With an increasing emphasis on fuel economy, some manufacturers are treating turbocharging as the holy grail. Ford, for example, has dumped a lot of R&D into its upcoming "ecoboost" engines. Leaving aside the fun factor of turbocharged engines, is this a good idea?
IN THEORY:
Turbocharged engines should be more efficient because they recapture lost energy in the exhaust. They should also be more efficient because they let the engine effectively run in two states of tune--on boost when accelerating hard, and off-boost when idling, cruising, braking, etc. This should make it more efficient than a naturally-aspirated engine with similar peak horsepower.
IN PRACTICE:
It doesn't seem to be a huge advantage. It's certainly not a consistent advantage. Let's do some comparisons with ratings from the US EPA website:
Jetta 2.0T (200 hp): 22 mpg city, 29 mpg highway.
Accord i4 (190 hp): 21 mpg city, 30 mpg highway.
Accord v6 (271 hp): 19 mpg city, 29 mpg highway.
Mazda CX-7 AWD: 16 mpg city, 22 mpg highway.
Mazda CX-9 AWD: 15 mpg city, 21 mpg highway.
Toyota RAV4 v6 AWD: 19 mpg city, 26 mpg highway.
BMW 335i: 17 mpg city, 26 mpg highway.
Cadillac CTS (3.6L DI): 17 mpg city, 26 mpg highway.
So what's the problem here?