Nakjima Homare - what's the verdict?

[tomo pauk]

Supposedly it was a problematic engine. Eg. Wikipedia says: However, the tight design of the engine made it difficult to maintain quality in manufacturing, and unreliability in the field was a significant problem; actual output of early models at altitude was in the range of 1300 hp (970 kW), far below the designed capability.

Questions: what was the actual Japanese view on the Homare? Any Allied report on the engine that can confirm it to being unreliable?

 

[Mike Williams]

Supposedly it was a problematic engine. Eg. Wikipedia says: However, the tight design of the engine made it difficult to maintain quality in manufacturing, and unreliability in the field was a significant problem; actual output of early models at altitude was in the range of 1300 hp (970 kW), far below the designed capability.

Questions: what was the actual Japanese view on the Homare? Any Allied report on the engine that can confirm it to being unreliable?

A.T.I.G. Report No. 45 - Nakajima Engine, Design and Development is worth a read.

 

[Shinpachi]

Supposedly it was a problematic engine. Eg. Wikipedia says: However, the tight design of the engine made it difficult to maintain quality in manufacturing, and unreliability in the field was a significant problem; actual output of early models at altitude was in the range of 1300 hp (970 kW), far below the designed capability.

Questions: what was the actual Japanese view on the Homare? Any Allied report on the engine that can confirm it to being unreliable?

Design was good but production.
A Sumitomo's factory was manufacturing the cylinder head for Homare which required higher skills as dozens of thin steel plates had to be removed from the hot head mold before cooled down within limited time.
In 1945, most of the factory workers were high-school students volunteers. They had no such high skills but were required production on schedule.
Finally, they had practiced the abandonment of job. That was substantially end of the war.

 

[Howard Gibson]

The Nakajima Homare engine was 35.8litres, and it put out 1990HP at 2900rpm. This works out to a Brake Mean Effective Pressure (BMEP) of 1720N/m^2, or 250psi. This is about what you get running allied engines on 100/130 octane fuel. The Japanese had 92 octane. Perhaps they flipped on the water/methanol injection.

All the accounts I have seen show that the Homare engined Nakajima Ki.84 had a top speed of 388mph. The Americans tested one and they got it up to 427mph on "WEP" (War Emergency Power). Water/Methanol? The Kawanishi N1K and the Mitsubishi J2M also exceeded 400mph in American testing.

• Did the Japanese quote engines with WEP running and aircraft without?
• The performance charts on WWIIAircraftPerformance.com say "92 Octane". Did the Americans really have 92 octane fuel lying around? Why?

 

[Shortround6]

The US certainly had 91 octane fuel in the early part of the war.
By the end or shortly afterwards the US had 91/96 octane fuel as a standard grade.
Getting a batch of 92 octane fuel at a US center doesn't seem that impossible but getting that fuel to a overseas test center would be a lot harder.

 

[AerialTorpedoDude69]

To elaborate on what Shinpachi mentioned, Japan's conscription policy did not exclude critical workers, such as skilled machinists and mechanics who would otherwise contribute to engine production. As a result, as the war progressed, engine quality fell off the map as critical skilled labor was rapidly replaced with child and newly trained workers.

Additionally, it's mentioned in multiple sources that the Homare ran best on 100 octane fuel. I'm not sure why that is because that implies it was designed from the outset to use 100 octane fuel.

But pretty much all of Japan's high horsepower engines were maintenance nightmares. If you look at the serviceability of planes with the Homare, the Kasei, or early model Tenzans (B6N1), they're all reported to be terrible. Why else would pilots prefer the Kinsei 64 over the Homare?

 

[SaparotRob]

Howdy ATD6. Welcome to the forum.

 

[AerialTorpedoDude69]

All the accounts I have seen show that the Homare engined Nakajima Ki.84 had a top speed of 388mph. The Americans tested one and they got it up to 427mph on "WEP" (War Emergency Power). Water/Methanol? The Kawanishi N1K and the Mitsubishi J2M also exceeded 400mph in American testing.

• Did the Japanese quote engines with WEP running and aircraft without?
• The performance charts on WWIIAircraftPerformance.com say "92 Octane". Did the Americans really have 92 octane fuel lying around? Why?

