I know the sleeve taper relaxes at a certain temperature window for designed operation. I know it can begin as early as 200 F but does anybody know (research?) at what point is the engine too hot such that the expansion of all metals causes sleeve/piston seizure, essentially bringing back 'compression' again? Or is that not possible before we encounter some other problem?

Just wondering if there was a phase of: tight, relaxed, tight fit during the increase of temperatures.

Any other heat related damage besides bushings, bearings, oil starvation?

If you're going to try that, I'd just take the sleeve out and heat that alone to shrink it. I don't know what kind of temperature you'd need to get it to shrink the right amount, though.

Good luck with this.... LOL. The only effect of a seriously over heated engine I've ever seen is a worn out piston and liner. This 'research' makes me think there is no effect that you're asking for.

Metals expand when heated. The amount of expansion is different for each type of metal, and will stop expanding at a certain point. (like a broke-in motor) Metals also get softer at very high temps, which makes them wear out faster.

I bought a used engine for parts that needed rebuilding. The sleeve and piston were worn out were the piston goes all the way through the top of the sleeve. The motor wont start. After heating the motor with a pen torch, I was able to run a few tanks without letting the motor cool down.

So with this experience. I can assume that the aluminum piston expands more than the nickel plated brass sleeve. When heated, creates a temporary effect that creates a better seal in the combustion chamber, hence brings back some compression.

If you want to bring back compression. Have it re-pinched. Cost around $20-25 in specialty RC shops and even ebay. Have heard a lot of people done this and race with re-pinched sleeves and also lasts quite sometime.

(ebay / search user osrocket)

They don't, compression is reduced in a worn engine as they heat up.

Thanks for the experience input Associated Driver.

My engine doesn't have a worn sleeve however I was trying to determine if the ABC metals still continued to expand at the higher operating temperatures to bring about "more compression" and thus more power! Not necessarily trying to run worn engines here, but I can see that could be a band aid like you have successfully verified.

How about this... anybody tune their engines for power regardless of how high the temperatures get? I used to do this all the time with an older OS .15 cv. It would perform ~300 w/o problems. Sometimes peaking as high as 360's (probably due to the inefficient head)! But after I threw a rod at 4 gallons (high rpm parking lot bashing) the existing sleeve taper was still enough to stop the piston 1/8" from the top from going thru.

I would accept 300deg when I was pushing it hard in a race, but not under any other circumstances, and if it went to 310, I'd do something about it immediately. Personally I don't have such an issue with high temperatures as most people, these engines run ok up to about 290, but after that you're in the danger zone, and reducing their useful life dramatically.

actually
associated drivers experience may be unique
my experience with worn engines is that as they get hot compression falls away dramatically

why you would think that super-heating an engine would cause an engine to gain compression is beyond me
after all
for that to happen
one component would have to heat more than the others
as these engines heat
all the parts heat up at relatively the same speed
the difference is that different metals expand at different rates
so the piston and liner will continue to expand at their rates and they will not catch up to one another
and thus a abc engine will never really seize up from heat
not like an auto engine will
they will self destruct
and the lean condition that causes the heat will cause extra wear
but you just don't see a nitro engine that "seized" from overheating
in 30 years, i've never seen one

i suppose that you could artificially over heat individual components to gain the appearance of compression

but to answer your question
no
overheating an rc engine won't give you better compression at some point
it will just speed the engines death

If the cooling head on your engine is over 300 deg F then you are running too lean. If the temp reaches about 320 inside the cylinder the fuel will explode instantly, way advancing the timing and possibly blowing the con rod. You will get more power running the engine that hotter (to a point), but I would never recommend it. The piston will be greatly expanded from normal running temperatures and when you bring down the temps to normal levels (260 or below usually) you will have reduced compression. The same goes for heating the piston up way high - you are actually going to do more damage to the engine. If you can get a few more runs out of a dead engine, go for it, otherwise, don't.

If you already have a worn engine with low compression and you would only apply heat to the piston, in theory, you could get it to seal up and run. How would you just heat the piston though?

If you have a low compression engine you can just put a few drops of after run oil down the glow plug hole and it will fire up again. Creates a seal between the piston and sleeve temporarily. Usually will run for up to half a tank of fuel if you keep it rich. What is really the point though?

Last year toward the end of the season one guy at our track lost all compression on an OS engine. He pulled the sleeve out, heated it up, put a hose clamp around it and tightened it, then cooled the sleeve. He did this a couple of times until he was happy with the pinch he had between the piston and sleeve. Slammed it back together and it worked fine. A cheep re-pinch in a pinch I guess.

