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  • Cam profile mania

    As many of you know, I have just spent a month revisiting my Kerzel engine, in hopes of getting a better "hit and miss" action. One of the things I did was to make a new exhaust cam lobe based on the original drawing be Kerzel. Prior to that, I had been using a cam which I made based on the original Webster cam. I am now trying to wrap my head around the differences in these two cam profiles. in truth, I don't see the engine behaving a lot differently with the new cam than I did with the old cam.
    I am getting into the scientific end of cam design and operation, and some of it stretches me.
    The area on the mainly round portion of the cams at the bottom, which is a true diameter, has no effect at all on the valves. When the lifter or cam follower is riding on these surfaces, there is actually a gap called the "valve lash" between the lifter and other components in the valve train. For purpose of simplicity, we are going to overlook the effect of that "valve lash" in this discussion. It does have some effect on valve timing, but the effect is very small and can be safely ignored on these small engines.
    .......It is only the portion of cam that extends beyond the base circle profile that has any effect on the valve, and even then there is a trick. Since the camshaft revolves at only half the speed of the crankshaft, then the 117 degrees on the left hand valve actually has 234 degrees of effect on the valve as relates to the crankshaft, and the 84.22 degrees on the right hand cam has an actual effect of 168.44 degrees on the valve, as related to the crankshaft.
    .....The kerzel cam shown on the right is for a slow running hit and miss engine with an atmospheric intake valve, so there is no benefit in having the exhaust valve stay open during a portion of the intake stroke. When the piston moves from bottom dead center thru to top dead center on the exhaust stroke, the crankshaft revolves thru 180 degrees. The Kerzel cam as originally designed by Kerzel exhibits only 168.4 degrees of influence on the valve as related to crankshaft revolution. So ---what does that mean to us? Well, it means that if the Kerzel cam just begins to influence the valve when the piston is at bottom dead center, then by the time the crankshaft has advanced 168. 4 degrees towards top dead center on the exhaust stroke, the valve will have opened, expelled any exhaust in the cylinder, and then fully closed 11.6 degrees before the piston reaches top dead center. Consequently, there is no benefit in having the valve begin to open before the piston reaches bottom dead center.
    -----The cam on the left however, was designed for a faster running throttled engine (the Webster), still with an atmospheric intake valve. Again, since the intake valve is atmospheric, there is no advantage in having the exhaust valve stay open during any part of the intake cycle. However, that 117 of cam influence relates to 234 degrees of crankshaft rotation. Since the crankshaft rotates only 180 degrees getting the piston to move from bottom dead center to top dead center, and we want the exhaust valve to be closed by the time the piston gets to top dead center, then we have to begin opening the exhaust valve while it is still 234-180=54 degrees before the piston reaches bottom dead center on the power stroke.--and surprisingly enough, that is almost exactly what Webster asks for in his engine plans.
    .....I am still not totally clear as to why the Webster cam has a much wider "dwell" area at 0.218" wide as compared to the 0.050" "dwell" area on the kerzel engine, but I'm sure if I keep plugging away at this cam business it will become clear to me. Notice that both valves go from fully closed to fully open in about 37 degrees of movement, and both close in about 37 degrees of movement. It could very well be that the longer dwell time on the Webster cam is only there to extend the amount of time that the exhaust valve remains open on the Webster, since it is a faster running throttled engine and needs the extended time period to allow time for a full charge of exhaust to leave the cylinder and have no pressure remaining in the cylinder when the piston reaches top dead center and begins it's descent on the intake stroke....Very, very interesting stuff indeed.---Brian
    Last edited by brian Rupnow; 12-25-2016, 12:28 PM.
    Brian Rupnow

  • #2
    Interesting stuff Brian. During my "working" days, mainly on diesel engines we did not concern ourselves with any of this. The gears all came with timing marks on them and we just lined everything up and put it together.
    In this case it is a careful compromise. You want to open the exhaust valve as quickly as you can and hold it open as long as possible to purge the cylinder. But you also need to have it closed close to TDC on the exhaust stoke so you can start building a little vacuum in the cylinder to draw the new air/fuel charge in as completely as possible to try and produce as much power as you can from that size of engine. This is more important with your atmospheric intake valve which needs a certain amount of vacuum just to pull it open. Because the engine is running on a gasoline mixture, thus a fast burning fuel, most of the power is produced in the top end of the power stroke so you can safely open the exhaust valve quit early with out much noticeable loss of power. The piston is traveling at it's fastest speed half way up the stroke so you will get the greatest purge in that area. So at least in theory the lifter should be passing over the center point of the cam lobe half way up the exhaust stroke and then the valve should close near TDC. The slower turning engine probably doesn't need the valve open quite as long because the velocity of the gasses is much slower, thus more time to complete the purge. At least this is how my older grey matter see's it. I have been hiding away waiting for Christmas dinner to cock so I better get back out there and show may face for a while.
    Larry - west coast of Canada

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    • #3
      Very interesting conversation. Thank you for taking he time to type it and sharing.