I'm sorry for going off topic, but I'm constantly seeing the 388-mph figure as well for the Ki-84. As far as I can tell, the reason is that everyone sources Rene J. Francillon's book, "Japanese Aircraft of the Pacific War". Unfortunately, much of the information in it doesn't seem to match Japanese records or TAIC data.

There are numerous reasons why that book shouldn't be treated as a primary source. But above all other reasons, I can't find an appendix or list of sources in it. That means any source based on it is potentially false since we can't vet the information in it. I would be very appreciative if someone could locate the sources of that book, though.

Similarly, the claim that Japanese engines were designed to run on MW50 also seems specious and without a source.

Did the Japanese quote engines with WEP running and aircraft without?

As far as I know, the equivalent of WEP was "Overboost" or オーバーブースト, which is a transliteration of the English word into Japanese. A few primary-source Japanese documents include the Overboost figures, although I don't know anything more about them.

• The performance charts on WWIIAircraftPerformance.com say "92 Octane". Did the Americans really have 92 octane fuel lying around? Why?

Probably. There were a lot of refineries and oil sources in the US and Canada in the 40s and not all of them used the fluidic cracking method which produced 100 octane. So they definitely had supply of the fuel and it would be reasonable to conclude many air bases stored a wide range of octanes although I don't know for certain.

 

[Shortround6]

Similarly, the claim that Japanese engines were designed to run on MW50 also seems specious and without a source.

For some reason most countries that used water/alcohol injection used a 50/50 mixture...or 49.5 + 49.5 + 1 percent (roughly) oil, anti corrosion etc. A few may have used 30/70 + anti corrosion stuff. You can't use pure water or the tank will freeze at high altitude so the alcohol is pretty much an anti-freeze. On the other hand water (per pound) will suck up more heat out of the intake mixture so pure alcohol doesn't provide as much cooling per pound.
The Japanese may have intended some of their engines to run on Water/alcohol. Wither it was actually the MW50 exact mixture (or anti corrosion agent/s0 is certainly subject to question.
The US (Shell oil?) and Jimmy Doolittle had experimented with water injection in the early/mid 30s? They got a P & W R-1340 engine of 600hp (or under) up to 900hp on a ground test rig. At least one racing car in 1938-39 used about 10% water in it's fuel mixture (two kinds of alcohol) for it's highly supercharged engine.

There were a lot of refineries and oil sources in the US and Canada in the 40s and not all of them used the fluidic cracking method which produced 100 octane. So they definitely had supply of the fuel and it would be reasonable to conclude many air bases stored a wide range of octanes although I don't know for certain.

Most bases are only going to use a small number of different fuels. In part to cut down on fueling mistakes. A number of pilot's manuals will show the different throttle settings (RPM and boost limits) used for P-40s. P39s, etc when they were running 91 octane fuel for training. The manuals show only 91 octane and 100 (or 100/130).
Some of the transports ran on 91 octane.
The small trainers used mostly 80 or 87 octane fuel. Some of the really small ones would use 65 or 70 octane.

 

[Howard Gibson]

I'm sorry for going off topic, but I'm constantly seeing the 388-mph figure as well for the Ki-84. As far as I can tell, the reason is that everyone sources Rene J. Francillon's book, "Japanese Aircraft of the Pacific War". Unfortunately, much of the information in it doesn't seem to match Japanese records or TAIC data.

Be careful here. The BMEP of the Homare engine is consiistent either with the 100/130 octrane fuel the RAF and USAAF were using in Europe, or with water methanol injection. If all the Americans have to test Ki.84s is 100/130 octane fuel, they may have used those boost pressures. Note the 1946 date. This is an analysis of Japanese technology, not an analysis of an opponent in combat.

 

[AerialTorpedoDude69]

S Shortround6 thanks, that is consistent with what I've read although your recall is better than mine.

I was referring to the claim that Japanese aircraft had extra large MW50 tanks in order to deal with low-grade fuel. The claim, or so it goes, is that Japanese fuels were so bad that they had to rely on MW50 as a supplement to to normal operation in order to prevent pre-detonation from volatile fuels. In other words, they used MW50 all the time.

