Graphite IC engine piston?

[GailInNM]

Has anyone tried using a graphite piston in a low performance IC engine?

I have a 3/8 inch bore 1/2 stroke horizontal hit-miss engine using cast iron cylinder and piston. Using an oil cup at the bottom end of the cylinder for piston lubrication. It takes about 2 drops of oil every half hour to keep it happy. No problem there.

I am THINKING about a vertical version. The oil cup lubrication will not work out very well there. I have tried using a fuel/oil mix on the horizontal with all sorts of oil types and fuel types and all tend to carbon up the engine after about 10 hours or so. Not much of a problem with larger engines, but with such small valves it only takes a little bit of carbon to keep a valve from seating. The oil cup lube eliminated this problem so I really don't want to go back to fuel/oil mixes if I can avoid it.

So the question is will an graphite piston without lubrication work for this. Graphite has a lower coefficient of expansion than cast iron so it may loosen up a bit as the engine comes up to operating temperature, but on the horizontal the cylinder temperature never gets above 120 deg F so I don't think that will be a problem in this size. Besides the piston will run hotter than the cylinder so that will also minimize it. I have plenty of room to increase the piston crown and wall thickness to make up for the lower strength of the graphite.

Any one have any thoughts or experience on this?
Gail in NM

 

[Metal Butcher]

Hi Gail.

I used a graphite piston in my Sterling fan and it works well.

The only thing that might cause a problem is the strength of a graphite piston.

The material is on the brittle side.

-MB

 

[doc1955]

Gail
I'm thinking you will have a problem with the graphite holding up. With an IC engine the shock on the power stroke maybe more than the graphite will take. But if no one has tried it yet we will never know for sure. I'd say give it a try but be prepared to make a piston from more conventional material if it fails.

 

[Layne]

A long time ago in machining school one of our projects what a flame eater engine. One student tried to make a graphite piston for his, but I believe it ended up breaking before he finished. It was about a 1.25" bore and the piston design is very thin. I bet you can do it with your small bore if you keep it pretty thick. There are many grades of graphite though, some so coarse that big chunks fall out of it, and some very fine. I probably have some tiny pieces of the fine stuff laying around I could send you.

 

[Stan]

The temperature in the cylinder has to be at least equal to the burning temperature of the fuel. Since graphite is pure carbon, will it not burn?

 

[Layne]

No, it does not burn. I've made crucibles out of it for people at work to melt metals in. Temp should be no problem.

 

[ghart3]

Just tried the graphite piston.
Have a Gade hit n' miss with about 7/8" bore. Cast iron cylinder with cast iron piston. Made a new piston to same size as the cast iron one only out of graphite. Only difference is left as much meat in piston as I could for the wrist pin and in the head.

Graphite is working just as good as the cast iron one. Took piston out after one hour of running, engine firing at about 6 times per minute. Piston showed no sign of wear and wrist pin was still tight in piston. Engine is running again on the graphite piston and will report if any signs of problems show up. Gary

 

[NickG]

Gail,

Think you've got your answer in the last post. I was going to say I've just made the poppin flame gulper engine with a graphite piston. It requires a very close but very low friction fit to work and it was dead easy with the graphite - I left it as finished straight from the lathe. I would suspect the poppin gets very hot with the flame almost on the cylinder. As graphite is about 1/3 of the density of cast iron, you can leave lots of meat on the piston - I only milled a slot into it sufficient for the rod and drilled the gudgeon pin hole, no further lightening.

It's run for a while and haven't seen any signs of wear either. I am definitely going to try graphite when I do my first I.c. engine and I am going to make a piston and valve from it for my Jan Ridders flame gulper also.

Nick

 

[GailInNM]

Thank you to everyone who made comments.

MB: I have built several hot air engines using graphite pistons and all have worked well. Some are 20 years old and have yet to wear one out. IC will be more demanding.

Doc: I really don't have anything to lose as my test bed engine is working perfectly with the iron piston so I can just put it back in if necessary.

Layne: I don't expect any machining problems as this will have a fairly thick cross section everywhere. Thanks for the offer, but I have a large supply of graphite in various grades on hand.

