What’s so good about P2 Hybrid

With the rise of new energy, "electrification" of off-road vehicles has gradually become the mainstream. Not only does it add electricity to off-road vehicles, it even boldly cancels the traditional gearbox and transfer case and changes it to an extended-range or series-parallel structure. Four-wheel drive is also achieved by an independent rear-axle motor.

These hybrid models retain the gearbox and transfer case in design, and only use the P2 hybrid structure, which means adding an electric motor to the gearbox. Friends who are familiar with hybrid systems should know that the P2 motor is a parallel structure, and under some working conditions, the efficiency is actually not as good as extended range or series-parallel connection. So what is the reason why established off-road vehicle manufacturers still insist on the P2 hybrid structure?

1. More torque at the wheels

As we all know, the biggest difference between off-road vehicles and ordinary SUVs is that off-road vehicles need to face more and more difficult hill climbing scenarios. In vehicle design, regardless of the differences in body and suspension forms, the vehicle's climbing performance is directly determined by the wheel torque. There are two main ways to increase the wheel torque. The first is to increase the torque of the power system, and the second is to Increase gear ratio. But in practice, taking into account factors such as cost, weight, volume, etc., it is obviously unrealistic to blindly increase engine displacement and power. Therefore, automobile manufacturers mostly increase wheel torque by increasing the transmission ratio. On a pure fuel off-road vehicle, the torque output by the engine is gradually amplified through the gearbox, transfer case, main reducer, and wheels. Some off-road vehicles that focus on climbing, such as the Wrangler Robin Hood, have a torque amplification factor of up to 77.2 for its 8AT version of the transmission system. Ignoring the impact of transmission efficiency, calculated based on the peak torque of 405N·m of the 2.0T engine, the maximum torque that a single half shaft of the Wrangler Robin Hood can output is as high as 31266N·m, which can fully meet the extremely difficult climbing requirements.

It can be seen that the gearbox and transfer case structure can effectively improve the climbing ability of off-road vehicles. As for the P2 structure hybrid mentioned earlier, its biggest advantage is that it completely inherits the transmission system of a pure fuel SUV, and uses an electric motor to assist the engine and input higher torque to the gearbox. For example, in the Wrangler 4xe Robin Hood version, with the support of the P2 motor and engine, the system peak torque is increased to 637N·m. Coupled with the 77.2 torque amplification factor of the transmission system, the torque on a single half-shaft is as high as 49176N·m. Therefore, when the power is sufficient, an off-road SUV using the P2 structure will have greater wheel torque. This is also a major reason why established manufacturers prefer P2.

Different from the P2 structure hybrid, the newly launched extended range and pure electric off-road SUVs all rely on electric motors to drive the vehicles. Although the motor has high torque, the torque amplification capability is very limited because the transmission system only retains the reducer. Take the American Rivian R1T and HUMMER EV as examples. Although their main reduction ratios have reached such high levels as 12.59 and 13.3 respectively, it is still impossible to increase torque with just one reducer and have a gearbox, transfer case, and main reduction gear. It competes with off-road SUVs with a P2 structure that increases torque.

Take the Rivian R1T driven by 4 motors. The peak torques of its front and rear axle motors are 560 and 671N·m respectively. Calculated with a main reduction ratio of 12.59, the output torques of the front and rear axle shafts are 7050 and 8448N·m respectively. . Even if the two are added together, it is only 15498N·m. What's more, both extended-range and pure electric off-road vehicles have electric four-wheel drive systems, with no drive shaft between the front and rear axles and no central differential lock. Therefore, it is not like the P2 structure off-road vehicle equipped with 3 locks, which outputs all the torque to a single wheel to achieve escape in extreme environments.

2. Durable and stable power

Although the maximum efficiency of the motor can reach more than 90%, the maximum efficiency of the gasoline engine is only more than 40%, which is more than half the difference between the two. However, the tolerance range of motors to high temperatures is very small. Whether it is a permanent magnet motor or an induction motor, operating at high temperatures will cause damage to the motor.

In order to allow the motor to operate within a safe power range, the industry has formulated two performance parameters for the motor: [peak power/torque] and [rated power/torque]. As the name suggests, peak power/torque is the maximum power that the motor can produce, while rated power/torque is the power that the motor can continuously and stably output. According to the recommendations of the existing national standards, the peak power operation time of a general vehicle motor is 30 seconds or 1 minute. In other words, the eye-catching parameters that everyone sees on the motors of pure electric vehicles are actually usable for a very short time.

Therefore, under most working conditions, the vehicle's motor can only operate at the rated power, and the national standard requires the rated power of the motor to be 30 minutes. After this time, the motor will also be protected from overheating. Therefore, in some extreme environments, such as long-term climbing and full-load driving, the power of purely electric-driven off-road SUVs is not so long-lasting and stable.

In comparison, for an off-road SUV driven by an engine + P2 motor, the motor only provides short-term assistance, and it is the engine that maintains the rated power output for a long time. Therefore, vehicles using engine + P2 motor are more stable than motor-driven off-road vehicles in terms of durability, whether it is long-term high-speed cruising or high-intensity off-road climbing.

