One of the main aspects of two-stroke engine performance is scavenging. The way fresh mixture enters the combustion chamber and burned gases exit through the exhaust ports determines the quantity of mixture burned in each cycle of the engine (each rev for two-stroke engines) and the way it is burned.
Quantity of ports
A modern two-stroke competition engine for 100cc direct drive category is made of an inlet port, which differs for piston-port, reed valves and rotary valves engines. Transfer ports are then positioned at the end of the transfer ducts on the walls of the cylinder, these are usually two, symmetrically positioned (generally four on 125cc gearbox engines), and one called “TT” centrally positioned on the opposite side of the exhaust port. Exhaust ports can be of two different shapes, rectangular with curved angles and a central separating wall of 2-3mm thickness, or elliptical with two additional holes on the sides on the upper part of the central hole. These two additional ports are called “boosts”.
Functioning and aim of the exhaust ports
The importance and function of the ports is to define the inlet or outlet of fluid (mixture or burned gases) and the direction of the flow. Ports are shaped so that timing of their opening and closing is precise, in particular this works on transfer and exhaust ports, whilst inlet ports differ as said from the kind of valve used. The piston determines the opening and closing of such ports: the opening occurs when the piston is moving downwards and the closing when the piston is moving upwards. The “lines” that determine the timing are the piston ring (on “L” shaped section ones as on kart 100cc engines) or the piston crown (on piston with rectangular section rings as on 125cc gearbox engines), and the upper edge of the ports. This means that if the upper profile of the port is perfectly horizontal the opening of the port will be extremely immediate and precise, which is something that helps functioning of the engine and port timing. In fact one of the main aims of port timing is to obtain the inlet (in the combustion chamber) and outlet (in the exhaust duct) of the fresh and burned mixture as quickly as possible, but with the right timing. When the piston moves from top dead corner downwards and then opens the exhaust ports we want as much burned gases to exit through the exhaust duct as soon as possible, which means we want also a good total area of the exhaust ports so that more gases exit the combustion chamber in less time. This helps total evacuation of the combustion chamber from burned gases, which brings to depression of such volume and better inlet of fresh mixture from the transfer ports.
On the other hand though we must regulate the exhaust phase so that fresh mixture does not also exit the combustion chamber during exhaust phase, or if it does, it does so in limited quantity. This means a good area of the exhaust port sections, but not excessive.
Also another limit to exhaust port dimensions is that a perfectly straight and horizontal upper profile of the ports can determine, especially when ports have a good width, that the piston ring might get stuck inside the port with consequent ring breaking or bending, and possible final engine seizure.
For this reason we have the two solutions anticipated previously. A rectangular section port with central separation wall, or a more or less elliptical port with boosts (these can also be sometimes found together with the rectangular solution). The rectangular shaped exhaust port has surely a greater area of its section, but the central wall, used to avoid piston ring getting stuck inside the port, creates some interference with the gas flow exiting the port. On the other hand the elliptical exhaust port has a smaller area, but no obstruction to gas flow. This is because it needs no central separation wall, since the curved upper and lower edges of the port avoid ring blocking.
Upper area of the exhaust ports
The area of ports, and of the exhaust ports is extremely important for the exiting of the burned gases from the combustion chamber, but not “all the area” has the same importance. In fact the upper part of the exhaust ports is the area that is uncovered first by the piston. This means that when the pressure inside the combustion chamber is extremely high the exhaust port opens and gases exit through the opening of the first upper part of the exhaust port. The speed of the burned gases exiting the port is maximum since pressure is high and the open area of the port is still not complete. To have maximum area open as soon as possible the port must have a great area of its section on the superior part. This is why the best solution is the rectangular shape. But, as already said, such shape gives problems with reliability and piston ring damage, so a central separating wall is used. The great speed of gas flow in the final part of the opening phase of the exhaust port determines great turbulence of the flow especially because of the central separating wall. Such turbulence is also created by the sharp edges of the port. Turbulence means that the flow does not follow straight lines, but creates swirls. Such swirls reduce the flow of gas so that the result is an area of the port smaller than the real one, which means that the swirl and turbulence creates a block to the flow just around the central separating wall and the edges of the port.
How to reduce turbulence
To reduce turbulence we can work with sandpaper or machine the edges of both port and central separating wall. This must be done to eliminate sharp edges, but we shall not exceed because the edges of the port must always maintain a certain sharpness to give the right timing of port opening and closing and the right sealing. In fact if port edges are too much “rounded” gases will pass by piston and cylinder in the area of the port edge before the port is really open.
Also the central wall can be machined to reduce its thickness, but do not exaggerate this work as well since a reduced thickness of the central wall of the exhaust port can determine expansion of such area and a pressure between such wall and the piston, with possible engine seizure.
The same treatment can be done to the lateral boosts of the exhaust port that also work in the initial moments of the opening of the exhaust port, so gas flow is fast and easily produces turbulence. You will always find sharp edges in new engines since honing of the cylinder produces such sharp edges. Also from time to time repeat the machining and rounding of sharp edges after honing is done to the cylinder for example after engine seizure.