The combustion chamber is the heart of the engine; a sectional view is shown in the first figure. The inlet ports and transfer valve (exit) are at opposite ends. This is a very important design feature that is sometimes called uniflow. This feature allows the best configuration to eliminate all the combustion gases for the next stroke. The process of eliminating the combustion gases is as known as scavenging. Incomplete scavenging prevents fresh air from entering the combustion chamber. The fuel injector is also at the opposite end as the transfer valve. This minimizes the possibility of unburnt fuel being exhausted. Since the transfer valve is not restricted by the fuel injector, it can be very large and cover most of the end of the chamber. This allows maximum flow area and minimum pressure drops in gets the spent combustion gases out.
The throttle interacts with the combustion chamber and is shown in the second figure. Details can be seen in the photo galley for the second generation. The throttle has a small travel (about 8% of the stroke) but has a very big impact. The reason for the big impact is that the clearance volume of an engine is already small. When the throttle is moved inward, the combustion chamber and clearance volume become even smaller. Just before TDC (top dead center), when fuel is injected into the combustion chamber, the volume has been compressed about 20 to 1. So the small movement of the throttle is magnified by 20. But changing the clearance volume would normally change the compression ratio. Therefore a compensating change must be done to the supercharger boost.
The third figure illustrates the change in the clearance volume. The design in that figure is for earlier generations. Improved designs have a fixed fuel injector. Furthermore the throttle is the entire diameter of the bore. Another improvement is the combustion chamber near the transfer valve is shaped to reduce its volume. Now an 8% travel of the throttle changes the clearance volume by a factor of five.
The fourth figure explains graphically how the simultaneous change in clearance volume and boost provide a constant compression ratio and effectively a varying size engine. At low throttle and low boost, the YankeeDiesel is close to traditional two-stroke diesel. Moving the throttle out (increasing throttle) increases the clearance volume. Therefore an increase in boost is needed to compensate; the engine effectively becomes bigger. The values in the graph are approximate and were determined by thermodynamic state analysis.
The combustion chamber cycle diagrams compare the full and partial throttle settings. The compression stroke is longer for the partial throttle. This is because the 8% travel of the throttle closes the inlet ports sooner and hence the compression stroke is longer. Naturally the boost is lower to compensate. After ignition, the power stroke continues until the transfer valve opens. Then the power stroke is extended by the secondary expansion chamber. The purging is earlier in the full throttle setting because the inlet ports are opened sooner, again the 8% throttle travel difference.