Author Topic: Exhaust Valve Timing  (Read 6527 times)

Offline Brian Rupnow

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Exhaust Valve Timing
« on: February 07, 2014, 12:25:35 AM »
I must admit, I haven't put a great deal of science into the design of the exhaust cams on the two engines I have designed from scratch, the "Rupnow Engine" and the "dual opposed piston" engine. With the "Rupnow Engine" I copied the cam profile from the Kerzel hit and miss engine, and I got lucky.---It ran. The dual opposed piston engine runs, but not the way I ultimately want it to. First I am going to tell you what I know. Then we will delve into the arcane science of cam design. So---This I know. When you have built a new 4 cycle engine from scratch, be it from someone else's proven design, or from your own design the first step in setting it up to run is to set the valve timing. The valve is driven either directly or indirectly by a cam, which revolves at 1/2 the speed of the crankshaft. The cam gear is attached to the camshaft, or else it rotates freely on a stub-shaft with the cam attached to the face of the gear, but either way, it meshes with a gear on the crankshaft. The gear on the crankshaft is not keyed in place, but is generally held in place on the crankshaft by one or two set screws. We are only concerned here with rotational movement, not movement parallel to the centerline of the crankshaft.  With the cam lobe not providing any lift at all on the valve, use a feeler gauge and set the gap between the end of the valve and the lifter (or rocker arm) at about 0.010". The engine is then rotated by hand in the direction of rotation you want the engine to run.  The set screws holding the crankshaft gear are not tightened down at this point. You want the piston to be moving from top dead center towards bottom dead center. About 1/8" before the piston reaches bottom dead center, stop turning the crankshaft. Rotate the crankshaft gear in the same direction with your fingers (it is loose on the crankshaft) and as you do the cam gear will rotate in the opposite direction. When the lobe on the cam just begins to lift the valve pushrod (a point that you can definitely feel, as there is resistance to the valve lifting created by the valve spring), stop and lock down the set screws in the crankshaft gear. Now your valve timing is set. As the piston moves that final 1/8" to bottom dead center, the valve begins to open. As the piston travels from bottom dead center towards top dead center, the exhaust valve will fully open to let any exhaust gas in the cylinder be pushed out thru the exhaust valve to the exhaust pipe. As the piston approaches top dead center, the valve should begin closing, and by the time it reaches top dead center, the valve should be fully closed to begin the intake stroke. This means that the "lifter" has raised up on the leading edge of the cam, skated across the major diameter at the end of the cam, and rode back down the other slope of the cam and returned to the portion of the cam which provides no lift. Now--Here lies the mystery. On low speed engines such as mine, both sides of the cam are a straight line that lean in towards each other at the major diameter area of the cam which provides maximum lift. The secret of just how much they should lean in towards each other is directly related to the number of degrees the crankshaft has to rotate to carry the piston from bottom dead center to top dead center, which should, in theory be exactly 180 degrees. I have set the opposed piston engine up so the valve is opening at the correct time. However, I find that the valve is closing much sooner than it should as the piston travels up the cylinder towards top dead center on its exhaust stroke.--This means I am going to have to come up with a cam profile that has a wider area of "maximum lift" than the profile I am currently using. I hope this is making some sense. If anyone with experience in the development of cam profiles wants to speak up and educate me somewhat on this, I would greatly appreciate it.---Brian


Offline John S

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Re: Exhaust Valve Timing
« Reply #1 on: February 07, 2014, 01:04:58 AM »
Brian, you need something like exhaust opening at 50 degrees before BDC and closing 30 degrees after TDC.
John Stevenson, Nottingham , England

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #2 on: February 07, 2014, 01:10:00 AM »
John---50 degrees before bottom dead center? That sounds like an awful lot to me, but I will try tomorrow night to translate that into approximate piston position.

Offline John S

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Re: Exhaust Valve Timing
« Reply #3 on: February 07, 2014, 01:26:35 AM »
Remember Brian that gasses don't move instantaneously. It it was instant then you would fire a plug at TDC not before TDC
John Stevenson, Nottingham , England

Online gbritnell

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Re: Exhaust Valve Timing
« Reply #4 on: February 07, 2014, 01:42:50 AM »
Hi Brian,
Cam design is an extremely complex science and far too involved to get into here so for the sake of simplicity here's what I can tell you.
What you have described would be known as very mild cam timing and should work fine for a hit and miss engine.
Normally on a 4 cycle engine with both valves operated by the cam the timing would be different and calculated by the working rpm of the engine. Here again for the sake of simplicity the exhaust valve should open around 30 degrees before BDC. By this time the fuel charge has done it's work and with the valve opening at this point is allows the exhaust gases to get a head start on exiting.
Now the piston is heading toward TDC. About 30 degrees before TDC the intake starts to open. The reason for this is to allow the moving exhaust gases to help pull the intake charge into the cylinder. Now the piston moves past TDC to about 30 degrees and the exhaust closes. The reason for this delay in closing here again is the scavenging effect of the flowing exhaust gases and the fact that in 30 degrees of crank rotation there isn't much vertical movement of the piston. Now the piston is headed back to BDC with the intake open. It stays open about 30 degrees past BDC so that the inertia of the moving air/fuel charge has a chance to fully fill the cylinder.
As I said these numbers are only approximate due to the many variations of engine design, but you get the picture.
gbritnell
Talent unshared is talent wasted.

