The Easiest Way to Understand the Mystery of the Toyota Hybrid System (THS)

This article is written for those who have tried reading various materials to understand the Toyota Hybrid System (THS) but still find it confusing. By the end, we will move toward an understanding like the diagram below. Let's get started.

Introduction

Ichiken, who posts excellent explanatory videos focusing on power electronics (electronic circuits that control electric power), posted a video about the Toyota Hybrid System (THS) that I was involved with as an engineer. Although I have moved to the countryside and am living in retirement, it stirred my heart a little.

While commenting on this video and answering questions, I realized that even with Ichiken's explanation, it is difficult for the general public to understand. The cumulative global sales of Toyota's electric vehicles exceeded 20 million units by the end of February 2022. It is not desirable that THS remains incomprehensible to users even after such widespread adoption.

I studied mechanical engineering at university, but the laboratory I joined for my graduation research was a course studying "mechanical elements," which is the application of mechanisms using links and gears. There, I heard about and saw mechanisms like planetary gears and mysterious gears. In the early 2000s, during the latter half of my career at the company, when I was assigned to develop parts for hybrid vehicles, my heart fluttered when I saw the THS planetary gears. I remember immediately buying a Tamiya planetary gearbox set and making a similar model. However, no matter how much I spun the model around, I couldn't grasp an image of how the entire system, including the control, was working.

I was assigned to the narrow field of cooling the power control unit, but as a mechanical engineer, separate from my duties, I read books extensively to understand the workings of the entire system.

The THS, which debuted in 1997 as the first-generation Prius, has been around for nearly 30 years without changing its core power split device. This is proof that this basic concept was excellent. Many explanations and analyses of hybrid vehicle technology can already be found in books, papers, and on the internet. One can somehow understand individual technologies that make up a hybrid vehicle, such as driving electrically without using the engine in low-speed, light-load situations where gasoline engines are weak, the high-expansion-ratio Atkinson cycle engine, the reduction of pumping (throttle) losses, and regenerative cooperative braking that coordinates with friction brakes while recovering braking energy. However, the THS powertrain is a system composed of integrated machinery and electricity, and its operation cannot be understood from just one side. In particular, regarding the mechanical explanation of THS, the power split device, and its operation, there are unfortunately almost no explanations that provide an understanding that overcomes the resignation of it being "complex, strange, and mysterious."

This is the same even when looking at papers published by Toyota or new car manuals for mechanics. Orthodox engineers would likely describe and understand it using mathematical formulas, but I began to think that this is perhaps because it has not been translated into a form that the general public can understand, or because it lacks an accurate metaphor. THS is not a magical transmission, but a mechanism that allows for a more precise understanding. Since I was a (rare) engineer who was terrible at mathematical formulas, I would like to write down the diagrammatic understanding I had reached in my own way.

Configuration of the THS Transmission

The Prius drive system looks like this. Inside the transmission, a power split device using planetary gears and two MGs (motor-generators) are housed, and it is connected to a power control unit (PCU) and a high-voltage battery (main battery).

A motor-generator means an electric motor and a generator combined. The fact that a motor and a generator are actually the same thing was "discovered" at the 1873 Vienna World's Fair. In short, the only difference is whether it is pushing or being pushed. Moving in the same direction as the force is "pushing," and moving in the opposite direction to the force is "being pushed." Because electric power is injected, it pushes (drive), and if it is pushed, electric power comes out (generation).

"Power running" and "regeneration" are terms imported from railway technology. In the context of an electric train, if the current from the overhead line is positive, it is power running; if it is negative, it is regeneration. Power running is the vehicle moving while producing power. Regeneration means recovering and utilizing energy that would have otherwise been discarded, or recycling it; it does not refer to power generation or braking itself. However, there is confusion in terminology even among engineers, and power running is often used to mean drive, and regeneration is often used to mean power generation.

What is noteworthy is that there is no mechanical clutch in the THS. Whether it is a manual transmission, a multi-stage automatic transmission, or a CVT, a clutch is always used, and various modes are "switched." However, basic THS systems like those for the Prius do not have such a clutch for switching. (There is something like a manual car's clutch between the engine and the transmission, but this is called an input damper, consisting of a spring that absorbs engine torque pulsation and a torque limiter. Its purpose is to prevent excessive torque from the engine from being transmitted and straining the electrical system. The torque limiter is a clutch permanently fastened with a spring, and it only slips when something abnormal occurs.) For this reason, there are no parts that wear out, and there is no need for a hydraulic pump or control valve to generate clutch pressure. This is why there are very few THS failures. There is also no starter motor or alternator, which are common with engines, and these roles are shared by the MGs.

How does this extremely simple mechanism handle all of the diverse driving conditions of an automobile? The key to understanding lies in the planetary gears.

