Valve temperature of a small i.c. engine

[Metal Butcher]

I wonder what temperature the valves reach on a simple single cylinder air cooled i.c. engine. I can't find any information on this subject. It would also like to know what head temperature can be expected on an engine with a 3/4" to 1" bore. Has anyone used an infrared temp gauge to check this out on one of their small engines? Here's a link to what I'm talking about.

http://www.google.com/products/cata...og_result&ct=result&resnum=1&ved=0CCsQ8wIwAA#

-MB

 

[Brian Rupnow]

Speaking from todays personal experience----Hot enough to burn your fingers!!! ;D ;D ;D

 

[MachineTom]

Thats like asking, How big is a small piece of? I'll answer based on full scale engines I have known.
Head temp on Air Cooled engine should be not more than 4-450° F, this on gasoline, alch engines run cooler, Intake valve temp will be about head temp, exhaust valve temp will be higher than exhaust temp, so 11-1700° F. A turbo-supercharged aircraft engine will blow those #'s away, but a model engine will not be that hot running.

 

[Metal Butcher]

Obviously I know I'm asking a tough question. The reason I ask is that the build I'm on, calls for the valves to be made from two pieces press fit, which I prefer over one piece, and I'm thinking of using a brass head with a steel stem. I could use a shrink fit but their is concern that when the valve stem assembly heats up it could come apart. I could also silver braze the parts, and again the running temp at the exhaust valve is a concern. Ideally Loctite would make the most clean and neat assembly but the temperature would surely break down the bond. Also, the valve seat is 6061 aluminum which I believe has a lower melt point than brass.

-MB

 

[GailInNM]

MB,
I use one of the Duratrax IR thermometers for a variety of purposes. They are like any of the other low cost IR thermometers in that they use an average value for thermal emmisity of the material being measured. The high end IR thermometers can be adjusted for the material. This may give you misleading readings if you are trying to get any absolute temperature measurements.

I have "Tiny" running unloaded in hit or miss mode right now and it has been running for a half hour or so. Measuring the cast iron cylinder temperature indicates 105 degrees. Measuring the polished brass cylinder head shows 88 degrees. All well and fine, but I can put my thumb on the cylinder as long as I want and never get uncomfortable, but on the cylinder head it gets VERY uncomfortable after about 10 seconds. This is because of the very low thermal emmisity of the polished brass. Just the opposite of what the termperature reading would indicate.

As well as material, surface finish is also a factor. I just measured a one piece aluminum coupling on an air collet closer that has been running for about an hour. Part of the coupling is polished alumninum and the other part knurled. The polished part reads 89 degrees and the knurled reads 106 degrees.

The IR thermometer is still very useful around the shop for comparative measurements.

On valves, all I know is that I have never seen a burned valve on a low performance model engines. I have seen photos of burned valves on some high performance model engines as well as holes burned in the piston crown, but that is of a different class of engine than most of us on HMEM build.

Gail in NM

 

[Brian Rupnow]

On the Kerzel engine which I just built, the original plans call for two peice valves, where the head is silver soldered to the stem. As I am lead to believe, a number of people have built this engine succesfully. If so, then they must run at lower than the melting point of silver solder.

 

[Lakc]

I ran a couple of my 2 stroke commercial model airplane engines on my test stand with thermocouples for cht. Its been a few years now but from memory ~400F on alcohol/nitromethane.

The actual answer to your question is pretty hard to answer without quoting half a thermodynamics textbook. If your engine is not doing any real work and just happily thumping about at a no load idle then things will be considerably cooler. Alcohol fuels produce just as much heat per unit of power output as any other, but have a higher latent heat of evaporation so do run slightly cooler, or a whole lot cooler if you run it extremely rich.

(Disclaimer: although I have that thermodynamic textbook on the shelf and have actually read much of it, I reserve the right to be incorrect at any time)

 

[Metal Butcher]

Thanks Gail, very interesting information. It looks like I don't need to worry. If the internal temperature reaches double, or even triple the outside temps on your 'Tiny', then their won't be any problem. I just got to get one of those I.R. gauges, a great gadget for guys like me that just have to know.

