Diesel fuels must have good flow properties at low ambient temperatures. These fuels have lower densities than normal fuels. Fuel-injection pumps work on the basis of volume, and thus the fuel mass flow is reduced by density decrease. This leads to lower engine power. In Finland, for instance, the engine power reduction is about 10% when the fuel is changed from summer grade to arctic winter grade.The problem could be solved in future by using fuel density sensors in electronically governed injection pumps or by using fuel heaters so that vehicles could run on high density fuels also in winter.