Two-stroke engines have superchargers because there is very little time to purge the spent gases from the combustion chamber and introduce fresh air. The fresh air and combustion gases must be moved quickly. The YankeeDiesel supercharger surrounds the combustion chamber. It forces fresh air into the combustion chamber soon after the power stroke ends. The location is ideal and eliminates any piping or manifolds. This reduces the pressure drop in delivering the fresh air into the combustion chamber. The first figure below shows the supercharger at the start and end of its compression stroke.
The second figure shows a complete cycle of the supercharger. At full throttle, the compression starts at top center (TC). The compression ends when the combustion chamber inlet port is exposed. This occurs after the transfer valve has opened and reduced the pressure in the combustion chamber. The higher pressure in the supercharger pushes fresh air into the combustion chamber and any combustion gases into the secondary expansion chamber. This process is called scavenging. Then at bottom center (BC) the transfer valve closes and the cylinder reverses direction. The pressures in the combustion chamber and supercharger equalize until the combustion chamber inlet port is closed. At full throttle, the pressure will be about 6 atmospheres in both chambers. Now the supercharger volume is expanding and some the work in compressing the air is returned until the pressure in the supercharger equals atmospheric pressure. Then the continued expansion allows fresh air to enter the supercharger through the one-way reed valves.
At partial throttle, less air is needed so the supercharger is vented during the first portion of the compression stroke. This is done with a spiral vent on the throttle. When the throttle is moved inward, it also rotates. Increasing the inward movement increases the rotation and venting. This is shown in the cycle diagram. Also notice that the purging starts later at partial throttle. When the throttle is moved inward for partial throttle, the stroke increases and delays the exposure of the combustion chamber inlet port. Likewise the charge is done quicker as the inlet port is closed sooner. This works out nicely since the idea of a variable size combustion chamber requires less fresh air and less purging at partial throttle. The recovery is also over quicker since the partial throttle pressures are lower.
Third figure (top half) shows the supercharger pressure for a complete cycle at partial throttle. Starting from top center, the supercharger is initially vented. At reduced power, the venting can reduce the supercharger volume by a factor of five. We need only about 2.5 atmospheres of pressure and 3 times the combustion chamber volume. At minimum throttle, the combustion chamber will need only 1.6 atmospheres of fresh air.
The bottom half of the same figure shows the full throttle case. Compression starts soon and pressures are greater. Notice that purging starts sooner and last longer. Furthermore, charging lasts longer as does the recovery.
The YankeeDiesel supercharger is very simple. The variable capacity, can be seen the photo gallery for generation 9. The design was improved twice and can be seen in later generations. In all cases the supercharger takes in only as much air as was used in the previous stroke. Also compressor work for unused air is returned. Such a supercharger allows the opportunity to preheat the air for cold weather starting. By repeated cranking with a closed throttle, the air in the supercharger will heat up. Still another opportunity exists in design. The supercharger can be made much bigger to allow efficient operation at high altitude.