Turbocharging alone saves no fuel
To the Editor:
I've noticed that in Automotive News and other publications, turbocharging is generally lumped in with direct injection and cylinder deactivation as a fuel-saving technology.
Simply turbocharging an engine will not reduce its thirst for fuel. What turbocharging does do is make an engine more volumetrically efficient; i.e., it produces more power from the same displacement.
Effective use of turbocharging to reduce fuel consumption requires replacing an existing powerplant with a smaller turbocharged engine, generally of similar power output.
An excellent example of how this all works is the Ford Taurus. For 2014, Ford's largest sedan is available with two turbocharged EcoBoost engines (a 2.0-liter four and a 3.5-liter V-6) and one normally aspirated engine (a 3.5-liter V-6).
Comparing the V-6s (both with all-wheel drive, as the turbo V-6 comes only with awd) the normally aspirated mill receives an EPA combined mpg estimate 1 mpg better (18 vs. 19, respectively) than the turbocharged engine. The EcoBoost engine does produce significantly more power, however.
Comparing the EcoBoost four-cylinder engine to the normally aspirated V-6 (both with front-wheel drive, as the EcoBoost four comes only with fwd), we see how turbocharging can help reduce fuel consumption. While the smaller engine produces slightly less horsepower than the V-6, it does produce more torque. To me, the difference in power output is largely a wash. Yet the EPA combined estimate for the EcoBoost four-cylinder-powered vehicle is 3 mpg better than for the normally aspirated V-6 (26 vs. 23, respectively).
Turbocharging is an excellent tool to make new vehicles more efficient, but by itself it does not help reduce fuel consumption.
Consumer Guide Automotive