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I don't know where to begin...
Diesel engines are more efficient than petrol engines due to the fact that they can run higher compression ratios than petrol engines due to the onset of knock. Using idealised Otto and Diesel cycles at the same compression ratio, a petrol engine is more efficient
Diesel doesn not "literally explode" inside the cylinder, it slow burns, hence the lower engine speeds. By contrast, petrol combustion is always a very short process even when ignited via compression rather than spark ignition. This is due to the volatility of petrol and is also the reason for the compression ratio limit.
compression ignition petrol engines can run to high compression ratios, the problem being that as the combustion is very short, the intake conditions need to be controlled precisely to avoid detonation or misfire.
The noise of petrol engined LMP cars has nothing to do with burning petrol in the exhaust (how could this happen anyway given petrol's short combustion time?) but is due to the items placed in the exhaust. The diesel LMPs are turbocharged, which has an effect in quietening the engine noise and the particulate filters further quieten the sound. It's worth noting that the Peugeot is now almost as loud as the turbocharged petrol LMPs after allegedly removing their FAP filter.
The diesels dominate LMP racing for 3 reasons: 1) they were given a massive advantage in the original regulations, 2) as diesel is a new technology to endurance racing, the development scope is greater, 3) factory teams always beat privateers due to their larger budgets allowing more development. Case in point, in 2009, the top petrol non-factory car was the ORECA, which completed 12 fewer laps than the winning diesel. In 2003 and 2004 against factory Audi supported opposition, the top privateers were 17 and 18 laps down respectively.
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