It's possible to improve economy by improving the efficiency of an engine, but you end up fighting some other things. Let's say that you optimize your valve timing so that you have a lower BSFC at cruising speed, AND more torque. Suddenly, your throttle plate needs to be closed more, which in turn hurts BSFC - probably not as much as you gained through improving efficiency, but you have to pair greater power with taller gearing, or your throttle plate is going to fight you.
Food for thought: Why does Chevy shut off 4 cylinders in the Corvette, if decreasing power does not save fuel?
Hypermiling the C7 - Taking the 2014 Chevy Corvette on a Fuel-Economy Run
Quote:
The C7 coupe takes all the fuel economy advantages of the C6 and stretches them. For instance, the C6's 436-hp LS3 6.2-liter V-8 engine was rich in low-end torque, so it didn't need to turn very fast to sustain speed, but it lacked some of the fuel-saving tech we have now. The C7's new 460-hp LT1 version of the same-size V-8 has the new tricks, including variable valve timing, direct fuel injection, and a cylinder deactivation system that has the engine firing only four cylinders under light loads.
Plus, where the sixth gear in the C6's six-speed manual transmission was a 0.50:1 overdrive, the seventh gear in the C7's manual transmission is an even deeper 0.42:1 overdrive. And the low rolling resistance Michelin Pilot Super Sport run-flat tires that the C7 wears are more advanced than the C6's Goodyears.
|
^ Cut the engine in half and add an extra gear on top, and you're saving fuel, mostly because you're keeping the throttle plate as close to wide open as possible while driving on the highway, and reducing the amount of air the engine has to pump.
Quote:
|
...So I filled up the new Corvette; it took only 3.34 gallons to refuel the C7 after the trip. That's a stunning 37.3 mpg over that course...
|
~
Honda's idea with the Insight was to downsize from a 4 cylinder to a 3 and gear it like a V8 (or taller), with an electric motor to add power on demand, and it works:
Close to 130mpg @ 35mph
Close to 120mpg @ 44mph
Close to 100mpg @ 62mph