What is a collinear diagram?

A "collinear diagram" is used to quantitatively understand the planetary gear mechanism. The following chart is for the first-generation Prius. However, few people can imagine the operation just by looking at this.

There are no diagrams on the internet that carefully explain why actual gears and collinear diagram graphs correspond. So, I will start the explanation from here. If you already understand collinear diagrams, please skip to the next heading.

In a planetary gear mechanism, the sun gear in the center and the ring gear, which is an internal gear on the outer circumference, mesh via a pinion (or planet gear), which is an intermediate gear, and the carrier holding the pinion can also rotate on the same axis.

Conventional automatic transmissions use a combination of multiple planetary gear mechanisms, but the THS power split device is made of only one set of planetary gears. The number of teeth in the THS power split device has remained consistent since its introduction in 1997, with 30 teeth on the sun gear and 78 teeth on the ring gear. The diagram is drawn accordingly. The pinion is merely an intermediary, so the number of teeth is not important for understanding here. It is shown as 24 for drawing convenience, but it is actually 23.

The reason there are multiple pinions is for torque distribution and dynamic balance (matching the center of gravity to the center of rotation to suppress vibration). Taking advantage of the fact that multiple pinions can handle large forces by sharing the torque, they are sometimes used in construction machinery. The first-generation Prius had four pinions, but the fourth-generation Prius has three. More is not necessarily better, as fewer meshing points also mean less friction.

To understand the operation, we only need to focus on one pinion, so it looks like this when simplified.

There is a white dot mark where the teeth are currently meshing.

When looking at the movement of a planetary gear mechanism, we focus not on the pinion itself, but on the carrier that holds it. It is common to fix one of the three rotation axes—the sun gear, carrier, or ring gear—in place. (This is because it turns a mechanism with two degrees of freedom into one with one degree of freedom. This will be discussed later.) First, let's try fixing the carrier.

You can see that the sun gear and ring gear are meshing via the pinion. At this time, the peripheral speed of the gears (the speed at which the teeth advance) is the same everywhere, so the rotation angle is larger for the sun gear, which has fewer teeth than the ring gear, and the ratio is the inverse ratio of the number of teeth. In this case, the rotation angle of the sun gear to the rotation angle of the ring gear is 78:30. And the carrier is stopped.

A nomograph is used to represent this proportional relationship of angles in a graph. If you take the gear ratio on the horizontal axis and the counterclockwise angle as positive on the vertical axis, S (sun gear), C (carrier), and R (ring gear) will always lie on a straight line. Since the carrier does not move, it is at a height of zero.

Next, let's try fixing the sun gear.

If you stand on the carrier, you can see that the appearance of the gear meshing is the same, and only the reference point has moved. If you represent this in a nomograph, it looks like this: The sun gear is stationary, so it is at a height of zero. The entire assembly has been shifted upward in parallel.

Next, let's try fixing the ring gear.

The direction of rotation as seen from the carrier is reversed, but the proportional relationship is the same. The ring gear is stationary, so it is at a height of zero.

Now, let's release the ring gear and allow it to rotate. Even if none of the axes are fixed, the proportional relationship remains the same, with only the carrier reference point moving.

If the sun gear rotates in reverse, the ring gear is accelerated.

A planetary gear mechanism where no axis is fixed allows the movement of the carrier and the movement of the sun gear to be specified separately in this way. This is clear when looking at the nomograph. If you determine two points in a plane, a single straight line connecting them is determined. Being able to choose two points in this way is called having "two degrees of freedom." If you fix any one axis, the degrees of freedom are reduced to one, the nomograph can only choose a slope, and the proportional relationship of the remaining two axes is fixed. One reason the power split device is difficult to understand is that it is a mechanism with two degrees of freedom where no axis is fixed.

In the explanation so far, the animation has been swaying back and forth, but in an actual machine, the gears continue to rotate. When considering gears that continue to rotate, if we use the timing when the marks are in a straight line as a reference, and view the timing when the marks are fully open as the state after a certain amount of time has passed, the nomograph represents angular velocity (i.e., rotation speed in rpm). Usually, when we say nomograph, it is a diagram of the relationship of angular velocities, not angles.

To expand your imagination, let's look at it from another perspective. Let's try moving the planetary gears onto an angle scale. Look at the auxiliary lines in the diagram below. Since they are meshing with the same pinion, if the sun gear advances by 3 teeth, the ring gear lags by 3 teeth. Even with the same 3 teeth, the angle is different for the sun gear and the ring gear, which is the inverse ratio of the number of teeth. As a result, what was originally a rectangle is deformed into a trapezoid, and the intersection of the diagonals becomes the center of the pinion. In other words, a nomograph is the appearance of a pinion corrected into a trapezoid.

