One testament to biodiesel can be found atop Mount Washington, where hikers passing the famous Cog Railway hardly ever encounter clouds of noxious smoke anymore. Instead, they'll simply get an enticing whiff of French fries.

Some hikers have long considered the coal-burning locomotives on the Cog Railway to be a nuisance, since they are none too pleased to see black smoke as a part of the White Mountains vistas. Some even partake in a tradition called "mooning the Cog," baring their rear ends to the train to protest its pollution and mock the visitors who can't be bothered to reach the summit on foot.

This practice may be a little less common now that the Cog Railway has largely moved to cleaner practices. In 2008, it debuted its first biodiesel engine. The railway now runs the bulk of its trips using biodiesel, with six locomotives using this fuel and only two steam locomotives still on the line.

Biodiesel isn't just suitable for New Hampshire tourist attractions. It has proved to be an interesting addition to the world of alternative fuels, giving drivers of diesel vehicles the option to run partially on vegetable oils and recycled cooking grease.

Although the majority of most diesel fuel is petroleum-based, some portion is derived from renewable sources. John O'Dell, writing for the automotive site Edmunds.com, says biodiesel is often derived from soybeans, canola, or other vegetable oils. It can also be synthesized from cooking grease recycled by restaurants.

There are several different blends of biodiesel available. The Alternative Fuels Data Center, part of the Department of Energy, says the concentration is indicated by the number following the letter B. For example, B2 fuel contains only 2 percent biodiesel and 98 percent petroleum diesel while B100 is entirely biodiesel. The most common blend is B20, or 20 percent biodiesel and 80 percent petroleum diesel.

You can't simply put biodiesel into any tank. You need to have a diesel engine, and you'll want to make sure it is rated to use biodiesel.

There are a number of advantages to using biodiesel instead of 100 percent petroleum diesel fuel. The Environmental Protection Agency says it will reduce a number of emissions that can contribute to pollution and climate change, including carbon dioxide, unburned hydrocarbons, and sulfates. O'Dell says biodiesel is nontoxic, biodegradable, and has lower levels of the carcinogenic pollutant benzene.

Biodiesel is seen as an environmentally friendly option for other reasons as well. The Alternative Fuels Data Center notes how soybean crops and other vegetables used in the creation of biodiesel will absorb some of the carbon dioxide produced when biodiesel is burned. The fuel can also be readily produced in the United States, helping to avoid reliance on foreign oil.

Instead of a pungent stench of burning diesel fuel, biodiesel will give off a more pleasant aroma akin to fried food. The only downside is that it may make you crave a burger and fries more often than usual.

Biodiesel won't impede the performance of most engines, and may even help to improve fuel economy. An analysis by Consumer Reports found that a 2002 Volkswagen Jetta using B5 diesel fuel got 22.6 miles per gallon, compared to 20.1 miles per gallon using petroleum diesel. G. Tyler Miller Jr. and Scott E. Spoolman, authors of the 2012 textbook "Living in the Environment: Principles, Connections, and Solutions," say the fuel economy of biodiesel engines can be up to 40 percent better than that of gasoline engines.

Lubrication of the engine may be more efficient with biodiesel. The Alternative Fuels Data Center says biodiesel has a higher lubricity, or ability to lubricate the fuel injectors and pumps. It also has a higher cetane number, meaning it is easier to ignite the fuel and burn it more completely.

Despite this burning efficiency, biodiesel is also considered safer than diesel because it has a higher flash point. As such, it is less likely to ignite after a crash or other mishap. Biodiesel will also cause less environmental damage if spilled.

There are certain disadvantages to biodiesel, however. While it has been shown to lower emissions for certain pollutants, it also produces higher levels of nitrogen oxides than petroleum diesel under certain circumstances. However, O'Dell says some biodiesel producers have been able to create fuels that reduce nitrogen oxide emissions as well, and that overall improvements to diesel engines have also helped to reduce pollutants.

Other environmental disadvantages are less apparent. Miller and Spoolman say growing crops for biodiesel production typically requires the use of fossil fuels, thus reducing the benefit of its reduced emissions. Biodiesel production also has the potential to reduce overall carbon dioxide reduction if large areas of land are cleared for plantations to produce soybeans and other crops used for the fuel.

O'Dell says biodiesel is currently present in most diesel sold in the United States. However, it is also typically limited to a B5 blend. Many automakers recommend that drivers don't use a higher concentration of biodiesel, since they have not determined whether these concentrations are safe to use.

Older vehicles may encounter some engine troubles when using biodiesel. The Alternative Fuels Data Center says the polymers used in the engines of vehicles built before 1993 may break down under repeated biodiesel use. In vehicles with the model year 2007 or later, higher concentrations of fuel may accumulate in the engine lubricant under certain circumstances when using a higher concentration of biodiesel.

Some compounds in biodiesel can crystallize during cold temperatures. However, biodiesel typically has cold weather additives to prevent this from happening.

B100 biodiesel has a unique set of concerns. O'Dell says pure biodiesel can solidify or develop mold when stored. It can also clog parts of the fuel system, especially in older vehicles.

While there are resources available for drivers to make their own biodiesel, it is generally inadvisable to use homemade fuel in a vehicle. The quality of the fuel can vary considerably, and this will also affect your engine performance.