Diesel engine

A diesel engine looks like a petrol engine inside, with pistons, valves and a crankshaft, but it has no spark plugs. It relies on heat from compression instead. Only air is drawn in, and the piston squeezes it to a much higher compression ratio than a petrol engine would tolerate; the air becomes hot enough to light diesel the moment it appears. A fuel injector at the top of each cylinder then sprays a fine mist straight into the chamber, timed by the engine control unit.

• Intake: air alone enters through the open valve.
• Compression: the air is compressed hard and becomes very hot.
• Power: diesel is injected at very high pressure (common rail) and burns as it mixes; the pressure pushes the piston down.
• Exhaust: the burnt gas leaves, in most modern diesels through a turbocharger before the silencer.

Because combustion is controlled by how much fuel is injected rather than by a throttle, and because the compression ratio is high, a diesel extracts more energy from every litre. Diesel also carries more energy per litre than petrol. The result is the strong low-rev pull that owners of a Scorpio-N, Innova Crysta or Fortuner notice, and the high mileage of small diesels like the Nexon or Venue. The trade-offs are heavier internal parts to cope with the pressures, more noise and vibration, a higher purchase price, and an exhaust system with a particulate filter and SCR catalyst that needs the engine to warm up properly to stay clean. Virtually every truck and bus in India is a diesel; no two-wheeler sold here is.