If we use a lever as an analogy, the pinion is not the fulcrum, the effort point, or the load point; it is the lever itself. That is why we do not discuss the pinion in the nomograph.

Nomographs and Power Splitting

When a planetary gear mechanism is drawn as a single straight line like a balance scale using a nomograph, this balance has a metaphorical meaning beyond just its appearance, and the distribution of torque (rotational force) can be understood using the analogy of a balance scale.

Like weights hanging from the balance beam of a nomogram, torque is distributed in inverse proportion to the length of the arms. Since rotational speed was in inverse proportion to the number of teeth, the inverse of the inverse means torque is proportional to the number of teeth.

Here, let's represent the magnitude of torque with width.

Since the vertical axis of the nomogram is angular velocity, the 'area' obtained by multiplying by torque represents power.

The torque pushing the carrier (red arrow) is distributed by the gear ratio of 30:78 into 28% torque pushed by the sun gear (green arrow) and 72% torque pushed by the ring gear (blue arrow). The area of the rectangle (Ps, Pc, Pr), where the width is the torque magnitude and the height is the angular velocity, represents the power (work rate) of each shaft. While the torque distribution does not change, the power distribution changes depending on the relationship of the angular velocities.

Once you understand this, let's connect MG1 to the sun gear, the engine to the carrier, and MG2 and the output shaft to the ring gear.

This is the THS system diagram as I imagine it. Engine power is split by the planetary gears and distributed into a mechanical path from the ring gear to the output shaft, and an electrical path from MG1 through the PCU and MG2 to the output shaft; however, the power distribution is determined by the speed relationship. As is often said, it cannot be decided 'freely' (that is, arbitrarily or at will). The electric power generated by MG1 basically passes through the PCU (Power Control Unit) to drive MG2, and is combined with the output shaft via torque. This is not 'assist,' but the mainstream of a powertrain that uses the engine as a power source, and is the 'series' part of the series-parallel hybrid. Even when the energy monitor screen displays that you are 'driving only on gasoline energy,' the electrical system is always working. Assist comes from the battery, and it uses energy stored from regenerative braking or from generating excess power when the engine has surplus power (where throttling output would worsen efficiency), whenever needed later, whether for EV driving or acceleration assist. The 'electric energy' on the energy monitor screen refers to this part.

Various movements of THS

I have animated how the nomogram moves along with the flow of torque and power. I hope you can grasp the image of what the power split device is doing. The height of R (ring gear) represents the rotation speed of the output shaft, and thus the speed of the car. The height of C (carrier) represents the engine speed, or the tachometer. (The arrows representing the direction of force have been omitted intuitively.) Now, please take a look at the 'moving nomogram'.

Since the ring gear is connected to the heavy vehicle body via the output shaft, its rotation speed changes only gradually, whereas you can see that MG1 moves around busily for engine starting, stopping, and gear shifting.

When the engine is not running, MG2 controls the movement of the car, and EV driving is performed by the torque produced by MG2, whether moving forward or backward. MG1 rotates in reverse, but since it is idling, no power is required.

Engine starting is a collaborative effort where MG1 and MG2 carry the balance pole together. When MG1 starts the engine, MG2 generates additional torque to cancel out the reaction that appears on the ring gear. The engine cannot be used between 0 and 1000 rpm because it is unstable. Passing through this range quickly is the engine start/stop control. Those involved in the development at the time have spoken about how performing this smoothly was a difficult challenge from the development stage of the first-generation Prius. The key to overcoming this was decompression via VVT-i (variable valve timing). As generations passed from the second to the third, the control was improved, allowing for start/stop so smooth that it is completely unnoticeable, which speaks to the hard work of many engineers.

In conventional gasoline engine cars, I think you have the image that engine speed increases because the throttle valve is opened. Gasoline engines have a characteristic called pumping loss, where part-load efficiency significantly worsens when the throttle is restricted. To use the engine with optimal fuel efficiency, it is basically run at full throttle regardless of the accelerator pedal's command. (It is not that there is no throttle, but it is electronically controlled.) So, how do you restrict output? You shift to a point where the full-throttle torque of the engine matches the driving force of the accelerator command and suppress the engine speed. In other words, you use the engine at the lowest possible speed that provides the same output. (Fuel-efficient CVT cars also use this kind of control.) In THS, after the engine is started, MG1 controls the output. The full-throttle engine torque is applied to the carrier, which is distributed to the sun gear and ring gear in a ratio of approximately 3:7. MG1 receives about 30% of the engine torque, but if the power generation torque is made larger than this, the engine loses and the speed drops. Once it drops to the target engine speed, the power generation torque is returned to match the engine torque again to maintain the engine speed. While performing this operation, the torque distributed to the ring gear also fluctuates, but the torque of MG2 is increased or decreased to compensate so that it does not affect the driving force.

