A diesel kiwi

[cox24711]

hello Chaps
Recently I have been modeling on fusion 360 a Kiwi by the great ETW.


(to take to one of my local foundries to cast for me, due to the cost of the shipping the Hemingway kit, to Australia being truly ridiculous. and because my parents do not like the idea of me messing around with molten metal.)

but in a fashion of which I am intrigued and interested in further developing, trying to make it an ether-less diesel that is mentioned here http://www.modelenginenews.org/gallery/p8.html on Ron's great website.
I have a few tricks up my sleeve in improving upon the one mentioned in the link that was built by 'Malcolm Beak'. I have already significantly 'beefed' up the conrod, crankshaft, cylinder thickness, head bolts, and the cylinder bolts.

The first one of the improvements I have thought of was making it air-cooled instead of water-cooled like the one in the link, and make the cylinder, piston, and head entirely out of cast iron, due to its lower thermal conductivity to help retain heat in the combustion chamber. My other thought is to make a copper part that will screw onto the face of the contrapiston, so it will conduct the heat created by combustion, and be a somewhat 'controlled' hot spot to aid the ignition of the compressed charge, but will be easily moved away from the combustion chamber to prevent detonation whilst lowering the compression ratio and make it 'less accessible' to the compressed charge.

As always all comments advice and suggestions will be greatly appreciated!
Because the only way I have gathered the knowledge that I have is from the help and advice from those who have greater experience and knowledge than me.

Thanks Greg.

 

[cox24711]

hello again chaps
I am just in the final stages of modeling the cylinder head
I am thinking of also fashioning a glow plug out of an old spark plug and some nichrome wire to ease the cold starting of it like many modern full size diesels do, is this a wise decision? or is it a source of more problems than advantages? Also in regards to the contrapiston, is there any advantage of having a larger bore over a smaller bore? the only thing I can gather is that the smaller bore the contrapiston is the finer the adjustment in the compression ratio you have, but in conducting heat for the copper "hot spot" I presume that the larger the surface area the more heat it will conduct. So I am thinking half inch bore for the contrapistion as a compromise.
As always all advice is greatly appreciated.

Thanks
Greg

 

[cox24711]

hello again fellow machinists
I'd thought I'll provide you some pictures, to get an idea of the arrangement that I have been coming up with.

the conrapiston bore is a quarter inch, and the little copper 'thermal conductor' screws onto the end of the contrapiston as you can see above. I went with 1/4 inch because if the 'thermal conductor' gets too hot and detonation starts accouring under high load, it can be moved quite some distance away from the combustion chamber without dramatically decreasing the compression ratio like a half inch one would. I am planning of using a very small Villiers S10/1 carburettor that I have, and I think it would be suitable, especially due to the kiwi's crankcase design being very similar to a villiers 2 "smoke" engine.

 

[petertha]

...to take to one of my local foundries to cast for me, due to the cost of the shipping the Hemingway kit, to Australia being truly ridiculous. and because my parents do not like the idea of me messing around with molten metal....

Just curious about your plan in this regard. Will you produce a solid wax casting part from outsourced 3D-print based on your Fusion Cad model & the foundry takes it from there? I assume the resultant casting must work out quite reasonable cost wise if shipping/Hemingway is the threshold target to beat. Will the foundry do onsey-twosey parts or is the assumption making more as a kit design? (you sell 9 & 10th one is free). Casting seems to be the ubiquitous challenge, even when a person has access to CAD & 3DP type parts, but admittedly I live in the wrong part of world for 'local foundry'. Interested in your comments & progress.

 

[cox24711]

petertha, pete, peter?

There are multiple foundry's within 100km of me, some small, some large, some do non ferrous castings, some do ferrous castings, some do both and they all do castings from cad. The from what I have been told, the main cost of when dealing with small 1 off castings, is having someone at the foundry model what you want in cad, so if you model it yourself you cut the costs significantly. I don't know what method of casting they will use but I'll do whatever they will think is the most suitable, I'm pretty sure they will be sand cast, like the hemingway kits are. Also I will be able to cast things that are not cast in the kit, eg the crankshaft, so all I will have to do is turn down the surfaces a bit, instead of hogging it out of a bar, or messing around with building it up and making sure it stays square!

