The Garrett e-turbo has some more special sauce going on at the compressor side that is not mentioned anywhere that I can find easily. You can see this extra actuator on the compressor housing under the compressor discharge. I lightened up the picture a bit, and you can see some moveable feature in the compressor inlet by the inducer of the compressor wheel. Pure speculation on my part, but I am guessing it basically allows air to bypass the compressor wheel. So, my guess is that when the car is coasting down in speed, some moveable part in the compressor stage allows air to essentially bypass the compressor wheel so that no shaft power of the turbo is used to compressor the air. Therefore, the engine is pumping air through the turbine wheel which then extracts energy out of the flow to spin the motor of the e-turbo and charge the battery. Another use case could be when the battery is low and the car is just driving and there is low engine load, this hypothetical bypass feature is used to not waste turbo shaft power on compressing air and maximize the power going to the battery. Again, just a guess on my part.
You will probably hear people say, “442hp out of a 2.0L engine is no big deal. People have been doing it for two decades out of imports!” Yeah, well, try doing that while meeting modern emissions standards and having a warranty. To meet modern emissions standards, there is a massive catalytic converter right after the turbo and I am guessing a gas particulate filter after that. There are quite a few dimples on the exhaust pieces to maintain minimum OEM clearances between parts.
Part of the puzzle for powering the motor of the Garrett e-turbo is this battery pack. Based on the specifications given by AMG of 70 kW continuous power and 150 kW peak power from a pack with only 6.1 kWh of capacity, they are pushing the battery cells really hard. 70 kW corresponds to a C-rate of about 11, give or take assuming I did my math right and using 400V as the pack voltage. For reference, a Tesla supercharging is roughly at a C-rate of 5-6. If you want your cells to last as long as possible, you don’t go over a C-rate of 1. C-rate for batteries is like the equivalent of hp/L for an engine. The higher it is, the more stress on everything and typically shorter life. Running high C-rate means a lot of heat, just like pushing a lot of hp/L. Therefore, serious cooling is required and Mercedes AMG is using direct liquid immersion cooling; so, the battery cells are submerged in a dielectric fluid (non-electrically conductive). This is first-to-market with the technology that I am aware of in a relatively mass-produced vehicle. The McLaren Speedtail also uses direct liquid cooling and Faraday Future developed the technology years ago, but they have not launched their vehicle. We looked at some other battery cooling systems for reference when we dove into the Porsche Taycan a bit.
10 kW of cooling capacity is the stated number from Mercedes AMG, but you now how marketing always likes to inflate numbers and that is likely a maximum number. The question is under what conditions the 10 kW of heat removal occurs. If we assume that 10 kW of heat rejection occurs during 70 kW of continuous power, that’s ~12-15% of energy loss in heat. That efficiency for a battery cell would be relatively poor. I’m going to go with the 10 kW number coming after a burst of the max 150 kW power output.
This diagram shows the basic layout of the guts inside the battery pack. The coolant pump circulates the dielectric fluid through the battery pack. The fluid is likely a type of oil, similar to dielectric oil fluids used for cooling electrical transformers you see on power lines.