India, April 27 -- A few years ago, automotive writers were talking of the emergence of 'clean diesel.' With technologies such as high-pressure injection, diesel engines had after all moved from being seen as the dirty, smoke-belching fuel that we remembered two decades ago to being a clean, and heaven forbid, an environmentally-friendly option. As technologies made diesel engines more efficient, in many cases offering half as much fuel economy, some considered them a carbon-friendly alternative to petrol. Given similar engine sizes and cars, diesel models burnt much less carbon thanks to their efficiency. At the same time, these machines were not the old plodding engines of the past. Modern turbochargers and stored energy systems made di...