The camshaft layout of the engine is divided into OHC (overhead camshaft) and OHV (bottom camshaft). At present, automobile manufacturers in Japan and Europe prefer the design of the overhead camshaft; the bottom camshaft is usually only visible in American cars.
In understanding the difference between OHC and OHV, we first briefly understand and understand their respective structures and characteristics technically:
The OHC (overhead camshaft) has evolved into SOHC (single overhead camshaft) and DOHC (double overhead camshaft). Single overhead camshaft is to rely on a camshaft to control the opening and closing of the intake and exhaust valves. In general, the single top is designed with the two-valve engine. Because the two-valve engine has lower intake and exhaust efficiency than the multi-valve, the layout between the valve angles is limited. The dual overhead camshaft can optimize these problems because a single camshaft controls only one set of valves (intake or exhaust), so the valve's rocker is omitted, simplifying the camshaft to the valve. Transmission mechanism. In general, the dual overhead camshafts have low intake and exhaust efficiency and are more suitable for high-speed engine performance. For the Japanese and European manufacturers who are pursuing high power, the design of the camshaft roof is certainly the most appropriate.
The camshaft of the bottom camshaft is generally designed for large displacement, low speed and high torque output, because the bottom camshaft is driven by the crankshaft, and then the cam and the valve rocker arm are connected by a metal rod. The cam pushes up the connecting rod, and the connecting rod pushes the rocker arm to realize the opening and closing of the engine valve. Therefore, the excessively high rotation speed will cause the top rod to bear the pressure so as to be broken. However, this kind of design with a jack has its advantages, such as simple structure, high reliability, low engine center of gravity, and low cost. Because the engine speed is low, the emphasis on torque performance, so the bottom camshaft design is sufficient to meet this demand.
Since these two designs tend to be different, the former is the most demanding power, and the latter is the pursuit of high torque. We know that the speed of the car is fast and the traction is strong, and the maximum speed is the power. Here is a simple formula: power = speed X torque. The simplest way for the engine to increase power when naturally aspirating is to increase the speed, and the higher the speed, the higher the power naturally.
Why do only Americans love bottom camshafts?
In Germany, where the automobile industry is developed, many highways are open-air, and speeds of more than 200 kilometers are common. Therefore, high power is naturally their favorite. But Americans are different. They are pursuing highway cruising performance. The engine does not use high speed. This has a lot to do with the national conditions of the United States. The United States has vast territory and vast territory, and most of it is plain. The roads are straight and straight. After World War II, Americans have always been keen on styling, spaciousness, and comfort-oriented luxury cars. The weight is large, the suspension is soft, and the comfort of running in a straight line is very good. Moreover, the price of oil in the United States is very low, and large-displacement engines are favored by the public. Therefore, the engine popularity of this kind of dynamic 6.0, 7.0 is very high. When it comes to this, there is one more point that must be added. We often talk about the engine power, and use this to determine the operating efficiency of the engine. But have you heard the term power density? To put this concept in simple terms is to release the same power, the smaller the engine's volume, the higher the power density. The camshaft-mounted engine benefits from its low torque and does not require complex multi-valve, dual-camshaft, high-strength cylinder head covers like overhead camshafts.
Therefore, the US engine adopts a bottom camshaft design that is based on the national conditions of the United States. It is not that the Americans will not build a good engine. But this also led to the US Department of the engine is not suitable for markets outside the United States, so when developing overseas markets, the Americans need to develop an engine more suitable for other markets.
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