Garrett Motion electric turbocharger with power electronics and battery
Electric motors are undoubtedly becoming a larger and more important element of the automotive propulsion landscape with each passing year. Directly supplying torque to turn the drive wheels is the most obvious use case as in hybrid and electric vehicles. But motors are turning up everywhere including oil and coolant pumps and now turbochargers. Garrett Motion is preparing for what may be the first production application of an electric turbocharger in 2021.
Garrett, was spun off from Honeywell in late 2018, reclaiming part of the original name it had before being acquired by the same company in 2004. Garrett has been producing turbochargers since the 1950s and is one of the handful of market leaders along with BorgWarner, BMTS, IHI and Mitsubishi, all of whom are developing similar technologies.
Let’s back up with a quick primer on engine boosting. An internal combustion engine makes power by igniting a mixture of air and fuel in the cylinders. The more air and fuel that are burned, the more power that can be produced. A naturally aspirated engine draws in air through the open valves by the partial vacuum created by the piston moving downward in the cylinder. A supercharger is a mechanically driven pump that forces more air into the cylinder. These are traditionally driven by a belt off the crankshaft. They give excellent response because of the direct drive, but consume a lot of power to be driven at higher speeds.
A turbocharger does the same thing but it is driven by exhaust gases flowing out of the engine through a turbine. The gases spin up the turbine which sits on a the same shaft as a compressor wheel. As the turbine spins, the compressor pumps the air into the cylinders. Turbos are more efficient than superchargers thanks to lower parasitic losses but they can have lag in spinning up while they wait for exhaust pressure. That’s where adding an electric motor comes into the picture.
There are actually two distinct types of electric boosting devices for engines that have emerged in the past several years. The first type that is already in use by Volkswagen Group and Mercedes-Benz is an e-booster. This is essentially just the compressor side of the turbo paired with an electric motor. The size of the compressor is inherently limited by the size of the motor required to spin it at high speeds and e-boosters are used in a sequential combination with exhaust driven turbos. The e-booster gives quick low-end response from the motor and then as pressure builds, the larger turbo takes over to provide maximum boost.
The e-turbo as conceived by Garrett and its competitors adds the electric motor to a turbocharger in one unit. At lower speeds, the motor can rapidly spin up the turbo and build boost for excellent low speed torque and driveability. As the exhaust pressure builds, it takes over the job of driving the e-turbo. This allows engineers to use a larger compressor and turbine for more boost which in turn allows for a smaller engine displacement. The larger wheels on the turbo would normally cause even more response lag, but the electric motor addresses that problem.
The e-turbo layout also has another benefit. When the driver lifts off the accelerator pedal, the exhaust gases and inertia of the wheels keep the turbo spinning for a while. At this point, the motor becomes a generator that can charge the battery. In conjunction with a 48V electrical system, the e-turbo contributes to recuperating energy that would normally be lost out the exhaust pipe. In turn, that energy is then used to spin up the e-turbo the next acceleration is required. The e-turbo is less complex to package than the combination of e-booster and conventional turbocharger.
According to Geoff Duff, director of applications engineering at Garrett, depending on the specific engine configuration and e-turbo sizing, the e-turbo can contribute to a fuel efficiency improvement of up to 10%, although in most cases it will be about 2-4% better.
That efficiency improvement can be further amplified with more engine downsizing. The added responsiveness of the e-turbo overcomes the power loss of the smaller displacement. Typically turbocharged engines run with a lean air-fuel mixture at low boost to provide some extra power in that operating range. However, that raises the combustion temperature and produces more NOx. The rapid acceleration and boost build of the e-turbo allows the engine to run at the ideal air-fuel ratio more in this range.
Conversely, a conventional turbocharger with a smaller turbine for quick response gets hotter at high speeds. Typically, these configurations shift to a richer fuel mixture as speed increases that cools the turbine but also pollutes more and consumes more fuel. The e-turbo can be sized with a larger turbine that stays cooler but doesn’t sacrifice responsiveness and again maintains the ideal air-fuel ratio for lower emissions across the board.
When used on diesel engine, an e-turbo can contribute to a 20% reduction in NOx emissions. This will be particularly important for heavy duty applications that currently rely on diesel. While there are many efforts to electrify the engines in these larger vehicles, the bulk of batteries still poses a challenge because it eats into the payload capability. Batteries are especially challenging for long-haul trucking. Using e-turbos on those big diesels has potential to reduce the worst pollutants produced by those engines.
Garrett isn’t yet announcing what manufacturer or engine type its e-turbo will be used on in 2021. So far none of the other turbocharger manufacturers have announced specific production launches, but it wouldn’t be surprising if one or more arrive in a similar time frame to Garrett’s launch program or soon after.
Primary electric drive is projected to take an ever larger share of the new vehicle market in the 2020s with Navigant Research projecting that battery electric vehicles will account for more than 15% of global light duty sales by 2030. However, internal combustion engines paired with varying degrees of electrification are likely to be around for many more years to come and Garrett intends to be a part of the mix.
