On a normal petrol engine, the throttle controls the air flow into the engine, and fuel is added in the correct ratio either by injection (requires metering) or carburettors (self-metering). At low throttle you have a high restriction (the throttle butterfly), which causes a pressure drop (intake depression, from which brake servos and older ignition advance systems etc are run), and limits the air flow. This causes work to be done on the top of the piston, which reduces efficiency. The fuel is added to suit the air flow into the cylinder to maintain ~ lambda = 1
On a diesel engine, there is no throttle, so as much air as gets sucked in gets sucked in, and the 'throttle' controls the amount of fuel injected. At low 'throttle' openings you have lots of air, and not much fuel = lean.
However, if you can make an engine burn petrol at any concentration without overheating, detonating, pre-igniting, and burn completely, then you can get rid of the nasty, restrictive throttle as well, meaning that petrol engines can be more efficient. If you can also get it to burn well at higher compression ratios as well, then petrol engines will have NO DRAWBACKS over diesel engines at part load (they currently have no drawbacks at high load).
Lean burn, stratified injection, throttle-less petrol engines are the future. BMW currently has a de-throttled petrol engine on the market, but I forget it's name, and I've not had the chance to look at it in any great detail. It's only half (or even a third) of what is required, but it's progress.
The days of the dirty diesel are numbered. Hurrah for everyone!
Hope that makes sense.