      Errol Groff
      Errol Groff

      New England Model Engineering Society
      http://neme-s.org/

      YouTube channel: http://www.youtube.com/user/GroffErrol?feature=mhee

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      • #4
        Honestly, a bit over my head in the details. But, I do know exhaust valve timing on gasoline engines relates to exhaust tuning. My understanding is that engines require a certain amount of exhaust back pressure, best timed with pulses to help pull the exhaust gases out and then stop pulling to aid in filling on the intake stroke. Thus, a faster-running engine of your design might be taking that into account, expecting the exhaust pulses to work with the longer dwell. Just a though,

        David...
        http://fixerdave.blogspot.com/

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        • #5
          Do ic engines have lead and lap like steam, I remember comparing cams with a displacement diagram 0-360 along the X and lift on the y
          No help I know
          Mark

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          • #6
            Originally posted by brian Rupnow View Post
            Again, since the intake valve is atmospheric, there is no advantage in having the exhaust valve stay open during any part of the intake cycle
            Brian, I suggest you read some old books on 4 stroke tuning, this is really old news (at least 50 years old) to those working with 4-stroke engines, - there is a massive advantage in having some exhaust valve overlap with the intake, especially in normally aspirated engines, as the inertia in the flow in the exhaust acts as an "extractor" and assists in pulling through more fresh charge than is possible with piston vacuum alone.
            Overlapped valves are used in conjunction with tuned exhausts and length inlet tracts to provide optimum power in the required rev ranges,
            Regards,
            Nick
            If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

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            • #7
              No, I stand by what I said in my first post. Read the post slowly enough that the words "atmospheric intake valve" registers. An atmospheric intake valve (which has no cam to operate it) won't begin to open until the exhaust valve is fully closed and the descending piston begins to create a vacuum in the cylinder. That is when the intake valve opens. If the exhaust valve stays open after top dead center, the descending piston just pulls air thru the exhaust valve and the intake valve doesn't begin to open until AFTER the exhaust valve is fully closed. Normal 4 cycle engines with cam operated valves on both intake and exhaust definitely do have an overlap, on both valves both at top dead center and at bottom dead center. Engines with atmospheric intake valves gain no benefit at all from having the exhaust valve closing after top dead center.
              Last edited by brian Rupnow; 12-26-2016, 10:17 AM.
              Brian Rupnow

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              • #8
                Originally posted by brian Rupnow View Post
                ........I am still not totally clear as to why the Webster cam has a much wider "dwell" area at 0.218" wide as compared to the 0.050" "dwell" area on the kerzel engine........
                It's because both cams have the same lift. Greater duration moves the opening and closing flanks father apart and that gives you two choices in the overall profile...maintain the same lift with the resultant increase in dwell or keep the same dwell but increase lift. Increasing duration with increased dwell doesn't necessitate any changes to the valve train but increasing lift might necessitate some changes that Webster didn't want to do.
                Last edited by Arcane; 12-26-2016, 10:48 AM.
                Location: Saskatoon, Saskatchewan, Canada

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                • #9
                  Originally posted by brian Rupnow View Post
                  No, I stand by what I said in my first post. Read the post slowly enough that the words "atmospheric intake valve" registers. An atmospheric intake valve (which has no cam to operate it) won't begin to open until the exhaust valve is fully closed and the descending piston begins to create a vacuum in the cylinder. That is when the intake valve opens. If the exhaust valve stays open after top dead center, the descending piston just pulls air thru the exhaust valve and the intake valve doesn't begin to open until AFTER the exhaust valve is fully closed. Normal 4 cycle engines with cam operated valves on both intake and exhaust definitely do have an overlap, on both valves both at top dead center and at bottom dead center. Engines with atmospheric intake valves gain no benefit at all from having the exhaust valve closing after top dead center.
                  if the exhaust was well designed there would be enough depression to open the atmospheric valve, for a short period before the decending piston has enough vacuum, maybe just something simple like a little extra length of the exhaust pipe might help create the depression, this would give a longer filling period, maybe putting an intake runner onthe inlet will help with cylinder filling as well
                  My neighbours diary says I have boundary issues