The transmission efficiency of the mechanical system is said to be 99%. In contrast, the electrical system is about 90% per mechanical-electrical conversion, and since it passes through this twice, the transmission efficiency is about 80%. Starting from an idling power generation state and accelerating while keeping the engine speed constant, it begins with a distribution where all power passes through the electrical system at zero vehicle speed. Gradually, the rotation speed of MG1 drops, the electrical system decreases, and eventually MG1 stops and all power goes through the mechanical system. This is the highest efficiency point called the 'mechanical point'. At this time, MG2 becomes easier, but MG1 is holding on firmly. This is tough for the electrical system, like doing a one-legged squat (because current concentration occurs in one phase), so in practice, MG1 is not stopped completely, and it is used slightly off the mechanical point. Once past the mechanical point, MG1 begins to rotate in reverse, and the power distribution to the electrical system increases again. Therefore, the lower the rotation speed of MG1 near the mechanical point, the better the transmission efficiency. In the first-generation Prius, you could hear the sound of MG1 dropping in pitch as it accelerated, and some people called this the 'ghost sound' and found it unpleasant. This was the sound of the electric CVT shifting, and the sound of transitioning to a region where losses gradually decrease and efficiency is better.

The state where MG1 rotates in reverse beyond the mechanical point is called power circulation. Due to the reverse rotation of MG1, the ring gear, or output shaft, is overdrived, but at this time, MG1 is in a driving state because the torque direction remains the same while the rotation direction is reversed. Its energy source is taken from MG2. If you simply drove MG1 in reverse, the mechanical torque (direct torque) coming out of the ring gear would be too large for an overdrive, and the power balance would not match. Therefore, by converting the torque absorbed from the output shaft by MG2 into electricity to drive MG1 in reverse, the rotation speed of the output shaft is increased. Then, it makes sense as a high-speed, low-torque transmission. Since the power output from MG2 is returned to the planetary gears from MG1, it is called 'power circulation,' but in terms of power balance, it is not strange at all. (This is explained in another article.) The phenomenon of power circulation is also occurring in the backlash eliminator (scissors gear), which is a type of gear mechanism.

Reversing is basically done by EV driving, but if the battery level is insufficient, the engine is started to generate power. Then, the ring gear produces direct torque that moves the car forward, so MG2 must do additional work to resist this and go backward. Where does this additional amount go? It increases the speed of MG1 via the planetary gears and increases the power generation amount. In other words, this is power circulation where the electrical path from MG1 to MG2 is the forward direction, and the mechanical path via the planetary gears is the reverse direction. More power than the driving power flows through the electrical system, which can be said to be a disadvantageous usage with large losses, but since the purpose is to recover battery charge, it cannot be helped.

Engine braking is a state similar to engine starting, where the engine with a restricted throttle is forcibly 'dragged along'. This is also made possible by the coordinated operation of MG1 and MG2. In the first-generation Prius, when put into B range, regenerative braking was strengthened, and when fully charged, it transitioned to engine braking, but strong regeneration was not as effective as engine braking. Especially at high speeds, the braking force equivalent to engine braking cannot be produced due to the limitations of the battery's acceptance capacity. From the second generation onwards, engine braking was actively used in B range.

Controlling all these behaviors is a computer called the Hybrid Vehicle ECU. Based on the accelerator pedal command, it determines the driving force and required power, the power management algorithm determines whether to use EV driving or engine driving from the battery level, determines the power to charge/discharge the battery, determines the necessary engine output, determines the engine speed, and determines the target rotation speed of MG1 based on the vehicle speed at that time. Since the engine output torque can be calculated from the MG1 torque, the direct torque of the ring gear proportional to it is found, and it is determined how much MG2 torque should be added to reach the target driving force.

Another interpretation, lossless half-clutch

Let's think about the function of the planetary gears from another perspective. First, let's remove the MG.

When no load is applied to the sun gear, no matter how much you turn the engine, the sun gear just idles, and no force is transmitted to the output shaft. Since the torque applied to the three elements of the planetary gear is always in a proportional relationship, zero torque on the sun gear means zero output torque, and the engine cannot produce torque either.

When you apply the brake to the sun gear to stop it, torque is transmitted to the output shaft, causing it to rotate. In other words, the same thing happens as engaging a clutch.

When you release the brake, the sun gear begins to rotate, and the rotation of the output shaft decreases. This is the same as a clutch slipping or being in a half-clutch state, and the power that was not transmitted to the output is discarded as heat. In this way, the planetary gear allows the brake to function as a clutch.