In addition, even if the P2 hybrid system fails, the engine and gearbox can still operate normally, and will not be trapped due to a failure of the electric drive system. For example, Toyota said that the new Prado equipped with P2 hybrid, even if the motor system completely fails, it will still be a 2.4T+8AT off-road vehicle. And if it is a model with electric four-wheel drive, once the motor or battery fails, the vehicle may become stuck and unable to get out of trouble.

3. Compatible with various suspensions

In terms of chassis design, because the electric drive system is usually integrated with the reducer and other components, the volume is relatively large. At the same time, electric off-road vehicles will also use electric four-wheel drive solutions with front and rear dual motors or even four motors, which also leads to the electric drive system. Not compatible with existing integral axle rear suspension. Therefore, the rear suspensions of today's extended-range and pure electric off-road vehicles have become independent suspensions.

Compared with the independent suspension, the integral bridge suspension adopts a hard bridge design. When one side of the suspension is compressed, the other side will be stretched due to the lever principle, which can obtain a travel range that is far beyond that of the independent suspension. When dealing with crossovers, In off-road conditions such as axles and shell craters, the wheels can be kept as close to the ground as possible to obtain better escape capabilities. Because of this, hard-core off-road vehicles such as Big G, Land Cruiser, Wrangler, and Tank still insist on using rear integral axle suspension. The advantage of the P2 hybrid is that it does not change the transmission structure of the fuel vehicle, but only adds a motor to the gearbox. Therefore, models such as the Tank 500 Hi4-T and Prado that use the P2 hybrid can still use the rear integral axle suspension. .

Even for models that pursue the ultimate off-road capability, the P2 hybrid's front axle can also adopt an integral axle structure, and the suspension layout will not be affected by the addition of an extra motor.

4. Simple structure and low cost

In terms of power system layout, off-road SUVs that generally adopt an electric four-wheel drive solution require an engine + three motors if they choose extended-range/series-parallel power, while pure electric vehicles require at least two motors. And taking into account the requirements for climbing and acceleration, the motor power of off-road SUVs is also relatively large, coupled with the large-capacity battery, so the cost is not low.

For off-road vehicles using P2 hybrid, the engine is still the main force, and the P2 motor is only an auxiliary. Therefore, the P2 motor does not require particularly high power, and the cost of the power system will not increase significantly.

More importantly, the fuel-saving effect of the engine + P2 motor solution is actually pretty good.

 Of course, even though P2 is so good, it still has many shortcomings

1. Small space inside the car

Although currently considered comprehensively, the engine + P2 motor solution has advantages in terms of cost, reliability, and suspension structure, it also has some shortcomings. The first is the issue of battery layout. Since the P2 hybrid structure retains the traditional gearbox and drive shaft, the middle position of the chassis is occupied and the battery cannot be arranged here.

Therefore, most models have the battery placed under the trunk floor. For a model like the Prado, which has a battery capacity of less than 2 degrees, it will not have much impact on the interior space. However, for models using batteries, the third row of seats in the fuel version is directly sacrificed. seats, and the floor height of the trunk is also significantly higher than that of the pure fuel version, which affects the seating and storage space.

2. Low body stiffness and high center of gravity

In addition to the impact of battery arrangement on space, another problem with the P2 structure is that it does not help the body stiffness and center of gravity. First of all, most hard-core off-road vehicles use non-load-bearing bodies. This structure results in a high center of gravity and low frame stiffness. The battery of the P2 hybrid is located above the frame, which will further increase the height of the vehicle's center of gravity and affect the vehicle's control stability.

As for electric-driven extended-range/pure electric/series-parallel off-road vehicles, since there is no intermediate transfer case and transmission shaft, the battery can be directly arranged inside the frame, creating an effect similar to CTC battery-chassis integration. Not only does it not encroach on the trunk space, it can also lower the center of gravity of the body and improve the rigidity of the frame. Coupled with the fact that the electric off-road vehicles mentioned above generally use a four-wheel independent suspension structure, the vehicle's highway driving performance will have obvious advantages over P2 off-road vehicles.

After the analysis, everyone should understand why established off-road vehicles prefer hybrids with P2 structure, right? To put it simply, they hope to retain the reliability, durability, suspension structure and other characteristics of the old off-road vehicles, while at the same time alleviating the fuel consumption and power problems of off-road vehicles to a certain extent, so the P2 structure has become the most suitable choice.

But we also have to see the changes of the times. The flexibility and high efficiency of motors give off-road vehicles based on series-parallel, extended-range, and pure electric power more playability. Not only are they comparable to sports cars in performance, they also bring innovative functions such as turning around in place. With the continuous advancement of three-electric technology and chassis and body technology, the boundaries between hardcore off-road vehicles and electric four-wheel drive SUVs will become increasingly blurred in the future, and off-roading will gradually transform from a hobby in niche circles into a well-known car method.