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Skinner/Fuller valve gear.

Posted by sidrug 
Skinner/Fuller valve gear.
September 04, 2014 03:42PM
How can the lash be adjusted in this type of two-cam valve gear?
There is a clearance (gap) in the valve train at some times in the cycle.
So, hydraulic lifters will try and compensate for that gap, and thus not seat the valve completely.
Any good ideas for self-adjusting valve gear?

Also, to our previous discussion on turbine drivetrain for a piston / turbine compound, here is a neat CVT design for a supercharger.
Re: Skinner/Fuller valve gear.
September 04, 2014 04:30PM
Sidrug,

Sure. By putting shims under the roller followers and the valve stems like everyone else does..
Or use rockers and then that pivot shaft bearing is an actual eccentric like the OHV Cadillac V-16.

A CVT between the secondary turbine would work IF it could stand the torque for thousands of hours, be really small and easily packaged.
The variable inlet nozzle vanes do the same job of matching loads at nearly all speeds. A power recovery radial inlet turbine with such nozzles has a nice operating range while maintaining good efficiency.

Jim
Re: Skinner/Fuller valve gear.
September 04, 2014 05:32PM
Good points. But I thought most everyone else was using hydraulic lifters nowadays, due to wear, noise and service intervals?

You don't happen to have some variable nozzle turbine numbers, do you? The ones I have seen have been pretty bad outside of a not that wide range IIRC.
Re: Skinner/Fuller valve gear.
September 04, 2014 06:38PM
nsds_turbine_chart.pdf

Sidrug,

Nothing says you can't use hydraulic lifters as long as you can supply them with pressurized oil all the time. That begins when you hit the starter in a car. With the steam engine stopped when the car is stopped, no oil so what are you going to do now? Hydraulic lifters, or the more modern compressed air springs, could also be used. Or just forget trying to use them and just provide an adjustment screw, zero valve lash is just not needed.
Remember, unlike a gas engine, a steam engine temperature doesn't keep going up and up until something melts, It does not exceed something much lower than the inlet steam superheat temperature and remains rather stable as long as the cylinder block is well insulated. So once the thing is heat soaked, it goes no further and you set your clearances for that condition.
Scott has it right how hydraulic lifters operate.

Turbines are designed for the use they are to be put to. The Barber-Nichols chart says it all. (attached)
You choose your inlet and possible outlet conditions, the flow rate range in cfm, the hoped for rpm range and design what gives you the expansion ratio you want. Radials are extremely adaptable.
No one yet has used a radial inlet turbine with variable inlet nozzle vanes as the second stage expansion for a compounded steam engine that I can find. They used two stage impulse, we suspect because back then radials were black magic. Only when gas turbines got the nod and were developed did anyone pay attention. Click on Barber-Nichols, then on radial turbines a read their stuff, short, sweet and to the point. There are radial design books available if you are real good with three dimensional flow math.

Jim



Edited 3 time(s). Last edit at 09/05/2014 12:07PM by Jim Crank.
Re: Skinner/Fuller valve gear.
September 04, 2014 07:11PM
But, how can the hydraulic lifters work - they adjust the lash when the valve is seated and the roller is rolling, by filling in with oil until full.
The Skinner gear has three states -
valve open, both cams are tall
valve closed, one cam is tall and the other low
valve closed, both cams are low. There is a gap now, somewhere in the valve train.

In the latter state, the hydraulic lash adjuster would be overfilled, and next time one cam went tall, the valve would become unseated.
Am I wrong about this?

That n>0.8 is something to hope for, but again the numbers I saw for BorgWarner BV40 was a bit less enthusing.
Light text and charts.
heavier text with more charts
Re: Skinner/Fuller valve gear.
September 04, 2014 07:31PM
sidrug,

Hydraulic valve lifters have a controlled leakdown port built in. Oil pressure removes the space that makes noise. As the lifter is raised by the cam lobe, the leakdown port is covered, no more oil is bypassed, and the lifter (for all intents and purposes) becomes solid.

[en.wikipedia.org]
Re: Skinner/Fuller valve gear.
September 05, 2014 05:57PM
It seems I am not getting across here. What does the hydraulic lifter do when there is a gap in the valve train?