Thanks Brian, your correct, I just looked at the Kerzel plans and they do show a brazed valve stem assembly. The indicated "wick' of the brazing is something that can cause a problem. From previous experience I found out that a .002 clearance between the parts is needed, making the line-up on small parts a real problem. However I can center punch around the stem to hold the head in place and to also allow room for the silver braze to flow ("wick"). Brazing is going to make a mess out of the head, and the contact area on the valve stem head will need to be re-machined using a collet that I'll be making. Looks like brazing is the only way to go. I'm glad I only need to make about 16 of these to be on the safe side. :'(

While flipping through the plans you used I got a big chuckle (again) out of the drawing on the lower part of page 10. I got a head ache every time I looked at that drawing. :big:

-MB

 

[MachineTom]

In reading your post I missed if the plans call for a brass valve head. The exhaust valves when open will quickly hit exhaust gases temperture, and that is very hot, then it hits the valve seat and is quickly cooled to head temperture. Thermal shock loading is not a feature of copper based metals. At 750° f bronze is 26% of its tensile at 70°, 18% at 930 f, steel is 97% and 57% same temps. Just a thought.

 

[Ed T]

I worked for many years in an industry where measuring the temp of metal parts was a critical, day-to-day adventure. As mentioned above, the emissivity of the metal (or other material) is very important when using a non-contact IR thermometer. They are OK for RELATIVE measurements of materials with the same emissivity but it can be very difficult to get good readings of what is the ACTUAL temperature without a bunch of PITA calibration exercises with thermocouples and such. You also need that kind of exercise to establish the similarity of the emissivity between two samples and you still don't know if the actual temp is correct. So, we seldom used IR themometers for anything that mattered with the exception of looking at gradients in a single part where the emissivity was not a variable. We tried everything from cheapy hand helds to really high end FLIR systems and always had the same issues.

 

[Brian Rupnow]

"While flipping through the plans you used I got a big chuckle (again) out of the drawing on the lower part of page 10. I got a head ache every time I looked at that drawing. "----Yes, when I went to model those parts, it was like looking in a very large haystack for a very small needle to figure out what the heck he had drawn.

 

[Metal Butcher]

In reading your post I missed if the plans call for a brass valve head. The exhaust valves when open will quickly hit exhaust gases temperture, and that is very hot, then it hits the valve seat and is quickly cooled to head temperture. Thermal shock loading is not a feature of copper based metals. At 750° f bronze is 26% of its tensile at 70°, 18% at 930 f, steel is 97% and 57% same temps. Just a thought.

Tom, The plans call for both parts to be steel, pressed together or brazed. I asked the temperature question, not knowing what it could be, to see if Loctite could be used. And it can't due to its high rate of breakdown as the temperature rises. With the valve seat made of aluminum I never thought that a brass valve would be an issue since its harder and has a higher melting point than aluminum.

I don't under stand what "thermal shock loading" is, and what will go wrong with the brass valves.

-MB

 

[MachineTom]

The inside of a IC cylinder is a hot place, the head while it is AL, with a lower melting point than brass has the inside surface containing the combustion heat and pressure, and the other side exposed to air or coolant, this cools the mass of the head so it does not melt. The exhaust valve when closed is likely to be close to the temperature of the head, but when open heats to combustion gases temperature, which will lower the tensile strenght (soften) the brass, or even melt it at the edges. This now very hot valve head closes and hits the valve seat and transfers heat into the head, rapidly cooling the valve, ( thermal shock). Like jumping from a 200° sauna into a swimming pool, its a shock, the brass does no hold up well to that treatment. Steel is the stuff.

 

[Metal Butcher]

Machine Tom, thanks for the explanation. Since the valves are all ready finished I give them a try any way and keep my eye on them. Maybe the valve temperature and shock on my small engines in hit-n-miss mode won't destroy the exhaust valves. If need be, I can make up more valve stem assemblies using steel. Had I known, I could have ordered some drill rod instead of using the usual brass.

