Cam Timing

[Rustkolector]

What would be a good cam timing arrangement for a 4 cylinder, 1" x 1.25" b x s, throttle governed, constant speed (500-700 RPM) engine that would yield good smooth operation? The flywheel will be plenty heavy.

Although it may not be transferrable to models, I have reviewed early automotive (1912-1950) cam timing and found they almost always opened the intake valves 5 to 17 degrees ATDC, closed 28-50 degrees ABDC. The exhaust valves opened 37-55 degrees BBDC and closed 0-10 degrees ATDC. They all had little, or no valve overlap. However, some stationary engines of the same period operating at 250-550 RPM used up to 18 degrees of overlap. Lobe separations were all about 108-115. The only thing I can see is the relative size differnce of these two type engines. Any thoughts would be appreciated.

Jeff

 

[stevehuckss396]

I assume this is an inline 4 cylinder? Some more information on what type if 4 cylinder and how it will be used might get you some more answers. The more info the better. I tend to really cam up an engine so I don't want to comment if you want a "good smooth operation". I prefer a cam with lots of overlap for a choppy idle and robust sound.

 

[gbritnell]

Hi Jeff,
In past threads we have discussed this subject to no end. There are more factors to engine performance than just cam timing, like flow numbers, combustion chamber shape and compression ratios.
My Holt engine has a 1.00 bore and 1.25 stroke. The cam timing is quite mild but still has some overlap. The best idle speed I can get out of it is about 700 rpm. To have an engine throttle governed that low would produce virtually no power at all. Why are you shooting for those numbers?
gbritnell

 

[Rustkolector]

Steve & George,
Ok, a little more info. This is an in-line 4 model of a 1920 vintage stationary engine. C/R is 4:1 with excellent compression. The prototype engines were constant speed throttle governed natural gas engines running in the 250-500 RPM range. They only had one speed. My engine has been running for some time now. So, last winter I built a generator for the engine to power some demo lighting. It seems to work well as a gen set. The engine/generator will easily produce 80-90 watts of power at 700 RPM. This is where the smoothness requirement comes in. Every time the engine misses, the demo lighting load flickers quite noticeably. It's annoying! The engine will eventually smooth out if pushed in excess of about 115 watts (and it sounds great at this load), but that is pushing the engine way too hard, and over taxes the cooling system. My goal is smooth running at about 25 watts.

I have already explored carburetion, intake manifold volume, valve springs, plugs, ignition timing, etc so now I am looking at the cam timing. I currently have the engine torn down for inspection and to fix some oil leaks, so now is a good time for any changes. That's the full story. Any suggestions are appreciated.

Jeff

 

[gbritnell]

Hi Jeff,
I'm amazed that an engine of that size will run consistently and make power at that low of an rpm. My suggestion would be to connect it to the generator with some pulley ratio that would allow the engine to breath a little, maybe 900 rpm. This way if it missed it wouldn't affect the generating that much.
But to answer your original question, I don't know what type of cam specs it would take to get the speed down, and make power, that low.
gbritnell

 

[Lakc]

Overlap will change your effective compression ratio, so I wouldnt want to add a hot cam profile to an old style, low compression engine. 4:1 is often mentioned as the bare minimum for an IC design to work. They really didnt discover the wonders of cam overlap until static compression levels got high enough to compensate.

Where the overall size difference works against you is fuel mixture atomization. All the bends and twists the intake charge takes between the carburator and the spark plug can cause vaporized fuel to drop out of suspension, leaving less fuel to find its way into the spark plug gap. Thats a really big problem with side valve engines, and as a group they tend to like richer mixtures then their overhead valve counterparts. Giving a little less valve lift can sometimes help in this situation, possibly giving the intake valve a bit more venturi effect. The way to tell if that actually IS your problem, is to meter a slight amount of propane into the carb opening and se if the condition improves.

 

[Rustkolector]

George,
I am really not trying to make a lot of power. I want smooth operation at a light demo loading. This thing really consumes fuel when it is working hard. When working on the engine, I noted that it ran better the more you loaded it. That's how I came to be realize the power it would produce. One thing seems evident, the wider open the throttle the better it runs. This would mean breathing could be an issue as well as carburetion. However, even at 90 watts the throttle is barely cracked open so the manifold vacuum probably isn't changing that much.

The generator was built as a single bearing design so I can't belt it. Besides, I have a single cylinder belted generator/set and it will flicker if it misses, but it doesn't miss very often. Since I built this engine to run for an hour or so at a time, I want it to run at near a prototypical speed (if possible). I am gonna try to anyway.

