Roy Poynting column
The automatic, atmospheric inlet valve had a short-lived period of favour in engines, but there are still examples to be seen chuff, chuff, cough, chuff, chuff-ing away ... Just like they were designed to.
When you visit a country fair, you almost always see a row of chaps and chapesses patiently sitting behind small stationary engines which chuff steadily away, just ticking over, or perhaps doing some simple repetitive task like spinning a grinding wheel.
Actually, chuff is the wrong word, as these engines typically go chuff, chuff, cough, chuff, chuff and so on. In my ignorance I assumed the occasional cough was a misfire among the steady beats, but no, it's actually a single firing stroke while the chuffing indicates an ensuing free-wheeling period. The intermittent firing leads to these devices being commonly referred to as 'hit and miss engines; which describes them perfectly.
Not only was I unaware of their mode of working, but - until I discussed one of these engines with its knowledgeable owner last year - I didn't realise they relied on the almost forgotten principle of the automatic, or atmospheric, inlet valve.
Veteran motorcycle enthusiasts will know all about these archaic devices, but at the risk of teaching my grandparents to suck eggs, I'll briefly recap their mode of working. The inlet valve looks pretty much like one from a much later engine, except that it is kept closed by a lighter spring, and is not operated mechanically. What happens is that the descending piston produces a partial vacuum in the cylinder allowing atmospheric pressure (hence the alternative name) to push the valve open and allow air and fuel vapour to trickle in. As the piston rises again it pressurises the fuel/ air mixture which, helped by the spring, pushes the valve shut again. After combustion occurs, the exhaust valve (which has to be mechanically operated because it is working against cylinder pressure) takes over as normal.
So the automatic inlet valve does exactly what it says on the box, but it has some serious drawbacks. For one thing, the reduction in cylinder pressure allowing the valve to operate only becomes significant when the piston is well on its downward path, so half the induction stroke is wasted. Similarly, the valve doesn't close very quickly, so some of the charge may be pumped back out again. An even bigger difficulty is providing a valve spring just strong enough to close the valve when required, but weak enough to allow it to open reasonably smartly.
That's hard enough at low speeds, but more powerful, faster revving, engines demand stronger springs, and then the valve won't open at all when you try to start it! Even if you get a reasonable compromise on all that, it only needs the slightest bit of crud on the valve stem for the system to break down.
Small wonder, then, that for all their superficial benefits, atmospheric inlet valves gave way to camoperated ones soon after the turn of the last century. At least they did as far as motor vehicles were concerned, but that wasn't immediately the case in small industrial engines where they continued to be perfectly adequate. In fact it was a bonus that automatic valves only operated when circumstances permitted, because these motors were never intended to fire continuously.
What happened was that when a firing 'hit' occurred, the flywheel was speeded up enough for a centrifugal governor to operate a simple mechanism, which held the exhaust valve open. The flywheel then continued to spin, pumping air in and out of the exhaust port, while the negligible pressure changes in the cylinder allowed the inlet valve to stay shut and not draw any unnecessary fuel from the spray-bar which acted as a rudimentary carburettor. After several rotations the flywheel slowed enough for the governor to permit the exhaust valve to close, and - bingo - the automatic inlet valve opened ... er... automatically. Fuel was drawn in, there was a firing stroke, and the whole sequence started again. Cleverly, the engine's speed was more or less self-regulating, because once the engine was put under load the flywheel naturally slowed more quickly, so the firing strokes automatically became more frequent.
These early hit and miss stationary engines were simple and reliable, but they had the major disadvantage that their mode of operation necessitated a relatively massive flywheel, which seriously compromised their portability. Unsurprisingly they fell victim to advances in motor technology, and were retired (apart from their current role as country fair curios) in the mid-1920s, when smaller, more powerful, reliable and economical conventional motors were developed. With them went the good old/bad old automatic or atmospheric inlet valve, but only after it had faithfully performed its working role for a quarter of a century after most motorcyclists thought it had been consigned to • the dustbin of history.