I'm not looking to try and match the price of the hemingway kit, I will be trying to match the cost of the hemingway kit and the price of shipping (which is more than the cost of the kit in the first place! Ideally I would have all the parts cast in the same foundry, which to me makes sense in keeping the costs down. but also the idea is to establish a connection with a particular foundry to deal with my future projects. I currently have another project in mind to have a tackle at doing next (designing and building a prototype 4 stroke british seagull outboard motor, to revive the brand)

Something tells me I am quite an ambitious soon to be 16 year old.
Anyway thanks for the reply petertha
Greg.

 

[petertha]

Good for you! Keep us posted on your progress & look forward to seeing your cast parts & engine come to life.
Peter

 

[cox24711]

Almost finished the model
The camshaft and rockers are standard, but the rockers are adjustable so I can make the valves open and close at exactly tdc. Some slight flywheel changes, inspired by British seagull outboard motors of course. only the carby and exhaust and gears to do now. doing away with the original lubrication system, to splash lubrication and oil drip feeders to the camshaft.


Greg

 

[mu38&Bg#]

How will you change valve timing with the rockers? I think the exhaust valve will be the hot spot. Cold start is the hard part, once warm it will keep running. Setting for steady state running is easy. Variable load or speed becomes tricky, especially as displacement increases. HCCI isn't common for a variety or reasons.

 

[Mechanicboy]

And overlap must not be too large or the valves will hit piston.

 

[cox24711]

thank you guys for raising your concerns.
the timing is adjustable by changing the valve clearance with the eccentric bushing that the rockers ride on so that there will be pretty much no valve overlap at TDC so the valves won't make a hole in the piston.


diesel pilot
I know that RC diesels are out there and I agree that 1 would think that it would be difficult to achieve a smooth easy starter that is not very twitchy with the controls, but this video of the .60 size ~10cc RC Paw

has made me convinced that it is possible. And I know that I have run my mills.75 using 91 octane petrol, and 40% castor oil. so I think that it seems feasible to get it running somewhat well on ether-less fuel.

with your comment on the exhaust being the 'hot spot', I know that that you get "dieseling" with petrol motors after a long run, but my observation is that my mills.75 runs significantly cooler than a comparable engine of that size the cox.049 babe bee (I know it is not petrol but you get the point), and I personally don't think that the exhaust valve will get hot enough being that it will be made of steel. if I made a copper face for it it won't be very controllable either Ie ( you cannot simply move it away from the combustion chamber to cool it down) but I like the suggestion, I think it will be a good experiment to do.

 

[Mechanicboy]

Overlap is determined by camshaft (timing period) and can't changed by valve clearance. Valve clearance is calculated by expansion by heat in the engine. Warm engine = less valve clearance without to keep valve open by expansion by heat.

 

[cox24711]

Increasing the valve clearance decreases the valve duration and lift because it requires the rocker arm to move a longer distance without contacting the valve. So if I have the piston at tdc during the 'overlap period', I adjust the rockers so that they are just touching the valve stems, so therefore it 'cancels' out the 30 degrees of overlap at tdc.

 

[mu38&Bg#]

Yes, it's possible. Ether makes starting simple, and softens all tuning characteristics. Two strokes are generally less critical in my experience. I do fly RC diesels. Even glow four strokes I've converted to diesel, even without ether. Running an excessive valve lash will do as you expect, but it will be tough on components especially if not properly hardened. In real engines variable valve lift/duration is complicated. It would be better to modify the cam profile for the appropriate timing. The exhaust valve is typically the hottest part of a four stroke poppet engine. This is why development lead to high temperature stainless alloys for exhaust valves almost a century ago. To have a small piece of copper be much hotter than what it's installed in, it would have to be insulated.

 

[cox24711]

thanks diesel pilot, I don't think that the amount of valve lash would be too excessive due to the overlap being pretty minimal as you can

see in the timing diagram, if any problems arise I can always modify the cams as they are only keyed onto the camshaft and there will be material to take away, which is a lot easier than putting it back on.

about the copper hot spot, my idea is to make the threads that go into the contrapiston a 'sloppy fit' and then wrap teflon tape around the threads so the expansion of the copper will be taken up, and not result in the cast iron contra piston cracking.

