Since the 1990s, the compression ratio of turbo-diesel engines has been dropping. This is 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.