When we last looked at the Mercedes AMG M139 engine, it was the highest output 2.0L engine to go into production at 416hp. Increasing horsepower out of a small displacement engine typically involves using a larger turbocharger and that typically means more lag. Nobody likes turbo lag. The automotive industry has been shifting toward hybridization with high voltage electronics to improve fuel economy. Electrification has the side benefit of making things faster too. Formula 1 has been running hybrid systems with turbocharged engines since 2014 and that technology is finally becoming available in a car you can buy. The two key pieces of technology being used by AMG to get 442hp out of the new M139 engine are e-turbo and the battery pack.
The modern key to running a bigger turbo without the lag is e-turbo. It’s electric anti-lag. In order to use e-turbo, you need a high-voltage power source and that is where hybridization comes in. Garrett provides the e-turbo used on the new AMG M139 engine. If you didn’t know, Garrett is the turbocharger supplier to Ferrari for their Formula 1 program.
The Garrett e-turbo design used on the AMG M139 engine has the electric motor on the shaft next to the compressor wheel. Typically, electronics do not like too much heat, so the electric motor is kept away from the hot turbine side. The inverter/power electronics mounted underneath the turbo take the electricity from the battery pack and feeds it to the turbo’s electric motor to spin up the turbo. In Formula 1 speak, the e-turbo motor is also known as the MGU-H (Motor Generator Unit – Heat) and it can take energy from the exhaust gas and charge the battery pack back up.
The Garrett e-turbo does still use an internal wastegate, but Garrett did some performance optimization in this turbine stage as every little bit counts when squeezing 221hp/L in a modern serial production car. To minimize lag, the internal wastegate poppet valve has an angled seating surface like you are used to seeing on intake and exhaust valves. This seals better than the old school flat valve and therefore improves spool by having less exhaust gas leakage; all of the exhaust gas is getting to the turbine wheel to spool it up. The placement of where the wastegate flow is re-introduced can also improve performance. Here, it’s further away from the turbine wheel which should help with turbine efficiency as the wastegate flow will not interfere as much with the exhaust flow coming out of the turbine wheel. The last bit of design to help with turbine efficiency is a narrow angle diffuser for the turbine discharge. To do a narrow angle requires space which is why the turbine housing is so long. The one last characteristic of an e-turbo is that it allows the use of a larger A/R turbine housing for reduced exhaust backpressure. Typically, a larger A/R turbine housing significantly increases spool time and lag, but the electric motor compensates and then some.
The Garrett e-turbo uses a linear electric actuator for the wastegate, so it has this linkage to allow for the arc motion the wastegate arm travels through. A typical journal bearing system is used instead of a more costly ball bearing unit. Ball bearings are typically used to reduce lag and improve transient response, but the Garrett e-turbo has the 6kW (6000W) motor for that. 746W = 1hp, so you can do the math. Ball bearings are also relatively ‘noisy’; can’t have a noisy turbo in a passenger car! The round passage above the motor is probably for coolant to help keep the motor cool. The oil flow being between the motor and turbine side is another thermal barrier to keep heat away from the motor.