Iamnot - W/o looking it up, I'm assuming the sleeve's thermal expansion is greater than that of the piston to allow the relaxation of the taper during normal operation.

The last time I checked thermal expansion was isotropic. In theory as the metals expand, the inner diameter of the sleeve should also narrow (material grows inward as well) along with the outward expansion of the aluminum piston. Add to that the assumption that the sleeve has a higher thermal expansion rate and at a certain high temperature you should be back to an increase of part interference which would lead rise to an increase of compression = more power.

yeah pretty sure thats how it works.
my buddy rustler has the old .15 yeh its been a while since they made rustler like that but it has no compression whatsoever i mean i can turn it over w/ my pinky and we have to heat it up w/ a heat gun to about 270 to get it to start.

And the reason nitro engine seizures are far and between (I'm assuming seizure primarily attributed to oil starvation) I'm thinking is partly due to the benefits of caster oil. Some tidbits of caster I googled:

"Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature goes up. As it polymerizes, it forms ever-heavier "oils" that are rich in esters. These esters do not even begin to decompose until the temperature hits about 650 degrees F (343 deg C). Castor oil forms huge molecular structures at these elevated temperatures - in other words, as the temperature goes up, the castor oil exposed to these temperatures responds by becoming an even better lubricant!

Castor oil has other unique properties. It is highly polar and has a great affinity for metal surfaces. It has a flash point of only 445 degrees F (229 deg C), but its fire point is about 840 degrees F (449 deg C)!"

Then there's the benefits of varnish... but we get the idea of how good this stuff is.

So anyways I was seeing if anybody else out there had some high temperature experiences/research on our 2 stroke nitro engines. There's also the benefit that hotter engines are more thermodynamically efficient, which should in turn equal more power to the wheels.

well even as the engines get hotter and hotter the metals used to make our engines would heat up to a point that they would fracture under the stress of the useage. also what about all the other ingrediants how well to they hold up to these high temps you were discussing?

Modern Engine Design

Airplane Page Home Page

There are far more details to engine design than can be discussed here, however here are the basics for 2 stroke
engines, many of the principles apply to 4 strokes etc.



MODERN INDUCTION SYSTEMS

Almost all ‘sports’ engines feature ‘front’ induction, where the fuel / air mixture enters through the hollow
crankshaft. Most racing engines feature ‘rear’ induction, of several types. The more common is the disk
induction where an extended crankpin drives a ported disk which opens and closes the inlet.

The less common rear induction is the ‘drum’ system, which in turn has 2 types, the ‘normal’ and ‘reverse
drum’. Instead of the crankpin driving a disk it drives a smaller version of the hollow crankshaft.

The conventional drum has the fuel / air mixture enter via the drum centre and exit vertically in a manner similar to
a front induction set-up.

The reverse drum has the fuel inlet from above (or below) in a manner similar to a front induction set-up and
exits through the centre of the drum. This system has the advantage of providing fresh fuel (oil) to the crankshaft /
conrod which is why it is very popular for C/L team race engines.

LINER PORTING TYPES

Almost universal nowadays is the Schnuerle port liner as opposed to the old loop scavenged , cross flow or
piston port systems.

The loop scavenged system used one large transfer port and one exhaust port on the opposite side. Some times
these ports were split into several sections. The intake charge was prevented from exiting straight out the exhaust
by using a piston baffle.

The cross flow system which was employed on many diesel engines where a transfer port was used front and
rear combined with an exhaust used on either side of the liner.

Piston port induction as per the famous Mills engines is where the fuel induction is directly into the liner.

Several manufacturers have recently introduced a reed valve induction system, via the engine back-plate. Not
totally new as it was done by Frog many years ago (although in a different fashion).

Schnuerle porting uses a group of transfer passages arranged in the liner as to direct the incoming mixture away
from the exhaust port (infact often crashing into each other). Over the years the system has seen many
configurations of transfer port numbers etc, but the same principles apply. The classic Schnuerle port liner has 1
@ exhaust, 1 @ boost (opposite to the exhaust) and 2 @ transfer ports on either side.

PISTON & LINER MATERIALS

Most early engines and a decreasing percentage of modern engines featured a cast iron piston in a steel bore.
Many of these engines feature a range of different ring types to provide the seal.

A more recent and now more common engine is the ABC, ABN and AAC piston and liner. Some of these
engines are ringed.

The ABC engine has an aluminium piston operating in a hard chrome bore, brass liner.

The ABN engine has an aluminium piston operating in a special nickel plated bore, brass liner.

The AAC engine has an aluminium piston operating in a hard chrome plated bore, aluminium liner.

