The secondary expansion chamber surrounds the combustion chamber. The purpose of this chamber is to convert exhaust gas heat into additional power. Other inventors have attempted this and essentially they develop a topping cycle. In other words, they take the high temperature exhaust as a heat source for a secondary power loop with heat exchanger. Another approach is to use the exhaust gases to preheat the incoming air to obtain higher combustion temperature. In general, gases have low heat transfer ability so their results have been weak. The heat transfer problem was solved here by mixing the combustion gases directly with cold air.
The secondary expansion chamber is almost a mirror of the supercharger which is directly opposite the secondary expansion chamber. The fixed partition separates the two. During the power stroke, the secondary expansion chamber fills with cold air through the reeds valves (see the first figure below). Then near the end of the power stroke, the combustion gases rush into the secondary expansion chamber. The gases are trapped and try to expand and push the secondary expansion chamber piston. Since this piston is directly connected to the supercharger piston (which is in its compression stroke), the power is used immediately (see the second figure below).
Then at bottom center, the piston reverses direction. The secondary expansion chamber gets smaller. The exhaust valves open and exhaust gases are pushed out (see the third figure below). The exhausting lasts 180 degrees of crank rotation so the exhaust velocity and pressure drops are small. In traditional two-stroke diesels, the exhaust must be expelled in 20 degrees of crank rotation. Any longer and the power stroke is shortened with a loss in efficiency. The YankeeDiesel will also be less noisy. When the exhaust is vented, it is at lower pressure.
The cycle diagram below shows the secondary expansion chamber as a way to extend the power stroke. The extension of the power stroke begins when the transfer valve opens. Yankeediesel is more efficient because the combustion gases can expand more and exhaust gas heat is converted into extra expansion. The secondary expansion chamber is about four times bigger than the combustion chamber. The amount of combustion gases entering the secondary expansion chamber varies with throttle setting. At full throttle, the mass of combustion gases is greater than the cold air in the secondary expansion chamber. Therefore the secondary expansion chamber improves efficiency by allowing the combustion gases expand more. At low throttle, the efficiency shifts to heat extraction.
Farther study is required but the secondary expansion chamber may be the perfect place for a catalytic converter. When the transfer valve opens, hot combustion gases mix directly with cold air. The excess oxygen could react with some of the combustion products. The last figure summarizes the benefits of the secondary expansion chamber.