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brian Rupnow
12-25-2016, 12:09 PM
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
http://i307.photobucket.com/albums/nn294/BrianRupnow/KERZEL-WEBSTER%20CAM%20COMPARISON_zpson6j9jsm.jpg (http://s307.photobucket.com/user/BrianRupnow/media/KERZEL-WEBSTER%20CAM%20COMPARISON_zpson6j9jsm.jpg.html)

Cuttings
12-25-2016, 06:26 PM
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.

Errol Groff
12-25-2016, 09:08 PM
Very interesting conversation. Thank you for taking he time to type it and sharing.

Errol Groff

fixerdave
12-25-2016, 10:09 PM
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...

boslab
12-26-2016, 06:40 AM
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

Magicniner
12-26-2016, 06:54 AM
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

brian Rupnow
12-26-2016, 09:15 AM
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.

Arcane
12-26-2016, 10:41 AM
........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.

Yow Ling
12-26-2016, 05:46 PM
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

Magicniner
12-26-2016, 06:09 PM
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.

gbritnell
12-26-2016, 07:14 PM
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

Magicniner
12-27-2016, 07:19 AM
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".
:D

gbritnell
12-27-2016, 08:30 AM
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

A.K. Boomer
12-27-2016, 11:19 AM
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.

A.K. Boomer
12-27-2016, 11:27 AM
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...

Cuttings
12-27-2016, 01:24 PM
Ive been away for a few days so thought I should stop by to see how this discussion was going. Very good I would say.
I have something else to throw in the mix. Most of you probably remember the screw in little mufflers that Briggs and Stratten used to use on their 3HP engines that were used on almost everything from lawn mowers to rototillers etc. I had a lawn mower with one of those and the muffler was getting burnt out. The engine was starting to pop and fart quit a bit and was starting to get a bit loud so for about $5 in those days I bought a new muffler to stick on it. Immediately it ran perfectly again. I guess it needed a little exhaust back pressure to run properly. I never did take the time to think it out to figure out why, but used the same thing a few times after that when they started to run poorly.

A.K. Boomer
12-27-2016, 02:30 PM
You bet and good observation, engine cam profiles and duration engineered with flow dynamics that have a factory exhaust system with copious amounts of restriction can fall on their face if this restriction is removed and of course same goes for ones with huge flow then getting restricted...

On a single cylinder you don't have the advantage of another cylinder providing scavenging for you at a collector unit down the road but they can still benefit from this effect with the length of free flowing pipe before the unit goes into a cat or muffler or some other restrictive unit,
length of pipes before collectors are VERY critical on where you want your "pipe" to come on - as in start to pull good, and keeping the pipes all equal lengths is critical too - hence the name "tuned headers" resulting in mild bends here and there to eat up space and make the shorter runs as long as the long ones, also makes for some funky looking pipes to people who don't know the reason "Why"

The Artful Bodger
12-27-2016, 03:13 PM
Exhaust flow is worthy of careful study in a hit and miss engine. Not only is the power effected by exhaust dynamics but perhaps more significant is the exhaust flow during the miss cycle where friction in the exhaust flow contributes to the slowing of the engine between power cycles.

Cuttings
12-27-2016, 07:45 PM
I would think this is quit critical on the Briggs, because when you remove the muffler from the engine the exhaust valve is right there.
No muffler or a burnt out one would cause no restriction at all .
Brian - I was wondering how much trouble it would be to try a little muffler on your hit and miss engine?

brian Rupnow
12-27-2016, 08:04 PM
Cuttings--I took a little muffler off to put the vertical pipe on.

Cuttings
12-27-2016, 10:35 PM
Sorry, I guess my old memory had erased that bit of information.

