As carmakers downsize internal-combustion engines, they have to balance increased efficiency with the performance that many buyers demand.
Volvo is proving that there is plenty of performance to be wrung out of its four-cylinder Drive-E engines.
The Swedish carmaker has unveiled a prototype version of its new 2.0-liter gasoline engine, fitted with three turbochargers, that produces a staggering 450 horsepower.
That's more than 200 hp per liter, which for many years was viewed as the practical upper limit on power achievable from production engines.
One of the three turbos is electric, meaning the engine should be efficient and responsive as well as powerful.
Volvo Drive-E engine with electrically-driven turbocharger
This becomes more a problem with larger turbos, which are typically needed to get large amounts of power--say, 450 hp--from small engines.
The electrically powered turbocharger spools up far more quickly than those powered by exhaust gases, which allows that turbo to work effectively at lower revs.
This not only improves the responsiveness of the engine, but means drivers don't have to venture as high into the rev range--which increases fuel consumption.
While Volvo didn't specifically say so, it appears the electric turbocharger may only be used at low revs. The pair of exhaust-driven turbos likely takes over at higher speeds, where they can operate most efficiently.
To keep up with the volume of air being pumped into the engine, the powertrain also features a high-pressure fuel system that operates at more than 3,600 psi.
2016 Volvo XC90 R-Design
The triple-turbo four demonstrates the flexibility of Volvo's Drive-E powertrain family, which now includes the T5 turbocharged four-cylinder, the T6 four-cylinder with turbocharging and supercharging, and the T8 plug-in hybrid powertrain that will debut on the new 2015 Volvo XC90 SUV.
Volvo calls the engine a "concept" and hasn't discussed plans to use an electric turbocharger in future vehicles--but thus far, every Drive-E version that the Chinese-Swedish carmaker has shown has gone into production.
Audi demonstrated electric turbocharging earlier this year on its RS5 TDI concept; it will use the technology on the SQ7 SUV, a performance version of the next-generation 2015 Q7.
Meanwhile, Ferrari has suggested it may augment a future supercharged engine with an electric turbo for added responsiveness.