If you attach MG1 instead of a friction brake, generating electricity corresponds to clutch engagement, and the power that would have been lost as heat due to slipping in a half-clutch state is instead output as electricity. The more it slips under strong torque, the more electricity it generates. In other words, combining a planetary gear and a generator creates a "regenerative clutch" that recovers energy that would otherwise have been discarded, realizing a lossless half-clutch.

So, wouldn't it be better to just replace the clutch with a generator without using complicated planetary gears?

It is possible to conceive of such a generator that rotates as a whole, that is, a "differential generator" that generates electricity based on the rotational difference between the input and output shafts. This is a bona fide power-split device that divides engine power into shaft output and electrical output. However, to extract the generated electricity, a sliding component called a "slip ring" is required. A slip ring capable of withstanding a large current of several hundred amperes is large, and there is also the problem of brush wear.

A planetary gear is also a type of "differential gear," and regardless of the engine's rotational speed, what comes out at the sun gear (MG1) is the "difference" from the speed at which the clutch is fully engaged (the mechanical point), multiplied by the gear ratio. In other words, THS realizes a differential generator without using slip rings by extracting the rotational difference into a stationary system.

Either way, this is like an MT car with only top gear running in a half-clutch state; you can reduce speed with the half-clutch, but you cannot increase torque. In addition to the "direct torque" from the ring gear, the power generated from MG1 is converted into torque by MG2 and returned to the output shaft, which increases the torque by the amount the rotational speed was reduced, completing the transmission function.

In short, THS is a mechanism that "generates electricity with the surplus power of a half-clutch and uses that electricity for series driving to amplify torque." Since the series drive power is added to the output shaft in parallel with the mechanical path, it is understandable why it is called a series-parallel hybrid. It is not that it switches between series and parallel, but that the properties of both are working simultaneously.

By adding a battery here, power is managed to assist the torque of MG2, or conversely, torque is reduced to charge the battery.

THS speed relationship diagram

Based on publicly available information, I would like to approach a more quantitative understanding. You can calculate the rotational speed of each shaft from the number of gear teeth, tire size, etc. Based on the understanding of the nomograph, I have graphed the operation of the THS. The premises for the calculation are provided at the end of the text.

This figure was created based on the values of the 4th generation Prius (the engine output characteristics are the same type, but from the 3rd generation). The horizontal axis represents vehicle speed, and the vertical axis represents engine speed. The rotational speed of MG2 corresponds to the vehicle speed due to the reduction ratio of the reduction gear.

The rotational speed of MG1 is determined by diagonal parallel lines. As mentioned earlier, the values that can be taken within the three-dimensional space where the rotational speeds of the three axes of the planetary gear—the ring gear, carrier, and sun gear—are x, y, and z, are constrained to two degrees of freedom. This forms an inclined plane passing through the origin, and taking omega (ω) to represent angular velocity, it is called the ω-plane. The diagonal parallel lines shown in green are projections of the ω-plane onto the x-y plane as contour lines.

At any given time, the operating point is within the region shown in yellow. If we consider a scenario of starting from a stop and getting on a highway, it would look something like this.

Accelerate gradually in EV mode up to 40 km/h, start the engine and accelerate further to 60 km/h, then press the accelerator to merge and accelerate to just over 100 km/h, then ease off the accelerator and move to cruising. The red line in the figure shows the trajectory followed by vehicle speed, engine speed, and MG1 speed.

Since the engine cannot rotate in reverse, the operating range is only on the positive side. The horizontal axis at zero engine speed is EV driving (carrier stopped), and the vertical axis at zero vehicle speed is idling (ring gear stopped). The region shown in dark yellow below 1000 rpm is not used as power, but is only passed through during engine start and stop. To eliminate throttle loss and maximize thermal efficiency, the engine is basically always used at maximum torque with the throttle fully open, so engine output corresponds to rotational speed. (The exception is when the engine is rotating without producing power. Sometimes, idling with the throttle restricted is performed immediately after starting to warm up the exhaust gas purification catalyst. At this time, the planetary gear without torque applied emitted a "rattling sound," but in recent models, a slight power generation torque is applied to eliminate the sound. Engine braking is also an exception to this, where the engine becomes a power sink with negative output.) The line at zero MG1 speed (sun gear stopped) is the "mechanical point" where there is no power passing through the electrical path and transmission efficiency is highest.

The beauty of the THS design is that, as the red trajectory in the previous example shows, the gear ratio is set so that frequently used driving points are distributed near the mechanical point. As you drive faster, running resistance increases, but in the practical range, it roughly matches the slope of the mechanical point line.

The maximum rotational speed for both MGs is a surprisingly high 17,000 rpm in the 4th generation Prius. (It is unusual to use over 10,000 rpm in a mechanical setup. From 6,500 rpm in the first-generation Prius, it increased to 10,000 rpm in the second generation and 13,500 rpm in the third generation, and the MGs have been miniaturized accordingly.) There is a missing area at the top left due to the MG1 speed limit, but a large output like 60 kW is likely not needed at very low speeds or in reverse.

