The Cosworth DFV in an internal combustion V8 engine, produced originally for Formula One racing by Cosworth. The "DFV" stands for Double Four Valve. Four Valve is from the engine having four valves per cylinder, and the Double refers to the entire engine is a double version of a previous in-line, four cylinder engine made by Cosworth, called the "FVA".
For a number of years, the engine was the dominant power plant in Formula One, including winning every race in the 1973 Formula One season. It was succeeded by the Cosworth DFY mid-way through the 1983 season.
The DFW was a 2,500 cc version developed for the Tasman Series in 1968-70. Only four DFW engines were built, but they were a success in the series. When the series changed engine specifications, the four DFWs were returned to England, and converted to DFVs.
The DFX was a 2,650 cc turbocharged version, used in the CART Indy car series. This engine dominated Indy car racing for over a decade, including ten consecutive wins (1978-87) at the Indianapolis 500.