本文涉及的换气式空气热机实质上是一种单活塞热机，在气缸内与活塞刚性地连接上一个膨胀体（换位活塞），当活塞在气缸内运动时膨胀体也一同运动，同时设置了气缸与外界适时开启的导气孔. 膨胀体的作用是它的占位迫使气缸内的工质空气在高、低温两个热源之间变换. 在空气工质经过高温热源后开始被压缩时（活塞处于简谐振动平衡位置时）导气孔打开一下，气缸内的高温空气工质部分地被排出气缸，以减小对活塞运动的阻力；在空气工质经过低温热源后开始膨胀时（活塞处于简谐振动平衡位置时）导气孔打开一下，外界的空气经过小孔进入气缸，以增大对活塞的推力. 活塞在气缸中运动，每经过一个周期导气孔开启两次，综合效果为空气工质对活塞做了正功，在PV图上表现为空气工质的状态参量完成了一次热机正循环.

The air exchange air heat engine is essentially a single piston heat engine. An expansion body (transposition piston) is rigidly connected with the piston in the cylinder. When the piston moves in the cylinder, the expansion body also moves together. At the same time, the air guide hole opened timely is set between the cylinder and the outside. The function of the expansion body is that its occupation forces the working medium air in the cylinder to change between the high and low temperature heat sources. When the air working medium begins to be compressed after passing through the high-temperature heat source, the air guide hole is opened, and the high-temperature air working medium in the cylinder is partially discharged out of the cylinder to reduce the resistance to the movement of the piston; When the air working medium begins to expand after passing through the low-temperature heat source, the air guide hole is opened, and the external air enters the cylinder through the small hole to increase the thrust to the piston. When the piston moves in the cylinder, the air guide hole is opened twice every cycle. The comprehensive effect is that the air working medium does positive work to the piston, which is shown as the state parameters of the air working medium on the PV diagram, and a positive cycle of the heat engine is completed.