A hydraulic hybrid vehicle uses hydraulic and mechanical components instead of electrical. A variable displacement pump replaces the electric motor/generator. A hydraulic accumulator stores energy. The vessel typically carries a flexible bladder of pre-charged pressurized nitrogen gas. Pumped hydraulic fluid is compressed against the bladder storing the energy in the compressed nitrogen gas. Some versions have a piston in a cylinder rather than a pressurized bladder. The hydraulic accumulator is potentially cheaper and more durable than batteries. Hydraulic hybrid technology was originally implemented in Germany in the 1930s. Volvo Flygmotor used petro-hydraulic hybrids experimentally in buses from the early 1980s.