DTH VS Extended Range Hybrid: Which is the Ultimate Form of Plug-in Hybrid in the Future?

Recently,  the Wenjie M7  (  parameters  |  inquiry  ) has been controversial after its launch. Huawei's current executive Yu Chengdong first called for "eliminating fuel vehicles as soon as possible" at the 14th Automotive Blue Book Forum, and then posted on the social platform that "it is nonsense to say that extended-range vehicles are not advanced enough" and "the extended-range mode is currently the most suitable new energy vehicle mode." As soon as this was said, Li Ruifeng, CEO of Wei brand under Great Wall Motors, couldn't sit still, and posted 6 pieces of content in a row to diss, such as "Those who make extended-range vehicles should just make a fortune in silence, why bother to speak out", "Those who lag behind will be beaten", "Someone has to burst the bubble that has been blown up", "Use backward technology to reap profits", etc.

The quarrel between the two has put the extended-range and hybrid systems on opposite sides. Of course, we have no right to interfere with the verbal fights between the bigwigs. The actual situation is what we consumers care more about. So today we will talk about the extended-range hybrid system of the M7 represented by Yu Chengdong and the DTH plug-in hybrid system represented by Weipai CEO Li Ruifeng, to see which technical school is the ultimate form of the future.

Before we begin, let’s briefly introduce the three main types of oil and hybrid power modes currently on the market: HEV (Hybrid Electric Vehicle), PHEV (Plug-in Hybrid Electric Vehicle), REEV (Range Extended Hybrid Electric Vehicle)

To briefly explain, REEV extended-range hybrids are currently divided into large batteries and small batteries. Large batteries refer to those that can be charged with an external power source and can be issued with a green license plate (the national regulation stipulates that a green license plate can only be issued if the battery is greater than 10 degrees). For example, the M7 and Ideal ONE belong to the PHEV category. Small batteries cannot be charged externally and have a small battery capacity. Most pure electric drives are only suitable for starting status (blue license plate) and belong to the HEV category.

At present, the new generation of plug-in hybrid system in China is referred to as DHT (hybrid power dedicated technology), which specifically refers to the "series-parallel" PHEV plug-in hybrid system of the motor and the engine. Manufacturers have refined multiple working plans based on different technical types, mainly using the engine-driven direct drive gear as the distinction.

Engine single gear direct drive

Representative models: Honda i-MMD Rui E+, BYD DMI

Engine 2-speed direct drive

Representative models: GAC G-MC, Great Wall Lemon DHT

Engine 3-speed direct drive

Representative models: Geely Thor Hi·X, Chery Kunpeng DHT

Series Extended Range

The working diagram of the large-battery extended-range hybrid system is to remove the direct-drive part of the engine, that is, the pure electric mode and the series extended-range mode.

From the above diagram, it can be seen that REEV range-extending mode can realize: single/dual motor pure electric mode, series range-extending mode, parking charging mode, driving charging mode, motor braking energy recovery (not marked in the figure) and other modes, while DTH can also realize 1/2/3 gear engine direct drive mode and motor engine parallel drive mode in addition to the above modes. The most important thing is that DTH covers "series range-extending mode".

In a subjective sense, DHT only has a few more parallel driving modes compared to the extended-range type, so what is the actual technical significance behind it?

First: For the overall vehicle layout, the components of the extended-range hybrid system are engine (generator) → inverter → battery → motor arrow drive wheel. Generally speaking, the engine energy only needs to be transmitted through electricity, without the need for mechanical direct drive, so the position layout is relatively free and the relative frame volume requirements will be lower; DHT, on the basis of the above, also needs to consider the layout of the drive shaft, as well as the development and adaptation of the corresponding dedicated gearbox.

Second: The extended-range plug-in hybrid system has relatively single power intervention because it relies solely on electric drive; DHT also needs to consider the seamless switching between the motor and the engine in direct drive gear, which is probably similar to the concept of smooth gear shifting in fuel vehicles. It is best for the driver not to feel the intervention and exit of the direct drive gear, and try to ensure a pure electric drive experience.

Third: The extended-range model only has two modes: pure electric and coordinated electric and motor driving, while DHT needs to consider dynamic coordinated control: when the engine is involved, the hybrid system enters the parallel drive state, and the engine and motor output at the same time. At this time, the engine and motor should be controlled as a whole system.

