Halfa, a DOHC inline Twin

[RReid]

Completing my 2-Bits V-Twin and having it run successfully and well gave me the confidence and the desire to start designing another IC engine project. Thinking about what that should be, I eventually thought “How about a double overhead cam (DOHC) engine?” Why not? But what should I base it on?

My first car, bought while still in high school from money earned working as a “line boy” at the local airport, was a 1966 Alfa Romeo Giulia Spyder Veloce. Although it had been somewhat abused before I got it and the body had both some collision damage and some rust, it was ran well and strong, stopped well, and handled beautifully. Being lowered for Autocross (slalom) competitions, it could even be driven under a semi-trailer.


So that's what I decided to focus on, either a two or four cylinder DOHC inline engine, based on the general layout and outline of the Alfa Giulietta/Giulia series of engines. That means cast iron (or steel) wet cylinder liners in an aluminum block, and two fairly widely spaced camshafts driving two valves/cylinder in an aluminum head. There is even a precedent for a 2 cylinder version. In the early fifties, Project 13-61 was a proposal for an front wheel drive economy car using a 2 cylinder engine based on the Giulietta engine then under development. The project was rejected by management though and no prototype was actually built. If it had gone forward, it is possible that Alfa could have beat the Austin/Morris Mini to market by as much as two years!

Anyway, I started by drawing up a twin version, with 0.75” bore and 0.88” stroke, and perhaps an open crankcase as I had done with the 2-Bits. This I call the Halfa.


With that looking good, I decided to see about stretching it to 4 cylinders. That ended up with a block length of about 5 inches, which I wasn't sure wouldn't be just a bit too long for my Taig lathe if I wanted to use a between centers boring bar to line bore the main bearings. Not being at home to check, I decided to shorten it by reducing the bore slightly, which allowed closing up the bore spacing a little. This left me with a bore and stoke of 0.625” x 0.74”. This engine I call the Bialbero.
 

Now I figured it would make sense if the two variants shared most of their parts and key dimensions. So I redrew the first version of the Halfa by cutting the Bialbero design in half, keeping the smaller bore and stoke, etc. The only change was to relax the bore spacing just a tad. This is technically Halfa-v2, but as it is now the primary twin version , I just refer to it as the Halfa, and this is the version I'm going to start building.


For me at least, the design process is an iterative one, and this brief synopsis only touches on the biggest of the iterations! Enough for now though. In the next post I'll show first swarf.

[crueby]

Great looking project, should be a fun trip!!




One term I don't know, you mentioned 'wet liners' for the cylinders. What does that mean?




 

[john mills]

the cylinder sleeve or liner goes through the water space  lot of older full size engines have wet liners  .separate from the engine block
standard engine co engines in Ferguson tractor is one.
john

[Hugh Currin]

FUN. My dad had a Romeo Giulia Sprint Veloce if I remember the name correctly, (named Julie). Must have been early 1960s. Red and noisy. He received several ticket for being too loud. He'd go to court and be let off 'cuz it was stock and well maintained. Eventually the word went out to leave him alone.

Nice project, will be following as I can. Thanks.

1966 Alfa Romeo Giulia Spyder Veloce

[Brendon M]

Exciting project I love the pun on the name too

[RReid]

Thank you for the encouraging feedback, guys!

Brendon - Glad you like the name!

Hugh - I got a noise ticket in mine too, but in my case the muffler of unknown origin and largely shot. Muffler shop put on an MGB muffler, which quieted it enough to get signed off at the police station.

Chris - John Mills gave a good answer to your question. I also at one time had a Triumph TR-3. That engine was based on the Ferguson tractor engine, and also had wet liners. One advantage of this construction comes at rebuild time, as it's possible to simply replace the worn liners with fresh ones.

Here's a cross-section image of my engine. In the center you can see the conrod/pistion assembly within the cylinder. The dark black outline is the cylinder liner wall. The blue areas are the block. The grey between the block and the central region of the liner is the water jacket. Water will entirely surround the liners. The small grey squares just below the upper and lower flanges are not water spaces but will take o-rings (not shown) to provide a seal between the liner and the block. The cylinder head will compress the liners down onto the o-rings.

[crueby]

Very neat!   

[RReid]

When I got home from the recent travels, I still wasn't sure whether I should start building the 2 or the 4 cylinder version. Arguments could be made either way. But when I went out to the shop, I soon realized that I had a block of Aluminum that I had set aside for yet another potential project that was just the right size to get the oil pan (which also serves as the lower crankcase) for the 2 cylinder Halfa out of. That was enough to push the decision that way and get to work.


I started by getting the perimeter outline and the outline of the inner cavity marked out with guide lines.


Then I set it up in the mill vise and drilled the corners of the cavity with a short 3/8” drill. Then I chewed out the bulk of the cavity with a 3/8” endmill, staying about 0.020” from the edges and about 0.050” above the final depth. This is so I can go back in later with a smaller ball nose end mill (which I don't have yet) to leave all the corners with a nice fillet. The pics are all post clean-up, but that little carving session generated a lot swarf! Took a lot of crank cranking too. One good argument in favor of the little guy; twice as much of both for the bigger one.




I'm trying using larger images than I've usually used in the past. Let me know if this is good or if the smaller size is better. Thanks!

[crueby]

pics (and work) look great here!
 

[Vixen]

Hello Ron,

The larger image size works well for me. Good start on the oil pan.   

Regarding the Halfa engine cross section; how are you proposing to adjust the cam/ valve gap? That adjustment can be a problem with overhead cam engines, more so with models as there is so little room.

Are those four drillings in the cylinder heat part of the cooling system?

Cheers

Mike

[RReid]

Hi Mike, Thanks for the feedback.

how are you proposing to adjust the cam/ valve gap? That adjustment can be a problem with overhead cam engines, more so with models as there is so little room.

That's somewhat TBD. One thought is to machine the bucket tappets to fit, and count on never have to adjust for wear. Another is to use small shims, as done with the real Alfas. Any suggestions are welcome.

Are those four drillings in the cylinder head part of the cooling system?

Yes, that's right, and they are connected by crosswise runners at each end. Water is fed to the rear left corner from a passage leading up from the block, makes its way across the runner and through the four lengthwise passages to the crosswise runner at the front, where it exits to the radiator at the opposite corner. (or it may be better to run the flow the opposite direction?) They will all be drilled, with most of the ends either blind, plugged or otherwise covered and sealed. The passages are small and few, but I ran them as near to the combustion chambers and exhaust valves as I could. Hopefully they will provide more than just an illusion of cooling to the head.

[Vixen]

Hello Ron,
Bucket tappets are as good a way as any in a model,  forget shims. Make a handful of bucket tappets and select a size to fit. It does mean lots of camshaft removal/ refitting, but you get that with a full size engine anyway.

Head cooling is never as big an issue with a model compared to full size.

The aluminium head is a good heat conductor so you don't need the water passages close to the combustion chamber.  Bigger drilling further away may give better cooling

Cheers

Mike

[RReid]

Thank you for the tips and suggestions, Mike. Very helpful indeed. 

I suspected that what you say about head cooling at model size and usage might be the case, but it's good to get that suspicion confirmed by someone more experienced. I'll take a look at moving those passages up higher where they can be made larger, but the crosswise passages in particular may run into something else. It gets a little crowded inside that head.

[Kim]

Great start, Ron!  And I like the size of the pictures 

Kim

[RReid]

Thanks Kim. When you asked a short while ago about the size of the pics in your posts, and a larger size seemed favored, it made me re-think what I was doing.