Whittle V8 Question?

[mfarris]

Has anyone built this engine? If so, I have a question. I am in the process of building a "doubled up" version of the engine. The original plans say the plug holes, which break through the combustion chamber, should be drilled to where the bottom of the hole is .025 from the seat of the intake valve and the bottom of the plug when screwed in should be 1/16" from the bottom of the hole. In my doubled version, should the hole be drilled .050 from the seat and the end of the plug be 1/8" from the bottom of the hole? It would seem that perhaps on something like this, the plug and hole should still be the same distance from the valve seat regardless of size of engine.

 

[Swifty]

You had me going for a minute, I thought you were building a V16, but I realised that by doubled up you meant twice scale. I think that your on the right track about keeping the same distance from the valve, but are you using the original size plugs? This may have to be taken into consideration. I think that the design is trying to get the spark as close to the combustion chamber as possible.

Paul.

 

[mfarris]

Well, I had planned to use the original plugs, which were enya 3's, but it doesn't look like that plug will have enough threads on it to reach down that far into the combustion chamber if I go the distance that the original plans call for. Are there any glow plugs out there that are bigger than those, or have longer threads?

 

[Swifty]

Others will have to answer about the glow plugs, I know nothing about them.

Paul.

 

[mu38&Bg#]

I'll look over a CAD model I have. I think you'd just drill the seat for the glow plug deeper in the head. There is a lot more room in the head for the plug in a twice size version.

Greg

 

[mu38&Bg#]

There is a good amount of room to work with. You do have the exhaust port not very far away, but the plug could be angled and set deeper in the head. I didn't create the head model and didn't verify all the dimensions, but ti looks good. The plug in the image is an O.S. F plug.

Greg

 

[mfarris]

thanks for the info dieselpilot. What's your opinion on my first post? Do you think the plugs in my double sized version should still be the same distance from the intake valve port as they are in the smaller version?

 

[Jasonb]

This is taken from Mike Knight's articles about building his 2X one that was in Model Engineer mag. You may also want to look at teh OS type F plug which is designed for 4-strokes and is longer than the 2-stroke plugs

Note teh thread on a standard plug is 1/4 x 32 UNEF not the ME form he states but you can usually squeeze them into a ME thread.

Also there was a really nice normal size one at teh Model Engineer exhibition last year, the guy has a website with quite a lot of photos that may be of use

http://www.missiseipi.com/

 

[mfarris]

Thanks Jason. I have that article, but he doesn't say in there how far down to drill the plug holes, nor does he say how far the plug should go down in the hole. That's really the main thing I'm curious about right now. Once I find that out, then I can find a suitable plug.

 

[mu38&Bg#]

The tip that protrudes beyond the threads of the OS F plug normally extends into the combustion chamber in current production engines. This wasn't always the case. Early OS four strokes had a 5mm bore that roughly matched the tip diameter and the plug was recessed in the combustion chamber. I'd might try to angle the plug I were building it. Again both images are twice size parts with a standard OS F plug. This position has the plug centered between the valves and perpendicular to the exhaust flange. It gets close to the exhuast port assuming a 12mm socket clearance hole. Also you'd want to make sure there is enough material in the head for a full thread as it breaks though the simple counterbore combustion chamber. If you wanted to angle it away for the exhaust port it could give more room.

The last image. For simplicity I would consider original layout centered between valves. Adding a hole drilled in the combustion chamber to reach the plug.

Check compression ratios, and it wouldn't hurt to add some squish at a bore of this size.

 

[petertha]

I'll look over a CAD model I have. Greg

Greg, is this your own 2XWhittle rendition / cad model design?

 

[mfarris]

Thanks for the drawing Greg. Problem with angling them is I've already drilled them like the plans show, and I drilled them down to within .050 of the intake port (the original plans say drill down to .025). I know I at least have to go down as far as I have them now, but the question is do I need to drill them further to the .025 mark, and if so, is there a glow plug available that will reach that far. I know Mick Knight got his running. He used glow plugs also, so obviously there is a solution.

 

[mu38&Bg#]

All you really need is to expose the glow plug element to the combustion chamber. A few thousandths really won't matter. The original is close because it's very small.

Peter, I just scaled the parts 2x. A 2x design would ideally have some redesign as it's quite large, but nothing really stands out as problematic.

Greg

 

[mfarris]

That sounds great Greg. When I screw the plug in now, the glow plug element is definitely inside the combustion chamber.......but it's about 1/8" or so from the valve seat. Hopefully that will do.

 

[petertha]

Remember to factor in the overall CR effect by plug re-positioning. Be interested to hear your 'numbers' once its up & running.

http://www.homemodelenginemachinist.com/showthread.php?t=23705