The secret of hybrid fuel saving: in-depth technical interpretation of eCVT transmission

You may have noticed that many common hybrid models are equipped with eCVT gearboxes, such as Lexus' full range of hybrid models, Toyota Camry Hybrid, and Ford Fusion Hybrid. The hybrid Honda Accord sold in the North American market is also equipped with an eCVT gearbox.

For hybrid vehicles that pursue fuel economy, eCVT is actually the lowest cost and most efficient transmission technology. But it should be noted that eCVT is completely different from traditional CVT transmissions, and its technical principles are very different. It can even be said that eCVT is just a power distribution technology, not a real transmission device.

eCVT is also called PSD, which stands for Power Split Device. As the name suggests, it is a power distribution device. It is generally used in mild hybrid vehicles (Hybrid) rather than plug-in hybrids (Plug-in Hybrid). The latter is generally matched with an automatic transmission.

Mild hybrid models have a characteristic, that is, they can achieve maximum energy saving, while the power of the model is not eye-catching. Generally, they are more suitable for family cars and compact SUVs. Of course, the Lexus LS 600hL, which is priced at 1.5 million, is an individual case. The following is an explanation of the specific principle of eCVT:

First of all, we need to understand the powertrain of hybrid vehicles, which is generally composed of a fuel engine, two electric motors, and a battery pack. It should be noted that the maximum output power and torque of the fuel engine of hybrid vehicles are generally not too high. Most of them are naturally aspirated models and use the Atkinson cycle.

Take the Lexus NX 300h as an example. It is equipped with a 2.5L naturally aspirated engine with a maximum power of 114kW and a peak torque of only 210N•m. At the same time, it is also equipped with an electric motor with a maximum power of 105kW and a peak torque of 270N•m. In terms of the battery pack, the car uses a 1.6kWh nickel-iron battery.

In this assembly, don't think that the maximum power of the vehicle is 114+105=219kW, because in most cases, only the electric motor is driving the vehicle. Only when cruising at high speed will the system switch the power source and the engine will directly drive the vehicle.

The 1.6kWh battery pack has two functions: one is to power the electric motor; the other is to store energy recovered by braking. The battery power is generated by the second motor. The energy sources of this motor are the fuel engine and KERS (Kinetic Energy Recovery System).

The above picture is a schematic diagram of the planetary gears in the eCVT. The green gear in the middle is called the "sun gear", which is connected to the generator; the four orange gears around it are called planetary gears, which are connected to the fuel engine; the outermost yellow gear is called the ring gear, which is connected to the power motor, and the ring gear is directly connected to the power output shaft.

In this way, the power of the fuel engine and the electric motor are integrated through a planetary gear structure. In most cases, the car relies on the power of the ring wheel, that is, the power of the electric motor to drive the vehicle. Since the motor has a fixed gear ratio, there is no so-called gear ratio concept in the eCVT, and multiple sets of planetary gears or clutches are not required to achieve different gears.

Only when cruising at high speed will the eCVT output the power of the fuel engine directly to the wheel end, which is equivalent to the traditional automatic transmission running in the highest gear. The reason for this setting is that when driving at high speed, the motor speed is very high and the thermal efficiency decreases. At this time, if the battery pack continues to output high power current, the thermal management of the entire electric system will be a problem. At the same time, under high-speed cruising conditions, the Atkinson engine can also enter the optimal working range.

The Atkinson cycle is an engine stroke mode that uses a connecting rod mechanism to maximize the piston stroke, and its expansion ratio is greater than the compression ratio. In this way, the engine cylinder can fully utilize the exhaust gas pressure to work, further saving fuel consumption. However, it is precisely because of this design that the low-torque performance and high-speed performance of the Atkinson engine are not as good as those of ordinary engines.

So, you can see that vehicles with eCVT powertrains are either driven by motors or engines, and there is no so-called hybrid output. The real output power is not the superposition of the two. The Lexus LS 600hL is equipped with a naturally aspirated engine with a maximum power of 290kW and an electric motor with a total power of 165kW, but its acceleration level from 0 to 100 kilometers is not as good as the LS460L equipped with only a 285kW naturally aspirated engine. Because although the former has two power sources, only one is outputting power at a time.