The engine's multi-gear direct drive is more efficient

A major disadvantage of the extended-range hybrid without direct drive mode when the battery is low is the energy waste caused by multiple energy conversions.

The extended-range model requires the engine to continue to work and generate electricity when the battery is low, and the electricity generated can be supplied in two ways.

1. Engine power generation → inverter → motor

2. Generator → Inverter → Battery → Motor

PS: The following parameters are all optimized values

Correspondingly, the engine's power generation efficiency (the efficiency of converting mechanical energy output by the internal combustion engine into electrical energy) is 95%, the battery's charging efficiency is 90%, the battery's discharge efficiency is 95%, and the motor's efficiency is 95%. When combined, there is a maximum energy loss of about 25%. If the engine speed increases out of the optimal speed range during acceleration or uphill operation, the efficiency loss will be even greater.

For reference, the maximum power of the M7 is 92KW. Under extreme conditions, after deducting the loss, there is only 69KW of power left to tow a 2-ton vehicle. The actual performance is the performance degradation problem after the battery is depleted.

In addition to the above situations, DHT can allow the engine to start intervening, and in the case of mechanical direct drive with a DHT-specific gearbox (fixed gear ratio) instead of a traditional gearbox (torque converter, gear set, etc.), the energy loss may reach 90%, which is relatively smaller.

With the current flourishing state of DHT, unlike BYD's DMI which can only activate direct drive mode at speeds above 80 km/h, other manufacturers have added more 2nd and 3rd gears, which have more advantages and a wider speed range for direct drive intervention.

Better power: The speed ratio is relatively large, the power experience is good, and when it intervenes at a speed of 30-650 kilometers/hour (Geely Thor Hi·X can intervene at more than 20 yards), it improves both economy and power; Better fuel economy: The speed ratio is relatively small, and it intervenes at a speed of more than 65 kilometers/hour, which can improve economy and reduce engine noise.

In general, the power gear is used for direct drive at medium and low speeds, and the economy gear is used for direct drive at medium and high speeds to improve economy; regardless of medium and low speeds or medium and high speeds, the power gear is used for direct drive during sudden acceleration to improve power.

Under such circumstances, there are actually great considerations for cost (more gears, more mechanical structures, higher costs), reliability (the more complex the structure, the greater the possibility of problems), and smoothness (dynamic adjustment in multiple modes). Just like the complex AT gearbox, the DHT dedicated gearbox wants a better experience, which is a problem of the car company's R&D capabilities and technical accumulation.

The best mirror for the development of any technology is the market

From the perspective of market product layout

In the passenger car field, there are currently only 8 models on sale equipped with extended-range plug-in hybrid systems, including Wenjie M5/M7, Ideal ONE/L9, Dongfeng Fengguang E3, Seres SF5, Lantu FREE and Skywell ME5. Among them, Ideal L9 and Wenjie M7 are just in the stage of being launched on the market.

There are nearly 100 plug-in hybrid models. Even if we exclude the P2-end PHEV plug-in hybrid system still used in most joint venture cars and some domestic cars, there are nearly 20 models using the latest DHT technology and the number is still increasing.

This question is the same as the 3-cylinder engine. Can the 3-cylinder engine be used? Yes! Is the technology mature? Mature?

As a result, when you buy it home, you find that everyone is playing with 4-cylinder engines again. Will your subsequent maintenance, after-sales service, and accessories also be a problem?

Regarding what Mr. Yu said about the extended-range new energy vehicles using half the fuel of fuel-efficient vehicles, a simple calculation will show that the thermal efficiency of the Wenjie M7 engine is 41%, so one liter of gasoline can generate about 3 degrees of electricity.

We refer to the official energy consumption of the Wenjie M7, which is 24 degrees per 100 kilometers. (excluding the energy loss of charging and discharging of the car)

24÷3÷0.9 (motor efficiency) = 8.9L. It can be roughly calculated that the fuel consumption per 100 kilometers in the low-power state is about 8.9L.

According to Mr. Yu's statement of half a liter of oil, 8.5L×2=17.8L! The Luxgen U7, which is also a medium-sized car, probably has this fuel consumption! The actual fuel consumption of a conventional medium-sized car is generally more than 15L per 100 kilometers.

So it does save fuel, but under normal circumstances it is less than half of that of a fuel car!