Edited 1 time(s). Last edit at 09/05/2014 05:58PM by sidrug.
Re: Skinner/Fuller valve gear.
September 06, 2014 11:26AM
It takes up clearance in the valve train using the oil pressure as hydraulic fluid. See [www.answers.com]

lohring Miller
Re: Skinner/Fuller valve gear.
September 07, 2014 03:29AM
Indeed, that's what it does. And the Skinner gear intentionally makes a gap in the valve train every revolution. I'll see if I can't find some drawings.
Re: Skinner/Fuller valve gear.
September 07, 2014 06:38AM
Scott has the answer if you want hydraulic tappets but I agree with Jim, why bother? Modern large diesels as found in trucks and tractors make do with mechanical adjustment. Consideration in the tappet design is required to allow valve operation when there is no oil pressure, or an auxiliary oil pump will be required.
For the Skinner arrangement increased pretension would be required so that the valve doesn't lift with just one cam. From some of the drawings it seems 12 to 20 thou slack is allowed before the hydraulic tappet comes into operation, for a Skinner this would have to equal the lift of one cam before the second comes into operation.
I have attached some sketches of a basic arrangement. There are many solutions for hydraulic tappets and I have shown a basic form without one way valves etc.

This is a good article about the pitfalls of hydraulic tappets. [egaa.home.mindspring.com]

Brian


Re: Skinner/Fuller valve gear.
September 07, 2014 07:01AM
Thanks, brilliant. Will read.
Re: Skinner/Fuller valve gear.
September 10, 2014 07:02PM
One important consideration is missing from this discussion. There is a practical limit determining the velocity which a valve touches its seat. There are so called ramps ground into the cam profile to control that velocity, which, of course, is highest at the highest rpm. Some valve & seat are addressed after every race, others are designed to close gently to last forever.

With steam engines the max rpm is low and therefore the "ramp" can be steep. More importantly, steam engines love sharp cut off. Steep ramp minimizes the change in real cut-off With the slow-running engines, there is no rump at all and the valve drops "like rock" hurting nothing of course at the choo choo speed.

We do want to close the inlet valve sharply. Lash is good. There is absolutely no need for hydraulic lifters unless one operates the valve(s) hydraulically in the first place.
Stan Jakuba.
Re: Skinner/Fuller valve gear.
September 11, 2014 02:42AM
Attached is the cam profile of a three cylinder Bryan uniflow auxiliary engine.
The engine ran around 2000 rpm.
I never saw one run, just copies of the drawings.
I always thought it would be a nice engine to make as a model.
Rolly


Re: Skinner/Fuller valve gear.
September 11, 2014 07:11AM
Guys, what about noise? Truck and bus and lorry engines are LOUD!!!
Re: Skinner/Fuller valve gear.
September 11, 2014 07:53AM
I know what Stan means. I'd getting ready to try cutting an admission cam for a steam engine. It is designed with a cycloidal lift profile and a maximum of 500 gees acceleration at the valve stem. I'm trying to use a spreadsheet to fit tangent arcs with least squares to the lift profile so that I can cut it on a conversationally programmed Bridgeport with connecting arcs rather than plugging in a huge number of data points. Trying to whittle down some of the second order discontinuities so as to minimize jerk is turning out to be straining my limited math skills.....

Piston valve gear design is quite a bit easier, but it just can't do the same things.

Ken
Re: Skinner/Fuller valve gear.
September 11, 2014 03:13PM
Poppet valves are noisy in comparison to piston or slide valves.
Sentinel successfully used poppets for all their production undertype steam waggons and here are some videos of the last type. Sounds like a diesel.
[www.youtube.com]
[www.youtube.com]

And another of my poppet valve Bolsover simplex steam car engine.
[www.youtube.com]

Brian
Re: Skinner/Fuller valve gear.
September 11, 2014 05:56PM
Piston valves:
Perhaps someone will eventually sort them out, their problems are like those of the main pistons: Controlling leakage without too much friction, cooling without too much heat loss, and lubrication. Unlike the main pistons the pressure and temperature is maximum where the rings are passing the ports.
For minimum cutoff and gees, I think the piston valve gear should run at half the engine speed, the ports passing each other at the moment of their maximum speed. Not sure whether there is a reasonably simple mechanical linkage that could adjust it between 80% and minimum cutoff. Maybe that electrical gear Andy has shown is the better way.

Back to poppets:
Wish I could help with the math Ken, but I don't even understand the problem. Maybe you can write some more about it, would much like to learn.
Seems like your Bridgeport has some kind of splining in its controlling hardware, that you want to adapt your cam shape to? Or does it have anything to do with the resolution of the machine?

Brian,
the first one really does sound like a diesel almost. The second one sounds better on the video. But, light diesels and gas cars are much more silent than trucks, given a lot of attention to the problem I guess. Maybe the hydraulic lifters are part of the reason?


