30cc Flat Four (Puma)

[Jo]

Having an offset idler is standard on model engine designs : think about how far apart it would be necessary to space the cam and crank to fit a gear with a reasonable number of teeth in between and the spacing would be forced by the diameter of the gears rather than to fit the shape  of the engine crankcase.

One thing worth thinking about is making the idler out of brass as it will quieten down the gear chain. And try to make the idler such that it is not an exact multiple or factor of the other two gears as this can cause a hunting tooth  . What you are looking for is that different teeth engage each rotation, which also helps to achieve even wear and quiet running. A good trick is to have an odd toothed gear as the idler 

Jo

[doubletop]

Jo

Thanks for the advice. I had realised that it was necessary to include an idler and then compromise to be able to use standard gears. If I’d done a better design job I should have worked out what gears to use a single central idler and then adjust the lateral position of the two camshafts so everything lined up properly. The odd number tooth for the idler was new to me. Belting Direct do gears and they have steel, brass and Delrin gears available. I guess a Delrin gear would be quiet but would they work in this application?

I was coming to a similar conclusion about compromise when using synchronous belts. The next available sizes for the crank pulley, after 12t was 16t and then 18t. The corresponding 32t and 36t pulleys for the cams were getting too large to look neat. The suggestion of using a flangeless pulley appears to be the way to go. We'll see and if it doesn't a switch to gears could be on the cards.

All good stuff thanks

Pete

[doubletop]

I’m working through the various stages of making the cylinder heads. I’ve got to the passages for the inlet and exhaust ports. They are angled at 15deg. I was faffing about with protractors and digital angle gauges when I had an inspiration that the valve guides would be a readymade sine bar.

They are 12mm apart so the offset needs to be 12x sin(15) = 3.105mm. I used a slip gauge but a 3.1mm drill shank would do it.



Pete

[Roger B]

I solved a similar problem using my rotary table. In this case the central bore of the RT was the same as the valve guide holes so I didn't need to make an adaptor and just used a length of 6mm silver steel. With the RT centred I could Slip each valve guide over the 6mm ss, set the angle and machine the port.

[doubletop]

Roger

Very good; as they say 'many ways of skinning cats' I was following Malcolm Strides approach and was not happy with it. I can't remember how I did it 6 years ago, then had the epiphany. I later realised I’d done similar things when machining things like connecting rods.

A bit of an update. Last week was spent doing the four cylinder heads, many different sequences were required so they took a bit of time. They were finished over the weekend so couldn’t resist trying them out.



The bags contain various other parts that I’ve made on the way. I find doing a few small parts, here and there, helps reduce the tension that occurs when doing the more complicated stuff. It also means that over time everything things get done before it is required.

The piston rings were made when I did the Bobcat six years ago. I had never done piston rings before and expected problems, so made a few.  My concerns were unfounded and only used four of them so ended up with a few spares.

I’ve been updating the CAD and now have the approach to the hall sensors sorted. I plan to use two of the RCExel type, twin cylinder CDI units. So will need two hall sensors that trigger alternatively twice each half rev. Following Malcolm’s approach, I’ll have two magnets on a disc for each unit and a disc on each cam. Each disc will be 90deg out of phase with the other so 4 trigger events on each rev alternating between the two units.



The vertical bar just behind the crank will have a sensor set in the left and right hand edges.

When I did the Lynx I ended up doing a home made ignition system with a PICmicro controller that enabled the advance curve to be set on a running engine. In the fullness of time I may resurrect that project for a 4 cylinder version

Pete

[bruedney]

Looking good Pete

 

I have had some time off so I have got back to doing some work on my single cylinder IC engine. Had it running on Shellite 

Yesterday afternoon was spent making a base whilst I await a smaller spark plug to come (ordered the wrong Rcexl unit).

Cheers
Bruce

[doubletop]

Looking good Pete

 

I have had some time off so I have got back to doing some work on my single cylinder IC engine. Had it running on Shellite 

Yesterday afternoon was spent making a base whilst I await a smaller spark plug to come (ordered the wrong Rcexl unit).

Cheers
Bruce

Bruce

Thanks

What plugs were you planning to use? The RCexl plug caps can be changed. I've used 1/4x32 on all my engines and have some of the larger plug caps spare. I'm not sure if they are CM6 or 14mm but give me a call and maybe I can help.

Pete

[bruedney]

Thanks Pete

I have a CM6 spark plug but a NM8 (1/4 x 32 I think)) connector on the Rcexl unit. If you have a CM6 connector that would be great.

Unfortunately I am back at work tomorrow but I will touch base with you sometime to catch up.

Bruce

[doubletop]

Thanks Pete

I have a CM6 spark plug but a NM8 (1/4 x 32 I think)) connector on the Rcexl unit. If you have a CM6 connector that would be great.

Bruce

I have one of each of the larger type of caps.

Pete

[doubletop]

I’ve been working on the cam belt and ignition triggers. Much as shown in the CAD diagram. The one thing that I haven’t done in CAD is the belt tensioner. The CAD thinks the 145mm belt is too short but that isn’t the case with the belt I’ve purchased.

I had drawn up a tensioner but with bearings it looked too large against the overall size of things. Then one of those “let’s try this” moment. A stack of 630ZZ bearings that were to be used as the tappet bearings for the Bobcat did the job and with a long 3mm cap head bolt it went straight one of the existing holes in the rear bearing holder.  Job done.



The next job is to assemble the two camshafts but some guidance from you please. Given this layout:-



Do I make the firing order 1,2,3,4 or 1,3,4,2 or is there any other alternative? What are the reasons for selecting one over the other?

Pete

[Jo]

The firing order is defined by the cam lobes 

My Seal should fire 1-2-4-3 or 1-3-4-2 depending on if it was cut moving the pointer clockwise or anticlockwise on the cutting jig

Jo

[doubletop]

The firing order is defined by the cam lobes 

My Seal should fire 1-2-4-3 or 1-3-4-2 depending on if it was cut moving the pointer clockwise or anticlockwise on the cutting jig

Jo

Jo

I've not assembled the camshafts yet, I have 8 separate cams that are to be loctited to the shaft. So I can make it what I want. I thought I'd check with the experts before I committed.

Given the crank journals are shared and 180deg apart I can either fire the front pair (serially) then the rear pair (1,2,3,4) or the left pair followed by the right pair (1,3,4,2).  I guess I'm looking for the solution that is likely to be the smoothest running.

Pete

[bruedney]

Hi Pete
I think you have your cylinder numbering wrong. Since it seems your crank config is more like a V4 than a boxer I would say (after looking at google image search) that the firing order should be 1, 3, 4, 2 but the cylinder number should be like is indicated here http://www.woiweb.com/index.php/Firing_order

Cheers
Bruce

[doubletop]

Hi Pete
I think you have your cylinder numbering wrong. Since it seems your crank config is more like a V4 than a boxer I would say (after looking at google image search) that the firing order should be 1, 3, 4, 2 but the cylinder number should be like is indicated here http://www.woiweb.com/index.php/Firing_order

Cheers
Bruce

Bruce

My original cylinder numbering was like an integrated circuit going anti-clockwise from #1. I realised that could be confusing to some so for the purposes of the question used the convention in my diagram. For now I'd like to understand whether option A (1,2,3,4) or otion B (1,3,4,2) would be best dynamically. After that any numbering scheme can be adopted but will still run in the preffred order.

Pete

[Roger B]

With your manifold design there shouldn't be much difference. If you had one carb feeding 1 and 3 and one feeding 2 and 4 the first option would be better as it would give even intervals between suctions. The same would apply if the exhausts were joined rather than separate.