Automotive Electronics Technology: Understanding Three Thermal Cycles of Engines

In the last issue, we talked about the thermal efficiency of the engine. However, when it comes to thermal efficiency, we have to talk about the thermal cycle. There are three thermal cycles on the market: Otto cycle, Miller cycle and Atkinson cycle. Let's talk about these today...

Most engines on the market are Otto cycles. The four-stroke engine's intake, compression, power and exhaust are all accompanied by normal valve opening and closing timing. Each valve opening and closing is at the top or bottom dead center of the piston. Please see the GIF animation below. This cycle is relatively conventional, and the piston strokes for compression and power are the same. However, this does not meet people's demand for performance. Many people want to increase the compression ratio, but they cannot make the compression ratio too large.

Such a conventional heat cycle exists, where do the Atkinson cycle and the Miller cycle come from? This is about a parameter that we often pay attention to - compression ratio. Many people judge the quality of a vehicle by the compression ratio, and believe that the higher the compression ratio, the better it is. In fact, there is some truth in this. The higher the compression ratio, the better the performance. By compressing the mixer to achieve greater work, this has been the direction of development since the birth of the engine. However, gasoline has its own characteristics, and the compression ratio cannot be blindly increased. If it is too high, it will produce knocking, which will affect the engine. So a man named Atkinson took a different approach and chose a completely new way.

Atkinson cycle - expanding expansion ratio

By redesigning the crank-connecting rod structure, the piston suction and compression strokes are shortened, while the power and exhaust strokes are lengthened, thus achieving the effect of increasing the expansion ratio. However, the engine with this structure is relatively large and is basically not suitable for automobile use. It has made some achievements in ships and industry , so it is not widely used in automobiles.

Atkinson heat engine model

Miller Cycle - Atkinson in a different way

Since the principle of the Atkinson cycle is not wrong, Mr. Miller invented the Miller cycle in the 1940s. He continued Atkinson's idea by changing the valve opening and closing time and increasing the expansion ratio. After the piston runs to the bottom dead center, the intake valve is not closed in time, and the gas in the cylinder goes through two processes: inertial intake and intake reflux (exhausting the inhaled gas). By delaying the closing time of the intake valve to a certain degree after the bottom dead center of the piston, the actual compression of the mixture is less than the expansion after the explosion. This is the "Miller cycle". For engines using the Miller cycle, no matter how the actual compression of the mixture changes, the piston, crankshaft, and cylinder block are consistent with the traditional Otto cycle engine, so the maximum volume of the cylinder and the combustion chamber volume when the piston is at the bottom dead center are fixed and will not change.

Changing the intake valve closing time

If this is the case, why are there so many Atkinson cycles all over the world?

As mentioned above, the Atkinson cycle is not actually applicable to automobiles, and has basically never been used in automobiles, but why do people say that their engines use the Atkinson cycle?

It is said that as early as 1993, Mazda resumed the Miller cycle technology and registered the "Miller cycle" as a trademark. Naturally, other manufacturers were not allowed to use it at the product level. Although everyone used the principle of the Miller cycle, the general effect was similar to the Atkinson cycle, so everyone used the Atkinson cycle naming method.

Application of Miller cycle engine

The engine of the Lexus CT200h also uses the Miller cycle

Switching between the Miller-Otto cycles?

Comments: I believe that after reading the above article, you have a certain understanding of the three thermal cycles of the engine. The most common one is the Otto cycle, and the most mysterious one is the Atkinson cycle, because it seems to appear often, but it does not exist at all. The Miller cycle is firmly grasped by Mazda. Although everyone is using this method to achieve energy-saving technology, you want to use this name? Humph, no chance!