It is interesting that many people believe that the fuel actually explodes in the cylinder and drives the piston against the crankshaft. That is hardly the case. The IC engine is a heat engine just as the steam engine is a heat engine. The mileage depends on the efficiency of the conversion of fuel into heat. The fuel-air mixture burns: it does not explode. When the cylinder comes up it compresses the air creating an oxygen rich environment for the fuel mixture. The spark causes the fuel to burn and the heat from the oxidation of the fuel causes the fluid molecules inside the cylinder to expand rapidly and equally in all directions. The force acting on the piston head from the expanding molecules is what moves the crank shaft. The expanding forces against the side walls of the cylinder do not contribute to the force on the piston head and so contribute to the inefficiency. But the optimum fuel-air mixture for the most complete combustion is not used (BUT in MHO it could be). The mixture is always on the rich side to guarantee predictable and repeatable ignition without any dead cycles. The rich mixture is another inefficiency because it results in less than complete combustion. There are some other kinetic factors like the inertial forces and migration of the mixture through the manifold, which is why polished ports and manifolds yield more power. Increasing the compression would create a richer oxygen environment to produce a hotter burn (more heat) which is the whole objective of the IC engine. But too much oxygen would move the process from burn to explode which would damage the engine. There is the real limit on mileage and efficiency: How much heat you can get from the burn of the fuel without exploding it. In diesel engines the compression ratio is much higher and it is so oxygen rich that a spark is not required to initiate the burn. But here, again, the efficiency limit is imposed by the rate of oxidation; too fast and BOOM, you wrecked your engine. The reason diesel can tolerate higher oxygen ratios is because there is less BTU per unit volume than for gasoline, of whatever octane. So to get more mileage you need to figure out how to get more heat per unit fuel, a more complete burn and keep on the safe side of explosion. Now here is an interesting point. Some people talk about alternative energy on one side of the page and better efficiency on the other side. So they came up with ethanol. Ethanol has less BTU per unit volume than Gasoline. To save energy they advocate mixing ethanol with the gasoline and at some pumps it is already premixed. Question! How is mixing something with less energy with something with more energy going to result in increased efficiency, or better mileage, and so on. The heat produced by the burn will actually decrease, resulting on lower power. The problem of repeatable ignition cycles requiring a richer mixture remains. Is it an oxymoron? Or is it simply a moron idea? I am baffled by it. ----- Original Message ----- From: To: "Microcontroller discussion list - Public." Sent: Wednesday, June 27, 2007 9:35 PM Subject: Re: [EE] Simple fuel-saver, so they say > Knew there was a reason I kept my flak jacket.... > > Not rocket science, guys - You want MPGs, slow down. > Had my nose rubbed in it driving a VW Bus for 30 years; > @35 mph 24 mpg, 55 mph 16 mpg. It'd manage 70+ mph > on the flat, but the fuel consumption was intolerable, and > a cross wind would put you in the klong, so did'nt spend > enough time in that part of the envelope to get real numbers. > > Jack > > > ---- Nate Duehr wrote: >> Jinx wrote: >> >> > Jeremy Clarkson's London-Edinburgh-London trip in a V8 >> > on one tank of diesel and similar tests on the track by both >> > Mythbusters and a NZ motoring program. You can save >> > substantially by driving conservatively, including reducing >> > drag (windows and tailgate up) and weight (didn't Top Gear >> > calculate it costs UKP6 a year to haul a moustache around ?) >> >> A moustache? On your face? (???) >> >> Mythbusters also proved that tailgating large trucks helps a lot, but >> it's a pretty horrible way to die if you don't stop when they do. And >> illegal in most places to follow closely enough to get the big gains. >> >> Even following a large truck at "reasonable" distances at highway speeds >> helps a little bit. A nice long flat road, and a cruise control set to >> the speed of the truck ahead of you, and you'll save a few bucks during >> the trip... if the truck's not doing 90 MPH! (GRIN) >> >> Nate >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist