The British company Rotron, which specialises in unmanned aerial vehicle (UAV) applications of Wankel engines, has designed and built a unit to operate on heavy fuel for NATO purposes. The engines uses spark ignition. The prime innovation is flame propagation, ensuring the flame burns smoothly across the whole combustion chamber. The fuel is pre-heated to 98 degrees Celsius before it is injected into the combustion chamber. Four spark plugs are utilised, aligned in two pairs. Two spark plugs ignite the fuel charge at the front of the rotor as it moves into the combustion section of the housing. As the rotor moves the fuel charge, the second two fire a fraction of second behind the first pair of plugs, igniting near the rear of the rotor at the back of the fuel charge. The drive shaft is water cooled which also has a cooling effect on the internals of the rotor. Cooling water also flows around the external of the engine through a gap in the housing, thus reducing the heat of the engine from outside and inside eliminating hot spots.