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FORCING MORE air and fuel into the combustion process enhances power on demand, a great idea in these days of downsized engines for better mpg. Hitherto, forced induction has been of two types: A supercharger is a compressor deriving its spin directly from the engine’s crankshaft. A turbocharger’s spin comes from energy in the exhaust; there’s no direct connection to the engine’s mechanicals.
Each has tradeoffs. Because it depends on exhaust flow, a turbo’s boost cannot help but lag behind demand. A supercharger’s boost is more prompt, but it saps engine power for this quicker response.
French supplier Valeo is proposing a third option: driving the forced induction with electricity. Valeo is calling the device an electric supercharger. Some media prefer the term “electric turbocharging” (see Automotive News, August 4, 2014, http://goo.gl/gAWQjF), and it’s optimistic to think the technically precise EFI (electrically forced induction) will survive online terminology.
In any event, compared with conventional forced induction, EFI has its tradeoffs as well. Its electrical demand calls for upping the car’s electrical system from today’s 12 volts to 48 volts. Automakers have considered higher voltage before, with one advantage being lighter yet more efficient wiring harnesses. (Aircraft have been using higher voltage systems for years.) One tradeoff is serious arcing in careless maintenance.
The elimination of lag is EFI’s long suit. Valeo says the device is designed to idle at 10,000 rpm and can spin up to a boost-producing 70,000 rpm in less than a second.
The electricity doesn’t come free, but Valeo reports that net fuel consumption benefits of electrically forced induction are on the order of 7 to 20 percent. This latter improvement would accompany regenerative braking that captures the car’s kinetic energy in retardation.
Another benefit is underhood packaging. A supercharger requires access to its mechanical drive. A turbocharger needs proximity to the exhaust flow. But an EFI can be mounted near the engine’s intake manifold, an optimal location typically cooler than the exhaust and not as restricted a space as at the front of the engine.
On the downside, cost is a factor. EFI has the plumbing necessities of conventional forced induction, plus its own electric drive hardware.
Overall, automakers are interested. BMW and GM are exploring the concept.
Audi recently tested a RS5 TDI diesel fitted with electrically forced induction at Pikes Peak. This German automaker is expected to be first in bringing EFI to production by 2016. ds
© Dennis Simanaitis, SimanatisSays.com, 2014