We've written before about just how worried automakers are at proposals by the U.S. Environmental Protection Agency to increase the ethanol content of pump gasoline from 10 to 12 or even 15 percent.

They fear it will damage older cars whose engines and fuel systems were never designed to handle that amount of the corrosive alcohol, and then owners will blame the automakers--not the lawmakers who have mandated a huge increase in U.S. ethanol fuel consumption.

Now a study done by respected auto-technology consultants Ricardo says that, in fact, very little harm will be done to the oldest of cars still on the roads, those built from 1994 to 2000.

Old cars: 1994 through 2000

Earlier studies looked at the impact on cars from 2001 through 2009, but 63 million older cars--which are 25 percent of the vehicles on U.S. roads--had not been evaluated.

The study was for the Renewable Fuels Assocation, a group whose members--not surprisingly--support adding more ethanol, which many of them produce, to the gasoline supply.

Ricardo says that the study showed "minimal risk" to the 63 million cars on the roads from 2000 or before. The company performed its study by focusing on the highest-selling platforms from six automakers whose products made up the bulk of U.S. sales from 1994 to 2000.

With a few caveats--going from E10 to E15 "should not" have an adverse effect on the performance or durability of 1994-2000 vehicles as long as they maintain "normal specifications" and a normal "usage profile"--the study concludes older cars will not be an impediment to the EPA's proposed rule approving up to 15 percent ethanol in gaosline.

Emissions, fuel systems OK

Specifically, a study summary says that assuming the car's engine-management system maintains the proper mixture for optimum combustion, the catalyst should be able to process any exhaust emissions, and evaporative emissions will be the same or potentially lower, since ethanol has a lower vapor pressure than gasoline.

The study also says that the impact of weather, road salt, and other environmental effects on the fuel system outweighs any degradation from increasing the ethanol in the fuel mixture. In a selection of fuel system components from a 1998 Ford Taurus, minor increases in exterior corrosion were seen, but no functional degradation.

In the end, Ricardo says, “The analysis concluded that the adoption and use of E15 would not adversely affect fuel system components in properly engineered vehicles, nor would it cause them to perform in a sub-optimal manner, when compared to the use of E10.”

Fear of failures

Automakers have disagreed, in varying degrees of vehemence, with this conclusion. They fear the wrath of customers angry that their cars couldn't run on a fuel they were not intended to use. Vulnerable components include engines, fuel pumps, and various rubber seals.

The Alliance of Auto Manufacturers reported that fully half the engines it tested had problems on fuel blends that included more than 10 percent ethanol. One problem, it said, lies in the engine-control system that may reduce the fuel content to compensate for a perceived increase in exhaust oxygen due to an extra oxygen atom on the ethanol molecule

The AAM says that could make the engine run hotter, which might damage the catalytic converter, making the car's tailpipe emissions dirtier overall. It might also cause the car's "Check Engine" light to come on, requiring a service visit.

The alliance has not yet responded to the Ricardo study. If it does, we'll give you the blow-by-blow.

Questions remain, but Congress likes ethanol

Ethanol, so the theory goes, can reduce the carbon footprint of vehicle miles, since the plants it's derived from absorb carbon dioxide from the air, meaning that the CO2 released when it's burned is not "new" carbon but has already been removed from the atmosphere during growth.

There are numerous questions about ethanol produced from corn, as it is in the U.S. It is the method with the lowest yield, and the "wells to wheels" carbon balance of the resulting fuel has been widely debated. Concerns over food displacement in agriculture have also been raised.