volt systems. Diesel-powered vehicles, including mobile construction equipment and heavy trucks use 24 volt electrical systems, as do many military vehicles. Research is ongoing into adopting a 42-volt electrical system standard for automotive electricity, but the entire electrical system will have to be redesigned and new components manufactured to work with the higher voltage. The main advantage of higher voltage is that electrical components can be made with less metal, saving weight and cost, and improving energy efficiency. Most modern systems, such as power windows, power seats, and power door locks, are electrically powered. Electrically driven power steering systems have been developed and are used in numerous models. High-efficiency all-electric refrigerant compressors for air conditioning are starting to be used, especially in hybrid or all-electric vehicles. The cigarette lighter receptacle serves as a de facto standard for use of portable 12 volt equipment in or near an automobile. Hydraulic The engine generally has a hydraulic pump mechanically driven by the engine, but there may also be electrically driven pumps. In passenger cars, the most common use of hydraulic power has been the steering system. Convertible tops may be raised and lowered using hydraulics. Windshield wipers were sometimes hydraulically driven, although this use mostly ceased after the late 1960s. On vehicles with little or no engine vacuum, hydraulic systems are generally adopted in place of vacuum systems. The French company Citroën devised a high-pressure hydraulics system for cars which was used for all manner of systems, even power-adjustable seats. The 1999–2004 Jeep Grand Cherokee had a hydraulically driven radiator fan, powered by the SUV's power steering pump. In vehicles such as heavy trucks and tractors, hydraulic systems are much more common. Hydraulic rams are used for accessories such as dump truck beds, cranes, loaders, and three-point hitches. Vacuum A commonly available source of power from an internal combustion engine is the partial vacuum available at the intake manifold. The piston engine is fundamentally an air pump, and it produces suction and partial manifold vacuum. Manifold vacuum varies depending on engine load and throttle position, and automobiles use vacuum reservoirs or "vacuum canisters" to provide a usable source under varying conditions. Turbo charged and super charged engines do not always produce vacuum; the intake manifold is actually pressurized when the turbo is spinning above a certain speed. Reservoirs and devices connected to the engine through check valves allow pressure to reduce when