Re: Skinner/Fuller valve gear.
September 12, 2014 03:32AM
Lubrication is another issue even with single acting engines, Serpollet and Sentinel injected oil into the steam to lubricate the valve stems. Oil or water injection into the valve guides has been suggested.


Brian
Re: Skinner/Fuller valve gear.
September 12, 2014 10:53AM
This spread sheet might be helpful for cam machining.

Lohring Miller
Attachments:
open | download - Cam Profile Calculator.xlsm (41.8 KB)
Re: Skinner/Fuller valve gear.
September 13, 2014 11:16AM
Ken,

If you consider the cycloid curve and look carefully you can notice that it can be broken down into three approximate parts. The first 1/4 of it is all acceleration and very little movement. The second 2/4 of it (the middle of the curve) is mostly movement and the last 1/4 is de-acceleration and again little movement.

If there is much clearance in the valve train, such as in excessive lash then the roller tappet or cam parts will slam into the rest of the un-accelerated valve train. What is needed is a pre tightening ramp on the cam to take up all of the slop so that the valve train is all begining to accelerate together. On the de-acceleration the valve spring should hold it all together.

On my desmodromic setup I have to have a bit of springyness in the valve train it'self to absorb the slop. This is to be provided by the correct placement of the oil holes in the rocker shafts and higher than normal oil pressure of about 100 psi for this engine. Notice that a rocker on a rocker shaft has similar action to a piston wrist pin. If set up with similar clearances then there is elastrohydrodynamic lubrication which provides a little springyness.

Elastrohydrodynamic lubrication can be enhanced by placing a washer at each end of the rocker bushing. This washer fits tight to the shaft and rides tight to the edge of the bushing, increasing the traping effect of the high pressure oil inside.

Best,

Bill G.
Re: Skinner/Fuller valve gear.
September 13, 2014 03:48PM
Posts: 1,074

Hi Lohring. Thanks a lot, I will have to go through that spreadsheet and see what I can learn from it. It took some effort to put together a spreadsheet calculate cycloidal profiles and then use it to find close fitting arcs to approximate the curves and make them tangent at intersection.

Hi Bill,

It took a bit more arcs than that to make the cam work. The thing is, I have access to a 20 year old Bridgeport with ball screws and the "EZ Trak" conversational controller. That unit is limited to straight lines and arcs. I could try to program the cam degree by degree but there are so many data points that it gets problematical with such an old unit. So, I went for arcs.

The cam will be fitted into a Harbor Freight 420 cc "Predator" engine, so I was limited to my choice of base circle radii and other factors....the short lift profile really makes it tricky to work the profile on the nose.

Anyhow, attached are images of the cam including the arcs of the constituent elements and a graph for acceleration, velocity and displacement. The ramps are more gentle than they look on the graphs, that is an artifact of compressing the graph for easy viewing and the relatively long closed period. As you'll note, the design was tweaked to provide a maximum of 500 gees acceleration.

Regards,

Ken


Re: Skinner/Fuller valve gear.
September 13, 2014 06:36PM
Hi Ken,

As I recall, I used 0.3" lift and higher accelerations. What was fun was to figure how long it would take using a cycloid curve at those accelerations to send a valve to the moon and back. Not long!

Is this design for cam roller? For me, the design load given for the cam roller along with the steam pressure load determined the maximum accelerating force on the system. I chose then an Iskenderian roller of 3/4" dia.

Best Regards,

Bill G.
Re: Skinner/Fuller valve gear.
September 13, 2014 07:36PM
Hi Bill,

Peak acceleration is going to depend on a number of factors including how long you want the engine to live. For an overhead cam engine I would design at about 800 gees peak acceleration for an automotive engine intended for long life, a pushrod engine is a bit stressed at 500 gees....I got those figures from a gentleman who designs cams for GM, so they seem like a good bet.

The Predator engine is a single cylinder, 420 cc, pushrod engine having flat tappets. The valve guides aren't going to accept a roller tappet and I really don't want the hassle of designing a new overhead cam head, so I will just use the stock pushrod hardware for the exhaust valve and a different style admission valve setup. I expect this should work well as the acceleration profiles are almost exactly the same as those Art Gardiner designed for the PSL engine that ended up in Chuk's car. What REALLY irritated me was that after spending a fair amount of time designing a DA cylinder head that could be easily built with a mill and a lathe I saw the drawings of the PSL head and it turned out I had almost exactly duplicated Art's work. If I'd seen that drawing earlier I would have just gone that route, it really is a simple but effective beast.

