Thoughts on the Nakajima Ki-84 and Kawasaki Ki-100

BiffF15 said:

I understand too much compression or heat causing detonation (premature ignition)

Click to expand...

BiffF15 said:

I followed everything you wrote except why higher compression may help in cruise but hurt max power. Is that do to cruise power being close to max power in a higher compression example or is it do to heat rise?

Cheers,
Biff

Click to expand...

Shortround6 said:

That is pretty much it but not exactly.

The entire intake charge is right of the verge of exploding if the temperature is too high. The temperature is the result of heating in the supercharger and heating in the cylinder if the charge is compressed.
In ordinary premature ignition something ( a hot spot or carbon deposit or....) ignites the mixture before the spark plug/s do but the mixture burns somewhat normally across the top of the piston or the flame front collides with advancing flame front from the spark plug/s causing a high pressure/high temperature hot spot.

If however the entire intake charge is hovering on the verge of ignition you may not get a traveling flame front across the cylinder but a near instantaneous explosion across the cylinder and sometimes before the piston has crossed top dead center. This is when you get bent/broken connecting rods and/or radial engine cylinders departing the rest of the engine for new locations. Sometimes it is a complete surprise and other times there are warning signs (noises)

With an supercharged engine it is a lot harder to wreck the engine, For one think it is very hard to the intake charge up to the auto ignite zone if the engine is running right or even close.
from the other direction, a supercharged engine running at 14.7lbs of boost as a it more than double the amount of fuel/air in the cylinder that a unsupercharged engine does (unsupercharged engines rarely have 100% volumetric efficiency unless highly tuned racing engines).

As an illustration Hurricane II with a Merlin XX engine at 20,000ft may be taking in air that is at 248 degrees C Absolute ( 25 degrees C below freezing) and yet the intake mixture in the manifold/s is at 394 degrees C Absolute ( 121 Degrees C above freezing or 21 degrees above boiling. Now put that hot mixture into the cylinders/s and compress at the nominal 6 to 1 compression ratio (neglecting valve timing).

This was the whole secret to high octane fuel. The higher the octane/PN the higher the autoignition temperature of the fuel and the less likely it was to detonate. Which allowed for more boost to be used to cram more fuel into the engine.

Click to expand...

GregP said:

If you run a higher compression ratio, you make more horsepower at the same rpm (with the right fuel), but you can't compress the air more then the detonation limit. The engine makes good power all through the power band. If you run a lower-CR, the engine makes good power but requires more boost to make the fuel flow higher, resulting in more energy.

So, the lower CR engine makes very good HP at cruise whereas the higher CR engine makes better power up high, but generally will lose some mid-range power versus the higher CR-lower-boost engine.

We saw the same thing when we were racing Mazda 13B wankel engines. The engine made good power stock but, if you bridge-ported it and pinned the ring gear, you could turn it higher opm and make more power at a much more narrow rpm band. If you took the area under the HP-rpm curve, it didn't change much, but peaked at about 8,500 rpm stock. When bridge-ported and correctly carburetted, it peaked at 10,500 rpm and could go until about 11,500. The power band was more narrow, but the peak was higher.

Same with a Merlin. Stock it makes good power at 2.850 rpm at a CR of about 6.0 : 1. When you lower the CR to 5.0 : 1 and increase the boost by a factor to 2, it makes more than twice stock power at higher rpm (3,600 rpm or so), but nobody really likes to cruise at stock power levels because engine life is short and the entire reason for a racing engine is to go fast while the engine is running well. So, they install a stock engine to fly around (like going to an airshow) and a racing engine to race for money or go for a speed record.

Click to expand...

BiffF15 said:

SR6,

Thank you! You just cleared up several things for me and I appreciate that! Outside of this forum I've heard of carbon buildup but did not realize the ramifications of it. I would guess if it's on the piston top / dome or the combustion chamber the results of hot spots (premature ignition) would occur but the damage would still be to the weakest part of the rotating assembly (I assume the block and heads in most instances are stronger and failure of those is do to exiting of the moving parts).

If the intake charge is on the temp cusp of ignition, or it goes over is that detonation? I had a 79 Mustang turbo and it was sensitive to quality / higher octane fuel. Lower and I would get what I was told was detonation, and to let up when that occurred to prevent said moving parts from attempted exit of the engine. It was not intercooled. We also had a rail dune buggy that would detonate (VW engine) but I would go to the airport and put a 50/50 mix of avgas in the tank and it ran fine on that.

Cheers,
Biff

Click to expand...

Dawncaster said:

His argument could be said to be error because he overlooked several facts.

It was a common occurrence between different models that IAS read differently. According to Pilot's Handbook, Even if the Spitfire IX and P-47D were flying side by side, the IAS seen by the pilots would have been different. The problem was that the difference between F6F and F4U was very large, and it was caused by the F6F, not the F4U, and Grumman worked on it and the measurement location was changed. In conclusion, the difference between IAS readings was nothing special. The actual speed was shown in TAS after calibration. Since the calculation of TAS includes all errors of IAS, the maximum speed of the document can be accepted as it is. Things like the fact that the F4U was faster than the F6F.


View attachment 642047

The latest model F4U-1D he claimed was actually an outdated F4U-1A BuNo.17781. It didn't even have a fixed upper section of the cowl flap that even the birdcage F4U-1s had installed for that time. It wasn't an F4U-1D as he claimed, nor was it a lastest model, so it used an older type of propeller blades that was less efficient than the F6F's (As you can see in the F4U-1 BuNo.17781 photo above). According to the report of the US Navy F4U-1 BuNo.17930 and the F4U-1 Airplane Characteristic & Performance, the increase in Vmax was about 14 mph when the F6F type 13 feet 1 inch diameter propeller blades were installed. The test-bad F6F-3 of Grumman's later model (F6F-5) achieved a speed of 410 mph, almost as fast as the early F4U-1A. However, under the similar conditions, the F4U-1A achieved 431 mph. Although both type could not cleared that speed in the mass production models.

Click to expand...