Detailed explanation of Toyota E-CVT technology: 30 years of one-step hybrid industry, planetary gear set is the key?
In the mixed circle, there is a saying that there are only two kinds of hybrids in the world, one is Toyota hybrid and the other is others. This is enough to prove the recognition of Toyota THS hybrid system in the industry. Today, we will talk about the planetary gear mechanism, the core component of this world-renowned hybrid system.
Toyota THS hybrid system is mainly composed of Atkinson cycle engine, permanent magnet motor, planetary gear mechanism, generator, high-performance nickel-hydrogen battery, power control unit and power control unit.
The core of the whole system is this planetary gear mechanism, that is, the E-CVT transmission in the media population. However, although it is called a transmission, it does not have such complex mechanisms as clutches, torque converters or gear shafts in the traditional transmission system, and it is a power distribution mechanism specially prepared for hybrid vehicles (the official name of Toyota).
So what are the advantages and disadvantages of Toyota’s system, and what are the differences between them compared with other hybrid systems on the market? This article will be reflected one by one.
1. Why does Toyota develop the E-CVT transmission?
E-CVT transmission is the key to the whole hybrid system, and the original intention of all hybrid systems can actually be attributed to energy saving and emission reduction, as was Toyota’s launch of THS that year.
In the early 1990s, the words such as "sustainable development", "global warming" and "greenhouse effect" began to be known around the world. Under this background, Toyota began to study "automobile development model in the 21st century" and launched a project called "G21" (G stands for Globe 21 and stands for 21st century). Toyota Hybrid Power System (THS) also began to sprout at this stage.
In 1995, Toyota released the Prius hybrid concept car at the Tokyo Motor Show. Two years later, in 1997, Toyota’s first generation Prius hybrid car went on sale in Japan, which was Toyota’s first mass production model based on THS hybrid system and the first model born under the Toyota MC platform.
Here, Toyota’s hybrid world door is officially opened.
Second, at present, the THS hybrid system has developed to which generation, and which models are it mounted on?
As mentioned above, the core of THS system is actually the E-CVT transmission, that is, the planetary gear set, and the iterative upgrade of Toyota THS system is almost around the planetary gear set.
The first generation THS hybrid system was born in 1997, and it was first mounted on Prius. Its planetary gear set consists of three parts: sun gear, planet carrier and outer ring gear, which correspond to three power sources in turn: the engine is connected with the planet carrier; Sun gear is connected with the MG1 motor; The gear ring is connected with the MG2 motor.
Through precise and complex control, this planetary gear set can shunt the engine and use the motor to adjust the speed, so that the engine has been in an efficient working range, so as to achieve the purpose of saving fuel.
In 2003, Toyota introduced the second generation THS hybrid system, but the upgrade of the second generation THS hybrid system was mainly in the aspect of electronic control, and the structure of E-CVT transmission did not change much.
In 2009, Toyota introduced the third generation THS (officially called the second generation, but the industry prefers to call it the third generation). Compared with the second generation THS, it added another planetary row on the basis of the original planetary row. The second planetary row is connected with the gear ring of the first planetary row, the MG2 motor is connected with the sun gear, and the planet carrier is fixed, as shown below:
In this way, the second planetary row becomes the deceleration mechanism of MG2 driving motor, and the output torque is increased by reducing the rotation speed, so the third generation THS of Toyota has better power performance than the previous two generations. Not only that, because of the addition of a first-stage reducer, the cumbersome chain drive was replaced by a lighter and smaller gear drive, which greatly improved the transmission efficiency.
However, due to the limitation of planetary gear structure, the speed of MG2 driving motor is limited by the limit speed of MG1, so the speed of this system in pure electric mode cannot be very high (about 70km/h).
In order to better adapt to the development of the industry, Toyota introduced the fourth generation THS in 2015. In terms of structure, the most significant change in this upgrade is to change the planetary gear added by THS-III into a parallel shaft gear, and at the same time put the MG2 driving motor in the side position.
The obvious advantage is that the axial dimension of this system is reduced by 47mm, which is undoubtedly a technological innovation in the horizontal platform with an inch of land and gold. It can not only adapt to engines with larger displacement, but also make the power of MG2 driving motor higher.
In addition, Toyota also introduced a plug-in hybrid system based on the fourth generation THS in 2016. Its main change is to add a single clutch between the engine and the MG1 motor.