gbritnell
12-28-2016, 10:24 AM
A hit and miss engine is a very rudimentary internal combustion engine. It came in sizes ranging from a couple of horsepower to upwards of 100 or more. They were used to power everything from ice cream makers to oil field pumps.
Although the production lasted into the 40's they still kept the same mechanical exhaust, atmospheric intake configuration. The reason being this mechanism was used to control the engine speed. At the time there were many engines using cam controlled intake and exhaust breathing but these engines had some type of throttle control.
Ok so you say that you could still use this type of control on a mechanical valve engine, yes but what would happen. With the hit and miss control being used on the atmospheric intake engine the intake valve won't open until the exhaust closes and the piston creates a negative enough pressure in the cylinder to draw the valve open. If it were used on a mechanically operated intake valve engine when the engine went into it's coast or non-power cycle the intake would be opening and closing and therefore would draw fuel into the cylinder. Mind you not as much as if the exhaust were closed but enough over several coast cycles to flood the cylinder. Now when the exhaust valve was released the engine would have to fire all this extra fuel, maybe yes but quite possibly no. You have to consider the fuel and ignition systems of the era.
The simplicity of the system is quite ingenious not counting the fact that a different cam would be needed along with an extra pushrod and guide and more space required to install them.
There had to be some merit to the design because of it's longevity and the vast number of engines produced by so many manufacturers.
These engines started out as basic power sources and that's all they ever were. In most cases they were quite reliable and very tolerant to the environment they worked in considering they had no air filters, were hand lubricated and lived in dirty conditions. They were truly inefficient as far as internal combustion engines go so I'm sure there wasn't much thought about exhaust characteristics and flow numbers. I can't think that some engineer at John Deere in 1912 said to his boss you know if we put a tuned muffler on this baby we'll really get some power out of it.
Just my two cents worth.
gbritnell

A.K. Boomer
12-28-2016, 11:17 AM
Well said GB - they served a purpose and did things better than other engines of the same era due to conservation of fuel,

sometimes the oil wells produced gas too and they would just run off of that so not that much of an issue - but other times fuel had to be brought in and the engines would run on either propane or gas, they were misers and if properly designed and tuned were tough to beat that way.

J Tiers
12-28-2016, 11:32 AM
Yes, pretty much stone hammer simple. And they needed to be, because they were operated by farm folks to whom the engine was extreme high technology.

In other cases the motor would be operating out in an oil field for weeks without attention other than maybe some lube every so often. I recall seeing them still operating on "grasshopper" pumps when I was a kid.

It's not as if the state of motor development was so rudimentary, they were still being designed as hit and miss at a time when relatively sophisticated aircraft engines were being made, and Lindbergh flew to Paris using one of them*. So there were reasons for making the engines as they were made. Simplicity, ruggedness, easy repair, etc.

Now, the full scale engine may be significantly different from a model. Scaling down reduces masses by a cube law, but dimensions only linearly, so many things change relation. A perfectly scale model, or scale motor that is not a reproduction of a real motor, may need many things changed in order to operate well.

The start of the whole issue here was to get better slow speed hit and miss action, something not improved by the cube law reduction of mass. The flywheel dimensions may have to be larger to store enough energy to get over compression etc, and the engine as Kerzel designed it and Brian made it does have fat flywheels, as one would expect. If directly scaled from a full scale engine, the engine likely would have to operate much faster.

* as an example of general technological thought at the time, the field effect transistor was described and analyzed in 1929 by a Frenchman, but the means of making it physically was not developed until much later. And the jet engine as well as liquid fuel rockets were not far off. Not a dark age of technological backwardness in any way.

BCRider
12-28-2016, 01:32 PM
A couple of thought occur to me on the timing.

If there's anything that is not good it would be running too short a timing that results in a late opening point and rounding BDC with a lot of pressure still in the cylinder. That's a real no-no in any discussions on engines I've read. It would both rob power and creates unnecessary load on the various bearing points. Even a valve that just starts opening right at BDC will see a lot of pressure still acting on the piston for some time after BDC until the valve opens up far enough for long enough to let the pressure out. These and more is why I've always seen any engine timing start opening the exhaust some amount before BDC is reached so that the cylinder pressure can start to leak down before BDC so there's less pressure robbing power away as the piston starts the upward exhaust stroke. And again even at the slow speeds we run on these model engines there's likely much to be said for starting to open at some angle up to perhaps as much as 20 before BDC to ensure that as the piston passes BDC there's only some residual pressure which is in the process of dropping fast.

On the timing for closing there's some issues to consider too. Again there is very little linear piston travel during the 10 of rotation either side of TDC. So there is nothing wrong with a cam timing that finishes closing the exhaust a few degrees after TDC. On the other hand a cam that closes the exhaust a little earlier than TDC will also be in the process of closing for quite a few degrees leading up to that point and producing a restriction in the flushing of the residual exhaust. So a cam that closes the valve right at or at all before TDC can result in some slight compression being present as the piston moves towards TDC. And any residual pressure will prevent the intake from opening until later in the intake stroke. In addition there's a slight braking action due to closing the exhaust too early so that some slight compression is created. So all in all I feel that even in a slower turning engine like this there's some advantage to an exhaust cam timing which finishes closes at something around 3 to 6 AFTER TDC.

