[hawk2100n]

From a thermodynamic perspective, diesel is assumed to be a constant pressure expansion where the slow rate of burn for diesel causes the gas to expand at almost the same rate as the piston falls and expands the volume in the cylinder. This constant expansion is due to the heavier weight of diesel and its reduced volatility, which is the speed which it turns to a vapor. So diesel injects into a very hot cylinder and burns slowly. Gasoline has had time to vaporize, especially riding in through intake and compression cycles so it burns nearly instantaneously. This allows the engines to easily rev much higher, see 14k rpm sport bikes that you see tooling around town.

The comparisons of diesel engines cannot really be made because BMW's amazing 6 cylinder diesel engines have rev lines of 6000 RPM now. What enables this is careful metering of fuel delivery at EXTREMELY high pressures (30,000 psi +) which allows the fuel to atomize smaller and vaporize quicker. This allows them to make 300+ hp in a 3.0L 6 cylinder under extreme emissions restrictions. On the other hand, the JCB diesel max used the JCB 444 engine as its base, a 4.4L 8 valve industrial power plant which in its most potent form develops 77 HP. It's amazing that they made it make 750 hp however that was at over 250 psi of boost, 12 to 1 compression and clouds of white smoke and constant glow plugs at anything less than full throttle. Watch the videos. And a Mercedes 5 cylinder uses per-chambers which allowed MB to get by with relatively low injection pressures (2000 psi range). All of the air would be compressed into a small external chamber to allow sufficient heating to burn the fuel. The burning mixture did its best to expand out of that chamber and into the cylinder to make power. Since there is restriction preventing the rapid expansion of gas, the engine cannot rev high. So bottom line is that the Mercedes engines really don't have any power to rev high but this has been overcome with modern direct injection systems in high performance applications. I had a w123 300TD turbo for a while and I am very familiar with that engine specifically. A legend for endurance but not so much for power or efficiency. It got 27 mpg on a good day. I tried and I don't think I ever got to 4000 rpm because I was feeling guilty for the enormous cloud of black smoke spewing out of the back from an otherwise smokeless car. And I had about a minute to think about what I was doing while the car accelerated from 3000 to 4000.