A gyrobus is a special bus which does not use a normal engine. It has a big flywheel of steel or other materials (weighing about one ton) rotating at very high speed (RPM). By rotating at such high speed, the flywheel stores large amounts of kinetic energy. This big wheel moves the wheels of the bus. At special stations, electric engines accelerate the flywheel so the bus can still run. There are not many buses of this kind because they are very expensive.