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With gas prices hitting their summer peak, conversation quickly turns to what might replace rotting fossils as the source of propulsion for vehicles of the future.

The crystal ball is understandably clouded, what with politics, international relations and other issues all having a bearing. But generally the engineers within car companies are talking about six or seven possibilities.

The hottest topic, but probably the furthest from widespread availability, is hydrogen.

The problems are production and distribution. Producing hydrogen or turning it into a usable gas or liquid, requires copious amounts of energy, so much so that much of the development work is centring on the use of nuclear or solar energy. Burning fossil fuel to create the energy needed to make hydrogen and save fuel isn't terribly efficient.

The distribution of hydrogen will require a completely independent and new infrastructure -- an expensive one.The most likely alternative to come into play in the immediate future are fuels from renewable sources: such as biogas and biodiesel -- sometimes referred to as sun fuels because they are made from plants or compost, animal waste or organic matter.

This is already a proven source of energy throughout Europe. I've driven bio-fuelled Volvos in Sweden, refuelling them at public stations.

Public transportation and commercial trucks using the fuel are common in that country and elsewhere in Europe. Brazil has been producing and using biofuels -- methanol produced from sugar cane -- for more than 30 years.

These fuels emit fewer toxic nasties, and can be mixed with existing fossil-based fuels to improve their emissions and reduce the higher costs of producing biofuels.

Compressed natural gas (CNG) is in use in many areas of the world. It is a practical and fairly efficient source of energy from a huge supply. CNG is commonly used in vehicles designed for dual-mode use -- able to switch from conventional fuel to CNG at the touch of a button.

It burns more cleanly than other fossil fuels and is common in Europe, where the more densely packed population makes distribution more efficient.

There is some experimentation with combining natural gas with other substances, such as naphtha and diesel fuel, but the issue is that the base product remains fossil fuel.

Liquefied petroleum gas (LPG) has been in use in Europe for years but is falling from favour due to problems with its volatility (explosiveness) and the reliability of engines using it. Although a byproduct of the refining process, LPG does not mix with other fuels and is unsuitable for dual-fuel use.

Synfuels hold considerable promise -- these are synthetically manufactured fuels which are commonly a mixture of conventional fossil fuel (gas or diesel) and other fuels, including biofuels, methane etc.

The problem is that the main source of energy is the traditional fossil fuel.

In all of these, the case becomes stronger every time the price for a barrel of oil goes up. With oil at $40 to $45 a barrel, these alternative fuels were not considered feasible. At $60 a barrel or more, they bear more study; at higher prices they just might fly.

rrussell@globeandmail.ca