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

In article ,
Dave Eadsforth writes:


Good day, good people,

I wondered if anyone out there (in all probability, Peter!) could help
me understand more fully the process by which the Merlin engine was
enabled to use 150 octane fuel; one feature of which was the addition of
the Xylidine anti-knock compound. (This was touched on in a thread
last year, I recall.)

If you can wait, I've got to dig out some files & make sure of some
references, but I'll have a comprehensive answer in about a day's
time. In the meantime, I'll chuck out a tidbit or two.


I understand that 150 octane fuel became available during 1944 - I don't
know when (a date would be welcome), but it was available in time to
boost the performance of those Spitfires that were assigned to knocking
down the V1s. What I would like to find out is what might have been
done to the Merlin to allow it to run on the stuff?

As far as engine modifications go, Not a lot, really, other than
changing the settings in the Manifold Pressure Regulators and possibly
tweaking the jets in the carburetors.

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.) The energy
content's really not much of a difference, though, so we can skip it
for now. One of the consequences of increasing the Manifold Pressure
is a higher Charge Temperature (The temperature of the compressed air
in the Intake Manifold). If that temperature gets high enough, the
fuel-air mixture will self-ignite - Detonation - "Knock" is too kind a
word for what happens at +25 Boost when uncontrolled burning, and its
attendant shockwaves get going in a Recip's cylinder. The higher the
Performance Number, the more resistance to detonation, and you can use
the extra power that the higher manifold pressure gives you without
having to walk home.
Note that there are other alternatives as well, such as Anti Detonant
Injection, or Water Injection, where an aerosol of water, or a
water/alcohol mix, is sprayed into the air as it's benig compressed to
absorb some of the heat, and lower the Charge Temperature.


Was the use of 150 octane restricted to particular marks of engine?

As far as I know, it was mostly used on Spit Mk IXs with Merlin 66
engines. There were also some experiments with 150 PN fuel in Rolls
Griffons, and Packard Merlins, as well.

I read somewhere that it was supposed to provide an effective increase
in power of about 15 percent - by allowing a higher manifold pressure.
Is that figure of 15 percent correct?

I'll have real numbers for you in a day or so, but that sounds about
right. Of course, the increase in power comes with a decrease in
Critical Altitude - The supercharger can only compress things so much,
after all, so to get a higher Manifold Pressure, you've got to start
with thicker air.

Was the conversion to 150 octane done by merely adjusting the existing
arrangements for the supply of fuel, or was there a need for new fuel
supply components (carbs etc.)? And would the permitting of the
additional boost have mandated the exchange of some internal engine
parts (bearings, crankshafts, etc.)?

The bearings/cranks/conrods/pistons, etc, were stock components.
Teh carbs were teh same - but I don't know if they got tweaked. The
automatic Boost Pressure Regulators would, of course, need to be
reset. It was the sort of job that could be done at the Squadron,
rather than Depot, level. You didn't have to go in & rebuild
anything. I'd prefer to start with an almost new engine, however. An
engine with a little running time has worked out all its initial
stress relief.

Would the use of 150 octane have automatically permitted a higher
ceiling for the machines that used it? Or don't things work quite that
simply! (I understand that specially prepared Spitfires had been able
to fly to at least 44,000 feet by 1943. Would I be right or wrong to
simply assume that 150 octane would have enabled them to go higher?)

No, As with anythig else in Aviation (Or any other Engineering), it's
a balancing act. You can only get so much of a compression ratio out
of a Supercharger, for any given drive speed. In order to get more
boost, you've got to start with thicker air, so the Critical Altitude
actually decreases. When you're chasing V-1s, though, or fighting
against Me 109s, or Fw 190s, that's not a bad thing. The Daimler Benz
engines in the 109, by virtue of their variable-speed blowers, which
didn't require as much power to run at low altitudes, gave a big
advantage down low. The BMW 801 on an Fw 190 had a geared blower, but
the critical altitude for the low gear was very low, down near Sea
Level.
In order to improve altitude performance, you've got to increase the
compression ratio of the induction system, or add an axidizer to the
fuel-air mix to help it burn. This can be done by adding supercharger
stages (Basically one supercharger feeding another, like, say, a
Merlin 60 series engine, or the turbosupercharger/engine driven blower
setups on the P-47 and P-38, or piping something like Nitrous Oxide
into the induction system, as the Germans did. The drawback is that
it takes more of hte engine's power, in the gear-driven examples, to
compress the air that much more. That means that at lower altitudes,
you're at a disadvantage. Or, you've got got to haul around a bunch
of tanks, regulators, pipes, valves, & all that for teh Nitrous
system. You've only got a limited quatity of Nitrous aboard, and you
can pretty much guarantee that it'll run out right when you need it.
Or, worse yet, the storage bottles could get damaged. Leaking
Oxidizers is a Bad Thing, especially when somebody's shooting at you.

More later, with real numbers attached.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster