We have looked at a number of scavenging solutions used in the past as well as currently in 2-stroke engines, focusing on petrol competition engines for karting. Now we need to understand how the combination of exhaust and transfer ports leads to certain types of scavenging and how modification of the transfer ports leads to changes in the gas flow inside the combustion chamber. Transfer ports and their positions Transfer ports are the windows machined into the cylinder wall that give the final orientation to the gas flow entering the cylinder. With tangential flow scavenging there are usually two or four ports symmetrically positioned at the sides of the cylinder wall with respect to the exhaust port. In competition engines built in recent years a new central transfer port positioned opposite the exhaust port has been added. This port, generally called the ‘TT,’ has the fundamental task of smoothing the area between the flows coming from the left and right and directing the flows upwards towards the cylinder head before they come together. So 2-stroke competition engines usually now have three or five transfer ports. An engine will be successful from a performance point of view when all the geometrical parameters of its ports are well balanced. Sizes and heights of transfer and exhaust ports must be defined after considering their balance and the gas flow (scavenge) as a whole.
We cannot have transfer ports with extremely large areas if the exhaust ports are small. That would mean a large fuel/air mixture entering the combustion chamber to burn but only a small exhaust area allowing burnt gases to exit. This would give the situation where not all the burnt gases exit the combustion chamber during one cycle. The fresh mixture would find the cylinder still full of burnt gases, preventing the mixture from entering the combustion chamber and resulting in very low torque and power output. Of course the reverse, with small transfer ports with large exhaust ports, would lead to the opposite situation but still gives poor performance. Ideal conditions for scavenging are obtained when the fresh mixture, induced through the transfer passages, arrives at the exhaust port without turbulence at the moment when the piston has closed it. Only then is pollution of the fresh charge with burnt gases avoided. This is the ideal situation that we can only aim for without ever reaching it fully. Angles of transfer ports Immediately before they enter the inside of the cylinder, the transfer passages are most suitably angled for the incoming mixture flow. The inclination, perpendicular to the axis of the cylinder and directed towards the wall opposite the exhaust, is usually kept between 25° and 45°. Sometimes it is convenient to aim at the upper wall of the cylinder but very rarely at an angle greater than 25°. Both these angles are decided on during the design stage to ensure the effective meeting of the incoming mixture streams takes place at a distance from the top of the piston equal to about half a stroke.
As a general rule the first angle diminishes and the second increases as stroke-bore ratios increase. If good flow is required, it is essential that the streams coming from the two ports meet at a very acute angle, avoiding harmful turbulence, and this can only happen in the upper part of the cylinder. Mixing the flows earlier would also detach them from the cylinder walls and divert them from the theoretically ideal route. The zone immediately above the piston and between the transfer ports cannot therefore be involved in the mixture flow, even with the piston on its way down when the flow is directed towards the cylinder wall where the exhaust ports are. This drawback may be overcome by means of the third (or fifth) transfer port (TT) placed between the two main ones in a position creating a third flow which is routed towards a meeting point with the other two flows. Duration of the transfer phase The difficulty of having a geometrical balance between the sizes and heights of exhaust and transfer ports comes also from the fact that engine revs vary. Two-stroke competition kart engines of 100cc capacity are usually direct drive and work best between 8,000 and 20,000rpm, a very wide interval. This means that at 8,000rpm a complete cycle of the engine will last double the time of a cycle at 16,000rpm and also means the scavenging time at 8,000rpm will be double that at 16,000rpm. However, the distance to be covered by the gas flow is always the same. Of course the speed of the pump pushing the gases increases, since that pumping is generated by the piston and the crankcase. This does accelerate the gases but not enough to have the same flow rate at 16,000rpm as at 8,000rpm. This means that when the engine is performing at its ideal speed, its scavenge rate is at its best and performance (torque) reaches its maximum. At different speeds from the ideal the scavenging parameters change and get worse, as does engine performance.