Model Engine Maker

Help! => Specific Engine Help => Topic started by: Misterg on October 09, 2020, 02:14:21 PM

Title: Carb / mixer design questions for hit and miss engine
Post by: Misterg on October 09, 2020, 02:14:21 PM
Hi folks - I hope this is the right place to ask...

As you might have seen (http://www.modelenginemaker.com/index.php/topic,9965.0.html), I've just completed an open crank hit and miss engine of my own design.

I've noticed that it has to be adjusted to run quite rich in order to fire reliably when the governor is active (i.e. firing then coasting for a number of cycles). I'm guessing that this is normal behaviour, as I'm sure I've seen comments to this effect previously. Mostly out of curiosity, I'd like to understand why this is, and if there's anything in my design that's exaggerating this effect.

When running without the governor, it is very, very difficult to get the engine to fire every cycle (when it does, it takes off like a chainsaw!)- it is happiest firing every two cycles (4 crank rotations).

I though it might be struggling to clear the exhaust gasses, but I've taken some slow motion video which shows the (atmospheric) inlet valve being opened every cycle, so presumable the cylinder pressure was cleared before the inlet stroke.

From about 1 minute in:

cKQwi_WaV3w
(Compare the sound of firing with the cam/rocker movement).

I *believe* the ignition is OK (it is the Hobby King module (https://hobbyking.com/en_us/replacement-complete-ignition-set-for-single-cylinder-gas-engines.html?___store=en_us) with static timing at TDC - firing every crank rotation).

Which brings me back to fuel and the carb (mixer)...

It is very conventional, and while not a direct copy of any particular design, it has a single 1mm / 0.039" hole in a spray bar directed down stream which is common to a lot of the designs in circulation. It also has a check valve in the fuel supply in the form of a 2mm dia S/S ball in the elbow at the end of the spray bar - the 1.7 dia hole is actually 2.5 (other details as below).

The needle has a smooth taper and needs to be ~2/3 of a turn open for the engine to run - there's a range of about 1/4 of a turn where the engine will run in some fashion or other, which doesn't seem unreasonable, but obviously a finer adjustment is better.

So: Does this behaviour sound normal for this style of engine?

Any hints or tips to reduce the need to run rich?

Are there any changes I could/should make to the carb/mixer to improve it?

Carb working drawing:

(https://www.misterg.org.uk/photo/images/2020/10/09/Carb-sketch.png)

Thanks in advance for any thoughts or advice :)

Andy

Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Roger B on October 09, 2020, 03:31:24 PM
How are you judging that it is running rich, black smoke, sooty plug?

The mixture you will get with a relatively steady flow through the carb will be different to with an intermittent flow.
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Misterg on October 09, 2020, 05:35:27 PM
How are you judging that it is running rich, black smoke, sooty plug?

The mixture you will get with a relatively steady flow through the carb will be different to with an intermittent flow.

Hmmm... I decided it was running rich because I had to open the needle valve more for hit and miss operation that I did for free running. If I let it free run at the same needle setting, it does indeed run very sooty.

I took the firing every other cycle (8 stroking?? - I would call it 4 stroking if it was a 2 stroke!) as a symptom of rich running too.

However your second point would probably explain all of that.

If it's all "normal", I'm happy  :Lol:



Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Roger B on October 09, 2020, 05:41:41 PM
With an intermittent flow the mixture can become leaner as the inertia of the liquid fuel is more than that of air. This could explain your need to open the needle valve further when in hit and miss operation.
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: coulsea on October 10, 2020, 02:11:35 AM
If the intake valve spring is too strong you will not get use of the whole intake stroke as the valve will only open when the suction is great enough to overcome the spring, if this only occurs near the end if the stroke you will not get enough charge to fire every time.
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: kuhncw on October 10, 2020, 03:25:18 AM
Your mixer throat may be a bit large for a 20.4 mm bore hit and miss engine.
You could try reducing the mixer bore from 5 mm to perhaps 4mm.  The reduced flow area will raise the velocity of the air passing over your fuel jet which will help draw fuel into the air stream.


Regards,

Chuck 
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Misterg on October 10, 2020, 11:36:31 AM
If the intake valve spring is too strong you will not get use of the whole intake stroke as the valve will only open when the suction is great enough to overcome the spring, if this only occurs near the end if the stroke you will not get enough charge to fire every time.

Thanks - I was wondering something similar before I took the video. It *seemed to me* from the video that the intake valve is open for a reasonable length of time, but a more experienced eye might spot something - the lift is limited to 25% of the valve throat diameter (1.5mm lift for 6mm throat).

The intake spring is quite light - 12 turns of .013" wire (guitar 'B' string) with 6mm/0.236" ID which calculates out at 0.22lb/in or 0.05N/mm (using the spreadsheet from here (https://sites.google.com/site/lagadoacademy/machining---lathes-mills-etc/tools---spring-making)). By way of comparison, the other designs that I looked at ranged from 0.19lb/in (Kerzel) to 1.28lb/in(Upshur farm engine) calculated using the same spreadsheet, with a lot of spread in between. (Incidentally, the variation didn't really seem to follow valve diameter).

