[Pacific Auto Network Technology Channel]What is it? Can realize the automobileSpeed change, torque change, direction change and on-off powerThe device is the transmission. The first car in the world was born in 1886, and the first transmission appeared soon after. It can be said that the development of transmission and automobile are synchronous. At first glance, E-, maybe everyone will naturally think of CVT, that is, CVT. Functionally speaking, E-CVT does have the function of stepless speed change, but structurally speaking, the difference between them is too great. It can even be said that there are two types of transmissions, one is E-CVT and the other is other transmissions.
● Introduction of E-CVT
At the beginning, we mentioned that E-CVT is not a simple "electronically controlled continuously variable transmission", so of course it is not circulated on the Internet. It is an electronic version of CVT. Toyota’s official definition is "power distributor", because it does not have complex mechanisms such as traditional gearbox system or gear shaft set, and it is a power distribution mechanism specially prepared for hybrid vehicles. Therefore, there has always been a debate: is E-CVT a transmission?
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Whether it is a transmission or not, we will discuss it later. What we can see is that the THS( Hybrid System) is becoming more and more mature from 200h to the later hybrid version. As a key component of the whole system, E-CVT undoubtedly plays a vital role.
● E-CVT structure
If you look at it from the perspective of traditional transmission, the structure of E-CVTIt’s really, really simple. Many times, the more perfect things are, the simpler they are. I think this must be the truth. As a masterpiece of hybrid cars, Prius has been carrying the core hybrid technology, so the next analysis takes Toyota Prius as an example.
The structure of E-CVT is very simple, only made up ofExternal gear ring(The outer gear ring here connects the No.2 motor and the output shaft at the same time) andplanetary carrier(connection) andSun gear(connected to No.1 motor), that is, the power transmission flow is:No.2 motor — External gear ring — output shaftAt the same time, due to the existence of planetary gear sets, there are also:Engine — Planetary group — External gear ring — output shaft. The following physical picture was taken from a foreign video.
If you look closely, you can’t help wondering: Isn’t this the structure? Yes, E-CVT is made up of a planetary gear set reduction mechanism (The red box in the picture above.）。 There is no chain or steel belt in CVT, let alone the torque converter in AT or the double clutch in DCT. So, the problem is coming. . . Ahem. Can an E-CVT with such a simple structure really achieve stepless speed change? The answer is of course!
● E-CVT principle
Overview:
In fact, E-CVT is not very complicated only from the perspective of mechanical motion principle. How complicated can the motion of a planetary gear set be? What is complicated is its electronic control unit, that is, the logical principle of electromechanical cooperation. Before introducing the working principle, briefly list some noun notes that will be used below:
PCU：Power Control Unit (Power control unit) This is an essential part of a hybrid car, which contains a voltage converter and an inverter, and can adjust the output voltage of the battery pack. For example, high voltage (about 600V) must be used to supply power to the motor, and the voltage of the battery pack is up to about 200V due to the size limitation, so the transformer is essential. butThe function of inverter is to change DC to AC or vice versa.Because the high-voltage AC motor has the advantages of small size, high efficiency and large size, and the battery pack emits direct current, an inverter is necessary between the motor and the battery pack.
1：That is, motor generator 1 is connected with the sun gear.
MG2：Namely motor generator 2, andExternal gear ringConnect.
Attribute diagram of two motors:
The engine of Prius is Atkinson engine, which is characterized by good intermediate speed performance, insufficient high and low speed performance and high energy utilization rate;
To facilitate understanding, put a simplified E-CVT structure diagram:
Next is the power distribution diagram.
First of all, we need to be clear about a concept:Power is the product of combination with rotational speed.(power = torque × rotational speed),
Then because there are four that canFree revolutionPlanetary gear, it can be imagined that when the gear ring is fixed (that is, the wheel is fixed), the sun gear (MG1) can only drive the planetary gear seat (that is, the engine) to rotate (that is, start the engine process) when the planetary gear rotates;
With the rotation of planetary gears, when the ring gear (MG2) rotates forward, the sun gear (MG1) can also rotate backward. Conversely, when the ring gear (MG2) rotates reversely, the sun gear (MG1) can rotate forward again.
For example, when the outer ring gear and the sun gear rotate in the same direction, the planetary gears can not rotate, but only revolve, thus driving the gasoline engine to rotate. When the planetary gear seat does not rotate, the ring gear and the sun gear can still rotate freely. It is precisely because of this ingenious characteristic of the planetary gear set,Only when the engine, wheels and motor are connected together can they run without interfering with each other.Therefore, the clutch structure is omitted.
S=C×3.6－R×2.6
In the formula, s = sun = sun gear, c = carrier = line constellation, and r = ring = outer gear ring.
