Downsizing saves fuel, so by all means shop for a 6 instead of an 8 or a 4 instead of a 6 cylinder engine. Inline 4s above 1.8L benefit noticeably from inertial compensation, as do all engines with fewer than 4 cylinders (found in microcars and perhaps, future hybrids).

For added power or more aggressive downsizing, consider a turbocharger. Porsche is so far the only manufacturer to offer a full VTG turbo on a gasoline engine; the exhaust gases are hotter than in a diesel so the VTG requires special materials. Other options: twin turbos (parallel in the case of a V6 (BMW), potentially in series w/ bypass in an inline 4, Citroen PSA), or a turbo plus detachable supercharger (VW 1.4L TFSI). All of these strategies minimize turbo lag - not high to begin with these days - to effectively zero.

Thanks largely to diesel engine development, turbos have come a long way since they more or less disappeared from the line-ups of gasoline cars. Drive a modern turbocharged car before you pass judgement on the technology and shell out for a bigger naturally aspirated engine.

Robert,

The following textbook will answer your question about the fundamental limits of efficiency:
Thermodynamics: An Engineering Approach (Hardcover)

Summary (paraphrased from what I remember from my Physics 1005 course): It's theoretically possible to make a heat-engine (like a gasoline engine or a steam engine) about 60% efficient. Some really good engines (say, a steam-driven power-plant or a diesel-locomotive) come surprisingly close to this limit.

Many engines are constrained by things like weight and performance requirements -- a 3000lb gasoline engine that turns 60% of the heat from the burned-fuel into force&movement will make a less-efficient vehicle than a 400lb engine that turns 40% of the heat into force&movement. Personally, I'd put the 3000lb engine in an electric generating plant and the 400lb engine in my car.

Engineering is fascinating discipline!

Sjc -

FEV recently studied a 1.8L turbocharrged quad valve w/ GDI and hybrid IMA with supercaps for Audi. The objective was to match the stationary and especially the transient performance of a 3.0L NA engine from the same manufacturer. It was an engineering success but we'll have to wait and see if Audi delivers it in a series production vehicle.

http://www.all4engineers.com/index.php;site=a4e/lng=en/do=show/alloc=3/id=3252

Toyota and Subaru are partnering on a similar concept. That said, it is possible to achieve substantial gains at modest cost even without hybridization, and this is likely to be the preferred route for the compact and subcompact vehicle segments.

Perhaps now GM will roll out their downsized engines with turbos that have been kept in Europe, or a few low volume mid/upscale brands. Redesign the IMpala, Cobalt, and Aveo, improve quality, improve fuel economy/tech, plus consolidate markets. Then you may have a credible challenge to the Japanese/European car makers. GM is also trying to fall back on the Up and coming China market. If they do not pay attention, the Chinese car makers (indigenous and joint venture partners) will eat their future.

Downsizing is probably the most common sense thing to do.

Smaller engine means you could have more engine bay room for a larger crush zone, use slightly less stout suspension components, brakes, & wheels (which each incremently adding up to enough saved weight to accomodate all of the other parts being downsized when everything is taken as an entire system).

Pleased to see Mercedes mechanical sophistication in US vehicles. Hope to see modern engine/trans instead of V8 Hemy and pushrod Dodge Caravan V6 dinosaurs. But no hope to see reduction in chronic to Mercedes and Chrysler vehicles obesity….

With obesity reaching close to 60% and growing fast in North America it will be difficult to over downsize the cars we drive unless we manage to lose a lot of weight. At 600+ lbs per couple, using lighter + stronger materials for the family cars or trucks may be an acceptable alternative solution.

Hybridization with much smaller, more efficient ICE gen-set + powerfull high torque electric motor + light weight super caps or quick charge-discharge batteries to assist accellerations and climbs may be a good interim solution.

Patrick, you are exception not a norm. On any given day just look around you and see how many obese people are there are. 60% is not really high number.

Biking to work would work wonders for a lot of people.

Rafael:

I wrote “Mercedes sophistication” not “Mercedes engine”. I know that it is not DB design, so no bubble burst there. Still great powertrain with CVT and with AWD.

Patrick:

V8 configuration is bad by itself. 6 cylinders is better from any respect, not to mention that these monstrous V8 gasoline engines are totally unsuitable to any application – you want power – use turbocharger, you want torque and towing capacity – buy diesel.
By the way, engine by itself could be of resent design, but cast iron block, pushrod valvetrain and two valves per cylinder, max power at 4000 RPM – sorry, I call it a dinosaur.

Drag is a big factor in fuel economy especially with all the highway driving in North America. Boxy styling on grills and hoods that looks like it is designed to create as much air turbulance as possible. Vehicles of all sorts are getting taller and wider, increasing the frontal area which is as important as Cd. Very driveable, attractive prototypes have been made for 20+ years with drag coefficients of 0.2 or less. Consumers need to be educated on how important drag is to their wallets. Dedication in the wind tunnel as opposed to the styling studio is a big part of the success of the Prius and Insight. I'm surprised there hasn't been a huge aftermarket parts market created to fix aerodynamic mediocrity left by stylists.