What I was unable to find was the amount of 'preload' needed - i.e. spring rate is only half of the information). It seemed to be minimal, so I went with the same, so I could stretch the spring later if necessary. I was worried that it might be too light, but that doesn't seem to be a problem. When I turn the engine over by hand, the intake 'farts' continuously through the intake stroke (if you understand what I mean!) but still easily seals on compression.

Your mixer throat may be a bit large for a 20.4 mm bore hit and miss engine.
You could try reducing the mixer bore from 5 mm to perhaps 4mm.  The reduced flow area will raise the velocity of the air passing over your fuel jet which will help draw fuel into the air stream.

Thank you - this is something I may well try - does the size of the hole in the spray bar influence this too?

Thanks for the responses. :)
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: gbritnell on October 10, 2020, 01:37:06 PM
While almost any type of mixer/carb will work on a hit and miss engine the thing that I see with your construction is you have the body with a 5mm bore. It is cross drilled 3.4mm. You then have your fuel tube inserted into that 3.4mm hole. Because there is a reduction in the area where the fuel tube sits in the bore it will create some what of a venturi effect which is needed for better atomization of the fuel. Now I have seen carbs like this work I don't know how efficient they are. I prefer to insert a tube with a tapered end from one side of the carb. This will extend to the center line of the bore. This tube will be drilled for the tip of the needle valve. From the other side of the carb body is a short threaded post onto which the needle valve threads. Attached is a picture of a simple mixer for a hit and miss engine. You will see it has a venturi area machined into the carb body. For your application you could use the design but alter the dimensions to suit the size of your engine. For a 20mm bore engine you should be able to use a carb with a 2.5mm bore.
gbritnell
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Misterg on October 10, 2020, 07:54:03 PM
Thank you very much, Mr. Britnell! :)
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Misterg on October 27, 2020, 12:00:19 AM
Ok, something of an update...

I haven't made a new carb/mixer. A 2.5mm bore *sounded* small, so I checked how it compared to what I had - 2.5mm would be ~ 4.9mm^2 choke area (less a bit for the needle / jet). My current carb has an effective choke area of 4mm^2, so 'in the ballpark'.

I wondered if the proportions of the spray bar to the choke diameter might cause problems, but got to reading some stuff about aero engines on modelengineeringnews.org (link (http://modelenginenews.org/tbirds/as_venturis.html)) which reassured me somewhat (I acknowledge that they are *very* different engines!)

One of the other things I wanted to do was lighten the governor weights - I had to fit quite a stiff spring and screw it right down to get the cut-off RPM high enough for the engine to run. Unfortunately, I did two things at once - replaced the cylindrical weights with lighter brass balls, and also shortened the lever arm - with the upshot that I now need to back the spring right off to get the governor to work at all.

It now runs slightly faster than it used to, but it made a big difference to the needle valve settings. It will now run free, or with the governor at the same needle valve setting. This also means that when running with the governor, it transitions beautifully from hitting and missing to firing more and more frequently as you load it down without stalling / bogging down as it did previously.

I am guessing that it was just running too slowly for the carb/mixer to work properly, so required the needle valve to be opened to compensate for lack of vacuum at the lowest RPMs.

I need to wind a new governor spring to get some adjustment back so I can experiment some more with running speeds (and maybe try blocking off half the venturi as a test as mentioned in the modelengineeringnews article), but the direction may lie in an even smaller choke area.

Thanks.
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Misterg on March 29, 2021, 11:00:41 PM
Just for posterity, I think I have a resolution to this.

I preferred the engine to run more slowly so I made a smaller mixer in line with gbritnell's suggestion above: 2.5mm diameter throat with a central 'needle jet' rather than the spray bar:

(https://www.misterg.org.uk/photo/images/2021/03/29/IMG_6685.jpg)


(I retained the check valve in the fuel inlet)

Old on the left, new on the right:

(https://www.misterg.org.uk/photo/images/2021/03/29/IMG_6690.jpg)

The engine started and ran well with this and the un-governed speed was (as expected) much lower. However I was back in the situation of needing to adjust the mixture between free running and when it was running as a hit and miss (which meant it wouldn't transition nicely between the two).

I believe the reason is due to the relatively low compression ratio of the engine (4:1) and the differences in scavenging (or lack of it) between repeated firing and the fire + coast of hit and miss operation: In repeated firing, the unswept volume in the cylinder contains exhaust gas, so the air:fuel ratio only depends on the intake air. When coasting, the exhaust gasses in the unswept volume gradually becomes diluted by air drawn back in through the open exhaust valve, so that the air:fuel ratio through the mixer needs to be somewhat richer to compensate for the intake air being mixed with the air in the unswept volume of the cylinder. (Hopefully you can follow my thinking!).

