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Figure 1 Timing belt drive of the engine 
Figure 2 Engine flexible timing chain drive
Although the new technology of the engine is constantly developing, and in recent years, the high pressure common rail is used on the diesel engine. The gasoline engine adopts advanced technologies such as supercharging, intercooling and direct injection, but the timing transmission of the engine, that is, between the crankshaft and the camshaft The transmission relationship (two turns of the crankshaft ensures that the camshaft turns one turn) remains constant to accommodate the entire working cycle of the engine's intake, compression, combustion, expansion work and exhaust.
The timing of the engine, on most diesel engines, the crankshaft and camshaft are placed on the cylinder at the same time, the center distance between the two is moderate. Since the gear transmission is simple and reliable, the gear machining has been programmed, and it is relatively reliable and economical, so gear transmission is adopted.
For the car engine, in order to improve the intake efficiency and power, a double camshaft four-valve or five-valve overhead cylinder head is adopted, and the center distance between the crankshaft and the camshaft is greatly increased, and the crankshaft hole and The camshaft hole is not on the same part, and the positioning error and the machining error overlap multiple times, and the center distance accuracy of the gear cannot be guaranteed. Another reason is that the size of the front cabin of the car also limits the size of the engine, and the gear transmission can not be adapted. Therefore, the car engine usually adopts a flexible transmission, that is, a timing toothed belt transmission and a timing chain transmission.
Rigid timing gear drive
In the original diesel engine, a rigid gear transmission was used, and a gear chamber was arranged in front of the cylinder. A gear chamber cover and a gasket were provided in consideration of sealing and gear lubrication requirements. Diesel engines often have a crankshaft and a camshaft disposed on the cylinder block. The camshaft gear uses the intermediate gear to rotate in the same direction as the crankshaft gear. The camshaft of the diesel engine realizes the opening and closing of the valve on the cylinder head through a series of parts such as a valve tappet, a valve lifter, a valve rocker arm, a valve, a valve lock clamp and a valve spring. Since the gear transmission has strict requirements on the center distance, in the cylinder processing line, the crankshaft hole, the camshaft hole and the idler shaft hole are often used to adopt a positioning clamping process on the three-axis combined boring machine, in order to ensure the gear transmission pair. The strict requirements of the center distance also limit the design space of the crankshaft and camshaft. However, the arrangement of the crankshaft and the camshaft on the cylinder body will inevitably increase the outer dimensions of the diesel engine body, making the diesel engine machine very cumbersome.
Flexible timing belt drive
The rapid development of China's automobile industry, especially in the passenger vehicle field, requires high power, low noise, low emissions and light weight for gasoline engines for passenger cars. At the same time, most of the engines in the car are arranged horizontally, and the size of the engine is adapted to the size of the front compartment of the car. Due to the high power required, the engine speed (6000r/min or more) and the efficiency of intake/exhaust are adopted. The four-valve (or five-valve, increased intake air) is used, and the cylinder head structure of the double camshaft is placed. To improve the efficiency of engine intake / exhaust. The intake cam and the exhaust cam are separately arranged, and a parallel structure of the intake camshaft and the exhaust camshaft is arranged on the cylinder head (the diesel camshaft has the intake cam and the exhaust cam on the same shaft).
The camshaft directly drives the opening and closing of the valve through the valve tappet. Since the transmission chain is greatly shortened, the transmission efficiency and the transmission accuracy are improved. The camshaft moves from the cylinder to the cylinder head, which increases the distance between the camshaft and the crankshaft. To use gear transmission, it is very difficult or even impossible to increase the number of intermediate gears to realize the transmission of the camshaft and the crankshaft at a large distance. It also does not meet the requirements of the car for light weight and compact structure of the vehicle engine. To this end, the car uses a flexible drive instead of a rigid gear drive.
At first, most of the gasoline engines used in the car used a timing belt drive (see Figure 1) instead of the timing gear drive. The timing pulley is iron-based powder metallurgy high-temperature pressing, and the sprocket mounted on the exhaust camshaft of the crankshaft timing pulley is driven by the timing chain, and the gear is driven between the exhaust camshaft and the intake camshaft. The ratio is the ratio of the number of camshaft belt teeth to the number of crankshaft timing belt teeth, which is 2:1. In order to ensure that the flexible timing belt has a certain tension during transmission, a timing belt tensioning mechanism is often used to pre-tension the timing belt. The flexible timing belt drive is feasible for the transmission design of the car engine with a double camshaft overhead cylinder head structure, and the design space becomes more free.
However, there are also shortcomings in the timing belt drive. Since the timing belt drive is a dry drive without lubricating oil, it is easy to generate heat, and the material of the timing belt is generally formed by hot pressing with cyanide nitrile rubber and nylon fiber. At high temperatures, the rubber rapidly decreases in elasticity, the texture becomes hard and aging, and the timing wear of the teeth is accelerated, and the timing belt breaks individually, causing a major engine shutdown. At the same time, due to the sensitivity of the rubber to the lubricating oil, when the seal between the cylinder head cover and the cylinder head fails, when the lubricating oil leaks, the elasticity of the timing belt will rapidly drop once it contacts the lubricating oil, and it will quickly become hard and brittle, and the timing belt The tensile strength drops, causing the timing belt drive to fail quickly.
Flexible timing chain drive
Nowadays, the high-end car engine is developing in the direction of high power and low noise. For the problems of high temperature difference, dry transmission and no lubrication of timing belt drive, the designer adopts flexible timing chain transmission. (See Figure 2). The crankshaft timing sprocket drives the sprocket on the exhaust camshaft shaft through the timing chain, and the chain drive is also adopted between the exhaust camshaft and the intake camshaft. In addition to the advantages of timing belt drive, timing chain drive also overcomes the shortcomings of timing belt drive. To this end, a front chain chamber and a front chain cover are provided at the front end of the engine block to form a sealed space, which not only satisfies the lubrication requirements of the timing chain transmission, but also improves the life of the timing chain and achieves lubrication. The cooling cycle of the oil makes the engine cool well, and the front chain cover also prevents the intrusion of dust from the outside. Compared to the timing belt drive, the timing chain drive has a longer life and lower noise. Timing chain drives are becoming more and more widely used in high-class car engines.
Conclusion
China's automobiles are developing in the direction of hybrid, pure electric and clean energy. However, it is now expected that the timing chain drive will have a longer life cycle. As for the current car engine, due to the advantages of timing chain, especially in the in-line engine and high-power V-shaped engine, the timing chain drive is becoming a trend, and the timing chain drive has become a trend. It has become the first choice for timing drives on today's car engines.