Alternative fuels

The environment is a topic of ever-growing concern. As every expert in the field will agree, the need to control all forms of pollution has become critical. The transportation industry is doing its part, making efforts in recent decades to develop viable alternatives for petroleum-based gasoline as fuel for motor vehicles. Scratch beneath the surface, however, and you’ll find that while the future holds promise in this field, some of the proposed substitutes aren’t as eco-friendly as they might seem.

Energy density
Before looking at the particular characteristics of the various fuels, let’s compare their energy density (also called specific energy, that is, the amount of energy stored per volume). It is expressed in BTUs (British thermal units) per litre.
 

FUELS	BTU
Diesel	35,620
Gasoline*	29,025–33,245
Propane	23,000
Ethanol	19,790
Liquefied natural gas	19,390
Methanol	14,900
Liquid hydrogen	8,970
Compressed natural gas	7,650
* The specific energy of gasoline varies depending on the composition of the crude oil and distillation techniques.

Ethanol
Ethanol, also known as grain alcohol, can be extracted from corn as well as a variety of biomass sources, such as sugar cane. It is a less corrosive fuel than methanol; like the latter, it will help keep an engine’s combustion chambers clean, and it has a high octane rating. Ethanol can be used to fuel a standard vehicle without any major engine modifications having to be made. Finally, it produces less hydrocarbon (HC) and carbon monoxide (CO) emissions than gasoline.

Ethanol’s high cost—nearly double that of gasoline—and complex manufacturing process, however, make it less attractive as a substitute for fossil fuels, at least over the short term. And, as can be seen in the table above, its specific energy is only about two-thirds that of gasoline. In other words, you would have to fill up more often. Moreover, whether used as a simple gasoline additive or in its pure state, ethanol can contribute to the formation of smog if its vapour pressure is not adjusted to match that of pure gasoline: it vaporizes faster than gasoline, thus releasing more volatile organic compounds (VOCs), which are factors in smog formation. Finally, because ethanol is more corrosive than gasoline, some automakers fear it would cause problems in the fuel systems of vehicles that are not driven every day. 
 
There are environmental disadvantages further up the chain: the tractors that are needed to harvest the corn or other biomass that is fermented to make ethanol, along with the fermentation process itself, produce carbon dioxide (CO₂) emissions. In addition, the fertilizers used to enhance the plants’ growth contaminate streams, rivers and groundwater, and produce nitrous oxide (N₂O) emissions.
 
In other words, the green benefits of this alternative fuel don’t quite outweigh the disadvantages. This is especially true considering that recently built vehicles all feature highly effective antipollution systems; the benefits to the environment would therefore be seen only in older vehicles.

Compressed natural gas
Once compressed, natural gas has a low energy density, which means it has to be stored in big, very heavy, highly resistant tanks. These factors, along with the fact that engine modifications are required for a car to run on natural gas, and that the fuel contains no lubricants, make it a less-than-perfect alternative. Currently, compressed natural gas is only used for fleets of vehicles in urban centres where frequent refuelling is easy. Compared to gasoline, however, it does offer significant reductions in HC, CO and CO₂ emissions.

Propane
Like compressed natural gas, propane is suitable for commercial fleets in urban settings. HC and CO emissions are slightly lower than with gasoline, and CO₂ emissions are far lower.

Methanol
Methanol has various advantages that put it at the top of the list of fuels likely to replace gasoline over the near term. The benefits of this liquid fuel—also called wood alcohol or methyl alcohol—include a high octane rating and lower CO₂ emissions (one of the chief causes of the greenhouse effect). Unlike gasoline, ethyl alcohol doesn’t produce unburned hydrocarbons, which form ozone at ground level—a major culprit in smog formation. Lastly, burning methanol leaves no engine deposits.

Unfortunately, methanol has its downside as well: it is extremely toxic, and can enter the body by inhalation or absorption through the skin. It evaporates slowly, which means cold engine starts are more difficult. In daylight, a methanol flame has almost no colour, so it is virtually invisible; it’s odourless, so it can’t be detected by smell either. It is very corrosive to metals, so specific materials have to be used for the engine components. Lastly, because its specific energy is near the bottom of the table, a methanol-powered vehicle requires a fuel tank about twice the size of that in a standard gas-powered car to cover the same distance.

Chrysler, Ford and GM have overcome some of these difficulties—like cold starts and the colourless, odourless nature of the methanol flame—by introducing gasoline/methanol blends. Unfortunately, producing and burning methanol are both harmful to the environment, as they create CO₂ and aldehyde emissions. Aldehydes generally are severe eye irritants, while formaldehyde is a major concern because it is thought to be a carcinogen.

Another issue is that production of methanol from coal releases twice as much CO₂ as does burning gasoline. If methanol is produced from natural gas, the net effect is approximately 10% less CO₂ emitted. These are issues that engineers will have to conquer before methanol becomes a viable fossil fuel alternative.

Liquid hydrogen
It may be the ideal fuel for a rocket booster, but liquid hydrogen poses serious problems when it comes to its use in motor vehicles. To start with, it is very difficult to liquefy. It must be stored at a temperature of -253°C, which requires a complex system. Marketing it as an alternative will require construction of an entirely new distribution infrastructure. The benefits of hydrogen include the fact that it produces practically zero pollution, and does not require major changes in engine design. Automakers including BMW, Ford, General Motors, Honda and Mercedes have been working for some time to perfect hydrogen-powered vehicles.

Until the future arrives…
Until the day comes when hydrogen and fuel-cell technology at last offer sustainably viable alternatives for the fossil-fuel-powered internal combustion engine, consumers do have an attractive, efficient option: the gasoline/electric hybrid system.

By combining a gasoline engine with an electric motor, designers can use smaller cylinders in the engine, considerably reducing fuel consumption and emissions. When combined with a continuously variable transmission (CVT), the benefits, which include greater driveability, are significantly increased.

There is still a long road ahead before we perfect production of a fuel that enables vehicles to be powered efficiently and economically—without creating new health and environmental problems as a result of the effort to eliminate certain of the disadvantages of gasoline. There are several technologies that show promise, and we can be confident that the experts are working hard on developing the ideal fuel of the future. And it’s not hard to imagine the future that lies ahead for the person who finally does develop the miracle solution!

© August 2008. CAA-Quebec, all rights reserved.