Before talking about Atkinson cycle, we should know about Otto cycle. 1862, a French engineer first put forward the four-stroke cycle principle. 1876, a German engineer, Nicola Otto, invented the engine by using this principle. Because this kind of engine has excellent performance such as smooth rotation and low noise, it has great influence on industry, and this cycle is named Otto cycle. A cycle of Otto cycle consists of four strokes: inhalation, compression, expansion, work and exhaust. First, the piston moves downward, so that the mixture of fuel and air enters the cylinder through one or more valves, and the intake valve is closed. Then the piston moves upward to compress the mixed gas. When the compression stroke reaches the peak, the spark plug ignites the mixture, and the thrust generated by the explosion of combustion air forces the piston to move downward to complete the working stroke. Finally, the burnt gas is discharged from the cylinder through the exhaust valve. Therefore, Otto cycle, also known as four-stroke cycle, is a thermal cycle of internal combustion engine and an ideal constant volume heating thermal cycle.
Although Otto succeeded in improving and manufacturing the four-stroke engine, he did not apply this machine to vehicles. 1886, karl benz applied the Otto engine to the improved carriage and invented the automobile. At present, the basic working principle of fuel engine still follows the original design concept, and there is no essential change. Note that in the above description, constant volume heating is an ideal thermodynamic cycle, in which constant volume means that the position of the upper and lower dead center of the engine piston will never change, so that the displacement of the engine, that is, the volume in the piston motion trajectory will remain unchanged. ?
At that time when the internal combustion engine was just invented, engineers were still curious about this new machine, and many people tried to improve it. Just six years later, British engineer James Atkinson (James? Atkinson) put forward an idea in 1882, that is, it is hoped that in the above four strokes, the piston can increase the stroke when it expands to do work, so as to obtain more kinetic energy and improve the rotation speed. Having said that, we must understand that the engines at that time were basically single-cylinder horizontal structures, and did not adopt the later crankshaft connecting rod structure. A large flywheel is pushed behind the connecting rod, and the working stroke can indeed be greater than the compression stroke through the movable clamp pin located at the joint of the connecting rod and the flywheel. However, because the output power of the engine was very small at that time, this cycle reduced the power density, so it could not be the mainstream scheme. Atkinson cycle entered people's field of vision but soon entered history.
Speaking of the power of the engine at that time, look at the engine of the first Mercedes-Benz car, which was very poor, only 0.9 horsepower, just enough to push the vehicle to walk slowly, so the main job of the engineer at that time was to improve the output power and conversion power of the engine. The following year, the combined structure of crankshaft and connecting rod appeared, which made the piston's motion stroke a fixed value, and Atkinson cycle could not be realized by mechanical mechanism.
After decades of development, the Otto cycle engine made great progress in the 1920s and 1930s. In addition to more and more cylinders and larger displacement, they also have complex intake and exhaust structures controlled by camshafts. At the same time, the fuel supply structure of carburetor and the thermal management system of water cooling are becoming more and more mature, and the current engine power is no longer a problem. However, in the process of increasing engine power, it is found that it is easy to knock when the compression ratio is increased, and the engine is prone to pumping loss when the load is low, which has become a problem that engineers have to solve.
At that time, the automotive frontier technology center had been transferred from Europe to the United States. In addition to General Motors' infamous Midgley who used leaded gasoline to solve the knock problem, American engineer R.H. Miller works in 1947? The strategy of early intake closing (EIVC) was first put forward in 2006. Closing the intake valve before the start of the compression stroke is equivalent to reducing the volume of air mixture in the cylinder and reducing the power lost by pumping. This is the so-called Miller cycle. Miller cycle is characterized by that the effective compression ratio of the engine is less than the expansion ratio, which improves the thermal efficiency, effectively suppresses the engine knock and reduces the emission of nitrogen oxides. From here, engineers finally found a new idea to solve the different schemes of compression ratio and expansion ratio, that is, to advance the valve closing time by using the intake valve timing, so as to reduce the compression ratio.
Although the theory is mature, it is still very difficult to operate in practice, because the valve control technology is not mature enough to achieve accurate control effect. It was not until the variable valve timing system composed of solenoid valve and variable cam phase adjuster appeared that Miller cycle was actually and effectively applied to the engine, so that the engine could naturally switch between Miller cycle and Otto cycle under different working conditions.
What I want to say here is that the core meanings of Miller cycle and Atkinson cycle are different. The starting point of the former is to reduce compressible gas (reducing detonation and pumping loss), while the real intention of the latter is to increase expansion ratio (doing more work). However, because both of them have goals, the power stroke is greater than the compression stroke, so there are similarities. Because the patented technology of Miller cycle was obtained by Mazda, other brands such as Toyota, Honda and Volkswagen can only call Miller cycle Atkinson cycle to avoid patent monopoly.
So, is there an engine that can really realize the Atkinson cycle? For a long time, it didn't, or didn't achieve mass production. It was not until the maturity of Nissan VC-Turbo technology recently that the Atkinson cycle was really brought back to reality. We have a chance to talk about this technology in detail.
Back to the current Miller cycle technology, we will find that Toyota and Volkswagen have adopted different strategies. Volkswagen EA888 and other engines adopt the strategy of closing the intake valve in advance to realize the Miller cycle, while Toyota's hybrid system mostly adopts the strategy of closing the intake valve in delay. The former directly shortens the intake time to reduce the intake air volume, and the latter pushes a part of the mixture entering the cylinder back to the intake manifold. The former is completely consistent with the technical route put forward by Miller in those years, and also needs the assistance of technologies such as pressurization and intercooling, while the latter shares the burden of the engine at high load with the assistance of the motor, which can make the engine give full play to the advantages of Miller cycle at low load, which is equivalent to using the motor to make up for the problem of insufficient power at high load such as starting and accelerating.
Speaking of which, if the next time a boy meets a salesman to tell you about Atkinson cycle, he can confidently answer: You are wrong, in fact, it is Miller cycle.
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.