If you read my recent article about direct injection, you know that by allowing more precise control of fuel delivery, direct injection opens the door for engineers to design a bike with a higher compression ratio that will still run safely on pump gas. Direct injection doesn’t just allow higher compression ratios, though – it allows higher cylinder pressures overall.
For this reason, direct injection is a perfect fit with turbocharging (which I discussed in previous articles found here and here). The ability to design a motor to operate with higher overall cylinder pressures means that engineers can build an engine with a higher compression ratio (compared to the typical 8:1-9.5:1 ratio found on most turbocharged motors), which increases exhaust gas velocity and volume, thus ‘spooling’ the turbocharger at a lower rpm. At the sime time, they can still run the same boost pressure (in psi) used on a typical turbocharged production engine (5-11psi) – which results in the higher overall cylinder pressures I mentioned.
So what’s the bottom line? A direct-injection turbo engine will spool the turbo sooner (at a lower rpm), as well as have a more linear powerband – a result of the higher compression ratio, which helps the motor make more power when ‘off’ boost (i.e. before the turbo has spooled).
Apparently the results are good enough for BMW, as the US Automotive press report that the German company plans to offer a twin-turbocharged, direct-injection inline six in it’s forthcoming range of 3-Series coupes. BMW has traditionally eschewed turbochargers and the consequent laggard throttle response – linear and snappy engine response is a hallmark of the company’s ‘Ultimate Driving Machine’ slogan. Apparently, BMW feels that technology has advanced far enough to make this paradigm obsolete – company spokesmen claim the new turbo six is just as responsive as their traditional naturally-aspirated motors have been, with turbo lag all but eliminated by the higher compression ratio allowed by direct injection.
Does this mean that turbocharged sportbikes are just over the horizon? Right now, we would say it’s a distinct possibility that when the motorcycle manufacturers bring direct injection to the market, turbocharging won’t be far behind, at least for some models. However, the real question is: how long will it take for the OEMs to begin producing direct injection bikes? We’re not sure, but we expect it to happen quickly, probably with the next two to three years.