In addition, since the diameters and teeth numbers of the external gear, the sun gear and the planetary gear have been fixed, it can also be obtained according to the formula on the drawing:When the planetary constellation rotates in the middle, the torque ratio allocated to the outer ring gear and the sun gear is certain.. The specific value is about72%Allocated to the external gear (actually 2.6÷3.6),28%Allocated to the sun gear (1-72%). As long as the engine rotates to work, it will not change this basic fact.
Then we all know that under a series of different working conditions, the requirements for torque are greatly changed. Traditional power vehicles need a transmission to adjust the output power of the engine. However, in this specific structure of E-CVT, because there are two motors, under different working conditions,According to the above formula. As long as the different rotational speeds of these two motors are controlled, the engine power obtained on the outer gear ring can be changed infinitely, so that the automobile can achieve stepless speed change.
As for how the PCU of Prius can control the fuel supply of the engine and the voltage, phase and positive and negative on-off of the two motors in real time according to the running conditions of the car. The control logic involved and the data collection and processing procedures of many sensors are too complicated to be described here. Understand that E-CVT can realize stepless speed change.
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● Detailed explanation of working conditions
The following analyzes the speed change process of E- one by one from various operating conditions:
I. Idle running (hot car)
After the start command is issued, it starts (rotates forward) in an instant and drives the start. The whole process is extremely fast and smooth.
After the engine is started, it runs at idle speed, and the gasoline engine drives the planetary constellation to rotate forward. Because the wheel (outer ring gear) does not rotate, the forward rotation of the planetary gear set (engine) will drive the sun gear (MG1) to rotate forward through the planetary gears. MG1 no longer receives power from the battery pack, but becomes a generator.Emit alternating current, through the inverter and voltage converter in PCU, it becomes low-voltage DC and charges the battery pack. In short, at idle speed,The whole engine is used to charge the battery pack..
Second, start
After the start signal is sent, a small amount of power will be connected to the No.2 motor, and MG2 will start to rotate. Drive the wheel (outer gear ring) to start forward rotation, and the car moves slowly. When you press the accelerator pedal slightly hard, the No.2 motor will get more power and the Prius will accelerate. Because the power of No.2 motor is very high (50KW), the low speed is also very high. Under the control of PCU, the acceleration of the car is very soft.Even by MG2 aloneYou can accelerate the Prius to a considerable speed! This starting process gives full play to MG2.Low speed and high torsionTo make up for the embarrassing situation of insufficient low-speed torque of the engine.
It is worth noting that if the engine is not working at this time (starting in pure electric mode), with the increase of the rotational speed of MG2, the rotational speed of MG1 will also increase rapidly. Because the speed of MG1 has an upper limit, when it reaches the upper limit, the engine is forced to start to intervene. There is a critical speed at this time. Although this speed is fixed, it is certain that the greater the intensity of stepping on the accelerator at the start, the earlier the gasoline engine will intervene (actively intervene). If you step on the accelerator all at once, the gasoline will ignite immediately.
In most cases, MG2 can smoothly accelerate the Prius to a good speed, but in some cases, there will be a phenomenon of insufficient power. This is what the engine will pass after intervention.Driving MG1 to generate electricity for MG2.andDirectly drive the external gear to rotate to push the wheel.Two ways to output power, it is worth mentioning that these two ways can exist at the same time. As for the proportion of each other, it depends on many factors.
Third, small load acceleration
When accelerating under light load, the wheels are mainly driven by MG2. With the speed of MG2 increasing, the speed of reverse rotation of the four planetary gears gradually decreases. When the speed of the gear ring is the same as that of the planetary constellation (engine), the rotation of the planetary gears stops, leaving only revolution. At this time, the planetary constellation (engine) pushes the gear ring (wheel) and sun gear (No.1 motor) through the four planetary gears that are no longer rotating, and the three speeds are consistent. MG1 continues to supply power to MG2 and charges the power battery through PCU. Thereafter, the speed of MG2 continues to increase until the vehicle reaches the target speed. At this time, the sun gear rotation speed is less than the rotation speed of the constellation.
Fourth, heavy load acceleration
In the face of heavy load acceleration (such as load starting), the power of MG2 can’t keep up.Engine speed increaseEntering its economic operation range, the power of the engine is greatly improved. Because the torque increase of the engine further drives MG1 to generate electricity and the power obtained by the outer ring gear increases, at the same time, the battery pack will also supply power to MG2. Thanks to this, MG2 enters the "full firepower" mode.