Stan: Temperature is not a problem. Like Layne, I have used graphite for crucibles. It does oxidize at high temperature, but these are in the range above 1200 deg F or so.

Gary: Thanks for confirming that I am not totally nuts on this. I will be very interested how it works out long term.

Nick: My Poppin is one of my engines that has a graphite piston in it. I have only run it for a few hours total so don't have in long term experience with it.

Since no one expressed the feeling that I am totally insane on this I had decided to try it even before Gary beat me to it.

I fished out some fine grain graphite from the shed last night. I have a few hours work to finish up some other things on the lathe and then I will start my conversion. Won't take but a little over an hour, then a half hour to clean up the lathe. I probably will get started on it in the morning and and then get back with the results. Test engine has over 200 running hours on it so I know most of it's quirks already and can make an easy comparison.

Thanks again everyone.
Gail in NM

 

[NickG]

Excellent Gail, can't wait to see the results.Yours provide a great benchmark as you test things logically, one variable at a time.

Thanks,

Nick

 

[ghart3]

Gail, your idea of using graphite is a winner. Only got another hour of running on the graphite piston and was having problems with spark jumping to valve springs so decided to make new spark plug with longer insulator. Wanting to change spark plug thread, pulled the head and cylinder. First time had just stuck graphite piston in cylinder without cleaning cylinder. This time cleaned the cylinder bore out and left dry.

First time the Gade hit n' miss engine was firing about every 6 seconds. After cleaning the bore it now fires close to every 9 seconds. With little over two hours running time the piston shows no signs of problems and wrist pin is just as tight in piston when first put in.

Great idea, Gail. Thanks Gary

 

[GailInNM]

Thanks for the kind comments Nick. I come from the old school of science where you only change one parameter at a time and then measure what happened. Sometimes it's unavoidable to change more than one but it's still a good principle to follow when I can.

I am glad that it seems to be working out for you Gary. I did not do a proper job of thanking you for your efforts following your first post. I was in a bit of a hurry getting things ready for the storm rolling in. I hope to be able to get on to my conversion here today if I can get the shop warm enough to work comfortably. Has not been this cold for a decade or more and it is supposed to be 10 degrees colder tonight. But, a big thank you for confirming first part that it is at least possible. No way of knowing about long term until things have had time to run for few weeks.

I got my other project off the lathe so if the heater keeps up I will be trying things out. My cylinder is cast iron and has about a 5 to 10 microinch finish so I think that will keep wear down enough for some long term running. I think about brushed DC motors that have 20K hours on them with graphite brushes. They have higher contact pressures than this should have and operate under harsh conditions with arcing and segmented commutators. I think that this service will be less demanding.

Gail in NM

Gail in NM

 

[GailInNM]

I got my graphite plate turned into a 7/16 rod yesterday afternoon but it was too cold to make a piston. In the early evening I got anxious and made up the piston. Five minutes on the lathe and then 5 minutes by the heater and repeat. Scrubbed the cylinder down to remove all traces of oil when fitting the piston. I fitted it slightly snug, about the same as I do cast iron, figuring that it would bed down to a running fit after an hour or so. Got the engine running and ran it for about 20 minutes before calling it a day. It ran about as expected with the snug piston. Sensitive to fuel mixture and barely getting up to the 2000 rpm necessary for the hit-miss governor to limit the speed.

Left the heater on full all night and started running again early this morning. Still cold, but tolerable. The forecast was right - it was 10 degrees colder last night. Many 90 year old records fell in the surrounding area but mine did not. Still it was minus 12 degrees F outside when I started.

I now have about 4-3/4 hours on the engine and have collected some interesting data now that the piston has bedded in. I have not used any lubrication on the piston. The operation is quite different from the cast iron piston. With the cast iron piston the engine turned over very freely at low speeds which made for easy starting. The graphite piston has more drag at low speeds. I may need to take off another 0.0001 or so on it if it does not free up from running. However, at higher speeds the graphite piston has less drag. This is evidenced by the cylinder temperature being lower and longer periods between the "hits". I can barely feel any warmth from the cylinder fins. If I am running at governed speed and hold the exhaust valve open the engine coasts twice as long as it's cast counterpart until it gets down to about an estimated 500 rpm. Then it slows down more rapidly than the cast iron piston. I am guessing but I think what is happening is the viscous drag on the piston at higher speeds is limiting the high speed performance and that drops off rapidly at low speeds. The greater drag at low speed with the graphite piston remains more or less constant on up to the higher speeds. Not what I was expecting.