I haven't seen a single source that validates this claim. The classified report on Japanese fuels contradicts this claim because it states that only the Octane 95 fuel derived from pine root was volatile. There was one source that mentioned pine root distillate was added to other fuels but IDK much more than that. But basically, AFAIK, the Homare was not designed around perpetual MW50 injection.

Be careful here. The BMEP of the Homare engine is consiistent either with the 100/130 octrane fuel the RAF and USAAF were using in Europe, or with water methanol injection. If all the Americans have to test Ki.84s is 100/130 octane fuel, they may have used those boost pressures. Note the 1946 date. This is an analysis of Japanese technology, not an analysis of an opponent in combat.

Thanks Howard. I have seen people mentioning BMEPs for the Homare but I'm not sure which are true. Can you point me in the general direction of a source? Or were you talking about this source: http://www.wwiiaircraftperformance.org/japan/ATIG-Report-45.pdf

I did not see manifold pressures listed in it though, but I could have missed it.

 

[Howard Gibson]

Thanks Howard. I have seen people mentioning BMEPs for the Homare but I'm not sure which are true. Can you point me in the general direction of a source? Or were you talking about this source: http://www.wwiiaircraftperformance.org/japan/ATIG-Report-45.pdf

I did not see manifold pressures listed in it though, but I could have missed it.

I worked it out.

BMEP = 2 * BrakePower / RPM / Displacement

You have to do all sorts of unit conversions. 1HP=33000ft.lb/min. The factor two is required because work is done on every second stroke of a four stroke engine.

 

[cherry blossom]

I suspect that there were more problems with the Homare than we have mentioned in this thread and one problem seems to show up at medium altitude. I assume that test rigs gave the correct power readings for the Homare's sea level performance. Could the distribution of fuel between the cylinders become uneven as pressure reduced or was there an icing problem with the fuel?

The Homares supplied to Mitsubishi for installation in the A7M1 prototypes are interesting because they are produced relatively early and we might expect that they were checked over fairly carefully. The speeds of the A7M1 and the later A7M2 were measured and one source has 345 mph and 387 mph (Jiro Horikoshi via Martin Caidin). Horikoshi estimated that the Homare yielded no more than 1,300 hp at 20,000 ft (perhaps 6,000 m and another source gives the A7M2 339 knots, which is 390 mph). The MK9B engine was designed to give 1760 ps at 6,000 m according to Horikoshi and another source gives 1800 ps at 5,600 m at 2800 rpm for a Ha-43-51. The Old Machine Press article on the MK9 gives the 1800 ps figure for 5,000 m for a Ha-43-12 and says that was similar to the engine fitted to the A7M2 prototype.

 

[Kilkenny]

Question: What would have been the difference if Japan had more "toughening" metals (especially nickel) for high-temperature/high-stress applications? More reliable Homare? Longer lasting Type98 10cm AA cannon? Where could they have mined/obtained those metals during the war?

 

[DarrenW]

I'm sorry for going off topic, but I'm constantly seeing the 388-mph figure as well for the Ki-84. As far as I can tell, the reason is that everyone sources Rene J. Francillon's book, "Japanese Aircraft of the Pacific War". Unfortunately, much of the information in it doesn't seem to match Japanese records or TAIC data.

There are numerous reasons why that book shouldn't be treated as a primary source. But above all other reasons, I can't find an appendix or list of sources in it. That means any source based on it is potentially false since we can't vet the information in it. I would be very appreciative if someone could locate the sources of that book,

The 427 mph figure has been discussed ad nauseum here. It is the calculated maximum level speed of the Ki-84 using data such as aerodynamic drag coefficients and peak horsepower ratings. The captured aircraft was never tested to find these limits in performance.

However the 388 mph figure can be found in official Japanese data, for the prototype during service trials. A provisional pilot's handbook dated January 1944 captured at Clark Field Philippines concurs with this speed. Apparently 394 mph was obtained by one of the pre-production models with the improved exhaust design.