The function of the one-way clutch is that when the MG1 motor participates in the driving function (more often used for starting and generating electricity), it will not drive the engine to rotate together, and at the same time, it forms a dual-motor drive with the MG2 driving motor.
The structural diagram of this one-way clutch is as follows. The structure determines that it can only rotate in one direction.
From the year, Toyota E-CVT has gone through several iterations, as shown in the following figure:
(first generation THS)
(second generation THS)
(third generation THS)
(fourth generation THS)
In addition, there are hybrid systems applied to vertical platforms (the version code has not been found yet), such as Lexus LS and Lexus GS. Compared with the ordinary version of THS hybrid system, it connects a 4AT in series to simulate 10 gears. The result of this combination, on the one hand, is conducive to improving the speed, for example, the pure electric maximum speed of LS500h can reach 140km/h, on the other hand, it is more accelerated.
The four-wheel drive system applied to Erffa and Lexus LM is composed of an E-Four electric four-wheel drive with a separate motor in the rear axle, which is not much different from Toyota THS in structure.
At present, Toyota THS models that have been introduced into China include: Camry, Highlander, Saina, Willanda, Lei Ling, Visa, C-HR, Corolla, RAV4 Rongfang, Asia Dragon, Crown Lu Fang, Lingfang HARRIER, Yize IZOA, Erffa and Wilfa; Lexus brands include lexus ES, Lexus RX, Lexus NX, Lexus LS, Lexus UX, Lexus LM, Lexus CT, Lexus LC and Lexus GS(2016, 2014 and 2012 models).
(Toyota)
(Lexus department)
III. Detailed explanation of E-CVT (officially called THS-II version)
3.1, structural composition
The core of E-CVT is this planetary gear set, which consists of sun gear, planetary carrier (including planetary gears) and external gear ring, which are connected with MG1 motor, engine output shaft and main reducer in turn, as shown in the following figure:
We can find that as long as the state of one mechanism is determined, the rotation of the other two mechanisms can also be determined. For example, if the external gear ring is stationary and the sun gear rotates, it will inevitably drive the planet carrier to rotate, which is the starting process of the vehicle.
Moreover, it is worth noting that the diameters and teeth numbers of the external gear, the sun gear and the planetary gear are fixed, which means that the torque of the engine will always be distributed to the sun gear and the external gear in proportion, and the specific value is about 72% to the external gear and 28% to the sun gear. Once the engine is running, this basic fact will not change.
In addition, due to the structural limitation of the planetary gear set, the gear ratio between the outer ring gear and the sun gear is 2.6, that is, when the outer ring gear rotates once, sun gear has to rotate 2.6 times. As a result, the speed of the M2 motor is limited by the limit speed of the M1 motor of 6,500 revolutions, which is only 2,500 revolutions, corresponding to the speed of the vehicle of 43 miles per hour, that is, about 70 kilometers per hour.
3.2, working mode
Toyota’s THS-II system can essentially realize a variety of working modes, one is pure electricity, the other is hybrid (both B and C in the picture below can be regarded as hybrid), and the other is kinetic energy recovery mode, which we will explain in turn from easy to difficult.
First, the pure electric mode. In the pure electric low load state, the battery drives the MG2 motor, and the power is transmitted to the front axle differential by MG2, which finally drives the wheels. However, because the outer gear ring of the planetary gear is hard connected with the M2 motor, it will also follow the rotation. At this time, the engine connected with the planetary carrier has internal resistance, and the power will be transmitted to sun gear, that is, M1 generator with smaller internal resistance through the planetary gear, so that it rotates in the opposite direction, and the generator is in a state of idling without generating electricity.
In the pure electric load state, at this time, the one-way clutch of the engine output shaft intervenes, the planet carrier remains motionless, the MG1 generator starts to act as a motor, the power is transmitted to the outer ring gear through the sun gear, and the wheel is transmitted to the front axle differential through the parallel shaft, and finally the vehicle is driven forward together with MG2.
The kinetic energy recovery mode is better understood. When the vehicle is decelerating, the wheels will drive MG2 and MG1 in turn, making them act as a generator.
