At present, reciprocating piston engines are widely used in commercial vehicles. There is also a well-known but rarely used engine, which is the triangular piston rotary engine. Rotary engine is also called Miller cycle engine. It uses the rotary motion of triangular rotor to control compression and discharge, which is very different from the linear motion of traditional piston reciprocating engine. This kind of engine was invented by Figas Wankel, a German. On the basis of summarizing the previous research results, some key technical problems were solved and the first rotary engine was successfully developed. The general engine is a reciprocating engine. When working, the piston does reciprocating linear motion in the cylinder. In order to convert the linear motion of the piston into rotary motion, the crank-connecting rod mechanism must be used. Rotary engine is different, it directly converts the combustion expansion force of combustible gas into driving torque. Compared with the reciprocating engine, the rotary engine eliminates useless linear motion, so the rotary engine with the same power is smaller in size, lighter in weight, lower in vibration and noise, and has great advantages. The motion characteristic of rotary engine is that the center of triangular rotor revolves around the center of output shaft, while the triangular rotor itself rotates around its center. When the triangular rotor rotates, the internal gear ring centered on the triangular rotor meshes with the gear centered on the output shaft, and the gear is fixed on the cylinder body and does not rotate, and the ratio of the number of teeth of the internal gear ring to the gear is 3 to 2. The above-mentioned motion relationship makes the motion trajectory of the apex of the triangular rotor (that is, the shape of the cylinder wall) look like an "8". The triangular rotor divides the cylinder into three independent spaces, and each of the three spaces completes intake, compression, work and exhaust successively. The triangular rotor rotates once and the engine ignites to do work three times. Because of the above motion, the rotation speed of the output shaft is three times that of the rotor, which is different from that of the piston and crankshaft 1 of the reciprocating engine.