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What gear should the car use when upshifting or downshifting?

2020-01-06 00:52:40

Many novice drivers have a particularly incomprehensible problem, which is why they downshift when overtaking. Isn't the higher gear the faster the car? This lowered the gear by one, and the speed didn't become slower. How could it overtake? The explanation of many old drivers is: the gears are lower and the power is stronger, so the overtaking is faster. But many people still don't understand, why is the gear lowered and the power increased? Today we will analyze it in detail: Why do we downshift when a car is overtaking?

First let's take a look at what the car gearbox does. The gearboxes in automobiles can be roughly divided into two categories: manual gearboxes and automatic gearboxes. Automatic gearboxes can be divided into hydromechanical gearboxes (AT), continuously variable gearboxes (CVT), and dual clutch gearboxes DCT) and Automatic Mechanical Transmission (AMT). No matter what kind of gearbox, their most basic function is two: variable speed, variable torque.

Who changes speed? Whose twist? Of course, it changes the engine speed and torque. In fact, the speed and torque emitted by the engine during operation cannot directly drive the car, because its working range is extremely limited. The general working range of the engine is between 800 and 6000 rpm, the maximum torque is several hundred Nm, and the maximum power is one hundred and dozens of kilowatts. And the engine has a most economical working range, generally between 1500 ~ 2500 revolutions per minute. Only when the engine is in this speed range can the car have both better power performance and lower fuel consumption. Working in this area is extremely uneconomical.

However, the resistance that a car encounters while driving is changing at any time. The speed can be from zero to several hundred kilometers per hour, and the range of change is very large. The engine itself cannot fully adapt to this change. For example, the drag torque of a car when it starts is generally more than 1000Nm, and the maximum torque of the engine is only a few hundred Nm. It is obviously impossible to drive the car to start. At this time, the wheel speed is close to Zero, the engine speed is up to thousands of revolutions; while the car is driving normally, the driving resistance may be only a few hundred Nm, and the excessive power output of the engine is wasted. At this time, the wheel speed is several hundred revolutions, but the engine speed is still It is several thousand revolutions.

In order to adapt to changes in the driving conditions of the car, the car must use the gearbox to redistribute the power output by the engine. After the output power of the engine is transmitted to the gearbox, the gearbox is put into the appropriate gear according to the actual needs of the car, and the power of the engine is enlarged or reduced, the speed is reduced or increased, and then the wheels are driven through the drive axle; The resistance on the wheels is fed back to the engine through the gearbox, allowing the engine to adjust its operating conditions in a timely manner. For a manual transmission, the driver needs to determine the driving conditions and change the gear position manually, so the power and economy of the car depends more on the driver's technical level; for the automatic transmission, The gears are automatically switched, so the gear shift logic and how well it matches the engine is very important.

An excellent automatic gearbox can make the engine always work in the best working range through the shift of its own gear, to achieve the most powerful and lowest fuel consumption. This is like a family. The husband is like an engine, earning money outside, and continuously funding the family life; while the wife is like a gearbox, reallocating the money earned by the husband for various expenses in the family. If the wife is smart, capable and well-thought-out, she will be well-organized in the allocation of funds. It will neither waste nor put a lot of pressure on her husband, and her family life will be smooth and happy. But what if she encounters a "loser"? It must be a mess in life, a serious waste of money, and a great pressure on her husband. So some people say that there is a great woman behind a successful man. For cars, if the engine is the heart of the car, then the gearbox is the brain of the car. Obviously, the role of the brain is significantly higher than the heart. Everyone should pay attention to this when buying a car. An excellent gearbox is far more important than engine performance.

So how does the gearbox distribute engine power? This involves the concept of a transmission ratio. The so-called transmission ratio is the ratio of the speed of the transmission's active gear to the speed of the driven gear. For automotive gearboxes, the driving gear is the input gear of the gearbox, and its speed can be approximated as the speed of the engine; the passive gear is the output gear of the gearbox. Since most passenger cars are For front-wheel drive, the transmission and the main reducer are generally made into one body, and its speed can be roughly regarded as the speed of the wheels. However, when calculating the transmission ratio, the transmission ratio of the main reducer needs to be added. Generally, the gearbox transmission ratio and the main reducer transmission ratio are collectively referred to as the total transmission ratio of the transmission system. When calculating the torque and speed of the car, the total transmission ratio is used. The picture below is the famous Volkswagen MQ200 manual gearbox parameter table. The gearbox parameters of other models are similar.

Below we will use this parameter to calculate the wheel speed and torque when the engine speed is 2000 rpm. Assume that this engine can produce 100N.m of torque at 2000 rpm.