In this way, when driving a hybrid vehicle, although the engine is roaring, its main work is to charge the battery. Why is it fuel-efficient? This is because, since there is no multi-stage gear ratio, the power is mainly output through the motor, so the energy loss of the eCVT powertrain is not as large as the traditional automatic transmission in energy transmission.

At low speeds, the vehicle runs directly in pure electric mode, and the engine uses the Atkinson cycle, which further reduces fuel consumption. This is the principle of fuel saving for hybrid vehicles.

One thing to note here is that the Atkinson cycle engine of a hybrid vehicle is not actually the complex structure designed by James Atkinson in history. Thanks to the progress of the engine electronic control system, the engines on hybrid vehicles such as Lexus now delay the closing of the intake valve, allowing part of the mixed gas in the cylinder to overflow after the stroke, thereby achieving a disguised expansion ratio greater than the compression ratio.

However, you may ask, if this is the principle of eCVT, then this type of hybrid vehicle cannot achieve four-wheel drive? Of course not, Lexus NX 300h has both front-wheel drive and four-wheel drive versions. But its four-wheel drive version is different from the traditional four-wheel drive.

It should be noted that the traditional four-wheel drive structure, that is, the four-wheel drive mode in which the rear axle transmits power through a drive shaft, is realized by relying on the gearbox. Whether it is a manual gearbox or an automatic gearbox, the version that can match the four-wheel drive system has two output shafts in opposite directions.

The eCVT is not a gearbox at all, so how can it provide two power output shafts at the same time? It doesn't need to. Lexus's four-wheel drive mode is to install an electric motor on the rear axle of the vehicle to achieve four-wheel drive, and there is no need for a drive shaft in the middle. This design is somewhat similar to the concept of Volvo V60 Plug-in Hybrid.

For hybrid models, fuel saving is the ultimate goal; but eCVT is actually just a conservative solution. Not all mild hybrid models use eCVT, because as mentioned above, although eCVT saves fuel, it lacks power performance. Some hybrid models that focus on performance, especially rear-wheel drive vehicles, still use automatic transmissions.

For example, the Infiniti Q50 Hybrid is equipped with a 3.5L naturally aspirated engine and a 50kW motor, which is matched with a 7-speed gearbox. This is because eCVT is only applicable to front-wheel drive or four-wheel drive, and is powerless for rear-wheel drive vehicles. The 7-speed gearbox equipped in the Q50 comes from the Mercedes-Benz 7G-Tronic, which can be applied to the hybrid system.

For this hybrid mode, there is only the situation where the engine and the motor drive the vehicle at the same time, or the motor drives the vehicle alone, and there is no situation where the engine drives the vehicle alone as in eCVT. This is because the engine and the motor are actually connected to the gearbox through an output shaft.

The significance of this hybrid mode is that the motor supplements part of the power on the basis of the engine, allowing the vehicle to further increase the maximum output power on the basis of the original high-horsepower engine. Therefore, the motor power in this mode is generally not very large. Under certain working conditions, it can also be adjusted to pure electric mode, which has a certain fuel-saving effect. But obviously, its fuel-saving effect is not as good as the eCVT mode.

As a winner of Toutiao's Qingyun Plan and Baijiahao's Bai+ Plan, the 2019 Baidu Digital Author of the Year, the Baijiahao's Most Popular Author in the Technology Field, the 2019 Sogou Technology and Culture Author, and the 2021 Baijiahao Quarterly Influential Creator, he has won many awards, including the 2013 Sohu Best Industry Media Person, the 2015 China New Media Entrepreneurship Competition Beijing Third Place, the 2015 Guangmang Experience Award, the 2015 China New Media Entrepreneurship Competition Finals Third Place, and the 2018 Baidu Dynamic Annual Powerful Celebrity.