This makes for a little bit harder starting as it is hard to spin the engine up to starting speed by hand, but once running it makes for a much better display engine where the engine is running with out load. Very smooth hit-miss action. Conversely, when the engine has a load clutched in , read calibrated thumb, it does not recover as rapidly as the cast iron piston. I want to be able to do this on an upcoming project.

I think a happy compromise can be reached, perhaps just by a few more hours of running. So far I have used 15 ml of fuel after the adjustment runs. Three runs with 5 ml each. First run was 1 hour 15 minutes. Second run was 1hour 25 minutes and the last run was 1 hour 45 minutes. I was getting a fairly consistent 1 hour 25 minutes with the cast iron piston. So it is obviously still getting bedded in.

Overall I am pleased and encouraged with the results so far.

Gail in NM

 

[NickG]

Great work and interesting results Gail. Do you think it did need to be slightly smaller? I was going to say a thou but I mis-read and you mean 1/10th of a thou!!!

Sounds like it's bedding in naturally anyway. :bow:

 

[GailInNM]

Thanks Nick.
Well now this is embarrassing. On the change only one thing at a time theme I made the new graphite to print. Never stopped to check the old piston that I took out that made the engine has been running so well on. It seems that the one I took out was not to print. My original piston when I first built the engine was to print. It was an aluminum one with an oring. When I changed to the cast iron that gave me such an increase in performance I screwed it up and put the wrist pin hole in referenced to crown instead of the skirt as shown on the print from Arv. Stupid mistake, but it was just habit as I always reference to the crown so any overall length errors are eleiminated on compression ratio.

I just discovered this about 10 minutes ago. Have not calculated the change in compression ratio, but it is considerable. I will be making a new piston (the wrong way) in a little while. I will recalculate the compression ratio also for the new one. This would explain a lot of the difference, maybe all, in the low speed performance. Need to make a fresh pot of coffee before I get started.

Gail in NM

 

[NickG]

Gail,

I probably would have missed that too and just assumed it was in the middle! :big:

Is it a higher compression ratio you mean or lower? Is that why it was harder to start by hand?

Well it all makes for very interesting reading anyway!

Nick

 

[GailInNM]

Nick,
The compression ratio that was giving me problems was calculated out at 5.8 :1. I have now made a new piston giving me a 4.7:1 ratio and the engine is back to being easy to hand start and having the load taking ability that I want.

While the engine runs with no lubrication, it runs much better with some oil on the piston. The oil requirement is way down from the cast iron piston however. With the cast iron piston I needed to put 2 drops of oil in the oil cup every half hour to keep it happy. With the graphite piston it starts to get sticky at about 1 hour 45 minutes.

Using the oiled graphite piston it has an excellent hit to miss ratio. Much better than with the cast iron. I have about 11 hours on the new graphite piston. After the first hour of running with no lube I put 2 drops of oil in the oil cup and it immediatly dropped in to hitting once about every 10 revolutions. After a couple more hours of operation it is hitting once and coasting about 14 revolutions before hitting again. It is slow enough that I can count the hits,, barely, at about 140 hits per minute with the engine running at 2000 rpm. I can apply a load to the engine and lug it down below 1000 rpm and it pops right back up to the governed rpm when the load is removed.

While I would like to be able to run without lubrication, I can live with the 3:1 reduction in lube. I have to lube the valve stems about then anyway to keep them from sticking. All in all I count the experiment a success.

I am not sure what is next. I may make up another cast iron piston with several relief grooves to reduce the surface contact withe the cylinder walls and form oil retaining grooves. This always worked well on steam engines. Not going to do that right away however. Back to my other projects for a while so I can think about it a little bit.

Gail in NM

 

[NickG]

Gail, that is unbelievable for an engine that size. I think a lot of it is how fine tuned you have it now and how well it has been made. :bow: