Well, i've been a killjoy on this subject so many times now that i'm starting to feel a little reluctant to post. But, i can't help reacting when i read about the glorious future of the automobile (as well you know, FW :-).
I guess my stance on the issue of biodiesel / cars running on veg oil is simple enough: "It'll not happen". The energy-systems analysis doesn't hold up to scrutiny.
As one analyst put it;
"I am afraid that support for biodiesel by environmentalists is ultimately motivated by the impossible dream of a guilt-free, environmentally sustainable car. Environmentally sound transportation must be based primarily on compact cities, walking, bicycling and public transportation."
It is no wonder Clarkson is a fan. It's no wonder the automobile industry is touting hydrogen and biodiesel and electric cars and water-powered cars. Personal automobile use is one of the most fundamentally wasteful things that humanity gets up to. The amount of energy we use doing it is staggering, and i suspect there's not enough practically accessible "used vegetable oil" to cover even 1% of our current diesel usage (let alone our petrol usage).
The reason i only *suspect* this, and don't have the actual numbers is because two companies set up to advise and develop the "used chip fat as fuel" refuse to answer my questions regarding this and other issues. They seem unwilling to subject themselves to an energy-systems analysis (and frankly that leaves me very sceptical).
Other issues:
Currently, used vegetable oil (in practically collectable quantities; i.e. from restaurants, etc) does not actually get wasted. This stuff is generally sold to make products such as soaps and animal feed. Any shortfall in the availablity of waste for recycling, is made up (certainly in the case of soap manufacture) by increased use of the primary raw materials - oil/coal-based hydrocarbons. The net effect is that you get next to no actual reduction in total fossil-fuels emissions.
Also, the vegetable oil we use is a result of high intensity (fossil-fueled) industrial agriculture. This means that under current conditions it is a net-energy loss. So it falls into the same category as hydrogen. Until we have a renewable, sustainable source of hydrogen / vegetable oil that doesn't depend on fossil fuels, any attempt to use an energy-sink as our primary fuel will only increase the rate of depletion of fossil fuels, increasing greenhouse gas emissions.
I would also point to http://wcs.org/humanfootprint
According to the latest surveys from the Wildlife Conservation Society, humanity already uses up to 98% of the areas where it is possible to grow rice, wheat or maize in significant quantities (according to FAO estimates). We already use 83% of the arable land on the planet. Could the biodiesel folks in their precious cars please tell me where the hell we're gonna grow all the extra vegetables needed to fuel them? All the while keeping the population fed using less and less fossil fuels.
This fact is alluded to by yourself FW, when you point out that: "... so many people are doing this in the north of England now that several supermarkets are rationing sales of vegetable oil."
The private car is dead. It just doesn't know it yet. Every report you hear to the contrary is just shaking the corpse in an attempt to make it look life-like.
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