Volvo recently unveiled a line of truck engines that can be driven without emitting any environmentally harmful carbon dioxide and are made from renewable raw materials, the automaker reports. The engines are powered by seven types of fuel, ranging from synthetic diesel to a mix of hydrogen and biogas. While none of the engines was being produced for the market, manufacturing could begin within a few years if the fuel was available, Volvo Chief Executive Leif Johansson said. Johansson told a news conference that politicians and energy firms needed to do more to make sure renewable fuels were developed further and made more available to consumers.