Anyhow, should this all work out there are two automotive engines that might make good conversions to test the idea out. These are the Chevy S-10 2.2 liter 4 cylinder and the Ford Ranger 2.3 liter 4. Both engines usually put out somewhere around 120 HP in stock configuration but both are routinely fitted with turbos capable of extracting upwards of 400 while remaining reliable enough for daily driving. The big difference is that the Chevy is a pushrod 4 and the Ford an overhead cam 4. I've worked on both and would generally say they seem relatively comparable. The overhead cam would provide the Ford conversion with some advantages but the pushrod Chevy would definitely be easier to convert and likely hold up a bit better as it would use more stock parts. Of course, if stupid amounts of power were the goal, there is always the small block Chevy V-8...just can't buy cheaper power.

Regards,

Ken
Re: Skinner/Fuller valve gear.
September 14, 2014 01:38AM
Ken,

So this engine will be a counterflow, which should be a whole lot easier to do than any conversion of an IC engine to uniflow?

The exhaust valve under low enough cylinder pressure at B.D.C. can open into the cylinder as standard IC practice, but then the inlet valve must open upward out of the cylinder?

An easy way to do this and one I was thinking of for my three cylinder double acting engine is to keep the valve spring on the valve where it is. Now use a negative cam profile that lets the spring open the valve. The cam then pushes the valve shut and the afore mentioned system provides enough springiness to the valve train to seat the valve and steam port pressure also helps.

Best,

Bill G.
Re: Skinner/Fuller valve gear.
September 14, 2014 10:59AM
Hi Bill,

Definitely a counterflow. The idea of punching holes in an IC cylinder gives me the collywobbles...getting wimpy in my old age...I assume the designer meant for the engine to take certain loads and have a given life expectancy based on the design as he executed it. Perforating the cylinder is going to make it weaker and interrupt lubrication to some degree, best use a cylinder designed specifically for uniflow if you can. If I were building a stationary gen set I would look at the same solution the White Cliffs project took....fabricating new cylinders and bolting them onto a Lister Diesel (or one of the knock-offs).

I thought of the negative profile cam but ultimately abandoned it. The stock spring isn't likely to be strong enough to resist admission steam pressure as the piston approaches BDC, strengthening the spring adequately would possibly require a heavier valve stem to cope with the higher stresses and all this potentially adds greater inertia to the moving parts which in turn leads to more issues.

If I were going to go with an overhead cam layout I would try to copy GM SES-101 as much as possible. You'd almost think the designers worked with poppet valve trains fairly often. It really isn't all that much different than Art Gardiner's PSL valve train except that the GM solution is a fair bit neater because they had the resources to fabricate the whole unit en bloc rather than build it up; compromises are what make the desirable feasible.

Actually, if someone really wanted to get serious, with cheap 3D printers today there are ways around even that. If one were to print a head in PLA rather than ABS (and build the structure up with cells) you would have a somewhat rough but acceptable shape for a casting, complete with all internal passages that you might want. As it happens, the PLA is soybean based and melts much more readily than the ABS and without the same kind of residue. This makes DIY investment castings approachable. Dip the model repeatedly in refractory and allow to dry between dips to build up a nice, thick cocoon. Then heat moderately and drain out the PLA through a convenient hole created by a spout on the top of the part that you didn't dip. Bake the cocoon to fully harden the refractory, pack in sand with a foundry flask and pour into the refractory.

Of course, if you aren't as poverty stricken as I you could have a very modern foundry make up a plastic bonded 3-D pattern using one of the new sand sterolithography machines. These lay out a fine, level sand layer on a table and then a print head moves over the sand laying down and adhesive. The table then increments down and the process is repeated building the cavity layer by layer.

Regards,

Ken


Re: Skinner/Fuller valve gear.
September 14, 2014 11:49AM
Bill,
If you really want a unaflow engine, then go look at the old 2-3-4-53 GM/Detroit diesels, now out of favor because of the smog stuff.
A very tough two cycle engine series with removable sleeves and if you use what were the inlet slots on the sleeves as exhaust, they are certainly big enough. Cam drive is 1-1 with counterbalance weights.
If you are lucky, you might find the aluminum version.
Until the Lear hardware suddenly became available, that was going to be the Bonneville engine.
Jim
Re: Skinner/Fuller valve gear.
September 14, 2014 02:24PM
Hi Ken,

How fast are you planning on turning that engine? Re-compression pressure should pop the valve up enough, but you might have to start throttle at lower pressure. There are just too very many ways to do anything!