To test this, I made a volume spacer to fit on top of the piston that raised the compression ratio to just over 6:1 (The gudgeon pin carrier is bolted through the piston crown, so an additional spacer was easy to fit):

(https://www.misterg.org.uk/photo/images/2021/03/29/IMG_6703.jpg)

(https://www.misterg.org.uk/photo/images/2021/03/29/IMG_6706.jpg)

This considerably reduced the unswept volume - both pictures are at TDC:

(https://www.misterg.org.uk/photo/images/2021/03/29/IMG_6698.jpg)

(https://www.misterg.org.uk/photo/images/2021/03/29/IMG_6711.jpg)

The head is flat, btw:

(https://www.misterg.org.uk/photo/images/2021/03/29/IMG_6700.jpg)

This change seems to have eliminated / greatly reduced the need for different adjustments between free running and hit & miss running. The engine can now be loaded up while running as hit and miss, and will transition nicely to continuous firing as the load increases, so I think I was on the right lines with the scavenging theory. For future reference, there is little other difference in the behaviour of the engine between running at 4:1 and 6:1 compression ratio - it is still easy enough to flip over compression to get it started. I think the 2.5mm choke diameter is a *little* small for the engine (maybe due to my chunky needle valve & seat), but very close. If anything, I think I need more flywheel diameter.

Anyway, just thought I'd update.... :)




Title: Re: Carb / mixer design questions for hit and miss engine
Post by: cnr6400 on March 30, 2021, 02:41:59 PM
That was valuable experimentation! Good to know about, thanks for sharing it.  :ThumbsUp: :ThumbsUp: :ThumbsUp: :popcorn: :popcorn: :popcorn:
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Zephyrin on March 31, 2021, 08:48:12 AM
thanks to share these results of experimentation as carburation in these little engines is never simple !
in addition the compression ratio is not that commonly tested !
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Admiral_dk on March 31, 2021, 04:43:53 PM
Quote
thanks to share these results of experimentation as carburation in these little engines is never simple !
in addition the compression ratio is not that commonly tested !

Amen to that - though the compression ratio, it has been debated earlier here and quite a few felt that a lower compression ratio makes the engine easier to tune ....
I would have thought going much under 5:1 would be too low - but can also see that this to a degree depends on the total swept volume etc. -> probably more the total pressure in the combustion head than the ratio.

Still very nice of you to post your results for the rest of us  :ThumbsUp:

Per
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Misterg on April 02, 2021, 08:47:33 AM
No problem! Thanks :)
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Roger B on April 03, 2021, 08:13:43 PM
Thank you for posting the update  :ThumbsUp:  :ThumbsUp: It is interesting that the lower the compression ratio with an atmospheric inlet valve the harder it is to properly purge the cylinder. With a mechanically operated valve no problem  :thinking:
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Vixen on April 03, 2021, 08:55:04 PM
 :ThumbsUp: :ThumbsUp: :ThumbsUp:

Thanks for posting your results. Now we all have some real numbers to work from.

Mike
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Misterg on April 05, 2021, 02:48:34 PM
It is interesting that the lower the compression ratio with an atmospheric inlet valve the harder it is to properly purge the cylinder. With a mechanically operated valve no problem  :thinking:

I think that the 'problem' comes from the difference in purging between free running and governed. It's such a slow revving engine, with conservative valve timing that I'm sure there's little/no effect from gas momentum, so the piston just works like a big syringe. When free running, any residual gas in the cylinder is the exhaust gas from the previous combustion stroke, so after the intake stroke, the gas in the cylinder is 80% fresh air/fuel mixture + 20% "inert" exhaust left over from the previous cycle. (I think 20% is  right for a 4:1 compression ratio?)

If the engine is governed by locking out the exhaust valve, the engine draws air back through the open exhaust valve when coasting which dilutes the residual exhaust gas in the cylinder; the mixture is expelled as the piston rises, then more air is drawn in on the next coasting stroke, etc. so that after a few revolutions of coasting, the cylinder contains mostly fresh air. When the governor trips back in, and there is an induction stroke, the cylinder contains 80% fresh air/fuel mixture drawn through the inlet valve + 20% residual air, meaning that the combustion conditions will be different than the free-running case (much leaner if the mixture hasn't been adjusted).

Increasing the CR narrows the difference - that's the way I'm thinking of it, anyway!  ;D


Title: Re: Carb / mixer design questions for hit and miss engine
Post by: Kim on April 05, 2021, 05:21:36 PM
Though I don't have any IC experience, I have found your experiment and this whole discussion on compression ratio and exhaust purging just fascinating.  Thank you for posting your experiment and for the whole discussion around it.

Kim
Title: Re: Carb / mixer design questions for hit and miss engine
Post by: 90LX_Notch on April 05, 2021, 08:38:38 PM
This has been a great post.  I have been wondering/considering adapting this for my “Tiny”.

-Bob