When it comes to fuel efficiency, the United States and Europe have been at opposite ends of the spectrum.  Automakers in the U.S. (and those based in Asia) have been touting the benefits of hybrids—using electric power to propel vehicles short distances in congested city driving and relying on conventional gasoline-powered internal-combustion engines to propel the vehicle at higher speeds—while the Europeans have been successful in convincing their buying public on the efficiency of diesel power as a way to boost fuel economy.

During a recent trip to Europe, I decided to put the European philosophy of fuel economy to the test with some time behind the wheel of the 2009 BMW 123d coupe.  While BMW has no plans to offer the 123d in the U.S. in the near-term, they suggested I give the car a try because of its “unique” engine: a variable twin-turbo 2-liter 4-cylinder that  produces 204 hp @ 4,400 rpm and 295 lb.-ft. of torque @ 2,000 rpm.  Yes, that’s more than 100 hp/L.   And it provides all of this with an estimated fuel economy of 54.3 mpg on the European test cycle, without any complex battery systems or other worldly technology to support. 

While it is true that the 2010 Toyota Prius attains an estimated 72.4 mpg (EU test cycle) average, it produces only 134 hp.  Admittedly, the 123 was designed to traverse the Autobahn at respectable speeds, not inch along the streets of downtown Tokyo.  BMW claims the car can go from 0-100 km/h in just 6.7 seconds, while the Prius requires nearly 4 seconds longer to reach 100 km/h, which makes all the difference if you want to maintain driving pleasure, while still sipping fuel. 

BMW admits its engineering team wasn’t solely focused on fuel efficiency when they developed the 123’s engine.  They wanted the car to be fun to drive.  That’s why they added variable twin-turbos—a smaller diameter turbo is used to provide added boost at low rpm, while a larger second turbo keeps the power flowing at higher engine speeds (BMW has applied this technology to its larger 6-cylinder diesels for several years, but this is the first time it has been deployed on a 4-cylinder engine).  Consequently, the turbo lag problem of many single-turbo systems is essentially eliminated..  And because diesels tend to be heavier than gasoline engines, additional attention was paid to weight reduction: the engine uses an aluminum crankcase along with thermally joined grey-cast-iron cylinder liners.

OK, enough about the tech, how does it drive?  BMW is right about one thing: there’s no turbo lag.  Torque is available on demand no matter what point of rev band you’re in.  Shift the engine into a lower gear when passing and the 123 just gets up and moves with little effort.  Attaining top speeds while merging onto the Autobahn is done with equally little effort.  During one of my longer driving sessions totaling 540 miles I achieved an average speed of 73 mph with a fuel economy rating of 35 mpg.  My overall fuel economy for the two weeks of driving came in at 37 mpg at an overall average speed of 58 mph.  What’s most important to note here is the fact that I didn’t skimp on using the accelerator during most of my driving—I was traversing the Autobahn at speeds in excess of 110 mph on several occasions, just to keep up with the other Bimmers and Mercs that were flying past me, and the 123 kept up with them with little problem, proving that you don’t have to sacrifice fun for fuel efficiency.