Lakc,
My primary fuel is propane. I like the cleaness, low odor, and it has worked very well in another similar single cylinder engine with and an atmospheric intake. I tried duplicating the atmospheric intake on this engine. It ran OK, but didn't improve the problem I am chasing. The engine is currently running no overlap. Intake opening 5 degrees ATDC, closing 15 degrees ABDC. Exhaust opening 35 degrees BBDC, closing at 5 degees ATDC.I have missed something somewhere.

Jeff

 

[Lakc]

Propane is definately a cure for a lot of evils, but if you have a misfire, something is still left to be improved.

A small intake leak, or unlucky turbulance, can still keep enough fuel away from the spark plug to cause a misfire. Reversion can be a problem, where exhaust goes back into the intake because of poor intake sealing or poor exhaust flow. You have to visualize both the intake charge and exhaust gasses as having mass, and thus momentum. For example, if you have a side valver and shut the exhaust too soon, the remaing exhaust will rush out of the cylinder through the intake valve when it opens. That is essentially what happens during overlap, and why you dont see cammed up race engines idling at 500 rpm. This should leave some visual evidence of exhaust around the intake ports. The physics of all these seperate masses starting, stopping, and turning are velocity dependant, and thus can be very rpm specific. Where higher rpm give less time, and thus less accrued momentum, lessening the problem. Thats much of what makes IC engines so fascinating, and frustrating at times. In this case, changing the volume of intake or exhaust manifolds will begin to make the difference, and perhaps changing the exhaust duration without adding overlap.

Pictures, especially of burn patterns, manifolds, ports, and combustion chamber design would be helpful. th_wwp

And of course, with high voltage, there is always a possibility of spark occasionally finding a shorter path to ground then you intended.

 

[gbritnell]

Jeff,
There is a mathematical point for a given stroke where there is virtually no piston movement, 10-15 degrees before and after TDC and BDC. To open the intake ATDC isn't going to change the performance that much. As Lakc pointed out, at very low intake velocities the fuel isn't going to be atomized effectively and the more bends the more the fuel will hit the port walls and convert back to droplets.
You initially stated that everything works fine until the engine misses and then you get a fluctuation in power. These little engines are finicky about so many things, fuel, velocities, ignition etc. that hoping to find a point where they operate almost perfectly is going to be quite hard. I would think that the occasional miss would be more than satisfactory. Some people can't even get them to run that well!
As an aside, is your engine configured something like a Holt? Mine has a huge flywheel on the rear and a smaller flywheel on the front end.
Here's another thought. Could you produce a little more voltage and then control it with electronics? That way if it went up or down by generation it would be more constant at output.
gbritnell

 

[MachineTom]

Have you tried fooling with exhaust tubing, muffler, the lenght can have a noticable effect on performance of the engine. Its simpler than just fooling around with can timing. If mine I'd add foot to the tubing same diameter, what happens, go down one size what happens. Much easier to expererment that way.

 

[Rustkolector]

Well, I see there are some issues still to consider. Somethings cannot be changed like the exhaust ports, the water cooled exhaust manifold, and combustion chamber. I am a little shy on spark plug electrode reach into the combustion chamber. I am just at the cylinder wall. I am using the Mini Viper Z2 plugs. I think for now I will extend the duration a little on each cam without overlap, and put it back together with new gaskets, and see what happens. I am thinking that no drastic cam changes would help. I have a feeling I have missed something obvious, as I have done before. So, I will revisit all the normal suspect problems again. Nothing with these little engines is simple. That's why I love 'em! Thanks for all the ideas. I will keep them in mind.

Jeff

 

[Rustkolector]

As an update to this thread, progress has been made almost to my total satisfaction. It wasn't the cam timing. A combination of carburetor modifications and ignition timing seemed to produce the needed results. The engine now runs fairly steady at about 650 RPM with a little load on the generator. It can be made to run fairly steady at no load, but runs better with a little load. Carb settings for no load, and loaded running are quite different. Running well on propane is very tricky, but I'm learning. I have ideas on a better propane carb design which I hope to try this winter. I will try to post a few pictures and hopefully a video of the completed project in a few days. Thanks again for the helpful suggestions.

Jeff

 

[Lakc]

Thanks for the update, glad you got it all sorted out.

 

[Rustkolector]

I have posted a video of this model in the videos section under "Bruce-MacBeth gas engine model". It is very near complete now.

Jeff

 

[metalmad]

And A very nice engine it is too!! :bow:
Pete