Any real motor-head out there has heard of compound turbos, and chances are most of them will think it’s a fantastic thing. If one turbo is good, two turbos is better, and if some boost is good, boost multiplication is way better than any times tables you memorized in third grade.
Typically, turbo compounding has been defined as using one turbo to compress air into a second turbo. Some set ups might even use two turbos to compress air into a third. Any combination of turbos is possible with the end result being higher volumetric efficiency when compared to no turbocharging, or even using a single turbo.
Since we know turbocharging is great for increasing the amount of air going into the engine, what if we could use this spinning turbo to do something else in addition to providing boost? With a compound turbo setup, one turbo does not always have to be feeding another turbo. What if the turbo could be attached to an electric motor/generator?
This is a technology that is currently being used in some performance applications, as well as being developed for regular cars and trucks. The 2014 season of Formula 1 featured hybrid powered cars that used a V6 engine with a turbo compounded with a motor/generator.
This system uses a gasoline engine that is a pretty typical engine. It also uses a motor/generator in a series/parallel configuration to help drive the wheels. This is typical setup for many hybrids currently on the road. The difference is in the way the turbo is set up with an electric motor generator.
As the exhaust exits the engine, it passes through the turbo driving the turbine wheel, the turbine wheel is attached through a shaft to the compressor wheel. The intake air going into the engine passes through this compressor wheel where it is pressurized putting more oxygen into the engine. No big deal, this is a normal turbo setup. Well, things get a bit crazy on the compressor side of the shaft.
The shaft in the turbo that connects the turbine and the compressor also connects to an electric generator. When the turbo is boosting is can also produce electricity. Turning the generator in this fashion produces electricity more efficiently than turning it directly from the engine. The energy in the hot, escaping exhaust gases would be wasted if not harnessed in this manner.
The turbo helps the engine have increased volumetric efficiency, and the electric motor at the drive axle helps to increase mechanical efficiency. If this technology were applied to a diesel powered engine the drivetrain would also have a higher thermal efficiency. An increase in these efficiencies always leads to more power on less fuel, with fewer tailpipe emissions.
Also, as a boost to performance, the motor/generator compounded with the turbo will also work as a motor. This can be used to get the turbo up to speed much earlier than the exhaust gases will. This would make it possible to run a much bigger turbo on much smaller engines. Also, there would be zero turbo lag. The turbo makes the motor/generator more efficient, and the motor/generator makes the turbo more efficient, and both of them make the entire powertrain more efficient.
The Mercedes F1 team blew away the competition in 2014 with this new compound hybrid turbo setup, but they did things a bit differently than the competition. One of the ways they made this configuration work so well is they split the turbo. They put the turbine housing on one end of the engine, and the compressor housing on the other end of the engine. In the middle, essentially down in the valley of the V6 engine, they put the motor/generator. The shaft that connects both sides of the turbo and the motor/generator together, runs through the valley as well. This is good because it keeps the very hot turbine side of the turbo away from the compressor side where the fresh air is being compressed to enter the engine. All of this make the turbo setup even more efficient.
With a quick Google search for “electric turbos” you will find all sorts of nonsense showing small DC motors attached to turbo compressor wheels, or even just small fans. All sorts of claims can be found regarding the amount of boost and power enhancement these little fans can affect. These claims are bogus. In order to get the compressor spinning fast enough you must have a significant amount of force. Escaping exhaust gases can provide this power, and a very large electric motor can provide this power. The electric motor needs a tremendous amount of current to keep the turbo spooled.
Using the compound electric turbo setup, we only need high current to get the turbo going, then the high volume and velocity of the escaping exhaust gases will keep the turbo going. The electricity generated by the fully spooled turbo will provide current to power the motor/generator attached to the drivetrain, and power to recharge any high-voltage batteries that are usually found in the system. Putting a little fan on the intake after the air filter will not do any of this because it just isn’t powerful enough.
Seeing all of this technology working in the high-performance world of motorsport is exciting. They can spend a lot of money developing, testing, and breaking these things out on the track. Then, as these things usually go, it’s only a matter of time before we start seeing these compound turbo, motor/generator setups in the cars we drive. If they can make them last on a race car, they can make them last on your car. The ways that engineers find to make cars ever more efficient seem limitless.