While there is no clear-cut answer as to which is truly the most efficient and cleanest – and there are many approaches to clean energy – fuels created from renewable resources, or biofuels, may very well have bragging rights for being the most green of all of the various options.

The term “biofuel” refers to a wide range of fuels that are produced from biological material such as wood, waste, hydrogen gas, and alcohol.  It is considered a renewable energy source and the two most common forms of biofuel are biodiesel and ethanol.

Biodiesel is typically produced from oils or fats (such as those  used in cooking) combined with alchohol.  Biodiesel is the biofuel of choice in Europe and it can be used in virtually any diesel engine.  It’s important to note that Rudolf Diesel’s original engine was powered by peanut oil.  They’ve been used in production automobiles since the 1930s.

________________

VOTE NOW FOR THE 2011 DIESEL CAR OF THE YEAR

SPONSORED BY THE DIESEL DRIVER MAGAZINE
________________

Diesel engines are very efficient in terms of converting heat energy into power.  They use the heat of highly compressed air to ignite a spray of fuel introduced after the start of the compression stroke.  Diesel fuel requires less refining than petrol and is generally cheaper, taxes notwithstanding.

First generation ethanol is produced by the fermentation of sugars (typically derived from wheat, corn, or sugar cane, among others) and second generation ethanol is made from agricultural waste products such as straw husks and leaves, which removes it from what has been a fuel v. food controversy in some regions.  It can be used in today’s petrol engines as a partial replacement to gasoline.  Blends of up to 15% ethanol are common.

While ethanol has a smaller energy density compared to gasoline, which means that more fuel is required to produce the same amount of energy, it does have a higher octane rating, which means that manufacturers can increase an engine’s compression ratio for increased thermal efficiency.

The Renewable Fuel Standard Program that was initiated in the U.S. in 2005 calls for 36 billion gallons of renewable fuel to be produced by the year 2022.  Current U.S. production of first-generation ethanol is 12 billion gallons and much of the balance is expected to be second-generation ethanol.

Second-generation ethanol is already available at the pump in Denmark and is sold alongside petrol and diesel fuel.  It costs (on average) 10 cents per gallon more at the pump but consumers there are purchasing it despite the higher price point.

Today, diesel fuel including biodiesel is popular in Europe where over 50% of new car sales are diesels.  In the U.S. that figure is in the single digits.  Ethanol is more widely used in the U.S., where several states including Florida, Hawaii, Minnesota, and Missouri, require ethanol to be blended with petrol.  Many cities are required to use ethanol blended petrol because they do not meet federal air quality standards.

E85 (85% ethanol) is popular in the Midwest, where corn, the primary ingredient in ethanol, is produced, but there were fewer than 2000 service stations in the U.S. offering E85 as of early 2009.   Unlike diesel fuel, which costs less to produce than petrol, ethanol costs more to produce and, since most ethanol is produced from corn and the electricity needed to produce it comes mostly from coal-fired plants, there has been considerable debate as to exactly how sustainable today’s ethanol product actually is.

The engine of a 2009 BMW 335d

It is highly efficient in converting heat energy into work.

A diesel engine differs from its gasoline counterpart in that the ignition of fuel is caused by compression of air in its cylinders instead of by an ignition spark: the high compression ratio causes the air in the cylinder to become hot enough to ignite the fuel. This is referred to as compression ignition as opposed to spark ignition.

Diesel engines burn fuel oil, which requires less refining and is cheaper than higher-grade fuel such as gasoline. During the combustion process, the stored chemical energy in the fuel is converted to thermal, or heat, energy. The temperature in each cylinder rises as high as 2,480 °C, thus creating pressures of about 100 kilograms per square centimeter. The pressure pushes against the tops of the pistons, forcing them to the other end of their cylinders. The pistons are connected by a rod to a crankshaft which they turn, thus supplying power to a vehicles or machinery.

Diesel engines were first used in automobiles in the 1930s. The first manufacturer to produce a diesel-powered car was Mercedes-Benz in 1936.

In Europe, ca. 50% of all new car sales are diesel. In the U.S. that figure is closer to 3%.