Sunday, July 24, 2011

The magic carburator

If you are around in the car culture long enough you will eventually hear about some old crusty guy who invented a carburetor. There are some of you who probably know where this is going and for others this may be the first time you hear this. This story is complete BS and I will explain why.

The Story
The story always follows this pattern:
Some guy tells you a friend's relative (or distant relative's friend) was working on some big pig old (Cadillac, Lincoln, Buick, Mercury, Chrysler, or some other land yacht), and came up with a new carburetor design in the (40's, 50's, 60's, or 70's). It was so good he drove this vehicle (half way or the whole way) across the country and used only (a half, a quarter, an eighth, or a whole) tank of gas doing so. Upon returning home he shopped the design of this carburetor around to (GM, Ford, Chrysler) and they were very interested. A few days later (the government, Arab sheiks, auto executives, oil executives) showed up and (killed him, paid him millions, disappeared him) taking his new carburetor. Currently the (oil companies, government, car companies, oil cartels, or any combination of them) are sitting on technology that could easily give up (100, 200, 300, 500, or 1000) MPG in our current cars.

The Truth
Personally I am sick of hearing this story. I don't know if people actually believe this story, like telling it because they are into conspiracy theories, or like messing with people who don't know much about cars. Very often the story teller will mention a specific type of carburetor, a fish carburetor, a catalytic carburetor, or a vapor carburetor. Basically it is impossible to create a vehicle of that size that gets that type of mileage. My other thought is that people are confusing the ultra high mileage cars that various teams build with actual cars. Now the ultra high mileage cars take fuel savings to new levels and basically these contests are about who can abuse the rules the most. These things are slow, extremely  light, seat one, and are started by pull sting. To achieve these extreme mileages the teams go and find about the smallest girl they can to have as the driver and then build the car body around them. Depending on the class that these cars a competing in they typically get between 500 and 3000 mpg.

Now if you are going to hold those types of "cars" up as proof that we can build cars that get extreme gas mileage and Detroit, the Government, or the oil companies are just withholding this technology they you are kidding your self. To improve on fuel economy there are only about 4 things that you can do and most you can't change once you purchase the vehicle.

Engine efficiency
This is one of the things you can change some on an existing vehicle. Internal combustion engines are typically between 20% and 30% efficient meaning that they only extract between 20% and 30% of the energy of the fuel. Now for those of you who think that this is some massive conspiracy and that they should be closer to 100% then you are kidding you self. The internal combustion engine that has the highest efficiency is a combined cycle GE gas turbine that is about 60% efficient and is used in power plants. For reciprocating piston engines the best is about 50% and that is only achievable in large engines like this one with a bore of 3 feet and a stroke of 8 feet. For those of you who think someday we might actually achieve 100% efficiency you are kidding your self. Thermo dynamics puts a limit on the maximum efficiency that can be achieved, this limit is the carnot cycle efficiency. So lets assume that it is possible to have a 100% what would one of those big pig cars mentioned earlier get. Well if we assume currently one of those engines is 20% efficient (this number will help our calculation) and lets also assume that the vehicle gets 20mpg (a gross overstatement, but it makes the numbers work out nicely they probably got more int he range of 12-14mpg) then by putting a 100% efficient engine in it it would get 100mpg which is pretty good but is would be a gross overestimate for this car which is frequently used in the story.

Mileage through lightness
Another way to get good mileage is to have a lighter vehicle. Those high mileage competition vehicles weight int eh 100-200 pound category, sometimes weighing under 200 pounds with driver. You can make some changes to your vehicles weight but don't expect any significant ones unless you start taking out everything you don't need including seats, and other trim pieces. Even then your vehicle will still weigh significantly more than those mileage competition cars, hell most motorcycles weigh more than those cars. The cars in the story usually weight in between 5,000 and 8,000 pounds or between 25 and 40 times the weight of the high mileage competition cars.

Better Aerodynamics
This is something you really can't change on your vehicle for the better, but you can make worse. If you look at those high mileage competition cars they are extremely aerodynamic. They also present a very small cross sectional area which also has a lot to do with how much energy is needed to move the air out of the way. Now compare that to another favorite car in the story and you can see how bogus thing really are starting to become.

Go slower
One thing all of those high mileage competition cars have in common is they are slow. Typically they will start the engine get up to speed and then shut it off and coast to a stop then repeat. These cars would be fast if they could go 30 mph. At these slow speeds you have less wind resistance. Now compare that to any production car made in the last 60 years. For the most part all cars made after WWII can reach 100 mph some can even go well over 200 mph.

So what gets really good mileage
Even today's best production cars don't get as good of milage as the best ones of all time. If you want to maximize your fuel economy I suggest reading this posting of mine but don't expect incredible gains at best you might get 5 or 6 mpg extra. The problem is most people don't want the cars that get close to 100mpg as they seat one maybe 2 don't have much power, and aren't very safe. If you are curious about these cars here is a list of some of the more famous or unique ones:

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