Double the thickness of the hard steel liner in the rotor housing.
This will result in a large reduction in fuel burn. BSFC.
A simple change that Mazda could do on the production line.
Paul Lamar
Hi Paul, How does a thicker liner give a reduced fuel burn; what is
the theory here?
Dave in Perth
One wants to keep as much heat (temperature) in the combustion
chamber as possible. The steel liner acts as insulation as unusual as
that sounds. The heat transfer rate of steel is much much lower than
that of aluminum. So one would want to increase the thickness of that
insulation to prevent the aluminum from carrying away all the
temperature.
As the combustion chamber rotates toward the exhaust port it is
exposed to new areas of the steel liner which are cooler and more and
more temperature is lost. When one cools off a high pressurized
container the pressure is reduced in that container. This pressure is
acting on the face of the rotor and that is what generates the torque
so potential torque is lost.
(P1 X V1) / T1 = (P2 X V2)/ T2
Change any one of those terms in the first part and that will
change the second part.
https://en.wikipedia.org/wiki/Gas_laws
It is a fundamental law of physics.
Note: in this attached chart stainless steel is 1/3 that of cast
iron. That is why Mazda needs to find out how to make the rotors out
of SS instead of cast iron. SS would be good for the liner as well if
it were possible to chrome plate it.
Also note: In the heat transfer units it is
watts / (meters X Temp)
so the thicker material the less heat is transfered.
Thermal details like this affect the BSFC of all engines.
Hold a piece of steel on a grinding wheel with your bare hands. Then
hold a piece of the same size SS and report back :-)
Paul Lamar
What is the heat transfer and cost of a ceramic coated rotor versus
a stainless steel rotor?
As temperatures increase of the combustion chamber and sliding
surfaces, what is expected to be the next weak link? I suspect
carbon buildup would go down but will seal lubrication become an
issue?
Speaking of seal lubrication, which paper addresses why the apex
seals aren't lubricated from inside the rotor?
Aron Buster
"What is the heat transfer and cost of a ceramic coated rotor versus
a stainless steel rotor?"
I have no idea. Who is doing ceramic coating on rotors? I would
hesitate to use it because if some of it came off it would destroy
the wear surface of an expensive rotor housing.
The proven Iannetti silicon nitride seals will tolerate double the
temperature of an iron seal. I have never heard of one breaking up in
17 years of this news letter. They are solid silicon nitride with
fiber reinforcement supposedly.
The main function of the apex seal oil is cooling the iron apex
seals. Synthetic oil will tolerate twice the temperature of organic
oils. Turbine oils are better than that.
If you use synthetic oil there is no carbon build up.
"Speaking of seal lubrication, which paper addresses why the apex
seals aren't lubricated from inside the rotor?"
You got me there. Do you know if there is such a paper? It might work
but the holes would have to be tiny and subject to plugging up if
some one used organic oils.
Also the amount of oil required is a function of horse power. I don't
know how one would control that with out a computer. Mixing with the
fuel already does that. More fuel, more oil, more power.
---------------------------------------------------------------------
BTW the stock oil "injector pump" is not really a pump at all.
It is an oil distribution device. The oil is transfered to the
compression chamber when the chamber is lower pressure than
atmospheric. The so-called injectors are open to the atmospheric
pressure.
If one turbo charges the RX7 engine less apex seal oil will find its
way to the apex seals. Mazda themselves made that mistake when they
turboed the rotary back in the late 80's and all of the 90's. If you
have one of those you better be mixing oil with the fuel :-)
Some people on the Internet get it wrong. They incorrectly assume
the RX7 "pump" AKA OMP is fed from the high pressure oil system. Not
true except possibly for the RX8. It is fed from a gravity fed
atmospheric pressure oil reservoir. To prove it take the RX7 "pump"
off and turn the engine over with the starter and see if oil squirts
out of the supply hole.
Paul Lamar
Hi Paul
I have looked quite a bit into ceramic coatings to act as thermal
insulation layer and as part of that I have talked to several world
experts on the subject matter
There are actually quite a few options for applying thermal coatings
onto metal surfaces and for the most part they can be applied to
many materials even some some low temp materials. Most are applied
with very high velocity impregnations. These nozzles are cnc
programmed to apply a pattern to get complete coverage
The issue is that I have not found any coaters who are able to
guarantee that the coating thickness will be uniform across the
surface being coated. This means than for the most part, the ceramic
surface will need to be lapped post coating process. This would make
this process coast prohibitive for low volume applications. Even for
high volume applications, it will be fairly expensive process because
the thermal insulating ceramics also tend to be extremely hard to to
lap.
With that said, if Mazda proceeds with releasing a new rotary, I am
pretty confident that they will utilize some sort of ceramic
insulating layer to help meet the more stringent regulations
Regards Simon Saba
Hi Simon, The rotor would only need to be coated on the three outer
surfaces, wouldn't it ? would they actually need to be lapped ?
doesn't only the apex seal contact the liner ? Chris Hoskins.
Don't do it Chris. You won't even notice it as it will be less than
1% lower fuel burn.
If a small part comes off you can kiss your engine good bye.
Also make sure you have a good air filter.
Paul Lamar
Hi Chris,
Paul is absolutely right You can coat on the opposite surface (ie
external surfaces surrounding the chamber) and indeed you will not
need to lap if on this surface instead of the internal surface. It
will still add insulation, however, because of the excellent
conductivity of the metal, the heat will conduct sideways (ie
radially) on the side plates, and axially and rotationally on the
housing. This will reduce the effectiveness of adding the insulation.
Certainly there will be an improvement but very small relative to
insulating the internal surfaces
The most effective, but unfortunately most expensive, is to insulate
the inside surfaces. On piston engines, there are several aftermarket
services that will coat the piston top and the inside surface of the
dome and it helps keep the heat inside by adding insulation and
reflecting radiated heat back in the piston. It would be an
interesting experiment for someone to coat the outer surfaces of the
rotor and quantify the impact.
Simon Saba
As long as there is no possibility of it coming off.
A piston engine will tolerate more debris.
GM patented a multi piece rotor.
Paul Lamar
A few years back I evaluated an automotive engine that had been
extensively coated with ceramics on the inside. The combustion chamber,
the valve faces and the piston were all coated. None of the coatings
failed but even a surface gap spark plug wouldn't survive. Any plug we
tried would literally melt in a very short time. Seems like all that
trapped heat had to go somewhere and the plug was the coolest point in the
chamber. The plugs were well cooled with a surrounding water jacket but
that wouldn't cool the tip sufficiently. We weren't able to run the
engine long enough to get any output numbers. The promise, from the
builder, was a significant increase in power and a reduction in BSFC,
which is a nice trick if you can pull it off. This approach didn't.
Gary Knutson
We had to get extra cold plugs for the TTC.
I think they are Iridium. The gap is .010" or so.
The melting temperature is near 3000 F.
All RX8's come with Iridium plugs.
"A member of the platinum group metals iridium is white, resembling
platinum, but with a slight yellowish cast. Because of its hardness
brittleness, and very high melting point solid iridium is difficult to
machine, form, or work, thus powder metallurgy is commonly employed,
instead. It is the only metal to maintain good mechanical properties in
air at temperatures above 1,600 °C (2,910 °F). It has the 10th highest
boiling point among all elements and becomes a superconductor at
temperatures below 0.14 K ."
Paul Lamar
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