I believe that many owners of CVT cars have experienced the embarrassment of low temperature protection and continue to maintain the first gear, and the speed cannot be improved. The period of removing low temperature protection is often closely related to the ambient temperature. It may be better in winter in the south, but it often takes more than 5 or 6 kilometers to remove the low temperature protection in winter in the north. What is the purpose of low temperature protection? What principle is used to achieve this?
In fact, as long as the transmission involves hydraulic components, there are certain requirements for temperature. Because low temperature will reduce the fluidity of hydraulic oil, ATF hydraulic oil used in automatic transmission is similar to engine oil. Low temperature will reduce fluidity, oil molecules will aggregate and viscosity will increase. Friends who have used hydraulic jacks in northern winter should understand that lifting weights can be lightly pressed at an ideal temperature, but in winter, they need to use more strength, or even can't be pressed at all.
The same is true of those hydraulic components in automatic transmission, such as locking clutch that can be easily pressed at ideal temperature, and conical disc that needs greater pressure at low temperature and can easily clamp steel strip or steel chain. If the ambient temperature is low enough, it may happen that the oil cylinder exerts the maximum pressure but still can't press the clutch or clamp the steel belt (chain). At this time, just like winter, even if you use all your strength, you can't press the jack.
What if the locking clutch plate is not pressed tightly, and the tapered Puli disc clamp is not only a steel belt, but also an excessive torque is transmitted from the engine end?
When the locking clutch plate hidden in the hydraulic torque converter is locked, the hydraulic drum forms a rigid connection and the transmission efficiency is improved. On the other hand, if the clutch plate is not locked, it depends on the pump impeller to stir oil to transmit torque (flexibility). When the fluidity of low-temperature hydraulic oil is poor, the locking clutch plate is not tightly pressed and slides (similar to the semi-linkage of manual gear).
What happens if the engine outputs a large torque at this time? When the clutch is not depressed, the output of high torque will cause the clutch plate to wear, just like slamming on the accelerator when driving the manual gear.
In the same way, what happens when the conical split disk is subjected to excessive engine torque without clamping the steel belt at low temperature? This will lead to sliding wear between the steel strip and the conical split disk. For CVT transmission, sliding between the steel belt and the disk once may lead to the scrapping of the transmission, so this sliding wear is fatal. There may be many friends who think it is normal for CVT transmission steel belt to slip? This kind of cognition is wrong. CVT is not allowed to slip, but it will be mistaken for slipping in many cases.
The low temperature protection of CVT is realized by limiting the upshift and not locking the torque converter lock-up clutch. If the locking clutch plate is not locked, the hydraulic drum is completely driven by hydraulic oil, and the transmission medium between the pump wheel of the hydraulic torque converter and the turbine is oil. Even if the engine speed is high and the torque is too large, it can be eliminated by the flexible sliding between the pump wheel and the turbine. The reason is simple, because there is oil between the pump impeller and the turbine of the hydraulic drum. For example, the pump impeller has 10 revolution, and the turbine can only turn 8 revolutions, and the remaining two revolutions are worn off by the sliding between the hydraulic oil and the turbine. Will sliding between oil and metal cause wear? number
Secondly, the transmission upshift is limited. The principle of deceleration and torsion increase of CVT transmission is similar to other types of transmission, but the realization method is different. The low gear transmission is to reduce the engine speed with a larger transmission ratio, thus amplifying the torque. CVT transmission uses small radius driving wheel to drive large radius driven wheel to realize deceleration and torque increase. The rotation radius of CVT driving wheel (driving wheel) decreases to the minimum, and the rotation radius of driven wheel increases to the maximum (at this time, the transmission ratio is the maximum, and it is simulated as 1 gear).
Maybe many friends don't understand, isn't the torque increasing effect of the transmission the greatest when the transmission ratio is the largest? It is true that the transmission maximizes the engine torque in the first gear, but the key is to minimize the engine output torque.
The transmission itself does not produce torque, and the torque that passes through the transmission and is amplified comes from the engine. Therefore, the significance of low temperature protection lies in how to limit the output torque of the engine, because the output torque of the engine is too large at low temperature, and the load borne by the gearbox is greater. When the transmission ratio is maximum, the torque output by the engine can overwhelm the minimum.
Why is the engine output torque minimum when the transmission is in 1 gear? For a simple example, at the same speed of 20 kilometers per hour, the torque output of the engine running in the second gear is greater than 1 gear. Because 1 gear amplifies the torque many times, while the second gear amplifies less. If you only look at the transmission ratio, the first gear can amplify the torque by 5 times, and the second gear can only amplify the torque by 2.5 times. You only need a torque of 100nm to join the car. How much torque does the engine output at 1 and 2? 1 gear (5x magnification) * Engine torque = 100nm, 2nd gear (2.5x magnification) * Engine torque = 100nm.
It also provides a torque of 100 mm, and the engine only needs to output 20nm in the first gear and 40nm in the second gear. The more the transmission magnification, the lower the engine output torque. Of course, if the torque is insufficient, the speed will make up for it (so the lower the gear, the higher the speed, the higher the speed but the lower the load, the smaller the impact). Conversely, the smaller the transmission amplification torque multiple, the higher the torque output from the engine end. It's as if the clutch plate wears more when starting in second gear than when starting in 1 gear.
The output torque of the engine is larger, and the engine speed can be reduced under the condition that the required power is unchanged. Although the rotational speed is low, the transmitted torque is large enough (heavy load) and the impact on the transmission will be greater.
Therefore, the low temperature protection limits the gear to 1, and upshifts are not allowed. Its significance is to let the engine use high speed to pull power and reduce torque. Because through the formula: power = torque * rotational speed /9550, we can know that the higher the rotational speed, the smaller the output torque under the same output power.
For a simple example, a car needs 20kw to run at a certain speed (even if the gears are different and the engine speed is different, the power required for running at a certain speed will not change). If the gear is 1 and the engine speed is 2000 rpm, if the gear is 2 and the engine speed is 1400 rpm, how much torque does the engine output between different gears? According to the formula: power = torque * speed /9550, 1 gear: 20kw= torque * 2000rpm /9550, and gear 2: 20kw= torque * 1400rpm /9550.
The calculation results show that the output of 1 gear engine is 95 Nm, and the output torque of the second gear engine is 136 Nm. Therefore, this is the reason why the gear of CVT transmission is limited to the lowest gear during the low temperature protection period. The fundamental significance is that only the output torque of the engine is firmly limited, and the maximum speed of CVT is limited within 25km when the transmission ratio is the maximum (simulated 1 gear state). The power required by the wheel end is only about 10kw (the power required by 100km/h is only 40kw at a constant speed). Therefore, even if the speed is stepped to the highest when the transmission ratio is maximum, it is limited by the output power.
Therefore, the continuous improvement of speed is only equivalent to the continuous reduction of torque (load), and the engine uses higher speed to make up for the low torque, which naturally has less impact on CVT. Higher rotating speed can accelerate the heating process of the circulating system, which is the principle and realization of CVT low temperature protection.
So does CVT transmission need to be warmed up? This logic can be seen as follows: CVT transmission needs preheating to reach the ideal state of hydraulic system, but many drivers can easily damage CVT without preheating or failing to do so. Faced with this problem, manufacturers have designed low-temperature protection for those drivers who do not warm up CVT in low temperature environment. Therefore, CVT equipped with low temperature protection does not need to be warmed up. Separating the lock-up clutch and limiting the upshift are all protective measures imposed on CVT transmission without warming up.