I know there are higher octane levels available for petrol engines, but are there higher levels for diesel?
As stated, octane rating is for petrol (the resistance to autoignition - high resistance=can advance more=more power) whereas cetane is effectively the opposite (high cetane=good ignitability) I believe Bristish Standards state Diesel must be a minimum cetane of 51, wheras V Power and BP Ultimate guarantee a minimum of 56 and 55 respectively. They may be higher.
Why do modern diesels have butterfly valves? And with fixed compression ratio and timing, how do they take advantage of higher cetane rating?
I think the butterfly valves are there primarily as a last ditch safety device to avoid engine runaway if the turbo blows and the car ends up running on its own oil. They may also be used in the strategy to reduce intake air pressure allowing very high levels of EGR, but I don't know if this is used much in production.
The advantage of high cetane is in the better ignitability, reducing the ignition delay and promoting a faster burn. The more fuel you can burn around the top of the power stroke, the more work you can do on the piston as you push it down, so greater power - as shown by 5th gear, I believe they were looking around 6bhp instant gains on a Civic CDTi, expected to increase with extended use and moreso with higher performance engines. Additives are also good for lubrication (as things like the fuel pumps are lubricated by the fuel flowing through them and a bit of bore washing can make the piston rings slide better) and cleaning out the injectors/reducing coke buildups. All in all this is good for emissions too. More complete, cleaner burn...
Shell V power diesel is made of NG (Natural Gas) in a GTL (gas to liquid) procedure.
The Germans used coal in the last 3 years of WW2 to make petrol
so there are many ways in which we can use internal combustion engines for many years to come
This is all true, but any such technology needs to be very carefully implemented, as the so called "well to wheels" emissions of generating diesel in this way are potentially huge in comparison to that of refining oil. The Germans used it as a last resort in wartime due to a lack of resources - if they had been invading Saudi Arabia the technology would never have been considered! It is possible to improve efficiency with advanced catalysts, but these (as anyone who has had to replace a cat in a car will tell you) are not cleap (think Platinum). No worries, we have a few hundred years of proved reserves yet - just they can be used for better things than producing smoke!