hit and miss

Chaps you seem to have got off on the wrong foot because the question and answer appeared to give the impression that the answer was already known. That too is an assumption :-) For those of us who are finding this thread interesting and enlightening please both take a deep breath and continue sharing your considerable knowledge with the rest of us. thanks Roland

Your inability to to comprehend that assumptions are not what facts are based on. You've assumed entirely to your own satisfaction, the raison d'être of the operating principles of all hit & miss governed engines. Well done!

Tom

I see, my apologies if it appeared I was trolling, I thought I was summarisng. At my age I am afraid my style is a bit fixed.

Cheers Roland, I hope others will chip in.

I did spend some time at the internal fire museum where it was mooted that some old engines, albeit diesel ci engines, were being considered for remanufacture for third world use because the technology was available to artisans with less than state of the art equipment.

AJH

1. Gas engine means fueled by gas, not gasoline, not petrol nor any other liquid fuel.

Tom

wrote (snip):-

But not sufficient to open atmospheric inlet valve - some H&M engines include a stop on the side rod to physically hold the inlet closed during idle strokes (eg larger Amanco)

The additional internal cooling was not generally seen as a problem. Optimising running temp for thermal efficiency had not really been thought about and indeed some engines had precious little other means of cooling (eg Fuller and Johnson Farm Pump, Aeromotor 'eight stroke')

The large clearance volume (ie low compression ratio) of early engines left a fair quantity of residual gas after the exhaust stroke to dilute the subsequent charge, many early inventors tried to improve on this (eg Atkinson) though 'improved scavenging' was more often than not used as justification for some patent dodging variation on the pure Otto cycle. Most small H&M governed engines have a very short and rudimentary exhaust system (again eg Amanco) and after a few idle strokes most of the exhaust would probably have been replaced by fresh air, though, as this would only happen at part load, I think it is a side effect rather than a reason for the adoption of H&M.

In most cases I think you are right - H&M governing is a rough and ready way of enabling an engine to cope with a varied load and particularly to avoid running away should the load disappear (belt falling off etc). It is this, rather than any efficiency benefits of running unthrottled, which lead to its retention for so long on 'farm' type engines where simplicity and cheapness were all important (compare a typical American 'mixer' to a proper carb!). Of course the story may be a bit different for something like the wonderful Gardner 4FHM at the Anson museum which you really should make an effort to see.

I really don't think the intention was ever to maximise volumetric efficiency by avoiding throttling, merely a simple way to control engine speed.

Not what you'd call advanced, but common throughout the 1920's, particularly in USA.

A bracket clamped to the side rod which lodged under the valve cap when the governor catch plates were engaged did quite nicely on the larger Amanco engines with atmospheric inlet. With cam control it would simply be a matter of choosing a sufficiently strong valve spring.

No. Octane values and cr's were uniformly low in the period.

I hope someone else can fill you in on the details, but the 'drum roll' as the valve gear alternates between hit and miss is quite something!

Do it!

Can you enlighten us Geoff?

Don't think so, but I am always willing to learn!

I think the Bentall Pioneer had H&M governing with mechanical actuatation of both valves, but I dont know if the inlet was held closed or not. Then there is an American engine which name escapes me for the moment in which the whole camshaft was de-clutched and remained stationary during the idle strokes! It is these many and varied approaches to achieving the same end which provide much of the fascination of engines for me.

Can't comment on the effective octane rating of town gas, but, if anything gas engines of the period seemed to have had an even lower cr than petrol versions.

Yes, I see it would not be a problem with an atmospheric inlet valve, but as I said this would defeat the advantage of better volumetric efficiency.

How advanced did hit and miss get? What period were they last common? I can see their development being curtailed because they were no good as motor vehicle engines compared with throttle control and it became more worthwhile to follow this line or all engines.

I also can see how a governor could hold the exhaust open with no great difficulty, holding the inlet shut on a cam controlled inlet seems more problematic.

Makes sense but the advantage of the hit and miss gas engine would appear to be in making use of a high "octane" fuel and thus ability to use both high cr and no throttling, do you know of any examples where these may have been features?

The picture of the 4FMH says it is a 1918 engine, no details, can you point out its salient features, I was expecting this sort of governing to apply to large single cylinder engines with huge flywheels rather than this 4 in line design.

It's probably a 5 hour drive for me to get there but I will one day.

AJH

Hopefully its curator will chip in but my remembrance of the wondrous 4FHM is that its not H&M but rather variable valve lift. The governer controls a stepped sector arm which is interposed between cam follower and valve stem. There is nothing new :-) regards Roland

Fair enough, after seeing from the semi diesel discussion that there were attributes to these that did not immediately spring to mind I was hoping for a similar revelation on H+M engines.

I didn't realise you were still referring to atmospheric valves, I was moving on to something more sophisticated where the inlet might be held shut to prevent fuel being drawn in, perhaps it never developed that far.

Well the octane value of town gas would have been as high then as it would be now, if available, a mixture of H2 and CO, CH4 and some nitrogen, presumably much better than the petrol then available, again perhaps these engines were never developed sufficiently to take advantage of this.

AJH

Stationary engines always lagged behind ( sometimes far behind ) whatever the current automotive state of play happened to be. An example of this is H&M which was - let's face it! - pretty quaint by the end of the first war. Reliability and ease of use by farm hands was far more important than volumetric efficiency or nett BHP per pound weight. Is there any evidence that after market pistons, pushrods etc for atmospheric inlet valves, better carbs etc were available to get more out of them?

It has often occurred to me that it might be fun to tune up a Lister D (cheap, plentiful and the only OHV in town!) by skimming the head or changing the piston by attaching a false crown, increasing the inlet tract bore and valve diameter, fiddling with the valve and ignition timing and fitting the poor thing with a real carburettor rather than the primitive fuel mixing device they wear as standard. You couldn't stop there, of course, but would then need pumped lubrication, a lighter flywheel and a balanced crank, but I reckon it could remain as a "street sleeper" with little externally to give it away except the carb. It would be a real cognoscenti device as few would understand the implication of the 10kva generator it was driving!

There's a project for Eric Brain's lads!

Regards,

Kim Siddorn,

Not quite, Kim, Bamfords, Fowlers, Lister X Type, probably loads more if I wasn't watching a HDD format while typing. Or was that even more tongue in cheek than usual? :-))

Peter

-- Peter & Rita Forbes snipped-for-privacy@easynet.co.uk Engine pages for preservation info:

Well, not quite, but don't see anyone wanting to modify a Bamford, Fowler, , let alone a Lister X Type - but a Lister D? Well, maybe ................

Regards,

Kim