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Is it true that industrial electric vehicles do not use gearboxes?Speed reducer to understand
- Aug 29, 2018 -

With the increasing popularity of electric vehicles, there is less talk about the gearbox.

In our past impression, the engine and gearbox are the two most important parts of a car.

For electric cars, the transmission is dispensable because the motor itself has a high output range of speeds and torque.

But is it true that electric cars don't use the gearbox?

Of course not, the reducer replaced the gearbox and became an integral part of the electric vehicle.

Today we will talk about the reduction gear.

Electric vehicles can do direct drive

Is an electric car driven directly by a motor or does it need to be driven by a reducer?

Just think if it is a direct drive can need not reducer, so requires, the characteristics of the motor with high torque at low speed and motor torque in certain cases, is proportional to the speed and power, the same power of driving motor, the lower the speed the greater the volume and weight, so the problem is the motor size and weight is large, the output power curve does not necessarily meet the requirements of the vehicle.

Why do electric cars need reducers

The motor + reducer scheme can reduce the overall weight and manufacturing cost of the driving system. At present, both schemes exist simultaneously, but there are many schemes for the motor + reducer.

For example, the scheme of motor + reducer + rear axle is used to replace the scheme of motor + rear axle. The overall weight of the scheme with reducer is at least 100kg lighter than that of the scheme without reducer, and the cost is reduced by more than 10,000 yuan.

If an electric vehicle does not have a reducer, the torque output to the wheel is too small, especially when high torque is required such as climbing.

In daily use, most of the time is on the road. If you choose a motor with high torque, the price will be very high, and the cost performance of the whole car will go down.

The common range of wheel speed is from 0rpm (start) to 1000rpm (speed around 100km/h), especially the usual speed of 50km/h in cities (corresponding to 500rpm of wheel speed). Now most civil motors have no high efficiency in this speed range. The choice is to use a higher-power motor, which usually means several times of cost increase.

The main function of gearbox is to slow down and increase torsion, which is a low-cost solution.

In addition, when the car is turning, the speed of the left and right wheels is different. At present, the car USES mature differential solution, and the differential can automatically adjust the speed of the wheel according to the change of driving resistance.

If the motor drives directly, not only complex algorithms are required to match the turning radius and wheel speed difference, but more difficult is to dynamically keep the actual speed of the wheel the same as the theory (two wheels, designed according to the theory).

In terms of cost and technology maturity, it is still the world of motor + gearbox.

So at present and in the future in the short term, it must be the high-speed motor plus reducer program to dominate the market.

Why is a two-speed reducer better

The original design of tesla's first model Roadster was 180kW motor +2 speed reducer. Later, due to the production quality of the reducer, the fixed gear ratio reducer was forced to be adopted. However, in order to achieve the same dynamic performance, the motor power was increased to 240kW to achieve the same performance.

To match the larger motor, the battery also needs to increase from 60kWh to 80kWh to provide greater output power.

With the tesla Model S P85, you can easily win at the start, but you can lose at the middle and back speed.

The problem lies in tesla's matching single-stage reducer: it keeps tesla in first gear, driving from start to top speed.

This is the equivalent of driving a fuel car, starting with a block and not changing gear until the speed is brought up to the red line, the engine cannot return to the optimum torque output range, and the acceleration is greatly reduced.

The single-stage reducer creates the torque output of the motor in one piece. The uninterrupted power output may be favorable for starting acceleration, but it is not conducive to the economy and comfort of the vehicle.

In particular, to pursue performance, the Model S of a high speed motor is adopted. It is equipped with a high power consumption of a high speed motor, and the large tooth ratio of a single-stage reducer makes the vehicle cruising state at a high speed critical point with low economy.

At present, most of the single-speed reduction schemes are adopted mainly because the characteristics of the motor are different from that of the internal combustion engine. The driving motor generally has the characteristics of low speed constant torque and high speed constant power, which can generate large torque at a very low speed.

However, when using single gear reducer, the dynamic properties of pure electric passenger cars completely depends on the drive motor, the demand is higher, the performance of the drive motor as drive motor can not only in the constant torque area provide higher driving torque, and can provide the higher speed in the constant power area, in order to meet with the requirements of high speed vehicle acceleration, climbing a hill.

When the speed of electric vehicles reaches the limit, there is no room for improvement, so the speed is restricted and the high-speed economy is not high.

At the same time, the use of single gear reducer for high power drive assembly system efficiency, this is because the single ratio often cannot at the same time balancing the performance and fuel economy of pure electric passenger cars, in most cases cannot drive motor drive in high efficiency working point, especially in the maximum or minimum speed and low load condition, the drive motor efficiency generally dropped below 60-70%, serious waste of the on-board power travel distance decreases.

Why do most electric cars in China still match single-stage reducers?

It's not that they don't understand the icing on the cake. It's that the transmission supply chain is weak, the integrated control of the vehicle factory is weak, and they don't want to use complex reducers.

Therefore, in the future, if electric vehicles can match a multi-speed reducer with a reasonable speed ratio range to optimize the timing of motor power explosion, the economy and continuous acceleration will be greatly improved.