It has been stated elsewhere that 400 mph was possible with the model 21 engine due to over-boosting but this is not how aircraft performance works. Anytime you raise MAP above rated levels it may increase horsepower but in return lowers the critical altitude thus never improving the absolute maximum level speed previously obtained by more than a couple miles per hour.

 

[AerialTorpedoDude69]

The 427 mph figure has been discussed ad nauseum here. It is the calculated maximum level speed of the Ki-84 using data such as aerodynamic drag coefficients and peak horsepower ratings. The captured aircraft was never tested to find these limits in performance.
However the 388 mph figure can be found in official Japanese data, for the prototype during service trials. A provisional pilot's handbook dated January 1944 captured at Clark Field Philippines concurs with this speed. Apparently 394 mph was obtained by the second prototype.

Thanks for addressing this question. I read the TAIC report as well as a summary of one of the two specimens recovered from Clark Field (as I can't find the original report). I've also seen the Francillon quote regarding the 388 mph speed on the prototype. There are a number of problems with this information but among these are:

1. The TAIC report doesn't mention how they performed their speed calculations, simulated or unsimulated. There's one sentence where it mentions maximum speed tests were not conducted, but it's not clear whether they meant in dives or level flight, or whether that was just one test flight or all test flights.

2. The Fancillon data is for the prototypes, which lacked thrust augmentation exhaust stacks, used an unspecified fuel type (probably 92 octane), had 1,800 HP engines (as you mentioned), used an unknown test methodology, etc... So these are not indicative of actual performance either. Also, I could not find any of Francillon's sources. I'm not sure where he got his data from.

3. The summary of two Ki-84s recovered from the Philippines mentions that speed tests were not performed as the landing gear collapsed after the first flight. It's mentioned there was a second aircraft but there's no test data from that aircraft and I can't find any records of it either. But basically, I can't find this document (unless it's the speed-at-altitude charts).

It has been stated elsewhere that 400 mph+ was possible with the more powerful model 21 engine and everything in good working order but the problem was this never seemed to be the case.

Getting back to the main thread, I've read numerous documents about horrendous issues with the Homare in combat conditions. For example, there was apparently an issue with the carburation system; during high negative-G maneuvers, IIRC and this could be apocryphal, the fuel system would cut out on occasion. The Homare 23 supposedly resolved the issues with the 21, by moving to a low-pressure direct injection system, but whether or not that's true, I don't know. But basically the Homare had a combination of quality control issues as well as design issues. When everything was running smoothly it was highly regarded by the Japanese. In a situation where there are supply shortages and daily air raids, the Homare became less well liked.

I had read that the Homare 23 was not a true direct-injection engine, but I don't know if that's true either. Whatever the case, the Homare 21 was an example of not enough development time, shoddy mass production standards, poor-quality materials used in place of components that required advanced metallurgy, and declining standards in metal quality. It goes without saying that a Ki-84 with a lemon engine was likely to have significantly worse performance than a prototype or well-built aircraft. However, I think most history buffs are interested in knowing the design speed as well as the operational performance of the aircraft. I think it would be like judging the late-war German aircraft by their operational performance, rather than what Allied testing found later on.

 

[Howard Gibson]

The 427 mph figure has been discussed ad nauseum here. It is the calculated maximum level speed of the Ki-84 using data such as aerodynamic drag coefficients and peak horsepower ratings. The captured aircraft was never tested to find these limits in performance.
However the 388 mph figure can be found in official Japanese data, for the prototype during service trials. A provisional pilot's handbook dated January 1944 captured at Clark Field Philippines concurs with this speed. Apparently 394 mph was obtained by the second prototype. It has been stated elsewhere that 400 mph+ was possible with the more powerful model 21 engine and everything in good working order but the problem was this never seemed to be the case.

Where does methanol water injection fit into all of this? The Ki.84 had it. It is the only explanation for the engine's rated BMEP.

 

[Kilkenny]

Thanks for addressing this question. I read the TAIC report as well as a summary of one of the two specimens recovered from Clark Field (as I can't find the original report). I've also seen the Francillon quote regarding the 388 mph speed on the prototype. There are a number of problems with this information but among these are:

1. The TAIC report doesn't mention how they performed their speed calculations, simulated or unsimulated. There's one sentence where it mentions maximum speed tests were not conducted, but it's not clear whether they meant in dives or level flight, or whether that was just one test flight or all test flights.

