Besides ice, what expands as it cools?

I want to make a temprature compensator out of some material that exspands as it cools.

I want it to work between 100 to 250* F.

It is a compression compensator for a model aiplane diesel.

On thease little engines the timing is adjusted by the compression ratio on the contra piston and they are very sensative to temprature.

I want to encase the materiall above the app. 1/2" contra piston and the adjstment screw to lower compression as the engine gets hot and increase it as it gets cold. The idea is similar to a wax pellot in a thermostat.

Maybe I can come up with a simple compact way to do this mechanicaly from an expanding material....

Any ideas?

Treven.

Good old Google came up with this stuff: zirconium tungstate ZrW2O8

http://www.dur.ac.uk/john.evans/webpages/zrw2o8.html

Probably not availibe at the corner hardware store though

perhaps its not what you need but...

Small Parts has some bismouth alloy that melts in boling water. Perhaps that will work in your case instead of the wax pellet?

Type metal expands as goes from the liquid to the solid.

Some of the CERRO alloys "grow" on cooling or after several hours

chevy43:

If you connect a material with a positive temperature coefficient to a lever arm with a pivot point and look at the motion on the opposite side of the pivot relative to the same ground plane, then you have a negative temperature coefficent for that motion.

Or use a cantilevered bimetal element.

.

Get a Bi-metal spring out of a carburator choke housing or from a home thermostat. They are very sensitive to a wide temp range. You could experiment with different lengths of the spring and customise it for your application.

The secret metal is antimony.

I have fiddled around with bimetalic strips and they very inefficient, i.e. they don't provide as much movement/torque as wax thermostat pellets. i.e. in the average heating thermostat, the bimetalic element is about 1 1/2" long by .25" wide and .030" thk, it moves about .050" for a 5 deg rise in temp but with the power of a Gnats c**k. The thermostaic element in a shower mixer is about 36" long .4" wide and .06 thick. It is all twisted and wound up and moves about 3/4" in an arc as it tries to untwist. But again it only rotates a plastic tube with holes in, not a stop cock!
Another source for smaller wax capsules are the temperature sensing heads for thermostaticaly conntrolled radiator valves. I have one that moves about 5mm fot 10 - 100 deg C and it seems very strong cos it actually shuts the water flow off. These are available in your nearest dumster in in a plumbers for about £7 in the UK.
regards Frank

chevy43:

How much motion is required for the 150 deg F temperature change, and how much force is required?

About a 64th or less of an inch or so from ambiant to 250*

The problem is it needs to be light, compact, adjustible with a screw and the temprature element has to be exsposed to cylinder head head temprature, not cooling air from the prop.
Not and easy thing to do.....

Here is a picture of the adjustable compression chamber

Dissasembled view

Installed in a 4 stroke cyl head in place of the glo plug

Thease were done by a talented engine modifier in Sweeden named Jens. He does great work and has shared his engine modifications generously.

[ 01-17-2007, 03:04 PM: Message edited by: chevy43 ]

Besides ice, what expands as it cools?

Click to expand...

Nipples?
Jim

Bismuth

Rich

If you succeed in developing a nipple-powered diesel, you'll be a very rich man.

All kidding aside, have you considered an electronic solution? I'm an RC car fan, and have noticed with interest that the latest & greatest radios - 2ghz, self-selecting channel switching - have the capacity for telemetry, with the data displayed on a screen facing the driver/pilot. Some of the "park flyer" or "electric indoor" servos are really stunningly tiny. Of course, they don't make a lot of torque.

It would be a fairly simple thing to equip the adjustment cylinder with a 28-pitch thread which moves 1/64th with less than a half-turn. That's important because you can't use a pushrod to reliably operate a bellcrank a full 180 degrees.

Neat project!

The Handbook of Chemistry and Physics shows the coefficient for many materials. A reasonable maximum for a useful material is 25 x 10 exp -6 length per unit length per deg C. Magnesium is in this range.