Jim,

What do you think the results may have been, in retrospect, if you had used the Detroit Diesel instead?

Best Regards,

Bill G.
Re: Skinner/Fuller valve gear.
September 14, 2014 03:48PM
Hi Bill,

Well, I have no idea if I'll ever convert a car engine, so hard to make an solid calls yet. One reason I liked the 1 cylinder Predator is that the displacement is actually significantly greater than a cylinder on my Chevy Sonic....although only rated about half as powerful. I assume the Predator is derated because Harbor Freight specifically says "not meant for vehicles" and because roughly comparable Honda's (of which this is a clone) put out more juice. If I can develop about twice the sellers rated power it means the basic concepts stand a chance with full blown automobiles.

I would like to see the Predator turn 5,000 peak rpm. As you might guess, I am doing something flukey with the valves and that is much of the reason for converting the engine.....it has niceties like electric start to aid light off and .......a balance shaft! That helps get away from the usual 3600 rev balance restriction on single cylinder engines.

As for the GM 2 stroke Diesel uniflow versus a converted gasoline engine counterflow? Depends on what you want to do. Those old Diesels are heavy unless you can score one of the more rare aluminum engines; way too heavy for use in most modern cars. You would either need to go with an older, more traditional sort of vehicle or use it in a truck. The aluminum motor is a different beast entirely. On the bright side, you should be able to find old Series 53 and 71 diesels for as low as maybe $2,000 for one in fairly rugged condition and maybe $5,000 to 7,000 for one that has been reconditioned. The navy used a lot of these monsters for all kinds of things and between naval fastidiousness when it comes to maintenance and the thoroughness of navy shipyard overhauls, you might find something really solid. The 4 banger car engines are lighter, power up nicely but need relatively high rpm to do this without overstressing the internals where the Diesels are lower revving machines. For a truck I'd want the Diesel. For a pickup truck it would come down to how I used the thing. For a mixed use, daily driver automotive conversion the gasoline engine is more my speed. For a long range highway cruiser the Diesel looks better. If I wanted to do something like go after the LSR I would say either is fine as you need to tailor the driveline and vehicle to the mission anyhow. By the same token, if the intent was to go around challenging Corvettes to road races, I'd pick the gasoline counterflow conversion....and still get clobbered. IC engines at that level of development are just too energy dense to compete with if you need to stick a boiler and separate burner in there. Nothing unexpected there, going onto the competitions turf and challenging them rarely works out well.

Regards,

Ken



Edited 1 time(s). Last edit at 09/14/2014 04:31PM by frustrated.
Re: Skinner/Fuller valve gear.
September 15, 2014 12:09PM
Bill, Ken,

The GM aluminum two stroke was strictly for the Bonneville car, not some road car. Because the big GM Diesel distributor here, Williams & Lane were the source for those nasty aluminum 6-71 offshore power boat race engines. The ones GM only guaranteed for three hours, then look out!!! They had a couple of used stripped lay down aluminum 3-53 s they had no use for and that dual engine streamliner had popped up and might have been available, the plot thickened.
Since finesse was not on the menu, just brute power, a new cylinder head was laid out that had three traverse valves where the cams operated them in sequence each with fixed cutoff like Serpollet, just because it was the simplest way to go. Then the Lear stuff came along, so the other idea came to a halt.

Jim



Edited 1 time(s). Last edit at 09/16/2014 11:33AM by Jim Crank.
Re: Skinner/Fuller valve gear.
September 30, 2014 08:30AM
Attached is an illustration of the valve lash with a poppet valve uniflow expander (the SES L4). Observing the valve lift trace, you will notice the "ramps" on both sides of the cam. The opening ramp is not all that important but the closing one is. It limits the velocity with which the valve hits its seat. We followed the standard IC practice for that velocity using also the steels of the IC engine practice.
Notice the change in the effective cut-ff with the lash. If the lash were adjusted to the minimum, the effective cut-off would be shorter. This "wire-drawing" was found to not affect the engine running to any significant degree. Because of the danger of the lash disappearing during the heating up of the various components, and the "no adjustment for the life of the engine, we opted for being "safe rather than sorry.".
Notice the steam pressure line and low it is at the valve closing - the engine lacked power at is max speed. Resonance in the steam column was the cause - see low pressure at the valve closing. The SAE paper written about the expander describes the resonance at length. Rearranging the inlet piping "de-tuned" the fluctuation.
Stan Jakuba


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