2. The Fancillon data is for the prototypes, which lacked thrust augmentation exhaust stacks, used an unspecified fuel type (probably 92 octane), had 1,800 HP engines (as you mentioned), used an unknown test methodology, etc... So these are not indicative of actual performance either. Also, I could not find any of Francillon's sources. I'm not sure where he got his data from.

3. The summary of two Ki-84s recovered from the Philippines mentions that speed tests were not performed as the landing gear collapsed after the first flight. It's mentioned there was a second aircraft but there's no test data from that aircraft and I can't find any records of it either. But basically, I can't find this document (unless it's the speed-at-altitude charts).


Getting back to the main thread, I've read numerous documents about horrendous issues with the Homare in combat conditions. For example, there was apparently an issue with the carburation system; during high negative-G maneuvers, IIRC and this could be apocryphal, the fuel system would cut out on occasion. The Homare 23 supposedly resolved the issues with the 21, by moving to a low-pressure direct injection system, but whether or not that's true, I don't know. But basically the Homare had a combination of quality control issues as well as design issues. When everything was running smoothly it was highly regarded by the Japanese. In a situation where there are supply shortages and daily air raids, the Homare became less well liked.

I had read that the Homare 23 was not a true direct-injection engine, but I don't know if that's true either. Whatever the case, the Homare 21 was an example of not enough development time, shoddy mass production standards, poor-quality materials used in place of components that required advanced metallurgy, and declining standards in metal quality. It goes without saying that a Ki-84 with a lemon engine was likely to have significantly worse performance than a prototype or well-built aircraft. However, I think most history buffs are interested in knowing the design speed as well as the operational performance of the aircraft. I think it would be like judging the late-war German aircraft by their operational performance, rather than what Allied testing found later on.

Perhaps the final verdict would/could/should have been a comprehensive fly-off between the Ki-116 and the Ki-84. I suspect that even with a well-manufactured and properly maintained Homare engine, the latter would have been 20-30mph faster than, but not as maneuverable and certainly not as available for use as, a Ki-116 with a properly manufactured and maintained Kinsei engine. Even a Ki-100 might have been a better all-round fighter.

 

[AerialTorpedoDude69]

Perhaps the final verdict would/could/should have been a comprehensive fly-off between the Ki-116 and the Ki-84. I suspect that even with a well-manufactured and properly maintained Homare engine, the latter would have been 20-30mph faster than, but not as maneuverable and certainly not as available for use as, a Ki-116 with a properly manufactured and maintained Kinsei engine. Even a Ki-100 might have been a better all-round fighter.

That's a good question. It may be that the Kinsei 62 was so well regarded because it needed fewer of the alloys that a high RPM engine required. After all, it was considered to be reliable by the Japanese and of superior quality by Americans. But I don't know. Shinpachi might know more.

I do know that the Kinsei 62 is regarded as one of the few (possibly also certain models of Kasei or Homare) Japanese WW2 engines that had full direct fuel injection. Because fuel injection requires very high levels of precision and technical sophistication, it may be that the Kinsei was just a technically superior engine compared to its stablemates.

 

[Kilkenny]

That's a good question. It may be that the Kinsei 62 was so well regarded because it needed fewer of the alloys that a high RPM engine required. After all, it was considered to be reliable by the Japanese and of superior quality by Americans. But I don't know. Shinpachi might know more.

I do know that the Kinsei 62 is regarded as one of the few (possibly also certain models of Kasei or Homare) Japanese WW2 engines that had full direct fuel injection. Because fuel injection requires very high levels of precision and technical sophistication, it may be that the Kinsei was just a technically superior engine compared to its stablemates.

Yeah... I have been wondering a lot about the alloys issue... What was the big difference in the engine materials?... And the more I read, the more I think that Japan should have concentrated its fighter engine development on the Kinsei (and its 18-cylinder derivative) than wasting all that time trying to beef up the Sakae, Ha-41, Mamoru, Homare, etc... Of course hindsight is always 20-20 or even 20-10.