Uneven running of "table" and "grasshopper" engines

[Coxy]

Anyone who has built a table engine or grasshopper beam engine, will know that they slow down on the up stroke and accelerate on the down stroke, due to all the iron-mongery hanging on the crank pin. I have a suggestion to mitigate this. These engine should all have at least a boiler feed pump, and probably and air pump. If a small tank of fluid were installed somewhere out of sight, the feed pump could draw from this unimpeded on the up stroke,  and then discharge back into the tank on the down stroke via an adjustable valve to provide a load. If using the  air pump, then air might work. The adjustment would probably only be correct for a specific setting of the steam valve but this is not really a problem. It would have to be an open system, as, if you try to connect the suction to the delivery to "tidy it up", then it would provide the same load to each stroke, which is not what you want. Of course, all of the above would only really be suitable if you are intending to run your engine on air. What do you think?

Regards    Maurice

[Jasonb]

Certainly noticed the problem on the E&A. That does have a large airpump which I decided to make a dummy but as it only works on the upstroke I suppose it would be possible to have it work in reverse and use a valve to restrict the rate at which air exits the pump which would regulate the drop of the beam.

I would also imagine that if teh engine were run under load the effect of the dropping beam would be less noticable that when just ticking over on display doing nothing

[Coxy]

Hi Jasonb, sorry but I don't quite understand about your air pump. You say it's a dummy, then say that it works on the upstroke I envisaged drawing air in under the pump piston on the upstroke,via a non-return valve, and then expelling it via an adjustable non-return valve on the down stroke. If the pump worked "the other way" then you would have to restrict the inlet instead of the discharge. I have a table engine that suffers from this inbalance. It is non-condensing, so has no air-pump, but it does have a feed pump, which I could connect to a small hidden tank, and use thin oil instead of air.

Regards   Maurice

[Jasonb]

Sorry, the drawings from Anthony Mount say the air pump can be made as a working one or just the top half made as a dummy, I went with a dummy.

The working pump is a single acting lift pump with diaphram type valves which open on the down stroke and close to pump the air on the upstroke. I just thought it would be easier to restrict the output so the internals would need to be reversed.

Your circulating system would work OK. It would also be possible to make something like a shock absorber, a cylinder with loosly fitting piston filled with oil. If the piston were also drilled and a diaphram placed below then on the upstroke the holes in the piston would be open so oil would easily flow past the piston. On the downstroke the diaphram would be pushed against the piston closing the holes so the oil would have to flow around the outside of the piston which would offer more resistance. You could then tweak the action with different weight oils or by trying different piston to cylinder gaps.

Though I still wonder if just adding a friction block to rub against the flywheel would give a load that would overcome the pull of gravity on the parts.

J

[Coxy]

I'm sure a dashpot as you describe would work. I have tried just having a brake working on the flywheel rim of my table engine. It did not help much. There are of course some variables such as the speed that it's running, and the pressure used . My engine could do with a good clean, so I think I will experiment a little more.

Regards   Maurice

[Coxy]

Sorry, I missed a bit; your dashpot could be built into the air pump. It would be much neater than my idea!

Regards   Maurice