A 150 deg F change equals an 83 deg C change (5/9). For this change in temperature over the entire length of a 1" Magnesium rod the length change is about 83 x 25 = 2075 or 0.002". The only way to increase this length change is to use some multiplier device. Also you have to change the sign.

Also I suspect you need a great deal of force capability.

The servo system suggestion by Racer Al maybe your most realistic approach.

.

If I understand this correctly, you want to increase the volume of the cylinder as it heats up thereby decreasing the compression ratio. And decrease it as it cools down; increasing the compression.

The thing you have to consider is the relative expansion of the compensating material vs. the material the cylinder is made of. As the cylinder heats up, it is already getting bigger. But then, so is the piston, connecting rod and crank, so the compression ratio should stay somewhat the same. I say somewhat because the temperatures of these elements will not be the same nor will they change by the same amounts as the engine heats up. So there is already some change in the ratio.

If you add another element to compensate, then what really counts is it's coefficient of expansion RELATIVE to the coefficient of the metal that the cylinder is made from. Clock pendulums were compensated this way, by using two different materials and adjusting their lengths to provide the same overall length when the temperature changes. (Or perhaps have it change in a way that exactly compensates for other elements of the clock design.) But the changes were not as great as 1/64" even with lengths of a foot or more. Hence the need for a material with a large negative coefficient of expansion. I suspect you will have to add some leverage to increase the effect you will get from any real material. And this leverage will decrease the force that the effect will produce and it may not be enough to overcome the force generated by the compression. You would have to do the math.

Frankly, I like the electronic suggestion. But you would need a small actuator (motor or what?) to drive the compensating element.

From memory of having a .8cc diesel some 40 years ago, don't they have a little lever on top which you rotate to increase/decrease the comprssion ratio? Changing the screw thread to a very coarse one could lead to a servo actuated solution.
Another solution (?) have a wax capsule mounted tranversly above the contra piston, with a bell crank pulling the CP up as the wax comes out.

bit of the wall , but instead of raising the contra piston alone, what would happen if the whole cylinder barrel was lifted with a wax capsule. It would have to transmit the flea power torque and would have some interesting problems around the inlet port.
Frank

Chevy43
Man you really peaked my interest with this one. I too have thought about a similar system for a FAI F2C 2.5cc racing diesel. Problem is with the compression ratio we run it is extremely hard to have something to control compression that moves easily most of our comp buttons are a interference fit .0001 to .0015 to maintain good compression at optimum temperature. In your situation I would try a servo driven affair with a temperature based resolver to control the servo. I assume you are working with an RC type model. So weight is not a big problem. What I would like to see is some real time inflight telemetry which would give temp., rpm, airflow pressure at the venturi, fuel flow etc. with the whole package weighting no more than 10 gm. yea I know, I am dreaming. As far as recording engine temp inflight there is a temp sensor you can purchase from one of the RC Car companys that will give peak temp. reading the hole unit weights less than 7 gm. Think about using the servo driven concept if you can find a method to read the temp. I would keep the resolution of such a device somewhere in the area of 5 degree increments to assure a even running engine.

On edit: Forget the 100 to 250 range the optimum temp range is between 220 and 280 at full rpm. I do realize that with a four stroke you are running at probably less than half the rpm our racing diesels generate (25,000 static to 32,000 inflight). The good thing is that once you get your needle set for the best fuel efficiency the compression is the only adjustment needed to keep from overheating and melting down an engine.
Scott

If the screw can be turned by a servo with a lever all you need to do is attach a (servo adjuster.The circle type with holes around it to the screw top)don't know proper name;then attach a thermostat sending unit to the compression compensator and have it lead back to a thermostat that you mount in the plane.Set it at the temperature you need and go flying.

What does it run like normally?Why do you want to add this?

Hitlist,
I would say that having the ability to adjust the compression is to control the temp and fuel efficiency with fuel effciency being the objective, with a high compression you get more ecomony with a lower you get less, the key is to maintain a given engine temperature regardless of the ambient temperature outside thus the reason for a variable compensation of the compression.

Scott