 

[radial1951]

petertha, pete, peter?
...I don't know what method of casting they will use but I'll do whatever they will think is the most suitable, I'm pretty sure they will be sand cast, like the hemingway kits are. Also I will be able to cast things that are not cast in the kit, eg the crankshaft, so all I will have to do is turn down the surfaces a bit, instead of hogging it out of a bar, or messing around with building it up and making sure it stays square!

I'm not looking to try and match the price of the hemingway kit, I will be trying to match the cost of the hemingway kit and the price of shipping (which is more than the cost of the kit in the first place! Ideally I would have all the parts cast in the same foundry, which to me makes sense in keeping the costs down. but also the idea is to establish a connection with a particular foundry to deal with my future projects. I currently have another project in mind to have a tackle at doing next (designing and building a prototype 4 stroke british seagull outboard motor, to revive the brand)

Something tells me I am quite an ambitious soon to be 16 year old.
Anyway thanks for the reply petertha
Greg.

Hello Greg

As you are well on the way to understanding the engineering requirements of internal combustion engines, I have no hesitation writing this on an engineering level. Well done, BTW.

A few things perhaps worth your consideration:

A sand cast crankshaft, built to the size of a steel crank, stands little chance of surviving in this application. The usual method of turning a crankshaft, with integral disc, from a steel billet and pressing-in a crank pin will be several times as strong as a sand cast version. Not to mention it will also be way less costly than having the foundry make a pattern, presumably by 3D printing from your CAD model, plus mods like core prints, shrinkage allowance etc, and then doing the actual sand casting. Some research into casting processes would be helpful.

The idea of tappet clearance is to ensure that the valves will actually be closed when the follower is on the base circle of the cam. Depending on various engine design features, and materials used, the tappet clearance may decrease or increase with the change from ambient to operating temperature. It may not be much in a smallish engine like yours, but it may impact the performance and life of your engine. Adjusting (increasing) tappet clearance to change valve timing will result in hammer loads on valve train components, substantially shortening component life.

You mention a cast iron cylinder head but don't show valve seat inserts. As you say cast iron will retain the heat, in this case to hopefully assist the initiation of combustion. Generally we are trying to get rid of heat so as to maintain a safe (engineering) operating temperature. Heat is the enemy of lubrication and, particularly in the case of exhaust valves, will accelerate valve seat and guide wear which are the main cause of valve recession. Perhaps not an issue at this size, but won't hurt to be aware of it.

I take my hat off to your ambition, and considerable skill with 3D CAD modelling. The above items may not be critical to your success, but as you obviously like to push the design envelope, they are worth knowing about.

Regards, Ross G.
radial1951

 

[cox24711]

thank you radial1951

I have completely forgot about valve seats, and I also will implement the the changes to the camshaft timing so it will run with standard valve clearance. I am aware of the issues regarding heat, from my experience with diesels is that they run very cool, my mills after a full tank isn't even to hot to touch, this might also helped by the high oil content in the fuel, but they do run very cool. Also the fuel will have 5-10% oil in it to provide the lubrication to the valves guides etc. thank you for the insight about having a cast crank, I thought it would be fine knowing that they cast full size engine crankshafts out of cast iron and steel.

thanks
Greg

 

[cox24711]

Here are the new cams. The intake and exhaust cams are identical, there is 4 degrees of over lap (2 degree for each cam) at tdc, and
the intake closes at 34 degrees after bdc and the exhaust opens at 34 degrees before bdc, the making duration of each cam is 216 degrees (of crankshaft rotation) and the lift is bog standard at 17 /32

and here are the old ones for reference


Greg

 

[cox24711]

Good news everybody!
I have found a shop that sells Kiwi Castings in Australia !
after some time searching on the interweb I found out that
http://www.ejwinter.com.au/
sell castings for the KIWI!
Bad news is that the castings aren't a stocked item and that to get them cast would take 6 weeks and I have just missed the boat because of the Christmas holidays the foundry will be closed. So they will have to be made after Christmas.

Greg