The region below the mechanical point where the MG1 speed is negative is overdrive due to power circulation.power circulationby overdrive.

There are no boundaries of "modes" in the operating region of the speed relationship diagram; everything is seamlessly connected. THS is called an "electric CVT," but technically it is an IVT (Infinitely Variable Transmission) that is more functional than a CVT (Continuously Variable Transmission). A belt CVT has only a finite range of gear ratio width (ratio coverage), so a torque converter is required as a starting device, but with an IVT, the ratio coverage is infinite, and it can handle situations without switching while keeping the gears engaged, even if the input (engine) is stopped or the output (axle) is stopped (even in reverse).

On the origins of THS

Mechanical technology is an eternal repetition; purely new ideas are rare. The job of an engineer is to revive old ideas with modern materials and propose solutions that meet current demands. Therefore, to think about the future, it is important to know the past and understand the history of technology.

In the 1960s, as air pollution became a serious issue in the United States, the Muskie Act was enacted, mandating automotive exhaust purification. To clear the strict regulations of reducing carbon monoxide and nitrogen oxides by 90%, automakers around the world began researching various methods. Among these were electric vehicles, which contained the seeds of hybrid cars.

A technological trend in the 60s was the movement to apply jet engines (gas turbines), which had become common after World War II, to automobiles. Gas turbines were touted for their ability to use any fuel and produce clean exhaust. The Chrysler Turbine Car is famous, but Toyota and Nissan were also researching them. Toyota prototyped this as a series hybrid car several times after the 70s, but the oil shock caused fuel prices to soar, and development of automotive gas turbines had mostly stopped by the 80s.

The idea for a power-split EMT (Electromechanical Transmission) was conceived in the United States around 1970, just before the oil shock. Three people from the automotive parts manufacturer TRW, Berman, Gelb, and Richardson, were researching EMTs as a technology for exhaust purification using EPA (U.S. Environmental Protection Agency) funding. The operation of the device assembled on a test bench is summarized in the EPA report "Analysis and advanced design study of an electromechanical transmission." A patent was also filed and established as U.S. Patent US3566717A. However, because the funding was cut as a result of the oil shock, this research stopped there.

According to the EPA report, a planetary gear was used as the power-split mechanism, with the engine on the sun gear, MG1 on the carrier, and the ring gear as the output shaft, where MG2 joined at a fixed gear ratio. (In Berman et al.'s naming, MG1 is called the "speeder" and MG2 the "torquer." I think this better describes the nature of the objects. MG1's role is to adjust engine speed, and MG2's role is to adjust drive wheel torque.) It also had a high-speed cruising mode that locked MG1. MG2 was a circuit capable of both driving and regeneration using thyristor chopper control of a DC series-wound motor. MG1 used thyristor chopper control after rectifying a three-phase AC generator, but in this configuration, MG1 could not be used for driving and was dedicated to power generation. The battery was a 200V lead-acid battery. The concept was far ahead of its time, but power electronics, motor technology, and battery technology were all not yet mature.

As time moved into the 1990s, the state of California enacted the ZEV regulation, which mandated the sale of zero-emission vehicles. To respond to this, Toyota developed and sold the RAV4 EV. By this time, elemental technologies leading to the Prius, such as nickel-metal hydride batteries, IGBT inverters, and rare-earth permanent magnet synchronous motors, had become available. Around that time, the G21 project began within Toyota to develop a car that would become the global standard for the 21st century. With a goal of doubling fuel efficiency, they simulated and examined various powertrains, and found that the electric hybrid THS showed the best fuel efficiency performance. Simple parallel or series types had many drawbacks, and the power-split type was superior. Furthermore, among power-split types using planetary gears, I heard they examined all 12 combinatorially possible arrangements of where to place MG1 and the input/output shafts, and whether to place MG2 on the input or output side.

Therefore, it was by no means an imitation, but rather they arrived at the same conclusion through technological necessity. In fact, the existence of the Berman patent, which was filed in 1969 and expired in 1988, becoming public knowledge, was a source of relief for Toyota. It made it clear that there was no danger of being sued for patent infringement by other companies regarding the basic concept.

