Otto cycle
Otto cycle can be said to be the basic cycle of four-stroke internal combustion engine. A working cycle of Otto cycle consists of four strokes: intake, compression, expansion and exhaust. In each stroke of Otto cycle, the piston stroke is consistent, which is also an ideal thermal cycle for constant volume heating.
It is also the basic working principle of mainstream internal combustion engines. Although different manufacturers have their own bright spots in engine technology, they are all based on Otto cycle.
The advantages of Otto cycle are stable operation, relatively simple engine structure, high power increase without special adjustment of valve train, and good power output of the engine at all speeds. However, it also has disadvantages, that is, its fuel efficiency is relatively poor, which can no longer meet the needs of contemporary society.
atkinson cycle
Atkinson cycle should be a kind of internal combustion engine cycle that friends have heard more about, especially on Toyota models. Before figuring out the Atkinson cycle, let's first understand the compression ratio and expansion ratio. The so-called compression ratio is the ratio of the cylinder volume when the piston runs to the bottom dead center to the cylinder volume when the piston is at the top dead center in the compression stroke. Expansion ratio refers to the ratio of cylinder volume at the end of power stroke to cylinder volume at the beginning of power stroke. In Otto cycle, the compression ratio and expansion ratio are the same, which has no positive effect on improving fuel efficiency. In Atkinson cycle, the expansion ratio will be greater than the compression ratio, that is to say, in the case of short compression stroke, there can be a longer power stroke, which is equivalent to getting more energy in the compression stroke. This directly improves fuel efficiency.
Atkinson bicycle was originally invented by British engineer James Atkinson. At the beginning of this cycle, Atkinson designed an ultra-complex connecting rod mechanism to make the power stroke of the piston greater than the compression stroke of the piston. Although his idea and design are genius, this complex linkage mechanism is not conducive to the high-speed operation of the engine, and the later maintenance is also very troublesome.
However, there are always some brands who like to show off their technology and are keen to realize this complex mechanism. After all, more difficult rotary engines can be mass-produced, not to mention relatively simple reciprocating engines. After Honda's repeated efforts, the Honda EXLink series engine was finally built, but unfortunately, the maximum speed of this engine can only be maintained at about 2000 rpm, and the engine cannot squeeze more power by increasing the speed, so the engine is not practical. In the end, Honda had to give up the Atkinson engine that was the most loyal to the Atkinson prototype.
In Atkinson cycle, when the engine starts the compression stroke, the intake valve will not be closed immediately, but will be closed after the compression stroke lasts for a short time, at which time part of the mixture will be ejected from the intake pipe. The real compression stroke will start from the closing of the intake valve, so the power stroke will naturally be longer than the compression stroke. This also greatly improves the combustion efficiency of Atkinson cycle.
But Atkinson cycle also has disadvantages, that is, low torsion and weakness. In the compression stroke, a part of the lean mixture is spit out, which makes the combustion more difficult, so Atkinson cycle is more suitable for the middle and high speed working conditions of low engine load.
Miller cycle
The principle of Miller cycle is the same as Atkinson cycle, which makes the expansion ratio of the engine greater than the compression ratio in order to extract more power, but the implementation method is slightly different. In the intake process of Miller cycle, by closing the intake valve in advance, the mixture actually entering the cylinder is less than the theoretical value, thus reducing the actual compression ratio of the engine, so that the expansion ratio will be greater than the compression ratio, so the combustion efficiency will be improved accordingly. Moreover, because the intake valve is closed in advance, Miller cycle can properly cool the air-fuel mixture in the cylinder, so Miller cycle can also effectively improve the knocking phenomenon of the internal combustion engine.
However, Miller cycle also has disadvantages, because the mixed gas entering the cylinder does not form strong turbulence, so the same problem as Atkinson cycle will appear, that is, insufficient power under high load.
However, at this stage, Japanese brands basically realize Miller cycle and Atkinson cycle in the same way, and both reduce the compression ratio by delaying the closing of the intake valve. However, Mazda registered Miller cycle, so in order to avoid patents, other brands will call their own cycle Atkinson cycle. The most loyal to the prototype of Miller cycle is Volkswagen brand engine, which often reduces the actual compression ratio of the engine by closing the intake valve in advance to improve the combustion efficiency of the engine.
But in the process of buying a car, friends don't have to worry about the shortcomings of various cycles, because major brands will switch their engines between Atkinson/Miller cycle and Otto cycle through complex valve train, thus making up for the weaknesses of each cycle and keeping the vehicle in a state of abundant power and high efficiency.
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.