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #5 on: February 08, 2014, 01:33:54 AM »
I thought of using my cad program to see what angle of advance the valve opening would have if the piston was 1/8" from BDC  last night, but was away on a job all day, and didn't have a chance to look until tonight. The engine rotates clockwise, and as you can see by the dimension on the drawing, the piston is 1/8" from being at bottom dead center. The angle of the crankshaft shows that this is almost spot on 50 degrees advance. Looks like John was "right on" with his advise!!!



Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #6 on: February 08, 2014, 01:55:37 AM »
On an engine which has a mechanically operated intake valve, I can see the reasoning for having the exhaust valve remain open for a portion of the intake stroke. On an engine with an atmospheric valve, I would think the exhaust valve should close right at top dead center. If the exhaust valve begins to open 50 degrees in advance of bottom dead center, and closes at top dead center that's a total of 180 +50=230 degrees. Now the cam revolves at half the speed of the crankshaft, so I think that translates to 115 degrees of "cam influenced movement" of the valve.--Stick with me folks, I'm winging it here. The cam rotates thru 360 degrees, so I think that means there should be 360-115=245 degrees of cam which should not be influencing the movement of the valve. (valve is closed when not being influenced by the cam). The drawing of my cam shows that the angle is about 235 degrees of cam which is not influencing the valve. So--what I have is pretty darn close. I am not sure of my reasoning here, but would a 10 degree difference make that much difference?--And am I correct in my thinking that  when using an atmospheric intake valve the exhaust valve should be closing at top dead center?

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #7 on: February 08, 2014, 01:15:26 PM »
I just got my feeler gauges out and checked, and found that my valve lash had slipped/changed to a whopping 0.023"!!! I am changing it to .005" to see what effects that has on when the valve opens and closes without changing anything else.

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #8 on: February 08, 2014, 01:41:49 PM »
Okay--That clinches it--There is something wrong with the machining of my cam, but not with the design of it. I set the lash at .005", and readjusted the timing so that the valve lifter just began to contact the valve at 50 degrees before bottom dead center, (which corresponds with the 1/8" of final piston travel before bottom dead center. I then rotated the crankshaft by hand until the piston began its travel up the cylinder on the exhaust stroke. The exhaust valve opened fully as it was supposed to--and then stayed open through most of what should have been the intake stroke as the piston travelled back down the cylinder. I have messed up the machining of the cam. I'm surprised that the engine ran at all. So--this weekend I will machine a new cam. I knew there was a reason I designed this engine to have a bolt on cam!!!

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #9 on: February 08, 2014, 03:38:16 PM »
Jeez--What a nasty little piggy to take the cam off of. I almost had to dismantle the entire engine!!! I can't really see anything wrong with the cam on visual inspection, but at this level there isn't really too much that would be visible to the naked eye anyway. Maybe if I had a visual comparator it would show up.--Oh well, it gives me a chance to clean up the engine.

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Offline John S

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Re: Exhaust Valve Timing
« Reply #10 on: February 08, 2014, 03:48:13 PM »
Yes massive duration on that from looking at it.
Best stuff it in the nearest Manx Norton   :cartwheel:
John Stevenson, Nottingham , England

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #11 on: February 08, 2014, 03:52:54 PM »
I just measured the angle between the flat faces of the cam with my machinists protractor.

 The angle measures 57 degrees. It should be 55.2 according to the drawing.

Offline John S

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Re: Exhaust Valve Timing
« Reply #12 on: February 08, 2014, 04:56:56 PM »
Re check Brian as 2 degrees will not cause the valve to stay open during the intake stroke.

You are working half angles are you not  ??
10 degrees on a cam is equal to 20 degrees engine rotation because of the 2:1 timing gear ??
John Stevenson, Nottingham , England

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #13 on: February 08, 2014, 05:00:31 PM »
John--the problem seems to be that the o.d. of the cam is not perfectly concentric to the center of the gear. I am taking steps to address this with the new cam.

Offline John S

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Re: Exhaust Valve Timing
« Reply #14 on: February 08, 2014, 05:31:18 PM »
Brian,
Maybe this will help, it's certainly a better shot of what a cam should look like. Yours has far, far too much duration.

John Stevenson, Nottingham , England