It would be nice to hear from someone that has tried valves from brass or other materials besides steel.

-MB

 

[compspecial]

Hi MB
Could you possibly peen or rivet the valve head and stem together? this is done in full size with complete success. As to temperature,in full size exhaust valves often run at red heat under load, I've seen this on deep well pumps where the muffler has broken off it cools quickly on contact with the seat when the valve closes but it soon opens again some makers fill the stem with sodium which better conducts the heat up through the stem and out to the guide. Of course in models everything goes down by the cube as the size halves.

 

[Metal Butcher]

Hi MB
Could you possibly peen or rivet the valve head and stem together? this is done in full size with complete success. As to temperature,in full size exhaust valves often run at red heat under load, I've seen this on deep well pumps where the muffler has broken off it cools quickly on contact with the seat when the valve closes but it soon opens again some makers fill the stem with sodium which better conducts the heat up through the stem and out to the guide. Of course in models everything goes down by the cube as the size halves.

Hi compspecial. Yes the stem could be peened at the top of the valves head. I don't know if brazing is really an issue, it seems to be a standard method indicated by at least some of the designers of model I.C. engines. The melt temperature of the brazing I used is about 1,150-F. On the type engines I'm building, the valve remains open after exhausting, and the piston cycles at least a few times before the valves closes again. This 'cycling' may be a factor in keeping the temperature down. I'm wondering if the exhaust valve's on these small model hit-n-miss engines normally get red hot. I used brass heads brazed on to the stems, and the brazing or the brass heads them selves won't take red hot heat for very long. Looks like the valves I made are questionable, and I need to start all over.

-MB

 

[Brian Rupnow]

MetalButcher---I wouldn't rush to start all over again. I have an old set of Upshur plans dated 1977 and it has the same note pertaining to the valves and stems being silver soldered , two peice const, etcetera. Surely over a span of 33 years, considering the thousands of Upshurs that have been built, if this was a real issue, the drawings would have been corrected or there would have been a big squack and other irate builders flooding the small engine websites with warnings that this is incorrect. In my opinion, and based on what everyone is telling me, once you get these engines into hit and miss mode, they don't get warm enough to matter because of the high volume of cold air they pump through the engine during the miss cycle.----Brian

 

[Metal Butcher]

Hi Brian, I have no doubt in my mind that the brazing will hold up to the heat that this hit-n-miss engine will produce at the exhaust valve. The plans call for this method of assembly with the engine being built as hotter running 4-stroke. The hit-n-miss option was added later to the plans without change to the valve assembly method.

MachineTom indicated that brass valve heads will not hold up to 'thermal shock' caused by the valve contacting a cooler valve seat, and become damaged. And he also said the edges of the valve would likely melt. I wonder if a much cooler brass seat would be subjected to the same thermal shock damage from contact with a hot steel valve.

Thanks Brian, I know your trying to save me some work by telling me to hold off and see if the valves do indeed become thermally damaged. I already started on the steel exhaust valves 'just in case', and they should be done in a couple of hours.

-MB

 

[Brian Rupnow]

Rick---In the Upshur plans that I have, it shows plans for a valve seat cutting tool thats not quite as complex as the one I built, and it looks like its all done on the lathe and mill----no Barbaric filing required. Do you have that drawing?---Brian

 

[Metal Butcher]

Rick---In the Upshur plans that I have, it shows plans for a valve seat cutting tool thats not quite as complex as the one I built, and it looks like its all done on the lathe and mill----no Barbaric filing required. Do you have that drawing?---Brian

Brian, yes I have that drawing in my plan set. It has only 2 cutting edges. Because of that, and that its hand operated, I don't think it will give as good of a result as a 4-flute cutter, that's more self centering. That's just my opinion, Maybe a 'cutting tool geometry specialist' with a doctorate in 'hand cut valve seats' will come along and re-torque my head bolts. :big:

-MB