The power-split transmission using planetary gears was already being developed within the group at Aisin AW (now Aisin) at the time, so Toyota took it over. In exchange, Aisin AW was allowed to sell systems similar to THS to other companies. The Ford Escape Hybrid adopted in 2004 used the HD-10 hybrid transmission made by Aisin AW. Mechanically, it used a small-diameter high-speed motor in a parallel-axis arrangement, which was almost the same as the 4th generation Prius Plug-in from 2015, and it was an excellent design that was 10 years ahead of its time. It felt like it incorporated every possible technology, such as an engine reverse-rotation prevention clutch to allow MG1 to be used for driving during EV mode, an MG1 lock-up clutch for high-speed cruising (only traces remained in the Ford version), and an inverter mounted directly on the transmission (what we now call an e-Axle) to minimize wiring length. Technologies from Toshiba and Mitsubishi, which had been 20 years ahead in industrial and railway power electronics, were invested in the electrical system. Supply to Ford ended with the second-generation Escape Hybrid, but it served as a foothold for Aisin's expansion into the North American market.

Various hybrid systems

Toyota first released a hybrid car two months earlier than the Prius, in August 1997, which was a microbus called the Coaster Hybrid EV. It was a plug-in series hybrid car with what we now call a range extender, using a 1.5L gasoline engine exclusively for power generation. Series types include the Nissan e-POWER and Honda e:HEV. If you use a series type, the engine can focus on power generation at optimal operation, and you can achieve the same seamless operation as THS without using planetary gears. The reason for going to the trouble of using a complex power-split type is to reduce the power passing through the electrical path, which is less efficient than a mechanical transmission.

Simple series types like the Nissan e-POWER and Daihatsu e-SMART HYBRID pass through electrical-mechanical conversion twice, so the transmission efficiency is about 80% across the entire range. While there are efficiency improvements due to EV driving and optimal engine operation under low-speed, light-load driving conditions, there is a problem where the electrical system gets in the way during high-speed driving, making fuel efficiency worse than non-hybrid cars.

On the other hand, a parallel hybrid only needs one MG to handle the assist, so it can be small, and if it is a gear transmission, a transmission efficiency of over 90% can be expected. However, simple parallel types like the Honda IMA have the problem that the engine gets in the way and prevents EV driving (the improvement effect on city fuel economy is weak).

These are problems that are two sides of the same coin, where the engine "cannot be directly connected" or "cannot be separated." Therefore, the appropriate countermeasure is to attach a direct-connection clutch to the simple series type and a separation clutch to the simple parallel type.

Honda e:HEV is a series type with a direct-connection clutch. Both the generator and motor are large enough to handle all the engine power, and in the space where a planetary gear set would be in a THS, there is a hydraulic clutch for direct connection, along with the necessary hydraulic pump and valve body. The Mitsubishi Outlander PHEV also uses this method. In direct-connection mode, it functions as a parallel hybrid, so some people call this a "series-parallel hybrid," but unlike THS, this is a selective use of series and parallel.

Renault E-TECH HYBRID is a series type with direct connection, but instead of a direct-connection clutch, it inserts a 12-speed transmission and mainly uses parallel operation even outside of high-speed ranges. It makes the two MGs work like an "electrical synchronizer" to match speeds, enabling the use of a dog-clutch transmission while preventing drive force interruption during gear shifts. By limiting series operation to low power only and keeping the generator small, it fits into a limited space.

As an example of a parallel type with a separation clutch, there was the Nissan Fuga Hybrid. There is the problem of how to start the engine during EV driving; since a starter motor has durability and noise issues, they use a method of starting the engine by push-starting with a half-clutch, which requires delicate control of the hydraulic clutch. Recent examples include the iFORCE MAX for large pickup trucks in North America, and the Toyota Dual Boost Hybrid System is also this type if you look only at the front side (the rear is series drive).

Honda's i-DCD is a type of parallel system with a separation clutch, but this one incorporated an MG into the odd-numbered shaft of a DCT (Dual Clutch Transmission), which was popular in Europe at the time, and was made by introducing technology from the German company Schaeffler. The entire DCT is 7-speed, but the MG ends up having a 4-speed transmission relative to the axle, allowing for powerful EV driving with a small MG.

An interesting combination of DCT and hybrid is the hybrid active shift transmission (HAST drive) developed by Hitachi for railways. It was a DCT that could shift using only dog clutches without using friction clutches by extracting the rotational difference between the odd and even shafts with a differential gear and supporting it with a motor. However, since there is no equivalent to MG2, this regenerative dual clutch cannot be used continuously as an electric CVT. Unfortunately, development has been suspended due to circumstances at JR Hokkaido.

Looking at it this way, we can see that various efforts have been made to counter the disadvantages of series and parallel types. In contrast, the essence of THS, which is a power-split type, is a "regenerative clutch" as mentioned earlier, so while it is mechanically simple, it can include both engine direct-connection operation (MG1 speed zero, i.e., mechanical point) and engine disconnection operation (such as during EV driving, MG1 generation torque zero).

