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Tianjin university internal combustion engine institute
Attention, upstairs

Mechanical efficiency and thermal efficiency are two different things.

Mechanical efficiency refers to the transmission efficiency of engine movement.

Thermal efficiency refers to the ratio of the energy effectively utilized after engine fuel combustion to the total energy released by fuel combustion.

When the engine is running, the combustion is insufficient and the mechanical efficiency may not be reduced.

Refer to the website of Internal Combustion Engine Research Institute of Tianjin University:

In an internal combustion engine, efficiency represents the relationship between input and output, and the efficiency of the engine is equal to the ratio of the output power of the engine to the power that can be obtained when fuel is burned. The efficiency of an engine has two indexes: mechanical efficiency and thermal efficiency. Mechanical efficiency is equal to the ratio of effective power to indicated power. The mechanical efficiency of gasoline engines is generally 0.8~0.9. Thermal efficiency is the ratio of the part of heat used to do work after fuel combustion to the total heat that can be generated. Part of the heat generated by complete combustion of fuel is taken away by engine cooling, part of it is discharged with exhaust gas, and only a small part is used for doing work. Therefore, the thermal efficiency of internal combustion engines is very low, generally 20%~25% for four-stroke gasoline engines, and even for high-performance engines, its thermal efficiency is less than 30%.

And according to the conclusion of numerical calculation of internal combustion engine principle, the mechanical efficiency will increase with the decrease of engine speed, so I think

Insufficient combustion of gasoline in the engine will certainly reduce the thermal efficiency, but the mechanical efficiency of the engine will be improved.