Espresso fuel (Day 89)

But there are other more obscure (and sometimes less practical ways) to power vehicles.

Air, waste cooking oil, waste vegetables, beer and spirits, chocolate, nappies (diapers), sawdust, nuts, styrofoam and other waste or co-products all have the potential to fuel cars.

In fact, finding ways to convert industrial co-products into biofuel always seems a sensible and sustainable way to re-use our raw materials - especially for high volume commodities like coffee.

Dr Chris Chuck, Whorrod research fellow in the University of Bath's Department of Chemical Engineering, says that: "Around eight million tonnes of coffee are produced globally each year and ground waste coffee contains up to 20 per cent oil per unit weight.

"This oil also has similar properties to current feedstocks used to make biofuels. But, while those are cultivated specifically to produce fuel, spent coffee grounds are waste. Using these, there's a real potential to produce a truly sustainable second-generation biofuel."

Oil can be extracted from coffee grounds by soaking them in an organic solvent, before being chemically transformed into biodiesel via a process called "transesterification".

Researchers at the Centre for Sustainable Chemical Technologies at Bath made biofuel from ground coffee produced in 20 different geographic regions, including caffeinated and decaffeinated forms, as well as Robusta and Arabica varieties.

The research found that there was a reasonably standard composition and little variation in the relevant physical properties of the fuels, irrespective of the source. This means that all waste coffee grounds are a viable feedstock for producing biodiesel.

The researchers suggest that while coffee biodiesel would be a relatively minor part of the energy mix, it could be produced on a small scale by coffee shop chains to fuel vehicles used for deliveries.

These same delivery vehicles could be used to collect spent coffee grinds and take them to a central biodiesel production facility to be processed. Companies such as London-based bio-bean already produce biodiesel and biomass pellets from waste coffee grounds.

The researchers estimate that waste and defective beans discarded by industry offer the potential as a sustainable fuel source. Even a small coffee shop would produce around 10kg of coffee waste per day, which could be used to produce around two litres of biofuel.

Times certainly have changed. One of the most successful advertising campaigns ever was Esso's 'Put a tiger in your tank'. Today, you could put just about anything in your tank, but 'Start your day - and your car - with a coffee' may just be around the corner for some.

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