Whew! Old thread. How did I miss this one for so long?
I think I'll jump in here, because this is a very interesting thread, and some errors need to be corrected. First off, let me say that the mythical high-mileage carb is something that I've been aware of and researching for many years. Charles Nelson Pogue's infamous carb was a reality, as well as a whole host of contraptions like the Nay box. Did they work?
One gallon of gasoline has 1.3 x 10 to the 8th power of energy in Joules. That's 166,000 btu's. I've been told by engineers that the btu output combined with the explosive nature of gasoline is sufficient that if one gallon of the stuff is appropriately vaporized and exploded, it would be sufficient to lift the Empire State Building (365,000 tons) one foot off it's foundation. Clearly, the modern combustion engine is leaving much on the table.
I'll also, right off the bat, dispense with conspiracy theories. If a 100 MPG engine was simple and easy, *every* manufacturer would be building it. Later on in this post I'll talk about what they *are* doing, though.
The problem of building a high-mileage vehicle (it goes so much deeper than just the engine, doesn't it?), I think can be boiled down to aerodynamics, rolling resistance, engine thermal efficiency and engine mechanical efficiency, which can be further broken down into energy extraction and friction.
Problem #1) Rolling resistance, and boy, do our trucks have a problem. Pretty simple to test, pop your F150 in neutral and try to push it (safely, please!). Then do the same with a Corolla. This issue is compounded with 3/4 tons. Truck running gear takes a bunch of energy to overcome, and unfortunately there isn't much we can do about it.
Problem #2) Aerodynamics. It's obvious that, say, an '86 F150 is a rolling brick when it comes to aero. The most aerodynamically efficient shape that can still occupy significant volume is a teardrop. Cars that start to look like a truncated teardrops are the most efficient (think Honda Insight or BMW Isetta).
Problem #3) Engine friction. Specifically the rings of the piston. Engineers have been trying to overcome this one for years. Piston rings account for the vast majority of the friction in an engine. Don't believe me? Put an engine together without rings and spin it over.
Problem #4) Thermal Efficiency. Peak thermal efficiency for current technology is about 40%. The rest of the energy is dissipated by fins, radiators, or goes out the tailpipe.
Problem #5) Gasoline. It's a horrible fuel with a distillation curve a mile wide. It's only redeeming quality is that it packages well.
-I'll be breaking these down one by one as I have time over the next couple of days. Feel free to comment or correct me if I make an error (I read it on the internet, so it must be true, right?
)
Corrections and comments (please don't be offended, I've believed much of the misinformation being quoted at one point, too).
"I've also read than when running fully vaporized, you can run a mixture as lean as 20:1." Run, yes. Ignite, no. Chrysler's lean burn ran at a max of 18:1, and only under ideal conditions. As those cars aged, they fell out of those ideal conditions and started running poorly.
"How would you measure a mixture as lean as 20:1?" The easy/cheap way would be a wideband O2 sensor. The Innovative LC1 with gauge sells for less than $200.
"Is there a point where the mixture will become too lean to ignite?" On the engines I've tested, it's about 16:1 where lean misfire sets in. Some engines are capable of a bit more.
"going any leaner than about 15:1 would get you into the realm of too much heat." At heavy or wide open throttle settings this is true, but under light throttle/high intake vacuum situations it is not. Remember that the only concern here is melting a piston or detonation. I've set up engines to run at a constant 15.5:1 ratio at cruise speeds of 75 mph with no ill effects.
"100mpg is not feasible in a speed going car (IE anything other that a wind tunnel designed body and faster than 70mph) " With current technology this is correct, but relatively small improvements in thermal efficiency and energy extraction could push us beyond this number easily. I remember a 1985 Ch*vy Sprint that could easily get in the 60's with a carb (55 mph commute).
"One other note, if you add a supercharger to this setup, you get what's called the Miller cycle, which Mazda has played with before and Toyota uses in their Prius hybrid, I believe." The description of the Atkinson cycle and the Miller cycle is accurate, but the Prius has no blower, thus is an Atkinson cycle engine. By the way, the lack of low-end torque matches the hybrid well, because of the high torque electric motor.
The biggest leap that manufacturers have adopted in recent years is gasoline direct injection and variable valve timing. It makes possible the concept of 'stratified charge' and variable compression. You can essentially have an Atkinson cycle when you want it and set the compression ratio wherever you need it. Heady stuff, but it still doesn't address piston ring friction and extracting energy from gasoline.
