A gas engine question

[Tim StevensParticipant @timstevens64731]

Another factor for a slow tickover is the valve timing. Modern engines all have 'valve overlap' – the inlet opens before tdc and the exhaust closes after. This promotes good scavenge and better breathing at high speed. But you need no overlap for a slow engine (as used in ordinary (non-racing) cars up to around 1930). Given this factor and a heavy flywheel (heavy in the rim, of course) then the CR does not need to be really low, especially if your carb (inlet tract) is small*. And if it is too low your generator will need to be really feeble, or it won't drive it. Although, of course, what you put into a generator is closely related to what you get out. Have it driving a couple of LED bulbs and you get impressive light for very little power.

* if the gas can't get in, you can't compress it so much. You can always lower your CR with a thicker gasket – as long at the rest of the geometry allows the head to move away.

Cheers, Tim

[MichaelRParticipant @michaelr]

Alan, I built a Centaur the cylinder head is machined as drawing but I made the conrod to bring the piston flush with the end of the liner, I haven't a clue what the compression ratio is but the engine seems to run OK, well for me it does.

[AJWParticipant @ajw]

That runs nicely Michael! Do you find your ‘cooling tower’ provides sufficient cooling?
I have various pieces of potential tower hanging around but was unsure how large it needed to be.

Alan

[MichaelRParticipant @michaelr]

The engine gets hot which heats up the circulating water in the tank, it really could do with a radiator or screen cooler to dissipate the water heat, I have now fitted a gear water pump driven from the side shaft, but it still could do with a cooling system for prolonged running, the water tank is a Bean Tin.

[not done it yetParticipant @notdoneityet]

Many stationary engines simply had a hopper above the cylinder head and slowly evaporated the water away – faster if under heavy load, when it would have reached boiling point. Some ran on kerosene, so needed to be hot.

That means that a large surface area for the cooling tank is needed for evaporative losses as well as a good radiating surface – better black than polished – and the pipe to the tank from the engine should slope upwards all the way (and not lose too much heat (so that the thermosyphoning is kept operating). I would guess that the upper connection would be below the tank water level, not as in the model shown above.

[not done it yetParticipant @notdoneityet]

Many stationary engines simply had a hopper above the cylinder head and slowly evaporated the water away – faster if under heavy load, when it would have reached boiling point. Some ran on kerosene, so needed to be hot.

That means that a large surface area for the cooling tank is needed for evaporative losses as well as a good radiating surface – better black than polished – and the pipe to the tank from the engine should slope upwards all the way (and not lose too much heat (so that the thermosyphoning is kept operating). I would guess that the upper connection would be below the tank water level, not as in the model shown above.

[JasonBModerator @jasonb]

The Centaur runs quiet a bit faster than the usual open crank stationary engines which will tick over at 200rpm or so which mans they generally run cooler. Add the fact that on a miss cycle they are just pumping cold air in and out of the engine and the water tends to me more like hot tap water than boiling.

I seldom run mine for more than 5mins at a time so don't even bother to fill the hoppers but for longer running or if making them work then they should be filled or if no hopper then a separate tank, screen or radiator is needed.

[not done it yetParticipant @notdoneityet]

Posted by JasonB on 05/02/2019 10:58:35:

….Add the fact that on a miss cycle they are just pumping cold air in and out of the engine and the water tends to me more like hot tap water than boiling.

…..

The originals would not be pumping cold air, I think, unless a small amount is drawn in through the open exhaust valve, but the main volume would be heated exhaust, just too-ing and fro-ing? Under a reasonable load,there would be few, or no, misses anyway. Pointless running a stationary engine with no load at all, if one wishes to emulate the originals. Farmers might have run with an intermittent or changing load, but they would rarely run an engine for no good reason – they were too tight with their money for that!

[JasonBModerator @jasonb]

Posted by not done it yet on 05/02/2019 12:18:55:

The originals would not be pumping cold air, I think, unless a small amount is drawn in through the open exhaust valve, but the main volume would be heated exhaust, just too-ing and fro-ing?

Engine fires and then the exhaust stroke will push all but a small amount of the hot gasses out, just what is left in the unswept volume and a bit in the exhaust tract. If the exhaust has latched then the next time the piston goes down it will draw in through the exhaust and the volume will exceed the unswept volume and what little is in the exhaust so it draws in air, next miss will dilute that so a higher percentage of air fills the cylinder.

You also get them with two exhaust ports, such as the air cooled Gade where any excess hot gas exits by the primary exhaust port at the end of the cylinder then the secondary valve controlled port only lets the piston return and empty what is in the cylinder at atmospheric pressure.

[AJWParticipant @ajw]

Fascinating, agree with regarding farmers ‘carefulness’ with money!
Going slightly off topic there are some wonderful stationary engines of all types to be found on the tube!

Alan

[JasonBModerator @jasonb]

Posted by AJW on 05/02/2019 13:14:52:

Going slightly off topic there are some wonderful stationary engines of all types to be found on the tube!

Alan

Don't look there it will cost you money! I'm just in the process of buying another casting set from the US and happened to look at the guys you-tube channel again to see he had added another video. I was forced to ask him for a set of drawings for that engine and one of his coils too.

[Nigel McBurney 1Participant @nigelmcburney1]

Regarding engine cooling,the water outlet must be from the top of the cylinder, then slope upwards the top of the cooling tank to ensure the thermo syphon operates effectively,when setting up an engine ,do not make a posh tank arrangement just try various size containers when an optimum size is achieved make a good tank of the same volume.old (non Diesel)stationary engines did not run on high compression ratios,lower ratios give nice slow running and do not produce so much waste heat. Now valve overlap,I have found ,even on early engines there is always some valve overlap,there was a comment that hit and miss engines would run hard and and not be allowed to hit and miss,which would allow air only to be drawn through the engine and help with cooling, there are numerous instances on work where the engine does idle then pick up again, instance log sawing the engine idles when the operator has to pick up the next length of wood,but you must wait for the engine to fire before pushing the wood into the saw otherwise it will stall. Throtle governing was used on petrol /paraffin engines to keep the cylinder head /vapouriser hot to ensure the fuel was vaporised. Most owners of full size gas engine prefer to use propane set at about 3/4 to 1 pound per square inch read on a gauge mounted close to the gas cylinder regulator and then a gas bag in the fuel pipe,setting up and adjusting the air gas fuel ratio can be difficult,unlike petrol ,gas will not run weak or rich so a petrol set up is far easier, i have 3 gardners,two are gas with hot tube ignition and the other is petrol and magneto ignition,the early 1906 gas engines runs beautifully but the lamp for the hot tube can easily get blown out on a windy rallyfield,the petrol engine keeps running if its blowing a gale,and it is noticeable that the 1910 petrol Gardner has a higher compression ratio and a lot more power.

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