It's like jelly. Well, actually, more like chewing gum.' As an experienced public speaker and award-winning television journalist, Briton Andy Pag doesn't usually have a problem with words. But he fumbles as he tries to describe the gloop that bungs up a motor when engine oil and cooking oil interact.
That's just one of a litany of mechanical problems Pag has faced as he journeys around the world in a 'biotruck' that runs on waste oil from restaurants - basically, chip fat. Pag is attempting to drive around the world producing no more than two tonnes of carbon emissions. For the mission, he bought a 22-year-old Mercedes-Benz 708D, formerly a 28-seater bus, he found in an English scrapyard.
Pag was in Hong Kong this week before flying to Los Angeles for the final leg of an epic journey that began in Britain in September 2009. He flew to Hong Kong from Bangkok, where his bus was loaded onto a container ship. He had hoped to join the ship rather than fly, but its captain wouldn't let him aboard for security reasons.
While the flights will have expanded the carbon footprint, Pag is philosophical. 'I'm not an environmental evangelist, I wanted to do it as an experiment.'
Pag is a seasoned traveller, having led overland trips across difficult terrain such as the Sahara Desert. He made headlines a few years ago when he took a team from London to Timbuktu in a van that ran on chocolate - out-of-date and misshapen waste from producers such as Mars. Then there was 'Grease to Greece' in the summer of 2008, a rally involving 10 cars racing from London to Athens with engines converted to take waste oil from restaurants and canteens.
At two metres tall, Pag needed a vehicle that he could stand up in and lie across to sleep for a trip of 38,000 kilometres running on old chip fat. He spend three months restoring the vehicle to ensure it was road legal and road safe. He then secured sponsors, including a company called TVT that converts exhaust systems. TVT installed particulate filters to capture much of the soot that would normally be produced when burning old cooking oil. The bus is able to cover five kilometres per litre, which is the same as it would on diesel, Pag says.
Pag refurbished the bus largely with discarded objects, including an old sink and offcuts of carpets. It was important to run it on waste oil as much as possible because of the ethical questions raised by using fresh vegetable oil. Rainforests are being cleared in places such as Indonesia for production of vegetable oil for biofuel, which negates the whole environmental aim, he says. But this does not come without problems.
'If it is used oil, it has a load of rubbish in it,' Pag says. 'So under the bed, I have a system of filtering and dewatering, so it dries the oil by heating it up and evaporating any water. It starts off sludgy, so after a day in the tank it becomes clear and see-through.
'And then I have a filtering system that is done with a centrifuge,' he explains. 'That spins the oil at 9,000 rpm and separates any solid waste, which goes to the sides and stays trapped in the centrifuge. The clean oil falls through and goes to the main tank. Then at the end of that process, I open up the centrifuge and clean it up - bits of sausage, chips and things like that.'
The problem with vegetable oil, when compared to diesel, is that it changes its viscosity. When it's cold, it is quite thick; when it's thick it doesn't burn efficiently; and if it gets too cold, it can solidify in the injectors and in the fuel filter, blocking the flow of fuel.
'To counter that I have a system where the fuel is heated up in between the tank and the engine to 28 [degrees Celsius], where it has the same viscosity as diesel. When the engine's cold I need to use biofuel, which is a fuel made from cooking oil. It goes through a reaction and that ... makes it a more viscous liquid so it behaves like diesel. So I have two tanks. When it's cold I start it with biodiesel. That warms up the engine for 10 minutes. Then I press a button on the dashboard to change to the used vegetable oil tank. And then, just before I switch the engine off, I have to run it on biodiesel to flush the used vegetable oil out.'
Sometimes the engine can get too cold. While Pag was in Tehran, the engine froze. 'The fuel system had frozen. So I had to start a bonfire under the engine to warm it up.'
Then there was the little hitchhiker. 'There was a mouse in the bus that would chew through the wires in the engine. So every morning I'd spend an hour trying to find which wire he had chewed through. In the end, I left out food so that he wouldn't eat the bus.'
The bus' advanced age also means there's plenty that can go wrong. 'Hardly a week goes by without doing something to it.' Which brings us back to the jelly and chewing gum.
'When you use biodiesel or cooking oil, there's a risk that it will come into contact with the engine oil. Over a period of time, and heating and physical manipulation, it causes the engine to go solid like jelly. But it just won't dissolve, it becomes like chewing gum.'
The bus engine was worn when he acquired it, with a gap between the pistons and the cylinder. 'On the underside of that you've got the engine oil sump, and on the top side you've got fuel that has been squirted in. So what was happening was fuel goes down the side of the piston and mixes with the engine oil.
'So the whole engine filled with this solid chewing gum. You just have to take the engine apart. Every bolt, every nut has to come out and be scrubbed with a wire brush.'
Another problem Pag had relating to the age of the bus was finding spare parts. The Mercedes-Benz design is licensed to India's Tata, which is a company that's hard to miss when travelling in the country. But when Pag needed to replace the bus' pistons, Tata did not have the right size. Mercedes came back with a quote eight days later for some pistons from Brazil, which would have cost Euro2,000 (HK$21,600). Luckily, Pag got to know a family of Sikh mechanics, who found a pattern in Agra for the pistons. 'So a few days later these pistons arrived and they were the same,' he says.
But pistons were the least of Pag's problems in India. He had arrived from Pakistan carrying a Thuraya satellite phone. What he didn't know was that the untrackable brand was the same used by the Mumbai bombers and now required registration. There were also rumours that the next terrorist attack would be carried out on paragliders - and he had one in the bus. As a result he was interrogated for two days and then jailed for seven more before being allowed out on bail.
'I then had to stay in India until the trial, which ended up being four months. So I went to south India and did paragliding there. The actual trial lasted three minutes and I was charged US$10 (HK$78).'
This won't be Pag's last expedition, even if his carbon footprint has been way smaller than the dent in his savings account.
For his next adventure, Pag will not reveal what he has up his sleeve, but he knows he won't be doing it in a Mercedes. 'I quite like the Toyota Hiace,' he says. 'And you can get Toyota parts everywhere.'