If you like what you read then clearly the original Kerzel cam fails due to having only 168.4 of total duration. On the other hand the Webster cam with 234 of total duration "might" be a little longer than ideal for the Kerzel engine. If we add up the 180 of the exhaust stroke with, say, 5 of overlap at TDC and even 20 before BDC we only have 205 that my guts say would be good. And in fact if the Webster cam was set to close at 5 ATDC as I feel is a good thing then it would be starting to open the exhaust at only 131 ATDC on the power stroke. That's not bad but it's opening up a lot earlier than is really needed for slow speeds such as on a hit n'miss engine. And as a result it wastes about 1/10 of the power stroke give or take. (note that the last couple of sentances are heavily edited from the original which was based on bad math-BCR)

So it would seem that even by armchair analysis that neither of these cams is at all optimum. You've got the "too warm" Daddy Bear's profile on one hand and the "too cold" Mommy Bear's profile on the other. What you need from where I'm sitting is the in between Baby Bear's solution of something around 195 to 215 total duration which provides whatever dwell is created in the process. Where to fit it in this range? I feel like with a closing point of 5ATDC that a 195 total duration would be a little tight with only the last 10 BBDC to leak off some exhaust. 205 would be better. And certainly a total duration of 215 would see the pressure all but bled off as the piston starts upward. Again this is my armchair engineer's view. But from where I'm sitting it seems like anything in this range would be better than either of the other two.

BCRider
12-28-2016, 03:51 PM
I was just checking in to see if there were any replies and was reading over my post. My math on the opening point of the Webster profile was WAY off. For some odd reason I subtracted the 49 excess of the Webster from only 90 instead of 180. So the 234 cam would not open at 41 as I posted but rather at 131 after TDC. So that's not bad at all and certainly not the sort of train wreck I made it out to be.

I've corrected my post above.

The Artful Bodger
12-28-2016, 04:07 PM
I guess I was one of those farm folks as we had a hit and miss engine to drive our sheep shearing plant. It was quite a small engine and was very reliable perhaps because it was only run for three weeks each year.

Unfortunately I do not have access to a H&M engine nowadays but if I did I would rig up a camera and strobe and try to get an idea of when the inlet valve actually opens and does it always fully open.

The Artful Bodger
12-28-2016, 04:49 PM
Thinking further (while wondering who has a H&M around here!) I am thinking that a solenoid operated inlet valve might make an interesting experiment.

The inlet valve could be opened at the very start of the induction stroke instead of waiting for cylinder depression to overcome the spring and open the valve which would stay open until the very last point of the induction pressure differential rather than have the spring start to close the valve before then.

BCRider
12-28-2016, 05:34 PM
Thinking further (while wondering who has a H&M around here!) I am thinking that a solenoid operated inlet valve might make an interesting experiment.

The inlet valve could be opened at the very start of the induction stroke instead of waiting for cylinder depression to overcome the spring and open the valve which would stay open until the very last point of the induction pressure differential rather than have the spring start to close the valve before then.

Or we could run BOTH valves on solenoids and skip the cam altogether. Or we could do what was done on the follow on designs and use two CAM controlled valves..... :D

Part of the beauty of the vacuum inlet is that it only opens when the exhaust closes and a vacuum is produced. If a solenoid run valve opened each time instead of only when it was needed we'd get a lot of spitting back through the carb. So the solenoid would need its own cam sensor, interrupter and other "stuff". Simpler at that point to skip the solenoid, power source and other junk and just run the inlet off a second cam lobe.

In the end the H&M is what it is and unless we are interested in some updated Steam Punk variation is likely best kept simple... at least to my thinking.

Mind you a jazzed up Steam Punk variation to the whole design might be a LOT of fun!

Lew Hartswick
12-28-2016, 05:38 PM
Thinking further (while wondering who has a H&M around here!) I am thinking that a solenoid operated inlet valve might make an interesting experiment.

The inlet valve could be opened at the very start of the induction stroke instead of waiting for cylinder depression to overcome the spring and open the valve which would stay open until the very last point of the induction pressure differential rather than have the spring start to close the valve before then.
I've been thinking this very thing for the last half doz. pages of this thread. NOTE! I'm not a model engine builder nor am I likely to ever be one. So just from the stand point of an observer, I think it would be an interesting thing for "Brian" to do. :-)
...lew...