Hybrid mode:
Compared with the first two working modes, the hybrid mode can reflect the essence of the whole system of E-CVT. When the motor power can no longer meet the demand, or the battery power is not enough, the engine needs to start and intervene at this time.
However, the engine of this system has no traditional starting mechanism, so at this time, the MG1 motor needs to turn from reverse rotation to forward rotation, so as to drive the planet carrier to rotate, and this speed will reach the efficient range of engine operation, and then the engine will start with fuel injection.
As we mentioned earlier, once the engine rotates, the torque of the engine will be transferred to the outer ring gear and sun gear, and at this time, the outer ring gear will transfer the power to the front axle differential coupled with it to drive the wheels together with the MG2 driving motor; Sun gear will drive the MG1 motor to generate electricity, which will be provided to the MG2 drive motor.
When we step on the accelerator deeply, or when we are at full load, there are three power flows. Except that part of the torque of the above engine drives the wheels and part of the torque drives the MG1 generator to supply power to MG2, the battery of the vehicle itself will also supply power to the MG2 motor, so that the vehicle can output the maximum power.
Another mode is in-situ power generation, in which all the engine power drives the MG1 generator to charge the battery. Perhaps some readers have found it here. Didn’t you say that as long as the engines move together, the torque must be distributed to the outer ring gear and sun gear in proportion?
(In-situ charging)
There is actually no conflict here, because in the in-situ power generation mode, the battery will supply power to the MG2 motor, provide a reverse torque, and lock the external gear ring.
3.3, control strategy
The structure of planetary gear set is not complicated, but the control unit of the whole system is difficult. After the engine starts and the system enters the hybrid, how to distribute the power?
First of all, we know that the MG1 motor can be regulated, and according to the relationship between planetary gears, the torque of engine torque distributed to the outer ring gear can be calculated. When the driver steps on the accelerator pedal, the system will judge the power required by the engine according to the accelerator opening and the SOC value of the battery. When the vehicle speed is low, most of the power of the engine will be distributed to NG1 for power generation, part of the power will be used to drive the wheels, and the rest will be supplemented by M2 motor.
When the vehicle speed is high, the engine will use most of the power to drive the wheels, and only a small part will be used to generate electricity, and the MG2 motor will assist in driving.
If the vehicle happens to be in the highest thermal efficiency range of the engine, the MG1 motor will get stuck, and the MG2 motor will idle, at this time, the power is completely provided by the engine.
From the above, we can also draw another conclusion: the engine is responsible for the main power output, which is used to maintain SOC balance and respond to the total power request; The generator is used for speed regulation, and its purpose is always to control the engine speed at the target working point; The motor is mainly responsible for supplementing the torque to ensure that the driver’s torque request is guaranteed.
IV. Advantages and Disadvantages of E-CVT
The E-CVT transmission has two advantages. First, the smoothness of driving. The planetary gear set replaces the transmission, and there will be no sense of frustration in shifting gears. At the same time, the system is simple in structure and more reliable in theory.
Secondly, this system can keep the engine in an efficient running range through the power split of planetary gear sets and the speed regulation of the motor, and make the fuel economy higher. We should know that the measured fuel consumption of the third-generation Prius was 4.3L/100km when it went on the market in 2009. It reached this fuel consumption level 10 years ago, which can be said to be ahead of the whole era.
Although the advantages are obvious, this set of E-CVT also has some inherent defects, mainly because of the restrictions brought by the gear structure. For example, the gear ratio of the planetary gear set limits the speed range of the engine, and at the same time, it needs to drive the generator to generate electricity, which cannot release all the power. Moreover, the more transmission paths of the gear set, the greater the energy loss; In the high-efficiency operation range of the engine, the power cannot be completely transmitted to the wheels, and it needs to be converted twice by charging or electric drive.
5. What is the current reputation of E-CVT?
Generally speaking, due to the special type of gearbox, E-CVT will not produce friction when working, and will not wear without friction, with better durability and low failure rate.
From the market reputation, we can also see the reliability of this system, and there are almost no complaints about the failure of E-CVT transmission, which also reflects its leading quality.
VI. Comparison of E-CVT Technology
Due to the limitation of patents, there are not many E-CVT with planetary gear sets as the core. Besides Toyota’s, the most famous one is the universal Volt dual-mode hybrid system (in fact, GM’s patent in this respect is earlier than Toyota’s), which also takes planetary gear sets as the core.