Fourth, driving at a constant speed
Driving at a constant speed occurs after the acceleration is completed. At this time, the Prius only needs to overcome all kinds of resistance, and the demand for power and torque is greatly reduced. After the throttle is relaxed, the speed of the gasoline engine decreases, and the speed of the gear ring (wheel) is higher than that of the planetary carrier (gasoline engine). At this time, the four planetary gears began to rotate forward. This forward rotation will drive the sun gear (MG1) to reverse.
There is something special here, as we learned from the previous article. When the power from the engine is transmitted to the outer ring gear and the sun gear through the planet carrier, the rotational speeds of the outer ring gear and the sun gear will satisfy the following formula.
S=C×3.6－R×2.6
When the rotating speed of the outer gear ring is higher than that of the planetary carrier, that is, when R>C, it is obvious that S < 0, that is, it is reversed. However, if the reverse rotation of sun gear at this time is idling (without any resistance), it will rotate so fast that all the power from the engine will be absorbed by MG1 and the external gear ring cannot be driven (because it is obviously much easier to drive sun gear than to drive the external gear ring). Therefore, MG1 will need to work in the motor mode here, so that the planet carrier can’t drive the sun gear to fly at high speed and waste the power of the engine.
But wouldn’t that be a waste of electricity? Therefore, in general, when cruising at high speed, MG2 becomes a generator to maintain the rotation of MG1.
In fact, during the whole driving process of Prius, the state of acceleration and uniform driving is constantly switching. The phase of voltage and current will be adjusted by PCU according to various operation actions such as stepping on the accelerator and releasing the accelerator, and various driving conditions of the car, and the output power and rotating direction of the motor will be changed instantly, and the functions of the two motors will be switched.
Fifth, slow down
In fact, the deceleration is relatively simple, with the engine turned off and the MG1 idling. MG2 is driven by the wheel and becomes a generator to absorb the deceleration energy of the wheel. And charge the battery pack.
Sixth, reverse the car
Thanks to the large torque of MG2, the torque MG2 required by the Prius when reversing can be easily provided. The battery pack supplies power to MG2, which drives the external gear ring to reverse, and the vehicle finishes reversing.
In the whole driving process of Prius, we can see that it is actually a process in which two motors and engines cooperate with each other to perfectly perform linear output under strict electromechanical conversion logic control. The basis of this perfect cooperation is the special structure of E-CVT. Because power = torque × rotational speed, the planetary gear set structure with proportionally distributed torque and infinitely distributed rotational speed enables the power of the engine to be infinitely distributed to the outer gear ring (i.e. wheels) at any time and place, thus completing all functions of the ordinary stepping gearbox!
The following table shows the running state of each gear of E-CVT during the whole process:
You can also refer to the following video to improve your understanding of the operating relationship between planetary gears, sun gears and external gear rings.
● Prospect of E-CVT
The appearance of gearbox was born in order to meet the different power requirements under various complex working conditions. Before it appeared, drivers needed to judge the road conditions and car conditions themselves to shift up and down in time. The advantage of this method is a strong sense of driving operation. After all, experienced drivers know what kind of power the car needs better than ECU. In addition, the advantages of simple structure, easy maintenance and small size also make the car with manual transmission have a large number of fans no matter where it is suitable.
The appearance of automatic gearbox is actually very early, but the real large-scale application was in the 1980 s, and it was also a free ride with the rapid development of electronic technology. Because automatic transmission is not difficult, it is difficult to change the speed more accurately. In the mechanical age, technology is obviously difficult to achieve, but nowadays, more and more intelligent ECU makes automatic transmission more and more able to understand your driving intention. But even so, the automatic gearbox still has defects:There is shift clearance.Even DCT can’t realize the real "stepless speed change".
As for the protagonist E-CVT in this paper, it can be said that it has reached the peak of the integration of electromechanical technology in the gearbox field. First of all, its logic control system is very complicated. In addition, it can effectively make up for the short board (low transmission power) of the traditional stepless gearbox, which integratesHigh power, quick response, stepless speed change and extremely high transmission efficiency.And other advantages in one. But it also has disadvantages:The support system is too complicated.We know that once any system is too complicated, the probability of problems will increase exponentially.
● summary
Let’s make a simple summary of E-CVT:
1. Very smooth, extremely smooth
2, the transmission efficiency is high
3, simple structure and small volume
Let’s go back to the question at the beginning of the article about whether E-CVT is right or not. Let’s put it this way: If other power sources were not needed to cooperate with it, I’m afraid E-CVT technology would have unified the gearbox field long ago. It is precisely because of this "fatal" defect that it is simply tailored for hybrid models. Writing this analysis is not entirely for the sake of popular science, but I hope that everyone can learn more about this somewhat heterogeneous and unknown transmission structure and its thinking. Because I firmly believe that in the future when fossil energy is exhausted, E-CVT will finally shine!
