How The Spitfire Mk XIV Compared to the K4 and Other Questions

[tomo pauk]

Sorry for such a late reply, my oldtimer laptop needed some overhaul

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The use of 100/130 fuel surely allowed the Merlin and Griffon base set up, ie compression ratio and supercharger settings, to be optimally configured for standard use of 100 octane fuel as opposed to 87 octane and so indirectly improved its power at altitude, when 110/150 came along the engine was already preconfigured for a higher octane fuel.

The RR design approach on the Merlin and Griffon (low compression ratio, comparatively big supercharger, among other details on a V12 engine) was a carry over from the Kestrel. There was no 'indirect improvement of the power at altitude' due to any of the better fuels available, the ever better altitude performance was due to ever better superchargers (and intercoolers, once installed). Sure enough, the low compression ratio helped out with power under the rated altitude(s), with manifold pressures going to around 3 ata with 150 grade fuel and no water injection.

The DB605L, with the two stage supercharger, seemed to require the higher octane C3 fuel. This is because pre-ignition is heavily a function of temperature which is a function of pressure ratio and compression ratio rather than manifold pressure and a two stage supercharger is likely to be producing pressure ratios of 5:1 at 10,000m and a rather larger temperature increase. Even an intercooler can't pull out all the heat.

The job of the intercooler was not to 'cool back' the compressed air, but to make sure the temperature is within the, for engine, manageable limits. Eg. for the Merlin 61 in Spitfire VII, the 40% of intercooling was found to give the best results (per Morgan Shackledy, pg. 269), while the US standard was 50% of intercooling (at least for turbo engines). The high compression engine, like DB 605L,will need the intercooler even more. The max manifold pressure (on 605L) being limited to 1.75 ata with MW 50. Not just at 1.75 ata, but also for 1.43 ata (30 min power, 'Kampfleistung') required MW 50 injection.
1.75 ata is under 51 in Hg, or +10.2 psig; 1.43 ata is around 41 in Hg, or +5.5 psig; for comparison sake, another comparable non-intercooled 2-stage engine, V-1710 in P-63, will do 55 in Hg in military power, 60 in Hg in WER (5 min, no ADI) and 75 in Hg in 'WER wet' (ie. with ADI).
Manifold pressure is surely a factor for pre-ignition, itself being the function of many things connected to supercharging, intercooling, ADI yes or not etc.

Had the Daimler Benz DB605 series had a fuel as good as 100 octane fuel available the DB605 would have been mass produced in the variants with a higher compression ratio (8.4:1 rather than 7.4:1) This would increase power with no increase in fuel consumption and with no decrease in Full Throttle Height.

The German C3 fuel was in the ballpark of what the Allies had when DB 605 was at the development phase. Increasing compression ratio, let alone increase of manifold pressure (and thus increase of power and stress generated) will induce even more reliability isues than encountered historically, that lasted more than a year.

The introduction of the intercooler on the Merlin 61 added about 120kg to the 620kg Merlin 20 series. A 2000hp engine consuming fuel at an rich setting and sfc of 0.55 will use about 1100lbs (500kg or 500L) and hour. MW-50 would be added at about the same rate as the fuel so an 86L tank i.e. 190lbs would add about 10-12 minutes of WEP available. It seems enough given the other time limits on the engine.

The rate between C3 and MW 50 consumption was around 4:1 in the DB 605L. The MW 50 tank held 70 liters, consumption was 150 L/h for 1.75 ata and 75 L/h for 1.43 ata. Basically, whatever the compression ratio saves, gets offset and more by need to consume MW 50 mixture. Intercooler stays with the aircraft, and DB could've opted for air-to-air intercooler (20-30 kg?) like Jumo did for the 211J/P. Of course, intercooler and lower CR and ADI are not mutually exclusive and give best results, as we know when looking in many ww2 engines, including the Jumo 213E.