Structurally, the MG2 of the universal Volt is connected with the sun gear, the output shaft is connected with the planetary carrier, and the power is output to the wheels, while the gear ring is connected with the shell or the motor MG1 according to the actual operation. Through the combination and separation of clutches C1 and C2 and brake B1, GM Volt can realize various operation modes, as shown in the following table.
Compared with Toyota’s E-CVT, although the Volt structure of GM is similar, the direction of power split is completely different, which is also the product of Toyota’s racking their brains under the blockade of GM’s patents.
In addition, it is worth mentioning that although the Volt is more similar to Toyota’s YHS in structure, consumers compare more with Honda’s i-MMD hybrid and Toyota’s THS hybrid, so we also make a comparative analysis of them.
First of all, in terms of structure, Honda’s i-MMD system consists of engine, generator, driving motor, power separation device (E-CVT), PCU and lithium battery.
One of the key differences lies in the E-CVT. Compared with Toyota’s complex planetary gears, Honda’s i-MMD completes the power split through very few gear sets and clutches, and we have to know that one more transmission path of power leads to one more loss.
Compared with Toyota’s THS, Honda’s i-MMD is characterized by high structural flexibility, which can realize pure electricity, series, parallel, high-speed direct drive (Toyota THS can’t realize direct drive) and other modes, and its efficiency is very high.
In addition, the hybrid power of Honda i-MMD is higher than that of Toyota THS. Take the third generation Honda i-MMD as an example, its total motor power reaches 135KW, while that of Toyota THS-IV is only 53KW.
As for why Honda i-MMD uses such a high-power motor, the main reason is that Honda i-MMD hybrid system can’t switch the speed ratio at will like Toyota THS, so this system is in series mode under most working conditions, and the motor drives the vehicle. In order to ensure the power of the vehicle, only a more powerful motor can be selected.
It also has some disadvantages: the battery uses lithium battery, the motor power is higher, and the corresponding cost will be higher. At the same time, because it is limited by the speed ratio of the internal reduction gear set, the working space of the Atkinson engine in the i-MMD system that directly participates in driving the wheels is very narrow. In addition, Toyota’s THS has been verified on a large scale in the market, and its reliability has been proved.
Seven, Toyota open E-CVT patent.
At the beginning of the article, it is said that there are only two kinds of hybrids in the world, one is Toyota and the other is others. The reason for this statement is that on the one hand, Toyota THS hybrid performance is excellent and reliable, on the other hand, it has applied for comprehensive patent protection, and it is even more difficult for latecomers to enter the market.
However, due to the timeliness of the patent protection period, in 2017, Toyota announced the opening of the first batch of hybrid patents, and it can provide technology to the outside world, and hopes to cooperate with other car companies in this regard, but there are few respondents.
By 2018, Toyota expressed the patent opening more clearly, claiming that it would provide the right to use for free, including about 23,740 patents of electrification technology such as motor, electronic control and system control.
In 2019, the opening of Toyota’s patent finally ushered in the harvest. First, Toyota "sold" THS technology to Toyota’s core component supplier Keliyuan in China at a price of "US$ 1" (Keliyuan’s major shareholder is Geely, so it can be understood as selling technology to Geely in disguise). Later, it reached a THS hybrid technology transfer agreement with GAC and FAW.
At present, the first Guangzhou Automobile Chuanqi GS8 twin-engine series equipped with Toyota hybrid in its own brand has been listed. The power is a 2.0T engine independently developed by Guangzhou Automobile+Toyota THS hybrid system, and the official fuel consumption per 100 kilometers is 5.3L/ 100 kilometers. In addition, Geely’s Zhiqing Hi·X oil-electric hybrid system is also inextricably linked with it.
Summary:
Generally speaking, as the core of Toyota THS hybrid system, E-CVT transmission perfectly realizes the function of power shunting through planetary gear sets, and makes the engine always in an efficient working range by using motor speed regulation. This is indeed a revolutionary innovation, which also makes Toyota’s hybrid technology almost dominate the world, and its cumulative sales volume is close to 20 million. At present, THS hybrid has developed to the fourth generation, and its structure has approached perfection. Moreover, in the context of pure electric vehicles becoming the general trend, whether Toyota will continue to upgrade THS or improve E-CVT may require a question mark.