Perhaps the most straightforward of biofuel's drawbacks is the most obvious: It isn't petroleum-based fuel, so it will operate differently in engines designed for petroleum-based fuel.
Corn-based ethanol, for example, has a higher density than gasoline; fuel injectors have to be larger in an ethanol-only engine to match the fuel flow of a comparable gasoline engine. And alcohol fuels (including ethanol) can corrode or damage some of the metal and rubber fittings used in gasoline-powered engines. The conversion from one fuel to the other, in some cases, requires a range of new injectors, gaskets and fuel lines. And once the engine's running, the differences in combustion properties between gasoline and ethanol means that the ethanol-converted engine needs to have its ignition timing adjusted to operate properly [source: Tsuneishi].
Biodiesel doesn't fare much better. Because of the higher-than-petroleum gel point of many biodiesel-producing oils, a biodiesel engine can be difficult -- if not impossible -- to start in cold weather. The problem is even worse for pure vegetable oil, used as fuel in so-called "greasecars." Drivers of vehicles using these fuels often have heating units installed to keep the fuel tank and lines free from gelled fuel, or install dual-fuel systems that flush the engine with petroleum diesel on start-up and shut-down. A number of manufacturers sell components for biodiesel and greasecar conversions, and intrepid tinkerers often find ways to overcome the gelling problem. But the conversions add time and money to the biofuel equation, something that can be off-putting for potential biofuel users.
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