Engine routines： cylinder diameter and stroke, who knows the heart of consumers better？

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As one of the core components of automobile, engine not only determines the quality of automobile, but also determines the temperament of automobile. Under the influence of manufacturers and media for many years, consumers generally understand what kind of output performance will be brought to the engine behind the parameters such as displacement, torque, horsepower and power. However, in the vehicle configuration table, the engine will have a label, engine cylinder diameter and stroke. What information is hidden behind these two parameters?

Influence of radius ratio on engine dynamic characteristics

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Simply put, the cylinder diameter refers to the diameter of the cylinder piston (inner wall diameter), and the stroke refers to the distance from the retraction to the extension of the piston rod. We all know that engine power is basically determined by engine displacement, and engine displacement = π× square of cylinder radius× stroke× number of cylinders, so the product of cylinder diameter and stroke determines engine power.

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Under the premise of constant displacement, engine cylinders can be designed into different combinations, which can be generally divided into three types: cylinder diameter > stroke, short stroke engine; Cylinder diameter = stroke, equal stroke engine; Cylinder diameter < stroke, long stroke engine. The ratio of cylinder diameter to stroke is called radius ratio, which is an important parameter to judge the high and low speed of the engine.

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The design of short-stroke engine with large cylinder diameter will increase the engine area and occupy a larger space in the engine room, but relatively, the engine height will not be too high, so it can effectively reduce the focus of the whole vehicle and ensure high-speed stability and handling performance. In terms of power output, short-stroke engines generally have higher rotational speed, because after the stroke is shortened, the stroke that a single work pushes the piston will also be shortened, and the number of times of doing work in the same time will also increase accordingly, and the output power will be greater, with the disadvantage of low torsion difference.

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The long-stroke engine is designed with a small cylinder diameter, so the engine occupies a small area, and the front of the engine can be designed to be shorter, giving space to the cab behind. However, the longer stroke leads to the higher height of the engine, which hinders the reduction of wind resistance and streamline modeling design, and the higher center of gravity is not conducive to high-speed handling performance. Generally, the engine with long stroke design will have a low speed of peak torque. Because the piston runs once in the cylinder for a long time, the dynamic acceleration is high, so the torque is easy to increase. The disadvantage is that the top speed is not high, and because of the long stroke, the jitter problem of the engine is obvious.

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The performance of constant stroke engine is between short stroke engine and long stroke engine. Although the short-stroke engine has high speed and stable output, it has no advantage in spatial layout, so the equal-impact engine is a compromise method, which keeps a certain cylinder diameter and a certain stroke, for example, Subaru 2.0L FA20 engine cylinder diameter = stroke diameter = 86 mm.

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Therefore, it can be seen that the longer the engine stroke, the greater the displacement of the engine, but if the stroke is too long, stability and bending performance should be considered, while the short-stroke engine has high speed, low vibration and stable operation, but the manufacturing cost will be higher. Therefore, at present, the cylinder diameter ratio of short-stroke engines is generally not more than 1.3: 1, and those that exceed 1.3: 1 are reinforced, and so are long-stroke engines.

Is it better to have a larger stroke or a larger cylinder diameter?

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A good understanding of the relationship between cylinder diameter and stroke can explain why some cars reach the red line at 5000 rpm, and some cars still have no fuel cut at 8000 rpm. Even if the same brand has the same engine with the same displacement and different models, its speed will be different. So, what effect does the speed have on consumers?

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From the performance point of view, the stability of high rotation output of short-stroke engine is king, the starting acceleration is not high, and the speed is its specialty. Of course, high speed certainly means high fuel consumption, because the piston runs up before a drop of oil is used up, so the engine with high speed has high fuel consumption and emissions, which is generally suitable for sports cars or sports cars.

Comparatively speaking, the long-stroke engine is more space-saving in design, and the layout in the engine compartment is more convenient and changeable. For car companies, even if a new model is introduced later, it can be easily adapted with a little optimization, so it is more popular, which is not necessarily a bad routine for consumers.

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Short stroke, small reciprocating inertia of the engine, suitable for high rotation; On the contrary, long stroke, large engine inertia and good low torque. Therefore, long-stroke engines that are good at emitting large torque in low-speed areas are more suitable for vehicles that require higher traction but lower speed, such as off-road vehicles, SUVs, trucks and so on. This kind of long-stroke engine generally has good torque characteristics and good starting performance, and the long stroke makes the engine burn more fully, so the fuel consumption is less, so it is very suitable for ordinary people to use. Now many ordinary cars also use long-stroke engines.

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With the development of science and technology, in order to save mold and design costs, some large companies use engines with the same diameter-to-diameter ratio for their sports cars, cars and off-road vehicles, that is, several engines with different displacement share the cylinder block, and only adjust the number of cylinders or ECU driving computer to meet the needs of different models. For example, Volkswagen does not name every engine with a displacement, but all engines that use a cylinder are called the same name, and the prefix of the name is the same. EA113 is the code name of the engine block. This series of engines have been used in the fifth generation GTI, Audi TTS, Passat B5 and other models of golf.

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Yaston’s nodule

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For Keyboard Man, it is the most basic skill to know the performance of the engine by looking at the parameters. It is impossible to determine the performance of the engine just by looking at the basic parameters such as displacement, torque and power of the engine. Cylinder diameter and stroke should also be considered as important information, which is also applicable to ordinary consumers.

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