The key to the next major advance in internal combustion engine fuel efficiency could well be the electric turbocharger. At a recent fuel economy technology showcase at the U.S. Environmental Protection Agency (EPA) National Vehicle Emissions and Fuel Lab in Ann Arbor, Michigan, Valeo showed off the motor-driven turbo it will supply to an unannounced automaker. The first production applications are scheduled to begin arriving in 2016, according to the company.
The aggressive expansion of fuel efficient technologies, such as electrification, multi-speed automatic transmissions, and engine downsizing, has played a major part in increasing miles per gallon. The average fuel economy of the American new light duty vehicle fleet has improved by almost 25% over the past decade. Meanwhile, gasoline direct injection and turbocharging have enabled engineers to cut engine displacement by 30% or more without sacrificing the performance that drivers have come to expect. As of the 2014 model year, approximately 75% of Ford gasoline and diesel engines globally are turbocharged while 85% of Volkswagen engines are boosted.
Part of the concept behind boosted engines is to use smaller engines with turbochargers that provide performance on-demand. There has always been an inherent time lag, however, between the time the driver presses the accelerator and the generation of enough extra exhaust gas to spin up the turbo and provide boost. Mechanically-driven superchargers eliminate much of the lag at the cost of substantial friction at higher speeds.
Replacing the exhaust-driven turbine side of the turbocharger with an electric motor provides a number of advantages, most notably in packaging, responsiveness, and operational flexibility. One of the fuel economy benefits Valeo highlights is the combination of an electric turbo with the cylinder deactivation – i.e., the ability to shut off multiple cylinders under light loads in order to improve fuel efficiency.
The fuel savings achieved by shutting off unneeded cylinders can be quickly lost when driving on roads that aren’t completely flat. Even a mild grade can cause an engine to switch back to running on all cylinders in order to produce enough torque to maintain speed. “With an electric turbo, the engine management system can request small amounts of boost on-demand to increase torque while climbing a grade while keeping as many as half of the cylinders inactive,” Ronald Wegener, application engineering manager with Valeo, told me. “This can yield up to a 10% improvement in efficiency.”
Valeo has developed versions of the device for both 12V and 48V electrical systems so that the turbo can also be used as part of a mild hybrid system during off-throttle conditions. Intake air flowing through the compressor drives the motor to generate electricity, charging the battery. Audi is using this as one of the two forms of energy recovery on its Le Mans-winning R18 e-tron race car. Many of the current crop of Formula One cars have also adopted this approach. Earlier this year, Audi announced that the next-generation Q7 TDI, scheduled for model year 2016, would be its first production application of the technology.
Electric turbochargers also provide packaging benefits to engine designers. Traditional turbos require complex plumbing to route exhaust gases to the turbine side of the turbo and feed the boosted intake charge to the other side of the engine. Disconnecting the turbo from the exhaust allows designers to place the turbo wherever it fits best for packaging and performance.
Executives and engineers agree that while electric vehicles will gain market share in the coming years, internal combustion engines will likely remain the dominant powertrain choice in the transportation space at least through the 2020s. With engines continuing to shrink, it seems likely that electric turbochargers will account for a growing share of the boosted engine market in the next decade.
Tags: Clean Transportation, Electric Vehicles, Fuel Efficiency and Emerging Technologies, Smart Transportation Program
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