List of technologies for low consumption on traffic jams/high power on demand dilemma
 

Feel free to include some other technologies (even if in development or theory stage) or discuss the ones mentioned here.

* Forced induction turbocharging/supercharging.
When feeding the engine with compressed air you get more power.
Of course the engine must produce more power to compensate for energy wasted compressing air.
Detonation limits power increases.
Supercharger - Wikipedia, the free encyclopedia
Turbocharger - Wikipedia, the free encyclopedia

* Cylinder deactivation
Deactivated cylinders don't use gas but they cause a bit of drag, the transition isn't that smooth and it's meachanically complicated. Also cylinders wear unevenly.
Variable displacement - Wikipedia, the free encyclopedia

* Variable compression
IMO this is the future as it not only varies the engine compression but it also reduces the combustion chamber size. Of course a smaller chamber uses less fuel.
Variable compression ratio - Wikipedia, the free encyclopedia
Saab Variable Compression engine - Wikipedia, the free encyclopedia

* Variable valve timing
You get the best of both worlds. An econobox-type engine with its low end power, smooth idling and low emissions for traffic jams and a sports-car like camshaft profile when hi power is needed.
Variable valve timing - Wikipedia, the free encyclopedia

* Electric hybrids
They use electric only for traffic jams, and gas engine for higher speeds.
The downsides? they are too complicated and require exotic materials like rare earth and Nickel. It is debated whether production of hydrids like the Prius pollutes more than the fuel saved.
Hybrid electric vehicle - Wikipedia, the free encyclopedia
Toyota Prius - Wikipedia, the free encyclopedia

* Kinematic energy recovery systems
They store energy usually wasted as heat braking the vehicle for later use.
There are many ways how they can store the energy. Electricity (either with batteries or supercapacitors), flywheels, even compressed air.
Then the stored energy can be used to move the vehicle directly (i.e. with electric motors), or to make the engine have more power like 2014 proposed F1 turbo tech.
Kinetic energy recovery system - Wikipedia, the free encyclopedia
Formula One engines - Wikipedia, the free encyclopedia
IMO the electricity recovered could also be used to electrolyse tap water and store Hydrogen, Oxygen or both to increase engine power output when needed.

* Lean burn
They intend to use less fuel relative to the air present in the combustion chamber volume.
Lean burn - Wikipedia, the free encyclopedia

I think that plug in vehicles are by far the best option, electric cars like mine don't have any exotic metals, just common ones like lead, copper, aluminum, my motor is all copper, steel, iron, brass, carbon with a little bit of plastic and varnish in there and auto makers that are making newer electric cars are finding ways to cut back on the size of rare earth magnets when they do use them.
The part of Lithium batteries that has the largest carbon foot print is the copper and aluminum that is used, it also has the most value and is the part most sought after when recycling them.

When an electric car is sitting still it's using very little energy, it's running the radio, lights, on board computer and HVAC system.

Crawling along in traffic most passenger vehicles are getting the worst mileage you will ever see, higher fuel use while sitting stopped while shifted in to drive then shifted in to park, that is why it's not uncommon to see gasoline vehicles sitting on the side of the road having run out of gas, something that happens way more often then electric vehicles running out of charge.

Human power. You can have your buddy get out and push you in neutral. There are also custom cars with pedals so your passengers could propel you at low speed with the engine off in neutral.

I think the answer is EVs with small ICE generators for when they need more range. I think if the vehicle is light enough, this backup generator can be portable so you don't have to waste energy lugging it around.

The volt could have been this vehicle, but, it is way too heavy and the ICE is much to large to be portable.

Another possibility is a trailer mounted generator. Imagine a mini-van type EV with a range of perhaps 50 miles. Great for many commutes, local errands or kid hauling. Add a trailer generator for weekend trips. This trailer could serve double duty. its primary purpose being to supply electrons, it could also be boat tail shaped so the minivan would be aerodynamically cleaner with it.

US patent #7677208. Crawling in traffic, the engine would run 5-10% of the time the vehicle was in that same traffic. It would only run at highest BSFC or not run at all.
Use any engine, or use and electric motor and a battery. The powertrain is the way to high efficiency, at least according to the EPA, Ford and the University of Michigan in their 2006 publication 80% improvement in fuel economy can be accomplished with powertrain configurations alone.

Add an engine designed to only run at best BSFC or not run (no throttle control), plus aerodynamics which lower overall demand, and LRR tires and you get to 120% improvement in mileage.

regards
Mech

The only answer; all else is nothing but band-aids:

http://upload.wikimedia.org/wikipedi...r_pleasure.jpg

Infinite valve timing using individual actuators instead of cams, see Koeniggsegg's R&D.

Pressurized air instead of combustion- I think a new PSA product does this.

Battery swapping stations for EV's- you swap your depleted battery for a charged one, like Netflix for batteries.

As someone already said everything is a band-aid.

Only solution is to get around the traffic.

Having said that compression is dynamic by nature in an engine; so variable compression ratios are already produced by variable intakes, super/turbocharing, variable valve timing, variable lift.

And the only technology for use in traffic jams is stop/start systems.

lean burn and multiple displacement systems don't work unless at constant speed.

no.

all other things being equal, shrinking the combustion chamber would increase compression ratio(which almost always has a positive effect to efficiency), but just the difference of a smaller combustion chamber alone will not use less fuel. a badly designed small chamber can do far worse than a well designed gigantic one.

EOC for nasty traffic.
Deep cycle battery to survive multiple restarts in case they are needed.

I'm merely addressing techniques to use in a conventional car (hopefully standard transmission) without redesigning the engine etc. Yes I do think the suggestions above have value but are more like R&D projects. In my humble opinion.