METALLURGY

The reason for all of the above liner configurations, apart from using materials compatible for wear and long use,
is to provide for the different expansion rates of the components due to heat (combustion).

If an engine used the same material for the piston and liner it would seize almost instantly as the piston is subject
to higher heat, hence expands at a greater rate than the liner.

In the case of the AAC engines, the piston is made of an aluminium featuring a high percentage of hard wearing
silicon in the alloy. This alloy expands at a lower rate than the lower silicon alloyed liner.

In theory the ABC, ABN, and AAC engines should never seize as the liner will expand away from the piston as
heat rises.

The major difference between good and bad engines is the metallurgy and quality of the component fits.

TOLERANCES & FITS

Whilst it is not easy to see without a trained eye, the quality of component tolerances, shape and fits is the most
important area of all engines.

It goes without saying, but often not the case, all the major components of an engine should be round, i.e. the
liner fit in the case, the bore, the piston, the head and the bearing housings. A close examination of a liner will
often show an uneven wear pattern, i.e. the piston and / or liner are not perfectly round or operating in a very
uneven cooling system.

The crankshaft often rubs on the case between the bearings due to the flexing loads imposed on the crankshaft.
All engines should have the case diameter of this area enlarged (most manufacturers do this).

The liner of an engine should not be a perfect cylinder. The cylinder should taper to a larger diameter at the
bottom (only the top of the liner is critical to the compression seal and power stroke). The average taper is
between .002 to .003 of an inch per inch of liner height.

The piston should also be tapered, although only the ‘better’ engines are tapered, or barrelled as it is often
referred to, because the piston should in a lesser way be shaped like a wine barrel, i.e. a lesser diameter at the
very top and bottom of the piston.

The reason for the piston to be barrelled is 4 fold.

1.To reduce contact friction with the liner.

1.It is best to have a small gap at the top of the piston to form a sealing
oil wedge.

1.The top of the piston is the hottest, causing the piston crown to
expand.

1.To help prevent a piston edge catching in a liner port. As the piston
travels up and down there are many side loads etc which can force
the piston sideways slightly. This can be a problem for racing engines
which generally use very large exhaust ports.



Crankcase vs. crankshaft

One of the biggest problems for engine designers is the expansion coefficient difference between the aluminium
crankcase and steel crankshaft. As engines are ‘fitted’ cold, under operating temperatures the case will expand
(lengthways) more than the crankshaft. This puts an uneven load on the bearings.

Many competition C/L team race engines have gone to great lengths to improve this problem, including the use
of all steel front section of the crankcase. Some of the new Webra engines use a special low expansion
aluminium crankcase, and it shows in excellent performance.

It is important that the ‘front end’ of an engine receive cooling air.

TIMINGS

The duration of the opening of the intake, transfer and exhaust ports (timings) is dependent on the end use of the
engine. Racing engines often have very high timings (generally quoted as the degrees of rotation for which the
port is open). The shape of the ports can also be very important.

As a general rule, the manufacturers do a good job of providing well designed ports, normally the only time the
end user can justify modifying a port’s duration is to match an engine to a tuned pipe. A good example of this is
some of the larger 2 strokes above .90 size, where the manufacturer has used low timings to increase torque.
For an engine to respond well to a pipe, the exhaust timing must be no lower than 145 degrees. Below 140
degrees, the pipe can actually deteriorate the engine’s performance.


i can quote the internet too...
and i've highlighted some of the pertinent parts

but
bottom line

why don't you run one of your engines up to 400 degrees or so
and report back
with your first hand info

on how much extra compression and power you get

and how much it costs to repair/replace that engine

Yeah I just thought of an experiment of how to test it (with adequate lubrication too). I'll see if I ever get around to it as it's interesting that nobody knows and I'd like to see documented data.

And guess what... I've already told you I've had my engine temps spike as high as 380's. And you know what the engine didn't break... it didn't break the day after either or the day after that. And what did that cost me... oh that's right... NOTHING!

Now why don't you report back to me on what you've actually done first hand so far as to actually contribute to this? What has your previous calculations shown? Oh btw next time don't bother to include non relevant induction and port timing content to this table.

You still failed to explain how a sleeve can be biased to expand only in the outward direction... esp when I'd think the combustion on the inside of the sleeve (hot side) would tend to make the material grow inwards just as well if not more (assuming there was minimal 'barrelling' to begin with and/or at the point where increased heat has already used up all the 'barrelling' tolerance; crankcase should be swelling up as well to add to that factor).