First gear: 125 rpm, 1596N.m

Second gear: 225 rpm, 887N.m

Third gear: 329 rpm, 606N.m

Fourth gear: 437 rpm, 457N.m

5th gear: 555 rpm, 360N.m

Reverse gear: 148 rpm, 1348N.m

From the above calculation data, it can be seen that when the engine speed is the same, the higher the gear position of the transmission, the higher the wheel speed, but the smaller the torque on the wheel. In fact, a graph can be made based on this data, which is the gear drive force map of the car. It can be clearly seen from the figure that the lower the transmission gear position, the greater the driving force, and the smaller the speed range covered; the higher the gear position, the smaller the driving force, and the greater the speed range covered; when the vehicle speed is the same, The lower the gearbox position, the greater the driving force available on the wheels. For example, the maximum driving force that can be obtained in the first gear is 5000 N, and the speed range covered is 5-30 km / h; the maximum driving force that can be obtained in the fifth gear is 1000 N, and the speed range covered is 40-210 km / h. When the engine outputs the maximum power and reaches a balance point with the driving resistance, the car reaches its maximum speed.

It should be noted that the top speed of some models is not obtained at the highest gear, but at the second gear. This is evident in many nine- or ten-speed automatic transmissions. This is mainly due to the high resistance of the car when driving at high speeds. The driving force is not enough to overcome all the resistance at the highest gear, and the maximum speed cannot be obtained. Force can run faster instead.

Seeing this, I believe many people have understood that the lower the gear position of a car, the greater the driving force, so lowering a gear position when passing is in order to obtain a greater driving force. So how much worse is this driving force? How much can the overtaking acceleration time be worse? Let's do a rough calculation. Assume that a certain model is equipped with an MQ200 manual gearbox and the maximum engine torque is 160N.m. The car runs at a constant speed of 2,000 rpm, the gearbox is in fifth gear, and a speed of 80 km / h. How long does it take to surpass the preceding car within a distance of 100 meters.

According to the previous calculations, the torque on the wheel is 360N.m. If the wheel radius is 0.3m, then the driving force of the car is 1200N. Since the car is driving at a constant speed at this time, it can be roughly regarded as the driving resistance of the car. If the driver fully depresses the accelerator at this time, the engine instantly emits a maximum torque of 160N.m (in fact, there is a process), and the driving force obtained by the car at this time is 1920N. Assuming that the car weighs 1,000 kilograms, the acceleration obtained by the car is 0.72m / s²; if the overtaking distance is 100m, the car's acceleration overtaking time is about 4.3 seconds.

What would happen if we downshifted by one gear and used four gears to accelerate overtaking? After the driver depresses the accelerator to the bottom, the engine instantly emits a maximum torque of 160N.m. At this time, the driving force obtained by the car is 2437 N, the acceleration obtained is 1.24m / s², and the acceleration time of the car is about 3.8 seconds.

You see, when the car is downshifted by one gear, it will shorten the time by about 0.5 seconds. Maybe some people think that 0.5 seconds is not much, but when the car is driving at high speed, it can pass a distance of tens of meters in one second, and about a dozen meters in half a second. This is very impressive, a distance of more than ten meters is enough to get you out of danger and avoid collisions.

Some people still have a doubt: the car's gear is lowered, does the speed drop? Why overtake? In fact, the speed of a car is not entirely determined by the gearbox gear. From the gear drive diagram above, it can be seen that the speeds of many gears overlap. For example, the car runs at an engine speed of 2000 rpm, fifth gear, and 80 km / h. When we reduce the gear to fourth gear, the engine speed increases to 2500 rpm, and the car still maintains 80. Driving at a speed of km / h, but the driving force of the car is greatly enhanced. If you accelerate at this time, the car will get higher acceleration and reach a higher speed in a faster time. For example, it takes 4.3 seconds to increase from 80 km / h to 100 km / h in fifth gear, and it takes only 3.8 seconds to increase from 80 km / h to 100 km / h in fourth gear.

From the above analysis, it can be seen that when the car accelerates to overtake, lowering a gear can get greater driving force and higher acceleration. It can surpass the preceding car in a shorter time and the shortest distance, allowing itself to quickly Out of danger, reducing driving safety hazards. On the contrary, if you continue to use high-level overtaking, it will cause too slow acceleration, longer overtaking time, and longer overtaking distance due to insufficient driving force. If the two cars are in parallel for too long, it will cause serious hidden dangers. Therefore, reducing one gear when overtaking is the correct and wise operation method.

For manual gear models, the timing of downshifting when overtaking is entirely dependent on the driver's level of operation. For automatic gear models, as long as the driver suddenly depresses the accelerator to the end, the transmission will automatically reduce one or two gears. At the same time, the engine control unit senses the driver's strong power demand and will immediately increase the fuel injection. The large engine power output makes the power transition smoother. Many automatic transmissions are smooth when upshifting, but they are frustrated when they are downshifting, because the downshift timing and engine fuel injection timing and fuel injection increment are not well designed. The "engine and gearbox matching" that many old drivers drew from their mouths also largely refers to the degree of adaptation of the gearbox's upshift process to the engine power change process.

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