The Mk103 was adequate to ranges equal to half its muzzle velocity, say 240 meters. It would be uncommon for greater ranges to be experienced and even with a higher velocity issues of the size of the target within the reticule become and issue.

The MK 108 you mean?

Not really that smart. Lets drop the pressure ratio of the 87 octane DB605A from 7.5 down to the same level as the Allison/Merlin ie around 6:5 (the merlin was even less) we've now lost about 13.33% of our expansion stroke and almost as much power while still burning the same amount of fuel.

The arithmetics in the bolded part are off by a large margin. Increase of compression ratio by 100% will not give 100% more fuel and do wonders on consumption:

So when we decrease the compression ratio by 13.33%, the power will go down by 3.3% for same RPM and manifold pressure. On the plus side, with CR of, say, 6.5:1, the 'Notleistung' regime (1.42 ata, 2800 rpm) might not be banned at all, instead for more than a year as it was, since the engine will be less stressed due to lower CR.

We can now increases the compression ratio of the supercharger but as this now forces more air and fuel into the engine and means that we must deal with much more waste heat and higher fuel consumption. The extra waste heat requires larger radiators and possibly higher coolant flows.

In order to cool the 3.3% increase of power? Not by a country mile.

The supercharger is now being used to over boost the engine and can no longer provide altitude compensation. We need a larger or a two stage supercharger. The heat increase at some point compels us to consider and intercooler. Our weight has gone up.

LW needs a larger or a two stage supercharger, that will be well served by an intercooler. Otherwise the Allied aircraft with multi-stage superchargerd engines will kill the LW. As historically.
As for 'the engine and can no longer provide altitude compensation' - care to elaborate on this, how much this was important, preferably with some sources?

Would it be reall woth it? could the little Me 109 frame cope with larger radiators and provide the space for an intercooler.
However, simply provide 100/130 fuel to the Me 109, increase the compression ratio with higher crowns on the pistons and it seems a win win: no decrease in full throttle height, more power with no increase in fuel consumption or thermal load. We only need to make sure the bearings can take the extra power.

LW was sticking all things on the Bf 109 that managed, among the benefits, to slow down it. The air-to-air intercololer can be installed under the engine, with oil cooler relocated in MC 205 fashion. Same for water-to-air intercooler's radiator. Or, install a bit deeper radiator, as a part of main cooling system.
As for the DB 605A with greater CR: no, no - the reliabilty is lacking even on historical CR.
The 'simply provide 100/130 fuel to the Me 109' tidbit is a little hard to swallow, BTW

 

[Koopernic]

Below is the efficieny (and therefore also power increase for various compression ratios.

7.2 What is the effect of Compression ratio?

The Merlin had a CR of 6.0:1
The DB601A had a CR of 6.9:1
The DB601N had a Cr of 8.2:1 (by installing flat top pistons)
The DB605A had a CR of 7.5:1 and 7.3:1 left right
The DB605D had a CR of 8.5:1 and 8.3:1 left right

The increase in compression ratio from 6:1 to 7:1 (Merlin versus DB601A) leads to an increase in power/efficiency of 25%->28% which is an relative increase in power 12% for an increase in compression ratio of 16.6%
The increase of CR from 6:1 to 8.0:1 gives a increase in power from 25% to 30% for a relative increase in power of 20%


The increase of compression ratio from 7:1 to 8.2:1 possible with use of early C3 fuel instead of B4 fuel allows an increase of just over 2% on 28% which is a 7.5% increase in power. However it the use of C3 fuel, because it is either 92/115 (for early C3) or 95/125 for late C3 has a high rich mixture response allows an additional 30% on top of that. You can see that when the DB605DC goes from 1.45ATA to 1.8ATA when using C3.

As you can see the use of C3 allows a significant increase in performance.

LW was sticking all things on the Bf 109 that managed, among the benefits, to slow down it. The air-to-air intercooler can be installed under the engine, with oil cooler relocated in MC 205 fashion. Same for water-to-air intercooler's radiator. Or, install a bit deeper radiator, as a part of main cooling system.
As for the DB 605A with greater CR: no, no - the reliability is lacking even on historical CR.

