Automotive Electronics: Let’s talk about engine thermal efficiency

A few days ago, I saw in my friend circle that the 2NR-FKE 1.5L engine installed in Toyota's new car has a thermal efficiency of "up to" 38%. I couldn't help but think that even the engine with the highest thermal efficiency in the world has a combustion efficiency of less than half, is it because the engine is too "unskilled"?

What is thermal efficiency?

Why should we care about thermal efficiency?

Why?

What factors affect the thermal efficiency of an engine?

The heat generated by the complete combustion of fuel is not completely converted into useful work. Only a small part of the heat is used to push the piston and drive the crankshaft to rotate. Part of the remaining heat is taken away by the engine cooling system, which is called cooling loss; part of it is discharged into the atmosphere in the form of exhaust, which is called exhaust loss, accounting for about 40% of the total energy. In other words, less than 1/3 of the useful energy generated by fuel combustion is effectively utilized, and more than 2/3 of the heat is wasted. The thermal efficiency of the engine itself is difficult to significantly improve, because if this excessive heat is not discharged, it will affect the engine operation.

Engine energy consumption distribution

In addition, incomplete combustion and mechanical losses are also important factors leading to low thermal efficiency. Incomplete combustion of gasoline due to insufficient intake often occurs. Nowadays, there are many countermeasures. Direct injection and turbocharging are both ways to improve thermal efficiency.

The automobile engine is a machine that converts the chemical energy of fuel into mechanical energy. There are heat losses and mechanical losses in the conversion process. The heat loss is mainly due to the fuel burning first to generate heat energy, and the high-temperature gas released by the combustion pushes the piston to do work. At this time, a considerable amount of heat will be transferred to the engine body and cylinder head, and dissipated through the cooling system. In addition, a considerable amount of heat energy is discharged with the exhaust gas; the loss of the mechanical part includes friction loss and motion loss. The friction resistance of the moving mechanisms such as the crankshaft and piston consumes power. As we all know, only one of the four strokes of the engine does work, and the other three strokes consume power.

Why is the thermal efficiency of a diesel engine higher than that of a gasoline engine?

Diesel engine perspective view

How will future gasoline engines improve thermal efficiency?

We have just mentioned several engine cycles. In the next issue, we will introduce you to the differences between the Otto cycle, Miller cycle and Atkinson cycle. Stay tuned.