A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Has a 200 mpg carburetor been suppressed by the oil industry?

July 28, 1989

Dear Cecil:

For years I've been hearing about fantastic carburetors that can give your car up to 200 mpg. But supposedly the automakers and Big Oil won't allow them to come to market because they'd wreck the industry. The people who tell you this are usually conspiracy buffs who offer it as an example of how the masses are duped by the Illuminati, so you have to be skeptical. But still I wonder: is the 200-mpg carburetor a complete fantasy, or does something like it actually exist? Do you have one on your car?

Cecil replies:

Nah. My main energy-saving strategy is to go only places downhill from me, so I can just put it in neutral and roll. Admittedly this system has its limitations, but it works better than "200 MPG carburetors," which at best are a fantasy and at worst a fraud.

Alleged high-mileage carbs are based on a beguilingly simple principle. Here's how one of my correspondents explains it:

"Detroit carbs put gas in the engine by spraying it in; much of the gas goes into the cylinder still in droplets and burns incompletely. High-mileage "vapor" carburetors pre-warm the gas using exhaust heat pumped through an in-line chamber. This enables the gas to evaporate quickly but thoroughly. More gas is burned and less goes out the tailpipe as pollution. Detroit seems to avoid these designs because they cost more. (Remember saving five cents per Pinto?) But better carbs are out there for the tinkering."

Someone also sent me a report that supposedly originated with the Carb Research Center of Oklahoma, which promotes vapor carbs. The report makes the astonishing claim that theoretical maximum fuel efficiency for a conventional auto is nearly 2,900 mpg. It goes on to tell the story of the original 200-mpg vapor carb, invented in the mid-1930s by one Charles N. Pogue. Another reader says vapor carbs work but they have a big drawback: backfiring.

While I don't want to belittle blue-collar ingenuity, the vapor carb's inventors are trying to solve a nonexistent problem. According to John Heywood, a professor of mechanical engineering at MIT and an authority on internal-combustion engines, incomplete burning of fuel is insignificant in modern cars. Fuel combustion today typically exceeds 97 percent. While it's true cars aren't very efficient — only 20-35 percent of the fuel energy is converted to useful work — that's mostly due to heat loss (through the engine block, out the exhaust pipe) and unavoidable energy loss during burning itself.

The theoretical (and unobtainable) maximum efficiency for a small car like a Honda Civic is around 200 mpg; for your big beaters it's much lower. Claims to the contrary are fraudulent, and I gather Professor Heywood said as much in a report he wrote for the Postal Service, which was investigating high-mileage carb vendors for fraud.

Carburetors in general are an obsolete technology now being replaced by electronic fuel injection, which offers superior emission control. Truth is, vapor carbs are the equivalent of the improved buggy whip. Forget 'em.

This is not to say super-high-mileage cars couldn't be built. On the contrary, there are plenty of proven energy-efficient technologies available, none of which has lacked for publicity or industry backing.

A survey in Technology Review listed ten experimental cars developed by seven major automakers that got highway mileage ranging from 71 to 110 mpg. A few years ago Renault trotted out a rig that got 121 mpg on the run from Paris to Bordeaux. I'm told the gas mileage record for motorcycles is about 400 mpg, and if you really want to go crazy, a GM subsidiary recently built a solar-powered prototype called Sunraycer that doesn't use any gas at all. Weighing only 400 pounds, it completed a 1,800-mile race at an average speed of over 40 mph. On the downside, it seats only one, has no trunk or luggage rack, and barely has room for a pair of fuzzy dice. But hey, life is full of trade-offs.

High mileage is never the result of a single miraculous component, such as a carburetor. Rather it's the sum of numerous small improvements. Among these are lightweight materials, low-friction tires, improved aerodynamics, flywheels to store and reuse energy now lost during braking, and "ultra-lean-burn" engines for more efficient city driving (already available in certain Toyotas sold in Europe).

Another improvement used in some high-mileage prototypes is the continuously variable transmission: instead of clumsily shifting gears, the cars shift transmission ratios gradually, typically using an ingenious (and conceptually quite simple) arrangement of belts and cone-shaped pulleys. Soon to come, it's believed, are ceramic diesel engines using turbochargers and perhaps stratified-charge engines that combine the best features of gasoline and diesel technology. While 200 mpg is pushing it, the experts think 100 mpg cars are within range of current technology.

Sounds great, you say? Well, don't rush down to the auto showroom just yet. It may be years, if ever, before the new technology becomes widely available. That's not because the automakers are conspiring to withhold it, but rather because they doubt the public will buy it. People today are less concerned about energy efficiency, and for good reason: corrected for inflation, gasoline today costs less than it did in 1973. Fuel-efficient cars tend to be little cars, and the size trend in recent years has gone the opposite way.

The only reason gas mileage has improved at all in recent years has been government-mandated fuel-economy standards, and God knows some recent administrations did everything they could to frustrate those. The superefficient cars now on the drawing boards don't figure to be cheap, and unless gas prices skyrocket, the fuel savings probably won't cover the higher cost. Barring a sudden burst of altruism on the part of the car-buying public, chances are you won't see ultra-high-mileage cars for sale until we're down to the last two gallons of Arab oil.

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