the late 1940s. In 1951, MAN built the K6V 30/45 m.H.A., 1 MW prototype engine, which had, for its time, an exceptionally low fuel consumption of just , equivalent to an efficiency of 45.7 per cent. This was possible because of the advanced turbocharger design, comprising a five-stage axial compressor combined with a nine-stage radial compressor and an intercooler. Use of turbo-diesel engines in road-going vehicles began with trucks in the early 1950s. The prototype MAN MK26 truck was unveiled in 1951, followed by the production model MAN 750TL1 turbo-diesel in 1954. The Volvo Titan Turbo truck was also introduced in 1954. By the late 1960s, demand for increasingly powerful truck engines led to turbo-diesels being produced by Cummins, Detroit Diesel, Scania AB, and Caterpillar Inc. In 1952, the Cummins Diesel Special became the first turbocharged car to compete at the Indianapolis 500 motor race and qualified on pole position. The car was powered by a inline-six engine producing . Research into smaller turbo-diesel engines for passenger cars was undertaken by several companies through the 1960s and 1970s. Rover built a prototype 2.5 L four-cylinder turbo-diesel in 1963, and Mercedes-Benz used a five-cylinder intercooled turbo-diesel engine in the 1976 Mercedes-Benz C111-IID experimental vehicle. The first turbo-diesel production car was the Mercedes-Benz 300SD (W116) saloon, which was sold in the United States from mid-1978 and powered by the OM617 five-cylinder engine. A year later, the Peugeot 604 D Turbo became the first turbo-diesel car to be sold in Europe. Turbo-diesel cars began to be widely built and sold in Europe during the late 1980s and early 1990s, a trend that has continued to the present day. Since the 1990s, the compression ratio of turbo-diesel engines has been dropping, due to better specific power and better exhaust-emission behaviour of turbocharged engines with a lower compression ratio. Indirect injected engines used to have compression ratios of 18.5 or higher. Following the introduction of common rail engines in the late 1990s, compression ratios decreased to the range of 16.5 to 18.5. Some diesel engines built since 2016 to comply with the Euro 6 exhaust emissions regulations have a compression ratio of 14.0. Characteristics Turbocharging can greatly increase the power output of a diesel engine, bringing the peak power-to-weight ratio closer to that of an equivalent petrol engine. Improvements in power, fuel economy, and noise, vibration, and harshness in both small- and large-capacity turbodiesels over