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                  • #10
                    Originally posted by brian Rupnow View Post
                    No, I stand by what I said in my first post. Read the post slowly enough that the words "atmospheric intake valve" registers. An atmospheric intake valve (which has no cam to operate it) won't begin to open until the exhaust valve is fully closed and the descending piston begins to create a vacuum in the cylinder.
                    A correctly tuned exhaust pipe and cam will create a partial vacuum and will initiate earlier and or better flow regardless of valve actuation mechanics. Again, that's old info and not very complex Physics.
                    Last edited by Magicniner; 12-26-2016, 06:11 PM.
                    If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

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                    • #11
                      In support of what Brian it trying to say these are hit and miss engines with atmospheric intake valves. I have never in my life seen an old farm engine with a tuned exhaust, generally a piece of 1-1/4 black pipe or a muffler with a flat face for the exhaust gasses to bounce off of prior to exiting through peripheral slots. That being said I don't think there is much exhaust scavenging going on, at least not enough to start sucking the intake valve open prior to the piston going down the bore and creating a negative pressure in the cylinder. It seems like the responders are all familiar with mechanically operated valve systems and what has been said it correct but we're talking engines that began and ended their day running anywhere in the range of 400 to 800 rpm and some of the smaller ones on their best day might have seen 1000 but I highly doubt it.
                      gbritnell

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                      • #12
                        Brian stated that exhaust valve timing and exhaust tuning would not work with atmospheric valves.
                        A search of this thread shows that your is the first use of the character combination 'rpm'.
                        Reed valves on two-strokes are "Atmospheric".
                        If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

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                        • #13
                          You're correct. Reed valves open by the negative pressure in the crankcase of the 2 cycle engine but aren't made for quite the same purpose. The reed valves are to prevent spit back as the piston is on it's down stroke thus reducing the fuel/air mixture to the transfer ports. Prior to reed valve use the piston ported 2 cycle operated this way and is still in abundant use in small utility engines.
                          The first event after combustion in the 2 cycle engine is the piston passing the exhaust port. At the same time the crankcase is being pressurized by the downward stoke of the piston. Now the piston uncovers the transfer ports and the pressurized crankcase pushes the mixture into the area above the piston, and somewhat out of the exhaust port (scavenging) but the reed valve at this point is closed. As the piston starts it's upward stroke it creates a negative pressure in the crankcase and pulls in a fresh charge through the inlet tract and the reed valves. If there is any exhaust gas flow strong enough to pull the reed valve open it's only due to velocity and rpm the later which is lacking in a hit and miss engine.
                          gbritnell

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                          • #14
                            It's an interesting discussion - there is still influence and Im sure there is a "sweet spot" to be had, but the engines are turning so slow and also the intake is atmospheric so "normal" overlap with the exhaust would probably not be "key",

                            might be closer to TDC than one thinks and yes comparing it to the reeds in a two stroke which in effect is "kinda" supercharged form the crankcase is not an ideal comparison,

                            My take on it is you both have a good argument,,, and there is a reason to stick closer to your guns than normal at the top of the expel stroke of the exhaust - however, even at very slow speed there might be a slight factor of overlap to be had...

                            as far as Brian's assessment about the Webster needing more duration due to it's RPM's he is correct, even though it has an atmospheric intake you have to start worrying about getting the exhaust flow out ahead of time when things are moving more quickly - so before BDC the exhaust should be opened, how much is depending on ideal RPM range,

                            General rule if you have the ability to control both the exhaust and intake valve and also have the added benefit of them being segregated (DOHC) is when the RPM running range goes up - You not only increase BOTH valves duration you also throw most of your duration at advancing the exhaust valve and retarding the intake, and fact is - is that overlap only takes up a small percentage of that total duration increase,

                            been around dynamometers most of my early years with formula engines and seen the hands on results.

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                            • #15
                              Another quick note --- the intake valve of most any IC engine is the dominant force --- so even on a SOHC if you want to make a little more power in the higher RPM range and you don't have the bennie's of being able to separate the valves the method is to retard the cam timing - your robbing peter to pay paul but paul with still give a larger dividend in the end,

                              Exhaust gasses will find their way out, getting the power charge into the engine in the first place is the dominant factor...

                              But again ideally DOHC rocks if you have a mean of adjusting them... now you have the best of both worlds, well not quite,,, VVT is making this all obsolete --- pull a stump out of the ground down low and come on like a race horse in the upper RPM range...

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