There are power-split systems even more complex than THS. A configuration where two regenerative clutches are connected face-to-face is called a compound split, and it has two mechanical point speeds. (I sometimes see comments asking why not attach a planetary gear to MG2 as well, and this is exactly that.) The Allison Transmission developed, and GM, DaimlerChrysler, and BMW released in 2007, the "Two-Mode Hybrid" used three sets of planetary gears, two MGs, and four hydraulic clutches. It was designed to increase efficiency across a wide gear ratio range by switching between two electric CVT modes and four fixed-gear parallel operation stages using clutches. It was marketed for its ability to handle high loads, such as towing a trailer up long hills with a large SUV, but it was not a commercial success.

Beyond electric types, there have long been hydraulic power-split transmissions. A series-drive transmission that goes from the engine through a hydraulic pump and hydraulic motor is called an HST (Hydrostatic Transmission), and it is often used in construction and agricultural machinery. A variant of this is the HMT (Hydromechanical Transmission), which uses power splitting in combination, and it is also used in aircraft power generation units (as a constant speed drive from a jet engine to a generator). As a pioneer of hydraulic transmissions, the Badalini type released by Honda in 1961 is well known, but this was an HMT that used a differential hydraulic pump rather than planetary gears. The idea of storing hydraulic pressure in an accumulator (a high-pressure gas tank) to use as a parallel hybrid vehicle was developed by Mitsubishi, Isuzu, and Nissan Diesel for buses. Therefore, if they had been motivated, it should have been possible to create a fully hydraulic THS, but I believe it could not beat the electric type in terms of efficiency, durability, and miniaturization.

Electric hybrid technology for buses and trucks has different roots than that for passenger cars, with Hino's HIMR, which appeared around 1990, being representative. This R stands for retarder, and it was an evolution of the electromagnetic brake for diesel engines that could not use engine braking. It was a parallel hybrid with an MG directly connected to the engine, realized using Toshiba's inverter control technology. The MG was an induction motor, a ring-shaped unit with the same diameter as the large clutch housing of a bus engine. Diesel engines are inherently efficient at light loads, so the importance of EV driving was low; the purpose of hybridization was to improve fuel economy through regenerative braking during deceleration, as well as to suppress black smoke generation by assisting with high loads during acceleration. (This was in the era before diesel particulate filters became common.) When Toyota and Hino formed a business alliance and Hino took charge of the truck business, this technology was inherited by Toyota's truck hybrids. Later, it was revised to a clutched parallel type that uses a high-efficiency permanent magnet synchronous motor and is also capable of EV driving.

Finally

When I started working on hybrid technology development, I was explained the extremely simple mechanism of THS and told that if we mass-produced this, we would surely be able to realize a car that was cheaper and more fuel-efficient than existing gasoline cars. But, as expected, electrical systems cost a great deal. Even after such widespread adoption, it takes over 150,000 kilometers of driving for the gasoline savings to exceed the additional cost of the hybrid. Looking at it from the perspective of CO2 emissions, it is empirically known that total emissions, including vehicle manufacturing, are generally proportional to the cost incurred. However, due to Jevons paradox, such as thinking, "If the fuel economy is good, I'll buy a bigger car" or "I'll go further," it ends up unclear whether it is actually eco-friendly or not. Above all, the mining of resources essential for electric vehicles, such as rare earth elements, copper, nickel, and lithium, all involve serious environmental pollution. While hybrid cars were a very interesting technical puzzle, when I realize that the SDGs of eco-car technology are trying to "sustain" the automotive society as it is, I think that technology is powerless against environmental problems and that engineers must be humble. I hope this will be a reference for young people who will lead the industry in the future.

References

1. Why are Toyota hybrid cars fuel-efficient? Explaining the low-fuel-consumption technology supported by the Prius's heart, the "planetary gear mechanism"! (Ichiken, 2025/03/28)

2. Toyota's cumulative global sales of electrified vehicles exceed 20 million units as of the end of February 2022 (Nikkan Jidosha Shimbun Electronic Edition, 2022.03.31)

3. Development of the Toyota Hybrid System (Shinichi Abe et al., TOYOTA Technical Review Vol 47, No.2 Nov. 1997)

4. History of Hybrid Vehicle Development at Toyota (Shoichi Sasaki, TOYOTA Technical Review Vol 54, No.1 Aug. 2005)

5. Electrical Systems of Successive Prius Generations (3) (Nagoya University Power Electronics Laboratory, updated 2021/08/21)

6. 2016 - 2022 Prius, Prius Prime Transaxle - P610 Deep Dive (P710, P810 Similar) (Weber State University, 2017/05/31)

7. Newly Developed 1.8L Engine Installed in the Toyota Prius (MOTOWN21)

8. The Secret Story of Prius Development (Takehisa Yaegashi)

9. The Era of Hybrid Cars (Yoshio Ikari, Kojinsha, 1999)

(Reference) Calculation of Speed Relationship Diagrams