The hot tube engine is a relative of the hot bulb engine with better timing control. The hot bulb engine only ran well at one speed- and a low one at that, typically 100 RPM. The timing of a hot tube engine is controlled by means of varying the length of the Hot-tube ignitor, which is longer and thinner than the hot bulb on a hot bulb engine. Length of the tube controls when the charge ignites, and allows different operating speeds to be selected. If made variable, this makes for adjustable engine speed, but also induces a mechanical weakness in the engine which tends to lead to failure. Both engine types are now replaced by diesels, which can be made to operate over varying loads and speeds in any size with modern methods. Hot bulbs are still used in remote areas where the extreme fuel flexibility is a major advantage. Hot tube engines may also be found there, if their difficulties with the adjustable tube can be overcome, or accepted and lived with.