also symmetrical. this allows simpler oil cooling of the piston crown The reduced number of ports (no exhaust ports), and reduced size of the scavenge ports (due to reduced air volume requirements), this reduces the problems of liner lubrication allowing reduced oil consumption.
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