Under those circumstances, I would think under high enough temperature, the crankcase sleeve's bore would narrow, eat up the barrelling relief, sleeve would narrow (because no more room to expand and it grows inwards just as well), piston would expand wide as well as in height increasing the compression ratio, connecting rod would grow (again increasing CR), maybe even the crank some what for a longer stroke (more CR you say?!), and the head combustion chamber could possibly grow into the volume, all adding up to a net increase in combustion ratio. That idea too far fetch?

DANG UR GOOD!

the funny thing here is you keep trying to refute what i say
with ASSumptions as thin as what you claim my info is

your refutation is riddled with "i think"
and spiking is far different from running at those temps

you don't have the nerve to put your theory to the test
but be sure to "document" if you ever do get around to it

and, yes, it is too far fetched

i've already told you what my 30 years of experience tells me
if you don't wish to believe me
fine, free country and all that

one last thought and i'm done with this thread

why do no engine manufacturers recommend what you're suggesting
and why do no professional race teams or racers do it?

engine life means nothing to a factory sponsored team
and they are after every little edge after all

and yet they still don't do it
wonder why???

I'm pretty sure from what I've already stated and somewhat experimented shows that I've already got more 'nerve' than your claims. In addition to that now it seems you somehow suddenly 'claim' to know more about extreme high temperature operations... hmm where is your facts and data?

Inferences mean nothing. Because you don't know yourself doesn't mean the things I illustrated out for you earlier don't exist either. Until you know for yourself you might as well go off and prove/disprove the existence of god while your at it.

Besides I'm making assumptions on the already known fundamental principles, which can't be refuted. If you really wanted to test this YOUR way, go find me an apparatus to hook up sensors placed within the engine vs temperature and spit it out the finite element analysis for us all. I'll be willing to bet you that... *gasp* things actually grow? Oh no... that can't be possible...

Based on what this search started of... let me restate and clarify for all. Based on tolerances and temperature growth, there should be a temperature threshold where tolerances reduce and thus compression climbs back up. I'd like to know if anybody knows when?

The fact that there is 2 people that claim to be better off starting heated engines further shows evidence of such things. On the old argument for a heated engine to even start would be no better off than a cold one. I do have an answer and I speculate it depends on which side of hyperbolic "compression" vs temperature curve you are on.

Themal explansion is just what it means, expansion from heat. If you heat a rusty nut on a bolt to get it loose, the reason the heat helped was because you thermally expanded the nut. Think about that. The hole in the nut grew because of thermal expansion, it did not shrink.

If you heat the engine, you are heating the hole she-bang and not just individual components. Heating all the components will cause all the components to expand. There are different thermal expansion factors for each type of material you heat. Maybe the aluminum piston is expanding more than the sleeve when you are applying heat because it thermally expands sooner than the sleeve? Maybe the piston starts to expand at a lower temperture than the sleeve.

In 2 stroke snowmobile engines, a common problem is cold siesing. This happens when a guy fires up the sled and hammers on it imeadiatly. The pistons expand faster than the sleeves to a point in which they get stuck in the sleeves. If you are easy on the machine when it's cold, it will warm up slowly giving the sleevs a chance to heat up and expand enough to allow an allowable clearance.

So your saying if I bake a donut from dough to bread (an example which has evidently a large thermal expansion coefficient) and measuring the inner hole diameter before and after the process you are proposing that the inner hole's diameter would stay consistent if not larger in the end?

Somehow people seem to be over simplifying that growth is only occurring in 1 direction, especially from an 2d plane. Here being only in the outward radial direction.

I still am sticking to my argument that growth by thermal expansion is isotropic, therefore if there was an imaginary cross-sectional centerline ring, growth would occur outwards 'in all directions', thereby reducing the 'inner diameter' as growth occurs. The obvious byproduct of this in engines is piston seizure. If a piston was ceramic but the sleeve had a high coefficient of expansion would you say engine seizure would be impossible because the sleeve could never reduce in inner diameter by thermal expansion?

Doesn't our sleeves have a higher expansion rate than the pistons? Eventually at a certain temperature I expect both sliding surfaces to start to feel the 'squeeze'. I'm after that value.

ttowntoolman I'm sure that's normal operation, but what about the 'grey' area where it's now in the upper temperature area and things start to 'grow' again and then some. That's what I'm trying to get after. If extreme high temperatures close up tolerances to bring back compression. Which the majority somehow already say no though they've never actually tested what should fundamentally occur.

No offense, but asking a question like this is "Will banging my head on a brick wall make me smarter?"

The answer to the question that started
this thread is,NO!
Enough with all the B.S., a simple NO, is all that
is needed. Nuff Said!!!