From the June 2016 issue
Turbochargers may be synonymous with big power and torque, but automakers have a very different motive for embracing forced induction. Smaller turbo engines fare better than naturally aspirated ones with similar performance on the EPA’s granny-like driving schedule. However, it’s not always clear if the fuel-economy advantage holds up on public roads with quicker acceleration and higher speeds.
To sniff out the real-world differences, we tested two Honda Civic sedans, each with the CVT, but one with the 174-hp turbocharged 1.5-liter four-cylinder and one with the 158-hp naturally aspirated 2.0-liter. By the EPA’s measure, the turbo Civic holds a 1-mpg edge on the highway with its 31/42-mpg ratings.
On a 300-mile loop of mixed highway, rural, and urban driving, the cars proved equally frugal by averaging 40 mpg. Digging deeper, we measured the steady-speed fuel consumption of the two Civics. Some of our results are astounding, such as the 50-plus-mpg both Civics achieve at 55 mph. The turbo wins across speeds ranging from 30 to 90 mph, with a 6-mpg advantage between 40 and 55 mph.
To generate the power required to maintain a particular cruising speed, any engine—small or large—must pump a corresponding amount of air. With equivalent gearing, the smaller engine requires a wider throttle opening to pump the same amount of air as a larger engine. Because pumping losses are lower with wider throttle openings, a smaller engine is more efficient.
Even at 90 mph, with the tach reading 2800 rpm, the turbo plays a minor role in cruise mode. This is precisely why nearly every carmaker will rely heavily on smaller-displacement, boosted engines to satisfy the fuel-economy mandate that requires a fleet-wide average of 54.5 mpg by 2025.
Spending the extra $1160 for the Honda’s turbo engine does have one clear advantage beyond efficiency: The additional power and torque clip 1.4 seconds from the zero-to-60-mph run and a full second off the quarter-mile time compared with the naturally aspirated alternative. But exceed the gentle, twinkle-toe throttle pressure we applied in our steady-speed tests and all efficiency bets are off. As boost rises, more fuel is injected and mileage drops. Precipitously.
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