has anybody thought of using an engine on them?
Yes, some gliders are fitted with small power units and are known as motor gliders.
Gliding
Although adding to the weight and expense, some gliders are fitted with small power units and are known as motor gliders. This avoids the inconvenience of landing out. The power units can be internal combustion engines, electrical motors, or retractable jet engines. 
how long do gliders stay in the air They can stay airborne for several hours. what is gliding? It is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. who created the first glider? The Germans when? In  the 1920s and 1930s how do they fly? Through air that is ascending as fast or faster than the glider itself is descending, thus gaining potential energy. what's the fastest they can go? UNANSWERABLE so you mean they use lift to fly? Yes, ridge lift, wave lift etc. how are they launched? Different methods, namely aerotowing, winch launching, bungee launch etc. what's the maximum distance they have flown? Up to 1,000 km what do you do to land? The pilot will 'land out' - choose a location where he can land safely without damaging the glider, the pilot, or property such as crops or livestock.
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Gliding [SEP] Use of engines or motors
Although adding to the weight and expense, some gliders are fitted with small power units and are known as motor gliders. This avoids the inconvenience of landing out. The power units can be internal combustion engines, electrical motors, or retractable jet engines. Retractable propellers are fitted to high performance sailplanes, though in another category, called touring motor gliders, non-retractable propellers are used. Some powered gliders are "self launching", which makes the glider independent of a tow plane. However some gliders have "sustainer" engines that can prolong flight but are not powerful enough for launching. All power units have to be started at a height that includes a margin that would still allow a safe landing out to be made, if there were a failure to start.