Gassie - National Gas Engine

[Jasonb]

It has been a while since I posted a build thread here but thought I may as well show one.

It all started when I said in another thread about Nattie*.

"Could not help thinking while I was running this that it would be fun to see if the cylinder could be sleeved to form a water jacket, gears changed to get 1:2 ratio, add a couple of valves into what was the exhaust block (turned on it's side) and a spark plug or hot tube in the head, about 24mm bore should work"

A few months later Graham Corry of Alyn Foundry sent me a pair of gears that he had been applying the special brown coating to for a number of years together with some Nattie castings so I could hardly refuse the challenge to make a sister engine called Gassie:-[

Although this engine does use castings I have put it under "Own design" as I did design the patterns for 50% of the castings and have modified the others together with designing all the remaining bits. This was done over a few evenings with Alibre along the same lines as Nattie which is to have an engine with the look of an early National Gas Engine.

I ended up keeping the exhaust block the same way up as that also kept the various bosses at the top of the cylinder in the right place making use of the rear one for a spark plug as I have seen on some full size engines. A liner allows for water cooling which is via the middle boss and a drilling through the underside. A new head houses the inlet valve and boss for the carb. A larger dia solid crankshaft takes care of the more powerful stroke and a thicker side rod carries the two cams and electrical contact for the ignition.

First job was to get rid of what was not needed.

*For those that don't know "Nattie" it is a flame licker very loosely made to resemble an early National Gas Engine

 

[GreenTwin]

Outstanding work for sure !

.

 

[Steamchick]

The flame-eater atmospheric engine is a 2-sroke, so you need different gears on 2:1 reduction for a camshaft for a 4-stroke spark ignition engine, methinks!
K2

 

[Jasonb]

Yes that is why I said "gears changed to 1:2 ratio" I class 1:2 as a reduction and 2:1 as an increase as one equals 1/2 (reduction) and the later 2 (increase to double). Although the teeth are not shown on my 3D images the skew gears are both of the same OD but helix angles different so you can fit twice as many teeth onto the cam gear as you can the crank gear. This also keeps the flywheels close to the frame rather than having to space them out a long way which would be needed if the cam gear was twice the diameter of the cam gear. common full size practice too.




With the unwanted metal sawn off the cylinder jacket casting it was held in the 4-jaw to face the end flange, turn the flanges OD and finally bored to accept the liner. I spent some time getting the sides of the casting as square as possible to the chuck face as well as either end running as true as you can get with cast surfaces. I also took as much off the flange as was practical as the images of full size engines I was using for reference looked shorter than the casting.

The casting was then flipped around and the head end faced back, again getting as close to the boss as possible, in this shot 0.5mm has been left to be skimmed off after the liner has been fitted.

One end of some cast iron bar for the liner was cleaned up and then held by that section to turn the OD and cut a waist that will become the water space. I used tailstock support while doing this.

Gripping by the other end the chucking diameter was taken to it's final size and the cylinder drilled to 22mm and then bored to the final 24mm finished size after which it was loctited into the jacket with #648

After a couple of days to make sure the loctite had set the 0.5mm was faced off the end of jacket and liner to give a true flat surface for the cylinder head to seal against. Then the 100mm vice was mounted lengthways on the mill table so that the assembly could be further machined. Firstly the exhaust block was milled to height and reamed to take the valve cage and it's two retaining stud holes drilled and tapped. A water inlet hole was also drilled and tapped and lastly at that setting one side of the exhaust block was milled flat to later receive the side shaft bracket.

Turning the cylinder the other way up the tops of the various bosses were milled to their required heights, the central one tapped for the water connection and the one at the head end tapped and counterbored for a 1/4" x 32 Rimfire plug. The smaller one will get drilled to suit what oiling method I decide to eventually go for.

Lastly the two stud hole patterns were done on the flange and head ends

 

[Jasonb]

The head was drawn up as if it were a casting with draft angles and fillets. After turning the spigot that fits inside the cylinder I used that to hold the stock and let the CNC take the strain. The video shows the initial adaptive cuts which remove the majority of the material which were done with a 4mm 3-flute carbide cutter. After that I used a "scallop" path which steppes the cutter over 0.2mm per pass as it travels in three axis at the same time, this was done with a 4mm 4-flute cutter with 1mm corner radius (convex) as I wanted the small internal fillets that this would leave.

It was then back to the manual mill to complete the inlet valve pocket, seat and guide as well as drilling and tapping for a cover plate. A thread for the carb was also tapped in from the side M6 x0.75.

I had an old casting with oval section bars that was not going to get used for it's intended purpose so cut that up and used the material to turn the exhaust valve cage from.

The other side had a small spigot to extend the length of the guide which was counterbored to locate the valve spring.

Then used the CNC to shape the flange along with a similar shaped cover for the inlet valve

Valves were cut from 303 Stainless using ctr support while turning the 3mm dia shanks. Then a CCGT insert with 0.8mm tip radius used to form a fillet behind the head and cut the seat at 45deg with the topslide set over to that angle.

The valves were cross drilled 1.0mm for some pins which were turned up along with some caps. Springs were sized from my collection by the tried and trusted squish between finger and thumb to see what feel right method.