The aircraft slowed down from 400.5mph (Me 109G1) to 387mph (Me 109G6) only due to poorly integrated greater armament and the loss of a retractable tail wheel. As soon as the 1.42 ata rating became available via the introcution of less preignition prone spark plugs the speed went back up to 397mph. C3 fuel would have had the same effect as better spark plugs. The evidence for serious unreliability is just not there outside of introduction of new variants of the engine during highly stressed wartime production. The fact that some engines were reliable and others were not proves a manufacturing system quality control problem.
The Merlin might have failed under similar pressure.

The 'simply provide 100/130 fuel to the Me 109' tidbit is a little hard to swallow, BTW

Only if you suspend the laws of physics for German engines but not for allied. You are saying that 100/130 fuel was of no benefit to the allies, they may as well have used 87 octane.

 

[tomo pauk]

Below is the efficieny (and therefore also power increase for various compression ratios.
The Merlin had a CR of 6.0:1
The DB601A had a CR of 6.9:1
The DB601N had a Cr of 8.2:1 (by installing flat top pistons)
The DB601A had a CR of 7.5:1 and 7.3:1 left right
The DB601D had a CR of 8.5:1 and 8.3:1 left right

The increase in compression ratio from 6:1 to 7:1 (Merlin versus DB601A) leads to an increase in power/efficiency of 25%->28% which is an relative increase in power 12% for an increase in compression ratio of 16.6%
The increase of CR from 6:1 to 8.0:1 gives a increase in power from 25% to 30% for a relative increase in power of 20%

The table is true for naturaly-aspirated engines. With supercharged engines, the CR is deliberately kept under 7:1 in order to make more power. A two-stage DB 605L (same CR as 601D) will make 1.75 ata with C3 + MW 50 and only up to 1.20 ata on C3 and without MW 50; a two-stage war-time V-1710 (CR 6.61:1; as 605L it has no intercooler) will make up to 2 ata with ADI, and up to 2 ata without adi. Jumo 213E (6.5:1 CR) will do above 2 ata on B4 and MW 50.
The Merlin with CR of 8:1 will never make 1500 HP, not with +12 psig, unless we bulk it up to the weight of Griffon.

The increase of compression ratio from 7:1 to 8.2:1 possible with use of early C3 fuel instead of B4 fuel allows an increase of just over 2% on 28% which is a 7.5% increase in power. However it the use of C3 fuel, because it is either 92/115 (for early C3) or 95/125 for late C3 has a high rich mixture response allows an additional 30% on top of that. You can see that when the DB605DC goes from 1.45ATA to 1.8ATA when using C3.

'Better' fuel = greater boost; nothing is curious with 605DC making 1.8 ata, same as Allied engines in 1941.

As you can see the use of C3 allows a significant increase in performance.

You know that I know that

As soon as the 1.42 ata rating became available via the introcution of less preignition prone spark plugs the speed went back up to 397mph. C3 fuel would have had the same effect as better spark plugs. The evidence for serious unreliability is just not there outside of introduction of new variants of the engine during highly stressed wartime production. The fact that some engines were reliable and others were not proves a manufacturing system quality control problem.
The Merlin might have failed under similar pressure.

Merlin was under similar pressure in 1940-41, yet the reliability was never in question.
I'd not agree with you that spark plugs were the main/sole source problem with DB 605A, the oil system was reworked too in order for 'paper' performance is met.

Only if you suspend the laws of physics for German engines but not for allied. You are saying that 100/130 fuel was of no benefit to the allies, they may as well have used 87 octane.

I'm not saying that better fuel was of no benefit to the Allies. Laws of physics (and those of economy) apply as ever - there might be enough C3 fuel for BMW 801D engines of the LW, but that does not mean that C3 is also available for Bf 109 units, in order to 'simply provide 100/130 fuel to the Me 109'.