Use of 150 octane fuel in the Merlin (Xylidine additive etc etc)

"Tarver Engineering" wrote in message ...
"Peter Stickney" wrote in message
...

snip
The higher engine output comes from the increased Manifold Pressure.
High Octane fuels tend to have a somewhat lower energy content than
those with lower Octane (or Performance) Ratings. (Technically, if
it's over 100 Octane, it's a Performance Number.)

No, higher octane fuel burns slower; but it contains more usable energy.


The octane rating or RON (Research Octane Number) of a fuel has
nothing to do with its energy content. All hydrocarbons have an
energy content of about 11.5kW.Hr per kg. (about 41Mega.Joules per kg)
Ethyle and Methyl alcohole for instance have a RON of about 130
(which is why it is used in indianapolis car racing) yet have around
half the energy content of gasoline.

The fuels physical density does vary with gasoline being about 0.73kg
per litre while diesel is about 0.78kg per liter. Military aviation
fuels for piston engines, gas trubines and rockets are generally
designed to be physically as dense as possible.

While, as you say, an increased RON means that more of the fuels
energy can be used in a piston engine because it can be given a higher
compression ratio and therefore expansion ratio without preignition or
knocking. If the same gasoline is burned in a multifuel diesel, gas
turbine or wankel the best or worst RON makes no difference at all.

Higher RON number do two things: First they eliminate pre-ignition due
to hot surfaces or the high temperatures caused by compression.
Second they prevent explosive combustion. Combustion should be a
controlled burn at subsonic velocities along a wavefront explosive
combustion (not the technical term) means that the combustion becomes
supersonic and is propagated by infra red radiation simultaneously in
the mixture.

The higher RON of Allied engines seems to have been used not to
increase compression ratio to obtain more power but to allow higher
emergency boost pressures and this practice would not increase fuel
efficiency just maximum power. Both the Merlin and the German Daimler
Benz and Junkers Jumo engines seem to have had a compression ration of
around 6.5. (varying between 6.2 to 6.9 and also varying as to which
bank of cylinders due to the con rods/king rod differences). I
recollect the distinct impression that the Merlin even had LOWER
compression ratios than the German engines.

Diesel engines are given a cetane rating. high cetane numbers are
generally desirable as this means the fuel is easy to ignite but slow
to burn. A centane number of 45 is considered good and 30 is low.

The German synthetic fuel fischer tropsch plants produced
extraordinarily high centane ratings of around 85 (catalysts produce
long linear chains). This was so high it meant that exhaust
temperatures went up by 25% and efficienciues down by 5% as the fuel
barely finished its combustion by the end of the power stroke.
Generally German diesel was a mixture of high cetane Fischer Tropsch
diesel blended with low cetane diesel from the hydrogenation plants.
This then gave an ideal blend.

Oddly despite the ease of producing diesel they often had to make
substitute diesel (maximum power suffered) by blending 95 gasoline
with 5% motor oil as gasoline production was emphasised. It was the
Russians that used the safer diesel in their tanks.