Spark engine - making it last?

[jzeeff]

Due to cost reasons, I have to use natural gas instead of diesel.  But there is no way I would expect a 3600 rpm lawn mower engine converted to NG to last very long.   Maybe if I slow it down to 1800 and live with the HP reduction.  Is there anything I can do to retune the engine to be more efficient at this lower than normal RPM?

1) is the stock camshaft timing ok?
2) retard the spark timing?
3) change compression ratio (natural gas can use higher)
4) run extra lean - shouldn't overheat the piston at the reduced HP output

What if I went even lower - say to 1200 rpm?

[wormshoe]

I'm a newbie. Just sharing some thoughts with you.

Some things that you might consider doing:

Advance the cam timing - (making sure that the valve-to-piston clearances are comparable to what you have now).

Ignition timing - retard (trial and error).

Changing the mixture - my first guess is that you would want to start on the rich side of things and then begin to lean it out, being very careful not to go too lean.  I would favor a richer mixture.

Piston pin offset - if your piston head is flat (does it have valve reliefs cut into the top ?) and if the pin is offset towards the major thrust side of the cylinder, one can reduce the skirt loading by removing the piston from the rod, turning the piston around 180 degrees and reinstalling the piston on the rod. This will change the angle at which the combustion forces work on the piston head and transmit more of that force to the crank. The piston will slap slightly until it warms up.  It is more critical that you allow the engine to warm up before throwing a load on it.

(Automotive engines in the U.S. during the fifties and sixties used to have a pin offset of .0625" or 1/16".  It was a tunning technique that one could use to make his car run quicker using stock parts - - read that as a cheap power upgrade .  The performance pistons of the day usually had a zero pin offset.)

Even though increasing the compression ratio sounds like a winner, my guess is that it is best to leave it alone - - the increased loads on the bottom end might not make the engine very happy and we haven't even begun to think about the increased heat being generated over a longer period of time and what that might do to the valves and valve seats or to the ability of the cooling system to handle the increased heat load.
( When leaded gas was available, it helped to lubricate the vavle face/valve seat interface which increased the service life of the top end. NG doesn't give you this.  )

I hope that some of the other folks chime in and give you some better advice than this  . 

wormshoe            

[jzeeff]

Thanks.  Wouldn't I retard the cam timing - at high rpm, valves open early to allow the flow to happen, but at low rpm, I can open a little later.  I found out that CNG burns slower and so usually needs more spark advance.  Also that lower RPM engines can use lighter valve springs and get longer valve life.

What instruments would I need to do this right?   Tach, O2 sensor?   Does one just adjust to get max rpm with a known load?

Perhaps there are guidelines for valve and spark timing on a 1800 rpm max engine.

[wormshoe]

Thanks.  Wouldn't I retard the cam timing - at high rpm, valves open early to allow the flow to happen, but at low rpm, I can open a little later.


I think the intake valve closing (trying to percieve it in relation to everything else that is happening) is the event that needs to be dealt with.  At a lower RPM there is a much reduced ram effect so one wants to close the intake sooner. One will actually end up with more air to compress if the intake valve closes fewer degrees ABDC.  Conversely, if you were looking to build a high RPM screamer, one would want to delay the intake valve closing so as to benefit from the ram effect.

Listen, I have been away from this stuff for a long time and the brain cells don't fire like they used to.     I'm probably taking you in the wrong direction.      I have noticed some very sharp gentlemen that contribute on a regular basis and I'm sure that they can tell both of us the truth regarding this matter.  I probably shouldn't have opened my mouth, but the meds worked their magic and compelled me to share my ignorance with you.   

wormshoe

[jzeeff]

I agree, valves could open later and close earlier on a low RPM engine.

I also found out that a hotter spark is generally used.
   

[Stan]

Jzeef.....I don't know for sure about nat gas, but with propane when I converted my last engine from gasoline to propane I had to retard the timing from 5 deg btdc (nrmal for a 440) to 21 deg btdc.  It was a chrysler 440 and I had to do other things due to the nature of the distributor.

My propane "expert" told me that the ignition qualities of propane make it a much slower fuel to ignite thus the timing has to be retarded.

Stan

[4x4_Welder]

That's not retarded, that's advanced.

The best thing would be to set up your cam timing, intake, and exhaust runner length based on a set rpm.  It is best to delay the opening of the intake valve and allow the engine to build a little vacuum in the cylinder so the charge carries more momentum coming in-  Basically, open the valve at about 25-30ºATDC, and close about 5ºABDC.  This combined with a proper length runner will create a sort of supercharging effect.  You can easily tune an engine like this for 110-115% VE, as long as you're going for a fixed RPM. 
I would start with an overhead valve V-twin of a lower HP rating (16-18) and keep the factory compression ratio. 
If you do want a single side-valve engine, go for a Wisconsin or OLD Kohler, and bump the compression up to about 7.5-8:1.  I think they both are about 6.5:1 originally.  This will give you a much more user friendly power curve, and also greatly increase torque.

[Stan]

 I know, sorry, my mind was thinking one thing and my fingers another.    Glad you caught my seniors moment.
Stan