The Artful Bodger
12-28-2016, 05:54 PM
No need to leave all the fun to Brian if you have a H&M do some experiments and try to advance the science just a little at least.

BCRider
12-28-2016, 05:54 PM
...... I think it would be an interesting thing for "Brian" to do. :-)
...lew...


Yep, no pressure there.... nope... none at all.... :D

Thanks for the good laugh when I read that part :)

wombat2go
12-28-2016, 09:44 PM
My B&S 280000series has burnt its starter motor contactors over its life. ( it is about 16 years in service !)
About 8 years ago i learned on internet that the valve lash has to be set within spec in order to decompress during starting.
So now I set the lash every year or so to in spec ( both 0.13mm =0.005 inch by the B&S maintenance sheet)
That certainly helps the starter to crank over in 20 Farenheit, , but both rockers wear loose every year.
There is no liquid oil up in the rocker box.
I would appreciate any comments on how the decompression works, about the lack of oil in the rocker box,
and,... "how much longer will she last" ?
Thanks

Cuttings
12-28-2016, 11:03 PM
Some good points there BC Rider.
One thing to keep in mind. Gasoline is a fast burning fuel, so the majority of the expansion takes place shortly after ignition. I don't know but I would guess that there is not much pressure left once the piston gets to the lower end of it's travel. So opening the exhaust valve early should not affect the operation much.
Only my observation - no facts to back it up.

wombat2go
12-28-2016, 11:38 PM
Some good points there BC Rider.
should not affect the operation much.
Only my observation - no facts to back it up.

Search " Otto Cycle " for pressures and efficiencies.
One ref here is: Eastop and McConkey: "Applied Thermodynamics for Engineering Technologists"

J Tiers
12-28-2016, 11:51 PM
Some good points there BC Rider.
One thing to keep in mind. Gasoline is a fast burning fuel, so the majority of the expansion takes place shortly after ignition. I don't know but I would guess that there is not much pressure left once the piston gets to the lower end of it's travel. So opening the exhaust valve early should not affect the operation much.
Only my observation - no facts to back it up.

More to the point, the piston does not move much for the last bit of rotation toward BDC, so the mechanical advantage is horrible, and there is little lost in terms of power from opening during the time the piston is not moving much, So if there is any advantage to be had from early opening, you may as well go for it.

A.K. Boomer
12-29-2016, 12:53 AM
Good understanding of it JT, and I might add the benefits of proper evacuation go beyond just that stroke esp. when higher RPM's are engaged, now the lower part of the stroke is really piss poor - so cracking the exhaust valve open way ahead of time and giving the gasses a head start and ALSO getting them moving in a fury will help the next exhaust stroke evacuate better esp. in multi-cyl. engines with header/collector systems,

you can see how this snowballs, now throw in the fact that many intake manifolds are also tuned to come on in a specific range,,, it's possible to exceed atmospheric pressures "stacking" up behind the intake valves backside in a pneumatic ram effect, couple this to what's going on in the exhaust side of it and there is allot of thought that goes on in normally aspirated engines that put out some respectable HP's

for back in the day the B-16 honda V-tec was the highest HP per CC car engine ever in mass production... they got the job done by going from a two valve per cylinder down low - to opening up all four at higher RPM's

4 valve heads actually flow too good down low, so if the cams profile is somewhat radical the gasses lack the momentum and will lack allot of previous mentioned details...
by creating a 2 to 4 valve head you can have some very radical profiles and not have to pay the price of the engine falling on it's face down low, but up high watch out, now your cooking,
I just have the D 1600 V-tec it only controls the dominant valve - the intake - exhaust are two valve throughout the range,,, My computer controlled hydraulic actuated intakes go from 1 valve to 2 @ 4,700 rpm's

I think the B-1600 actuates at 5,200 it has a redline of 8,200 rpms and stock created 160 hp out of 1600cc that's back in 1993 and very easy to get 200 out of them with just a few bolt on components...

BCRider
12-29-2016, 01:41 AM
More to the point, the piston does not move much for the last bit of rotation toward BDC, so the mechanical advantage is horrible, and there is little lost in terms of power from opening during the time the piston is not moving much, So if there is any advantage to be had from early opening, you may as well go for it.

Eggzactly.....



