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The long-forgotten mechanical transmission is plotting to return in the electric era.

The fuel car has retired from the C position in the market, one of the three major pieces that generations of fans like to talk about, and the transmission has also been left in the cold. But beneath the exaggerated appearance of the Mercedes-Benz CLA electric concept car at the Munich IAA auto show, there is an extra mechanical two-speed gearbox.

Of course, this is not the first time an electric car has turned back: a few years ago, Porsche Taycan equipped its rear axle motor with a two-speed transmission as an important part of its electric drive performance, and later a similar configuration appeared in the R + version of Lutes Eletre.

However, unlike these fairies who clearly pursue extreme performance, the CLA concept car rehearses the next-generation Mercedes-Benz mid-and low-end product line, and the motor power of the 175kW also illustrates the model orientation. For such a mainstream level, the meaning of mechanical transmission is completely different.

The Taycan minority report that the CLA concept car, which appeared in Munich this year, is closer to the production model of Mercedes-Benz's new platform MMA, and the technical indicators are not groundless, but indicate the technical direction that the production car may apply.

The two-speed transmission is included in the MB.EDU drive module of the CLA concept car. Mercedes-Benz does not focus on the transmission structure, we only know that MB.EDU is a series of electric drive integrated modules, while the CLA concept car carries the 175kW version.

For the powertrain of the CLA concept car, Mercedes-Benz emphasizes "defining the 'one-liter' of the electric era (ultra-energy-efficient fuel vehicles less than 1L / 100km)", that is, ultra-low power consumption.

Equipped with a non-exaggerated 89.6kWh battery pack, the CLA concept car with only 12kWh ultra-low power consumption of 100 kilometers, to achieve the WLTC life of 750km. Although it is a concept car after all, it at least shows an overall attitude of "no battery power consumption".

Mercedes-Benz says the energy conversion efficiency from the battery to the wheel end has reached 93% with the help of a new generation of silicon carbide and other efficient technologies. The two-speed transmission added to the MB.EDU naturally serves the big goal of ultra-low power consumption.

The previous Taycan and Eletre are completely different, the main purpose of using multi-gear is to better take into account both acceleration and extreme speed, and finally improve the limit performance. Also in order to adapt to strong power, Porsche chose the planetary gear set 2AT structure that has the best tolerance for large torque (Lutes unknown).

There are not many examples of electric vehicles adding mechanical transmissions, and there are even fewer mass-production models located in the mainstream. Before the CLA concept car, Roewe only used a two-speed transmission on the Marvel X / R, but there was no sense of existence in the end.

Although there has been a lack of enthusiasm for mass-production applications, suppliers have long prepared all kinds of special transmissions for electric drives. In addition to the AT structure of ZF's planetary gear set selected by Porsche, there are also electric drive CVT (Bosch) with steel belt drive and smooth two-speed transmission (NSK) with friction drive.

Because of the identity of German high-speed and high-performance cars, "one more gear to increase extreme speed" is easier to understand, while CLA as a mainstream electric vehicle to pick up the transmission appears to be somewhat "reactionary".

Then again, most people today know that electric cars no longer need the complicated transmissions of the fuel era, but the reasons and mechanisms behind them are not clear to everyone.

One of the advantages of motors over internal combustion engines is that "large torque can be obtained efficiently in a much wider speed range". This makes single-speed ratio transmission a realistic and feasible option for motors, while internal combustion engines are difficult to emulate anyway.

As a kind of machine that converts linear reciprocating motion (piston) into rotating motion (crankshaft), internal combustion engine dissipates too much heat at low speed and increases friction loss at high speed (of course, there is more, which will not be detailed in this article). So the torque curve is often low at both ends and high in the middle, that is, large torque is output only in the middle of the speed range.

"ideal torque and efficiency can only be obtained in a narrow speed range in the middle", so that the internal combustion engine needs multiple speed ratio "relay" of the transmission, so that the engine can operate in the "intermediate speed range" in the whole practical speed range, without the cost of acceleration, NVH and extreme speed.

Motor is a completely different species: power on, that is, the force, the size of the current directly determines the size of the force. Therefore, the maximum torque can be output at zero speed, and with the help of frequency conversion technology, the motor can maintain the maximum torque in a considerable speed range, and then still maintain the maximum power.

Today, the motor can provide enough power output to drive a car in the legal and practical speed range at fully acceptable costs and specifications, so the single speed ratio has become the mainstream choice of electric vehicles.

