Diesel engines do not have an ignition or carburetion system. Because of this, they run cooler with less maintenance. Diesel engines will run longer on the same amount of fuel, so they are less expensive to operate. Diesel fuel is also less flammable, so it is considered safer to use. Diesel engines are built heavier to withstand the stress caused by the high compression ratios required to ignite the fuel as well as the stresses caused by their heavy-duty use, so they are tougher and last longer. They operate most efficiently at 50% load for long periods of time. For these reasons, a diesel generator has the advantage on construction sites and mines where they are required to operate lights, heaters, pumps and so on day and night.