There are different ways to merge the two, and homogeneous charge compression ignition (HCCI) is one of these technologies we've reported on in the past. Ciatti, though, is taking a slightly different approach by burning a lower octane fuel (80 to 85 RON, or Research Octane Number). This gasoline is easier to ignite than regular pump gas, but no where near the level of diesel fuel. Using a modified diesel engine (still no spark plugs or throttle) the diesel fuel injectors are able to spray fuel two or three times before ignition occurs at top dead center. The multiple fuel injections spread the fuel more evenly in the combustion chamber, which lowers particulate matter and NOx emissions compared to traditional diesel.
The added benefit of a lower octane fuel is that its easier to refine, making it very attractive to oil companies and BP and ConocoPhilips have taken notice of Ciatti's work. There is a downside, however, to this technology. Peak power drops approximately 25 percent, so this technology wouldn't be useful where ultimate power density is needed. However, since the torque curve stays essentially the same, we're willing to bet many people will trade a little top end power for a more efficient gas burning engine.
[Source: Wired | Image: Argonne National Laboratory]