On the other hand, adding the transmission will also increase the cost, volume, weight and complexity, not only the advantages and disadvantages, that is, the net income after deducting the potential loss must be taken into account. Then when the total benefits brought by increasing multiple speed ratios are not attractive enough, the single speed ratio wins overwhelmingly.

For motors, the single-speed ratio is not necessarily the best, but it is also uncertain that there is a "better". At present, the single-speed ratio is enough, and there is too little temptation and motivation to seek "better", not to mention that the multi-speed ratio is not necessarily better.

We all know the relative superiority of the motor, but it does not mean that the motor "walking alone" is unassailable and there is no room for improvement. For example, with the characteristics of low speed and high torque, many electric vehicles can exceed the acceleration of fuel vehicles with a single speed ratio, but the acceleration ability is relatively weak at extreme speed and high speed.

However, this is not usually a problem today, especially in our country. A mainstream electric vehicle uses a single speed ratio, which can easily crush the same class of fuel vehicles within the legal speed limit, while 150km/h and 160km/h whose extreme speed is lower than fuel vehicles usually do not have a substantial impact.

So in the past, only the Taycan, which takes into account the ultimate performance such as the track and the demand for unlimited speed in Germany, would adopt a two-speed transmission, an alternative move in electric vehicles. The CLA concept car may also have a second consideration.

The range of large torque and high efficiency of the motor is much wider than that of the internal combustion engine, but there is also a low efficiency zone with a small range at the edge of the speed and load. For models with proper power matching, the probability of using inefficient areas is very few, so multi-gears may not have an advantage over today's single-speed ratio, and may even outweigh the losses due to weight gain and other reasons.

However, for those models with relatively insufficient power, such as those with smaller motors and larger and heavier bodies due to cost constraints, the multi-shift transmission is still unable to increase the motor. It is possible (but not necessarily) a feasible option to improve energy consumption and power performance.

This situation has already occurred in hybrid cars, if the focus is extended from pure electricity to the whole range of electric drive, in today's DHT hybrid technology clustered by independent brands, part of the DHT gearbox with a mechanical gear can "serve" the motor (rather than the direct drive of the internal combustion engine) in pure electric mode.

So this kind of DHT hybrid often has the condition to choose the motor with less power (single speed than DHT), so as to reduce the cost, or reduce the volume of the DHT transmission, in order to achieve its own design purpose.

The Qirui Kunpeng 9 model 11 speed includes the pure electric mode in different gears. "insufficient power" can also be extended to: the single speed ratio is sufficient in the daily use environment, but the requirements of extreme operating conditions are relatively higher. It is possible that all three models come from European brands not only by accident, but also by designing low-cost (small motor) electric cars for mountain markets (which require high climbing requirements).

Or when power consumption suddenly needs to be haggled. For example, suppose that power consumption suddenly becomes a hard indicator like fuel consumption, such as dazzling technology models that pursue long range, such as those that suffer from natural losses, such as small cars that are difficult to improve their battery capacity, square styling with high wind resistance, and so on.

In the past, "mindless" can directly ignore multi-gears, because even if the "net income" (including loss of weight gain, reliability, etc.) is actually positive, this energy consumption advantage may be too small, affecting only a little bit of battery life. But if the value of this little power consumption advantage suddenly increases, multi-gears may be reconsidered until it is verified which is the real optimal solution.

Porsche thinks the Taycan's two-speed transmission has improved its battery life by 5 per cent, which is certainly fine for many electric cars, which can be easily recovered by overcharging, recharging and stacking more batteries. But for some models with limited size or cost, the reduced energy consumption "out of thin air" will be more precious.

The introduction of a multi-gear transmission into the MB.EDU of the CLA concept car seems to be just the beginning: no one ever imagined an electric car going back to a mechanical transmission, but now someone is finally willing to consider it. (Taycan and Eletre are far away from mainstream markets and can be regarded as special cases. )

At present, it is not certain that the multi-gear mechanical transmission will make a comeback on a large scale, so that pure electricity consumers will have to immerse themselves in learning ancient knowledge such as AT, MT, CVT, DCT and so on. Don't worry, it's probably not the case.

But the beginning may also be a sign that multi-gear transmissions will not die out in the world of all-electric cars-as they are now. In the end, some electric vehicles may choose to introduce multiple speed ratios to solve the problem out of specific needs and conditions.

This change is not unthinkable, for example, German car companies may be more motivated, which is understandable; second, if medium-and low-end small-volume electric cars-- which are difficult to have large batteries and large motors-- face greater battery life or cost pressure next. Increasing the speed ratio may be another way to "roll".

This article comes from the official account of Wechat: autocarweekly (ID:autocarweekly)

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