The DiesOtto is, at heart, a 1.8-litre four-cylinder petrol engine with twin sequential turbochargers and direct injection. But the clever bit is that Merc engineers have incorporated both the conventional Otto cycle and a sparkless, diesel-style combustion. On start-up, the engine runs in standard petrol mode, with spark plugs igniting petrol injected directly into the cylinder. But once the engine is warm and cruising, it switches to a more efficient 'diesel mode', raising the compression ratio and deactivating the spark plugs.