A synthetic diesel fuel derived from natural gas, an excellent candidate for direct-injection engines. DME contains no sulfur and burns more cleanly than conventional diesel fuel, so it produces fewer particulates. It behaves much like propane, and its cetane number is 55–60, up to 20% better than modern diesel fuel. However, the viscosity of DME is 10 times less than that of diesel fuel, which makes it difficult to find pumps and valves that will not overheat or leak. Also, the entire fuel system must be kept under a pressure of at least 5 bars at room temperature to keep the DME liquid until it is injected.