The Brilliance of Electric Turbochargers

What is a turbocharger's job? In essence, it's to increase thermal efficiency. An electric turbocharger does this and more, which is why I'm a big fan.
Thermal efficiency is a measure of how much of the potential energy of a fuel is consumed to create power, versus how much of it is simply generating waste heat. In pure terms, an automotive internal-combustion engine is not very efficient. For example, Toyota made a big deal in the late 2010s when it achieved
40 percent thermal efficiency
in its Dynamic Force four-cylinder engine. Meaning it was
only
wasting 60 percent of its potential energy.
Welcome to
The Rabbit Hole
, a bi-weekly column where Senior Editor Chris Perkins explores his latest obsession with automotive technology. He speaks to the best in the business to understand how cars work and what the future of the automobile looks like.
Incidentally, this is why EVs have an appeal beyond zero local emissions. Thermal efficiency doesn't apply to electric motors because they're not directly powered by a heat source. But in terms of electrical efficiency—the ratio of electrical energy a motor consumes to its useful output—an EV's motor is about 75 to 90 percent efficient,
according to Renault
, at least.
So, in short, internal-combustion engines, especially on their own, aren't especially energy efficient. Electric motors are very energy efficient. Turbocharging can help narrow that gap. Mind you, it's still a big gap, but any little bit helps, right?
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Turbocharging 101: A turbocharger consists of a turbine in the exhaust system, a compressor in the intake, and a shaft connecting the two. The exhaust turbine spins up with the flow of exhaust gases, which in turn spins up the compressor, increasing the density of the air headed into the engine, boosting power. In terms of thermal efficiency, it takes energy that would otherwise be lost as heat and turns it into something useful.
Photo by: Mercedes-Benz
Turbocharging 201: An electric turbocharger adds a motor attached to the shaft between the turbine and compressor. This means you can spin up the turbocharger independent of exhaust-gas flow, which has all sorts of benefits. Most notable is the all-but-elimination of turbo lag, but also the lowering of boost threshold, and allowing for higher boost pressure. And simply knowing the shaft speed of a turbocharger—which admittedly can also be achieved with a simple speed sensor—allows the automaker to run the turbo more safely closer to its maximum speed.
But an electric motor works backward too, generating electrical energy if you use it to brake the turbine. An engineer from Mercedes-AMG once told me that in some cases, an electric turbocharger can be
energy neutral
; The energy the turbocharger's motor regenerates is enough to power the turbocharger itself.
There are big thermal efficiency gains to be had using electric turbochargers. Mercedes-AMG said in 2017 its electric-turbocharged Formula 1 V-6 exceeded
50 percent thermal efficiency
, which was one of the first times ever an automotive engine converted more of its fuel source into useful power than waste heat. Like all F1 engines, the AMG V-6 uses a Motor-Generator-Unit-Heat (MGU-H), which is simply another term for an electric turbocharger. AMG later became the first to offer electric turbochargers in a road car with the four-cylinder in the C43 and C63.
Photo by: Mercedes-Benz
Porsche then took things a step further with its hybrid system for
the new 911 Carrera GTS
. Its single BorgWarner turbocharger has a 14.7-horsepower electric motor on its shaft, and uniquely, no wastegate. Typically, a turbocharger uses a wastegate—a valve that opens to expel excess exhaust gas—to limit boost pressure. Porsche instead brakes the turbocharger's motor to control boost pressure, so it's not wasting any exhaust gas and generating additional electrical energy. That additional electrical energy can power either the turbocharger itself, or the 53.6-horsepower traction motor sandwiched between the engine and transmission.
Photo by: Chris Perkins / Motor1
A Porsche engineer also tells
Motor1
that using a large turbocharger and limiting its turbine speed with the motor reduces exhaust-gas temperature, and therefore, the temperature of the charge air going into the engine. That eliminates the need for fuel enrichment, which is often used to reduce combustion temperatures, but this practice now being banned with Euro 7 emissions regulations. Porsche's use of an e-turbo boosts the engine's thermal and fuel efficiency, and overall vehicle efficiency.
Broadly speaking, going electric feels like a natural extension for turbocharging. If the point of turbocharging is to boost efficiency, why not go for a solution that furthers that aim? Well, electric turbochargers are expensive, complicated, and heavy. Ferrari is using electric turbochargers for its F80 hypercar, but its closest rival, McLaren, uses conventional turbos in the coming W1.
McLaren engineers told
Motor1
that they didn't want the extra weight electric turbos would bring, and that they'd rather use the car's electrical energy to power the traction motor.
Adding weight and complexity is always a difficult decision for an automaker, one of the many compromises it must consider in the course of engineering a car. The complexity has to justify itself.
McLaren might also have a point on the electrical energy side of things. In the past, I've written about interesting internal-combustion engine technologies, like
Mazda's spark-controlled compression ignition
and
Nissan's variable compression
.
Both improve efficiency and performance, but not so much as augmenting internal combustion with a conventional hybrid system. Does electric turbocharging fall into the same category? Someone from one automaker might say yes, but then why would engineering powerhouses like Mercedes, Porsche, and Ferrari all embrace it?
Photo by: Porsche
Ironically, for a technology that was developed in Formula 1, the sport will soon abandon electric turbocharging. To attract more engine suppliers, F1 is changing its engine formula for next year to abandon the MGU-H, deeming it too expensive and not relevant to road cars… just as more road cars are embracing this technology.
F1 is also upping the electric portion of its hybrid powertrain to achieve about a 50/50 split between engine and motor power. And hey, F1 is expanding its engine supplier base with Audi, Ford, and GM all joining the fray. F1 is a sport and a business, not simply a technological proving ground.
In any case, turbocharging is, in spirit, about not leaving energy on the table. An internal-combustion engine is going to produce a ton of exhaust gas that is pure waste. Why not make something useful out of that? And why not generate additional electrical energy from it while you're at it?
Engineering at its best maximizes the potential of what you have in front of you. This isn't to say that cars that don't use electric turbochargers are bad, or that there aren't legitimate reasons to skip out on this piece of tech. It's possibly something that only justifies itself in higher-end performance-car applications. There's an admirable engineering ideal with electric turbochargers that satisfies the nerd in me. Isn't maximizing potential something we should all strive for?
Further Down the Rabbit Hole
Why Carbon-Ceramic Brakes Are Expensive. And Why They Might Be Worth It
Why BMW's B58 Is a True Successor to the Toyota 2JZ
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