For the 2-stroke secondary expansio

For the 2-stroke secondary expansion mode, it is more responsive to operate in high load condition. Compared with the 4-stroke working mode, the 2-stroke working mode does not have the compression stroke, which would help the expansion cylinder avoid the negative compression work in high load condition. Besides, the exhaust pressure of the combustion cylinder is high load condition, which could push the piston of expansion cylinder work again to recover the exhaust energy. Two expansion cylinders work together to make the burned gas fully expand. The working fluid was compressed in one combustion cylinder, but it was expanded again in two cylinders, so the expansion ratio was nearly twice the compression ratio. However, in the low load condition, the exhaust pressure was so low that it cannot make a pair of expansion cylinders perform a positive work, which can be explained by Fig. 13. In the low load condition, the in-cylinder pressure of expansion cylinder in the intake stroke was lower than that in the exhaust stroke, while the situation was positive in the high load condition. if the in-cylinder pressure is higher in the intake stroke than that in the exhaust stroke, the expansion cylinder would perform positive work, because the positive expansion work is greater than the negative compression work. However, in the opposite situation, the expansion cylinder would cost work. Therefore, the 2-stroke secondary expansion mode could achieve higher thermal efficiency in the high load condition.