Before mains electricity and the formation of nationwide power grids, stationary engines were widely used for small-scale electricity generation. While large power stations in cities used steam turbines or high-speed reciprocating steam engines, in rural areas petrol/gasoline, paraffin/kerosene, and fuel oil-powered internal combustion engines were cheaper to buy, install, and operate, since they could be started and stopped quickly to meet demand, left running unattended for long periods of time, and did not require a large dedicated engineering staff to operate and maintain. Due to their simplicity and economy, hot bulb engines were popular for high-power applications until the diesel engine took their place from the 1920s. Smaller units were generally powered by spark-ignition engines, which were cheaper to buy and required less space to install.