The world's metropolitan law-makers gathered in New York this week (14-17 May), at the C40 Climate Summit, with the aim of reducing global greenhouse gas emissions. One of the routes to do that is right under their noses, however – a diesel Audi A8, shown at the summit itself, that runs on squeaky clean fuel.The A8 TDI runs on Shell's Synfuel, a gas-to-liquid propellant manufactured from natural gas that's entirely free from sulphur and aromatics. When burned, Synfuel produces 35 per cent less soot, 93 per cent less carbon monoxide, nine per cent less nitrogen oxides and five per cent less carbon dioxide than conventional diesel fuel. Unlike diesel, it produces no sulphur dioxide at all. And, according to Shell, it can be used in any diesel-engined Audi model without any retrofitting measures.According to various fuel industry sources, Synfuel has various advantages compared to other alternative fuels. The raw material is available in sufficient quantities to meet global demand, it can easily be produced in petrol, diesel or kerosene form without additional steps like cracking or reforming, and it's compatible with existing engine and fuel distribution technology. It's also been proven reliable under extreme conditions, says Audi. The company's 2006 Le Mans winning R10 TDi racer ran on a Synfuel blend.So why isn't it available on the UK forecourts? Well, according to Shell spokeman Adam Newton, it already is, as a constituent part of the company's V-Power diesel fuel. "Gas-to-liquid diesel is already cleaning up cars in the UK, and we've committed to increasing our gas-to-liquid production ten fold by 2010," he told Autocar.