Recently, the State Administration for Market Regulation issued a recall announcement recalling some EQC electric vehicles with production dates from November 30, 2018 to December 15, 2021, a total of 10,104 vehicles. The reasons for the recall are as follows: due to manufacturing deviations, the cooling system of the electric drive module may have been undersealed, resulting in coolant leakage.
If the coolant is slightly infiltrated into the motor, the insulation resistance value of the high-voltage system may be reduced after long-term use, and the vehicle may not start in extreme cases. Coolant leakage does not meet the requirements of the relevant national mandatory standards on the sealing of the cooling system, under extreme conditions, the output power of the electric drive module of the vehicle will be reduced, and there is a safety hazard.
Figure 1. Recall of Mercedes-Benz EQC (Notice from the General Administration of Market Supervision)
Part 1 Innovation and iteration of electric vehicles by traditional auto companies
EQC is the first generation of Product Design of Mercedes-Benz in the field of pure electricity, and the electric drive system is also jointly developed from the industry to find resources, mainly with ZF and Mercedes-Benz joint design. As shown in Figure 1 below, the main consideration here is whether it is possible to provide enough power, the front and rear axles are each equipped with a power motor, the total power reaches 300kW, and the total torque is 760Nm.
From the structural point of view, this electric drive system adopts the arrangement form of motor reducer left and right + inverter above, reducer parallel shaft structure, motor and reducer share shell. Here, the motor shaft and the reducer input gear are one-piece structure, three-ball bearing support, such a design on the motor rear cover, motor housing and reducer rear housing, the coaxiality of the connecting components is higher.
Figure 2. The powertrain of the Mercedes-Benz EQC
From the perspective of cooling, the system integration cooling method is adopted here:
●The inverter and motor adopt direct connection type water pipe, O-ring seal;
●The motor stator and rotor shaft are water-cooled, the stator water jacket is sealed with O-rings at both ends, and is bolted to the casing, and the rotor shaft water-cooled sealing structure is complex (mechanical seal);
At present, the main problem may be in the rotor water cooling technology, which is the cutting-edge cooling technology in the motor cooling technology, from the market point of view, most of the motors use water to cool the stator, or take the oil-cooled method.
Figure 3. Explosion diagram and main overview of mercedes-Benz rear-drive systems
In this sense, we can look at the considerations of German engineers in electric drive technology. Audi's e-tron series, which also uses rotor water-cooled technology, is mainly aimed at making high-power motors on induction motors (less efficient than permanent magnet synchronization), hoping that the motor can maintain a high continuous power output without derating through cooling technology (an efficient rotor cooling solution is bound by this requirement).
From the perspective of engineers in Germany, the development of rotor cooling technology has brought many benefits, including an increase in peak power and a significant increase in continuous power output. The further reduction of the temperature inside the rotor increases the durability of the entire drive system, and engineers can further optimize the bearing clearance to improve the acoustics due to the lower bearing temperatures.
Figure 4. Audi engineers' design considerations for rotor water cooling
Part 2 The Cost of Innovation
In the current development of electric vehicle companies, the drive system is decomposed into motor design, inverter design and overall thermal management design, which are all in the core of the car company. The main purpose is to improve the efficiency of the electric drive system, so that the efficiency of the whole vehicle can be higher.
From the perspective of technology selection, Japan and the United States (hybrid models and Tesla) departments tend to electric motor oil cooling technology, while European (BBA) tends to electric motor water cooling technology, in the first generation of models, Mercedes-Benz and Audi are working together with german ZF, although the motor rotor water cooling technology is more difficult, but the benefits that can be brought are still objective.
The design threshold of this technology lies in the water seal, and we can see that because European car companies are currently in the process of technical iteration from the first generation of pure electric models to the second generation of pure electric models, the more complex the design technology, the original traditional automobile companies need a long time to iterate and analyze, and then control the overall failure rate.
My personal understanding is that a lot of technology rushes to the front, which is closely followed by comrades who need manufacturing processes and after-sales quality, and innovation from the organizational structure requires full participation and help to solve problems together.
Summary: Objectively speaking, in the trend that we tended to think that the drive system is gradually being dominated by car companies and taken back, the main engine manufacturer will also put a part of the purchase order outside - because it is impossible to fully cover all the risks. This time Mercedes-Benz can also pull ZF together to see where the problem is, if all the pressure on mercedes-Benz's motor engineers, this matter will be troublesome.