EGR, by introducing an appropriate amount of exhaust gas into the combustion chamber to participate in combustion, so as to reduce the temperature in the cylinder and reduce NOx emissions. Send part of the exhaust gas discharged from the engine back to the intake pipe and enter the cylinder again together with the fresh mixture. Because the exhaust gas contains a large amount of CO and Ho, which can not burn but can absorb a large amount of heat, the combustion temperature of the mixture in the cylinder is reduced, thus inhibiting the formation of NOx.