Ramon's ETA 15Ds

Some small diesel engines like the Mills 1.3 have a cast iron piston in a hardened steel liner. I have one which has run for hundreds of hours and still has good compression. Other engines have cast iron pistons in a cast iron sleeves so no expansion problems. Most ofl the diesels I have made have this set -up but I made an Amco 0.87cc with a steel cylinder and it too has had a long life. Some of the latest glowpiugs have plated cylinders and apparently require virtually no running in.

Hi Paul - I have just opened an email from Jason refering to your post - my apologies for any delay. My apologies to Mark Walker too as I have only just seen his query above. I do not visit this site very often these days my main posting being done now on the Model Engine Maker site. There is no other reason for this other than time - there simply isn't enough of it - even posting on MEM can take more than what's left of the day at times!

So firstly Mark, the milling machine I use is a quite old but in very sound condition Linley jig borer. That sounds better than it is - that was it's original function but it is definitely a milling machine now! It is a very sturdy machine but does have limitations - not much spindle clearance and does not have a tilting head. The Lathe is a basic Myford Super 7 - no gear box.

Paul - your question has been fundementally answered by Jason and Thomas but perhaps a little more ?

The 'diesel' or compression ignition to give it's correct term does require extremely good piston to liner fit relative to glow and spark ignition to acheive the kind of compression required to create the temperature for ignition. Any kind of clearance that would be acceptable to a degree in the latter two would give rather poor performance in the first. Aluminium pistons, as far as I am aware, have never been used in basic, run of the mill, 'diesel' engines for the reasons stated - mainly different expansion rates. They are used however in high performance diesels but then the liners are usually made of brass or ally which has been hard chromed and the aluminium used for the piston is a specialist grade - you would probably not get far with using HE30 or even HE15

Many early engines of my youth had very tight set ups with the piston runnning in basically parallel bores. These would take careful running in to achieve a good fit and getting the engine hot by over rev-ing or overloading (coarse pitch props) would soon lead to the engine becoming hard and eventually slowing to an abrupt stop. Long, slow and rich oily runs with larger than normal props but with a fine pitch was usually the initial approach. I can think of several well known engines that would take an age to 'settle down'. As Thomas states most were steel liners, many hardened though not all, and virtually all had CI pistons.

The 'tapered' bore came later and is now a well accepted way of providing better conditions - the bore is tight at the very top of the stroke where it needs to be, the liner expanding at that point where the heat is greatest to provide a much better running fit. Running in is much better carried out with short, lightly loaded but fast runs allowing the engine to cool between. Once the optimum bedding in has been achieved steady consistent runs are easily maintained in average weather conditions - really hot days can make a difference though.

This taper technique was/is used in the 'ABC' set ups - Aluminium piston - Brass liner - Chromed

These unringed pistons, are very tight approaching TDC but as soon as the temperature is up the running conditions are reached very quickly. To run an engine in in the traditional manner would soon wear the piston in the (relatively) cool upper cylinder so it is important to make those early runs much faster and leaner than previously carried out. This period is very short compared to standard bores - hence Thomas's remark on engines requiring very little running in.

Hope this helps a little more.

Regards - Ramon

Ramon,

Thank you for taking the time for your detailed reply. The high precision needed to make one of these apparently simple engines has convinced me to stick with steam.

Would I be right to describe the "running in" process as an improvement in surface finish, without changing the actual diameters, but leading to a fractionally increased clearance? This must be tricky as it involves rubbing without seizing solid. As for thermal expansion, it seems that the temperatures of the piston and liner must be quite close. The expansion coefficient of iron is 11 x 10-6 per degree C, or about 0.0002mm per deg C for your piston. So a temperature difference of only 10 C would take up a clearance of 0.002 mm, which I think is something like what you achieve with lapping? That is a surprise to me.

One reason for my interest is that I was involved in a discussion on another forum about piston rings for steam traction engines. I contended that the piston and cylinder were the same temperature in a traction engine (steam jacketed) so no expansion, but the others,with automotive experience, insisted the piston had to be hotter. No matter, thank you for the education.

Paul

Paul, you only mention the expansion rate of CI as being 11, this is quite close to that of a steel liner at 13 so this is why the two can be run without much change in clearance. Also worth remembering that a lot of these small diesels don't have the liner inside a aluminium crakncase for a lot of their length.

Now if the piston were aluminium then that has a rate of 22 so as its usually paired with an iron liner you can see that the clearances will reduce far quicker with this combination of metals until things sieze solid.

The same applies with a traction engine, if running with an iron cylinder and iron or steel piston and iron rings than things all move at a very similar rate as they expand so you only need say 0.002" clearance for the piston regardless of OD when cold.

There is a trend to use aluminiun pistons to get a smoother running engine as the solid weight is comprable to that of a hollow cast iron piston that would have been fitted to the full size engine. Due to the aluminiums higher expansion rate it has to have a clearance of around 0.003" per inch dia so on a large engine you could be 0.010" or more below bore size when cold.

So in some cases you do need more clearance than others, it all depends on materails and the sizes involved.

J

> The high precision needed to make one of these apparently simple engines has convinced me to stick with steam.

Have courage, Paul. Ramon wrote a short series in ME recently that gave excellent step by step instructions for making pistons and liners.

Neil

If a CI engine is a bit much, maybe a glow plug motor would be worth a try, not the same high compression, and just as noisy. Ian S C