STOCKHOLM, Aug 29 (Reuters) - Sweden's Volvo , the world's second biggest truck maker, unveiled on Wednesday a line of truck engines adapted to run on renewable fuels and called for more efforts to make such fuels commercially available. The engines are powered by seven types of fuel, ranging from synthetic diesel to a mix of hydrogen and biogas. These are made from renewable raw materials and do not add carbon dioxide to the ecosystem, the company said. Rising emissions of carbon dioxide are