Diesel-electric hybrid bus conversion. Engine of a diesel bus that is being converted to a hybrid diesel-electric technology. Here, the gearbox has been removed. This retrofitting project is being carried out by Transports Metropolitans de Barcelona (TMB), Spain. The conversion makes existing diesel buses cleaner, quieter and more efficient. Excess heat energy is converted and stored in high-capacity capacitors (ultracaps) on the roof of the bus. Electricity is also generated and stored during sudden braking. Photographed in 2012.