Cuttings, you mentioned that the burn occurs quickly. And yes, that is certainly the case. But the chamber is starting on the compression stroke with nearly a full stroke worth of fuel and air because the intake valve let's it fill as much as it likes with only a slight reduction from ambient pressure. And after it burns there's more volume thanks to the heat and production of the gaseous byproducts. So there would still be a pretty fair degree of pressure over ambient at the bottom of the stroke if the exhaust valve didn't open early enough to vent it down close to ambient before the piston reaches BDC.

This and the reasoning noted above about the rotational and linear combinations is why there's little to be lost and much to be gained by opening early. How early? I commented above that I was spouting armchair engineering. And that is the case. But I can't help but feel that the Webster cam is likely opening a touch earlier than it needs to open. Which is why I was suggesting the other range of values.

I'm pretty sure though that the Kerzel timing is NOT a good profile. At least not in terms of a free rotating sort of timing. Carrying the pressure past BDC with a valve that does not open until well into the exhaust stroke is not only inefficient use of the pressure but it's also going to be harder on the bearing surfaces of the piston pin, conrod and the entire lower end. Either that or we need to let the exhaust open a little before BDC and then close well before TDC. In which case it'll compress the chamber for the last portion of the rise and the inlet valve won't be drawn open until the complementary angle on the other side of TDC. So we'd have an overly short intake period.

But perhaps that is the intent? It would be nice to talk to the designer and see what he had in mind with that short a timing. And where it was intended to be centered and the resulting open and close times.

The Artful Bodger
12-29-2016, 02:14 AM
We should bear in mind that in the hit and miss engine an induction stroke does not immediately follow the exhaust stroke except when starting and under full load.

For a 'nice' model we want an engine that will consistently miss for a few cycles.

BCRider
12-29-2016, 02:20 AM
That's true. We only get an intake stroke after the exhaust valve is allowed to ride on the cam through the pushrod.

But it stands to reason, or at least seems so to me, that we still want good efficiency from that one power stroke so it speeds the engine up enough to coast for a good number of coasting revs.

wombat2go
12-29-2016, 04:01 AM
But it stands to reason, or at least seems so to me, that we still want good efficiency from that one power stroke so it speeds the engine up enough to coast for a good number of coasting revs.

With hybrid cars, the hit-and-miss energy storage idea is back.
Last week I was in Osaka, travelling in the latest Toyota Prius station wagon ( I forget the name),
about size of a mid range suv in USA.
The consumption was about 23 kilometre/litre over the week. ( approx 54 US miles/gallon )
The gasoline motor could certainly be heard starting and stopping.
The owner was pleased with the car, the only downside she commented was that it was a bit slow
pulling up the ramps to the freeways around Osaka.

A.K. Boomer
12-29-2016, 10:26 AM
There's so many factors to consider when trying to dial in any engine - even the slow moving ones,

bore and stroke can radically change the way you want an engine cam'ed,
long stroke and small bore creates greater piston speeds and more of a pressure "stall out" at at the bottom of the stroke, so even a lower speed engine may benefit greatly by getting the gasses on their way before BDC by at least a little amount.

compression ratio's and porting play just as large of a role,

what's incredible is how well NA engines work at higher RPM's given the fact that air has mass and it does take it time to get from point A to point B,
at your normal shift point of a smaller auto-engine the exchange between expelling the spent gasses and getting the new charge in is happening at a rate of 1/100th of a second! that's 1/200ths of a second to either evacuate the chamber or re-fill it,
You can see why duration becomes very beneficial due to the delay factor, you can see why even though an intake valve is left open LONG after BDC the fuel and air mix will continue to rush into the chamber even though the piston is actually traveling back upwards a great deal...
It also explains the larger overlap period, spent gasses still leaving in a fury out the exhaust even though it's at the end of it's cycle well after TDC and yet the intake got cracked open before TDC - with a pneumatic ram effect behind it and the exhaust well established and still dragging everything out with it - it's all just pulsating flow dynamics in delay and at work trying to play "catch up"

J Tiers
12-29-2016, 11:39 AM
A lot of that optimizing is fairly irrelevant to an engine at low speed that is essentially supposed to "putt-putt" nicely and is not really expected to work for a living.

The primary requirement is for it to "hit" once, and coast many....

