IMO variable compression ratio is the answer
 

Lets suppose your car has a 3.0L V6 and 10.0:1 compression ratio
then you have 300 cc that must be filled with air/gas mixture about an optimal 15:1 to ignite reliable and cleanly.

But during traffic jams or stoplights, it can switch to 30:1 compression ratio.
Thus you have only 100 cc to be filled with explosive mixture.

Of course an 3.0L engine at 30:1 compression ratio would guzzle more gas than a normal 1L engine at say 10:1 because of larger friction and pumping losses.

Do you think variable valve timing would be needed at all for significant consumption improvements?

Variable compression ratio - Wikipedia, the free encyclopedia

Variable Compression is ALREADY available by using a turbocharger, with the realtime adjustment of the "effective" compression-ratio occurring via computer-controlled dump-value!

It's a mechanically external "huff & puff" operation that 'recovers' some free energy from the exhaust...unlike complex, internal, VVT mechanisms which eat energy.

the answer is stop/start engine software and electric powered a/c, mild hybrids, etc.

also, in densely populated areas, mass transit/rapid transit.

I filed a patent application for a variable displacement-compression engine in 2004, but later it evolved into an infinitely variable in wheel drive with capacitive regeneration. The patent was issued in 2010.

regards
Mech

http://www.animatedengines.com/gnome.html

Yea variable compression is not the lowest hanging fruit, not even close. The practical limit for mechanical compression ratios is maybe 14:1, for full throttle. At say 20-30% load there is a benefit to increasing the compression ratio even if the valve timing mechanism allows minimal pumping loss, but why not just introduce a little (cooled) EGR to bump up combustion pressure/temperature?

Forced induction with a high mechanical "compression ratio" (we're really looking for the expansion ratio here), and say "Atkinson cycle" intake cams to increase low load efficiency and decrease detonation tendencies is essentially variable compression ratio.

I feel like the "upgrades" that engines are actually going to see on a large scale are direct injection (already happening), further upgradess to cam phasing systems (can get rid of most pumping losses if the range of movement for the cams is larger), forced induction (as a cheaper way to get most of the benefits of variable compression, variable displacement, and more complex variable valve timing), cooled EGR (already happening), and maybe laser ignition or something of that flavor. These things are highly reliable, simple, and relatively cheap to design in, compared to things like fancier valvetrains, variable compression, HCCI, etc. which have many more technical challenges.

Sounds like you want a diesel engine.

It's the expansion ratio that counts. If you could have 10:1 compression and 14:1 expansion ratios, that would be the key. Honda has a small engine that does this.

...and, Direct Gasoline Injection (GM's SIDI) allows the compression-cycle to happen WITHOUT any "bucking" counter-pressure occuring from advanced-ignition timing, because combustion ONLY occurs on the down-stroke/expansion cycle, when it's finally injected into the cylinder ala' diesel cycle operation.

Constant pressure (diesel) vs constant volume (spark ignition) combustion.

i think i read somewhere that someone has already planning for a variable compression car and has already developed a prototype. i think it was saab, not sure...

DI injector current shows injector timing relative to TDC
 

Sorry, I probably should've said "pre-ignition temperature". EGR increases the mass of gas in the cylinder, which increases the pressure and temperature before ignition occurs. That's not saying that it helps combustion at all, or that it increases combustion temperature, etc. Well, the charge certainly starts at a higher temperature but since it's diluted it ends at a lower temperature most likely.

Ya know a few weeks ago we went around about the variable valve timing, that essentially is a variable displacement method.

But....at the end of the day....all we are trying to cut out of the equation is the pumping losses, which on their worst day are costing us 4% efficiency. I came away thinking the variable valve timing was a pretty good solution.

60% of the energy losses in the engine are due to thermal losses. What can we do to even get 1/3 of that back? Pretty much double our mileage if you could.

Uh wait where are you getting 60% from?

Pumping losses aren't a big deal at the end of the day like you say, but friction is! Friction eats away several times more energy than pumping. When that's considered, the actual heat cycle itself isn't doing too bad at maybe 50% efficiency. Could be bumped up to 60% with higher expansion ratio and multiple ignition points or something, but cooling losses are hard to solve when the engine is car sized and material constraints (and those emissions regulations) limit the temperatures we can burn fuel at. A second stage heat recovery solution (TEG, steam turbine, etc.) can get some of it back but the economics isn't super great.

I read somewhere that Saab had built an engine with an electric supercharger, nothing new there but seems like a viable thing to me.

Friction losses vary as the rpm squared, twice the RPM, 4 times the friction loss. Variable valve timing does more then reduce pumping losses, it allows for more expansion on the combustion cycle. A small low speed turbocharged VVT lean burn adiabatic Atkinson cycle engine would be most efficient, probably running on alcohol, and put into a very light car.

A question, how much heat energy is a Hypermiler like Pale Melanasian loosing in his Honda when he is averaging over 80 MPG? At EPA ratings the heat losss would be 60% to as high as 67%, but when you see what can be accomplished through Hypermiling then that same heat loss has been converted into movement of his vehicle.

If technique can reduce heat loss by say 50% then that 60% becomes 30%, not due to inefficiency in the engine, but due to inefficiency in the vehicle as a system. With sustained power requirements at less than 20% of available capacity, waste is inevitable.

I also think a lot of losses attributed to friction are losses due to reciprocation of the mass of pistons, a portion of the mass of connecting rods, hydraulic losses in oil circulation, as well as cam drive losses. All of these losses are greatly reduced by Hypermiling which minimises inefficient engine operation. Is it really the fault of the engine when you incorporate it into a poorly designed system. To me it's more like running your hot water all the time so you will have some hoit water when you need it, without waiting for the water to warm up.

regards
Mech

ummm no not really
 

SAAB developed a kind of oil-filled head gasket which led to the compression variation, but it was quite a complicated design, and even more complicated due to the valvetrain. Would be easier to apply to a 2-stroke design due to the absence of a valvetrain, altough a turbocharger with overbooster would be a more cost-effective option.

You have to consider at what time in the cycle. Before ignition, the pressure and temperature are higher because a greater mass (or number of particles or whatever you want) of gases (at approximately the same temperature) has been compressed to the same volume. After ignition, the same amount of heat energy from the fuel is going into a greater mass of gas, so the temperature is lower.