50cc D-Cycle Performance Prediction
The D-cycle 50cc prototype starts with an 80cc 4-cycle baseline. It retains the same torque arm and is limited to the same peak combustion pressure. It would have the same full load torque capacity if it could retain the original stroke lengths. It can potentially drive like a 160cc 4-cycle without additional stresses on the parts. (The peak power is limited not only by the rated rpm but also by the cooling. The 50cc D-cycle is not producing more peak power, since the baseline engine already has excessive horsepower for its intended application.)
Figure 1. D-cycle utilizes the cylinder volume more effectively for power generation by stroking only the upper part (and saves the ring friction at the same time). The lower part of the cylinder is ineffective in generating torque/power. First, in power strokes the cylinder pressure drops rapidly as the piston move downward while the torque arm decreases to zero at the BDC. Second, in compression strokes the cylinder takes up to half of the lower part to get 2 times compression. This low-pressure compression can better be done by a blower.
The 50cc D-cycle power stroke is only about 2/3 of the crank strokes of the 80cc baseline, give-and-take (G-A-T). From the engine torque curve, it loses about 1/9 (~=1/3 squared) of the torque capacity of the baseline, G-A-T. This is because the engine torque peaks at about 15 degrees aTDC and drops rapidly to zero at BDC, due to rapidly decreasing pressure and torque arm. The lower end of the power stroke is very ineffective in producing torque/power, while it continues to consume a big share of the frictional energy with the piston rings. The frictional energy of the rings saved in power strokes is about 1/3, recovering some lost energy. Therefore, not only can we safely assume that no torque capacity is lost in using only the upper 2/3 crank strokes for power in most driving, it is more effective to not stroke the bottom end. 4-cycle engines have no such choice. Opening the exhaust valve early still continues to waste ring frictional energy.
The 4-cycle engine size is designed for full load needs such as passing. In regular driving it needs much less load, about 1/3 for most and up to 2/3 for some heavy driving. The full load capacity is rarely utilized in every-day driving. The engine is constantly throttled or choked from its full load capacity, which keeps it away from its most efficient sweet-spot performance. This is another reason for poor fuel economy. D-cycle corrects this shortcoming by assigning about 1/3 of the baseline stroke for intake, G-A-T, which meets about all the driving needs.
Without charging, this is equivalent to about 2/3 (2 times 1/3) full load torque capacity of baseline engine, G-A-T, which is adequate for most city driving. It should be kept in mind that the 50cc D-cycle still has doubled acceleration rate of the baseline. It feels a 160cc driving power!
With 1.5 times charging it makes up to the baseline full load torque capacity, G-A-T, retaining the doubled acceleration rate. This can be done with a blower or the like. With still higher (3 times total) charging it can produce the 160cc equivalent full load torque, G-A-T. It is worth noting that the baseline full load capacity with doubled acceleration rate is all that is needed for most city driving. A higher full load could be overkill.