So, yes, it needs to accelerate well on one hit, or at least to add enough energy to the flywheel in one hit to actuate the governor. As a practical matter, that means accelerating, since the governors are not very precise. So the lightest flywheel setup that gets it through several revs with enough energy to go over compression for the power stroke is the best. It seems that Brian's has that, as more mass did nothing useful.

But that is not a guarantee, because there is the other variable of RPM. A lighter flywheel may need to run faster to get over compression. Lightening may just require an increase of rpm.

The real issue is the ability of the engine to speed up the flywheel sufficiently on one hit. In many ways, the RPM is not too relevant so long as it hits and coasts with reasonable timing.

Some of that may be a design factor, a short stroke is not really good for that, a long stroke seems likely to be better. Long stroke tends to be slower rpm, which is what most hit and miss full size engines are. My Jaeger is rated something like 600 rpm at full power. That's a slow idle for many engines. I'd have to measure or look it up, but IIRC the Jaeger (Hercules model E) has a stroke that is considerably longer than the bore.

I do not recall what the Kerzel is as far as bore and stroke. It is known that they CAN do a very creditable hit and miss.

Cuttings
12-29-2016, 11:44 AM
I just ran across an interesting article about valve timing on a hit and miss engine with atmospheric intake valve.
http://www.oldengine.org/members/czerwick/timing.pdf
In step 6 I think there is a mistake and should read bottom dead center instead of top dead center.

BCRider
12-29-2016, 02:41 PM
Re step 6;

I got the same impression. But if we forget about following the rotational direction and are simply looking for the points of valve movement then it makes sense. He's aiming for the valve to either start or finish at TDC depending on which way you're jogging the engine rotation. But the key being that he wants the last/first motion to occur at TDC. Then as a secondary thing he's saying to check to see that the motion starts at or really near to BDC as a check that the cam's dwell is centered through that full up stroke. The check for movement right at TDC is very like what Brian described in post #1. But it sort of assumes that the lobe duration is long enough that the mirror movement will occur at or very close to BDC. Other than that it doesn't consider the cam's duration at all other than to infer that it should have a duration of pretty well 180.

The Artful Bodger
12-29-2016, 03:12 PM
Hopefully the exhaust duration is a lot more than 180 degrees extending over several revolutions of the miss period.

Which brings up another point, the cam turns once for 720 degrees of crank rotation and depending upon such variables as load, temperature and the phases of the moon the engine must coast up to two revolutions between the governor tripping and the beginning of the hit cycle. This variable delay may explain why the engine must hit twice sometimes which tends to spoil the effect of a H&M on display. Maybe there is some practical means of stopping the cam during the miss cycle and reengaging at the appropriate point on the next revolution rather than having to wait for up to two revolutions for the next cycle to begin.

BCRider
12-29-2016, 03:36 PM
Bodger, there are so many different designs for H&M engines out there that I have no doubt that at least one of them used a method that interrupted the cam in the max opened position for coasting then caught the cam when a firing cycle was needed. I'm sure someone will be along shortly with a name of such an engine.

The hit and miss times are not a precise sort of thing because the action of the flyweights are not tied to the rotational timing of the engine. So yeah, often the engine will fire twice in a row and that alters the coasting times. But that is just part of the enjoyment. If they sounded too much like a metronome they would not be as much fun. They are just way more fun when they have a more jazz like syncopation to their exhaust.

Cuttings
12-29-2016, 10:51 PM
My memory is not too clear on this but I do remember the old double-ended fish boats with an Easthope hit and miss engine. They would take a few chugs then it seemed like quit a long time before they cut in again and gave a few more chugs. There must have been a fair RPM difference between cut out and cut in. On those old slow turning engines that may not have been a lot.

BCRider
12-30-2016, 01:34 AM
You got me looking up the Easthope engine company. I had no idea that this was a local Vancouver BC company. All that history right in my own backyard!

Paul Alciatore
12-30-2016, 03:31 AM
Brian, I confess that I did not read all the responses, but I do not see any drawings of the follower(s) that you are using with these cams. Unless the follower has a sharp point that rides on the cam, the shape and width of the follower will have an effect on the overall action. You can not analyze the action of a cam with just the shape of the cam. For instance, your first drawing shows a flat surface for the rise and drop of the follower and they each occupy a 37 degree angle. But you can not assume that the follower just follows this profile as the edge of the dwell area will hold it open for some time into that 37 degrees on both the rise and fall. And there could be other effects due to the shape and width of the follower.

Perhaps you are discounting them, but they are there.