Model Engine Maker

Help! => Specific Engine Help => Topic started by: kev on September 24, 2014, 01:45:57 PM

Title: Scaling port dimensions
Post by: kev on September 24, 2014, 01:45:57 PM
Im making a 2x scale elmers #24 beam and an thinking of drilling the cylinder soon.

General question concerning port sizes, should I just double the linear dimension ie diameter or double the port area?

For this piston type valve im thinking double the diameter will keep the timing right but other engines maybe its the area.

Im sure for a toy it does not matter a jot but im generally interested
Title: Re: Scaling port dimensions
Post by: Maryak on September 24, 2014, 10:22:37 PM
Attached is a spreadsheet for steam port area.

The original info courtesy of Sandy Campbell.

Best Regards
Bob
Title: Re: Scaling port dimensions
Post by: Alan Haisley on September 30, 2014, 08:37:46 PM
Attached is a spreadsheet for steam port area.

The original info courtesy of Sandy Campbell.

Best Regards
Bob
Bob/Sandy,
This spreadsheet has an error. Please examine the formula in cell B9. This reads as =B13/12*2*B15. The B13 term should be B14. i.e., it needs to be the stroke rather than the bore.
In the example given the result is OK because the bore and stroke are the same.

Alan
Title: Re: Scaling port dimensions
Post by: Maryak on September 30, 2014, 11:08:10 PM
Alan,

Don't blame Sandy, blame me, it's my spreadsheet and you are quite correct.

So...........thank you for finding and fixing the error.  :NotWorthy:

Best Regards
Bob
Title: Re: Scaling port dimensions
Post by: SandCam on October 01, 2014, 01:49:54 PM
Hi Guy's,

Thanks for correcting that error Alan.

Bob,

I have taken the opportunity to add some other alterations to the spread sheet, specifically for the port dimension calculations based on standard slide valve design practice.
It will now calculate port length and width automatically for changes to BORE, STROKE and RPM.

Your fixed 0.094" width gave some great fun answers if you increased the stroke too much... try 27" with your original and you will see what I mean.

I have also added/expanded some of the notes.

Hope you approve.

Best regards.

Sandy. :old: :cheers:
Title: Re: Scaling port dimensions
Post by: SandCam on October 01, 2014, 01:56:37 PM
Im making a 2x scale elmers #24 beam and an thinking of drilling the cylinder soon.

General question concerning port sizes, should I just double the linear dimension ie diameter or double the port area?

For this piston type valve im thinking double the diameter will keep the timing right but other engines maybe its the area.

Im sure for a toy it does not matter a jot but im generally interested

Hi Kevin,

Yes, just double the port dimensions otherwise you will need to alter the piston valve dimensions and possibly the eccentric throw... so keep it simple.

If you were starting from scratch then you could design it in accordance with the general design rules as given in Bob's spreadsheet (now Modified).

Hope this helps.

Best regards.

Sandy :cheers:
Title: Re: Scaling port dimensions
Post by: Maryak on October 01, 2014, 11:40:42 PM
Hi Guy's,

Your fixed 0.094" width gave some great fun answers if you increased the stroke too much... try 27" with your original and you will see what I mean.

I have also added/expanded some of the notes.

Hope you approve.

Best regards.

Sandy. :old: :cheers:

Hi Sandy,

Thanks for your amendments much appreciated. (Downloaded and saved). The 0.094" was not "fixed" but could be changed at will to alter the other one to give good sized ports. It probably would have helped a lot if I had been smart enough to annotate that!

Best Regards
Bob
Title: Re: Scaling port dimensions
Post by: kev on October 22, 2014, 10:27:44 PM
Thanks guys, thanks Sandy, I have just gone ahead and doubled the diameters, should be fine but still a couple of weekends off trying to run it, here is the current status:

(http://i233.photobucket.com/albums/ee128/kevscraftshop/IMG_3239.jpg) (http://s233.photobucket.com/user/kevscraftshop/media/IMG_3239.jpg.html)

the engine block seen here is just being worked on now

(http://i233.photobucket.com/albums/ee128/kevscraftshop/IMG_3205.jpg) (http://s233.photobucket.com/user/kevscraftshop/media/IMG_3205.jpg.html)