Author Topic: Carb / mixer design questions for hit and miss engine  (Read 739 times)

Offline Misterg

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

As you might have seen, 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:


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

I *believe* the ignition is OK (it is the Hobby King module 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:



Thanks in advance for any thoughts or advice :)

Andy

« Last Edit: October 09, 2020, 02:20:02 PM by Misterg »

Offline Roger B

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Re: Carb / mixer design questions for hit and miss engine
« Reply #1 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.
Best regards

Roger

Offline Misterg

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Re: Carb / mixer design questions for hit and miss engine
« Reply #2 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:




Offline Roger B

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Re: Carb / mixer design questions for hit and miss engine
« Reply #3 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.
Best regards

Roger

Offline coulsea

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Re: Carb / mixer design questions for hit and miss engine
« Reply #4 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.

Offline kuhncw

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Re: Carb / mixer design questions for hit and miss engine
« Reply #5 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 

Offline Misterg

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Re: Carb / mixer design questions for hit and miss engine
« Reply #6 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). 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. :)

Offline gbritnell

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Re: Carb / mixer design questions for hit and miss engine
« Reply #7 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
Talent unshared is talent wasted.

Offline Misterg

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Re: Carb / mixer design questions for hit and miss engine
« Reply #8 on: October 10, 2020, 07:54:03 PM »
Thank you very much, Mr. Britnell! :)

Offline Misterg

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Re: Carb / mixer design questions for hit and miss engine
« Reply #9 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) 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.