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