hit and miss

From what I understand early gas engines operated on an open throttle and controlled revs by either having a spark or not. How did the fuel enter the mixture and did it just exit unused if the spark did not occur?

I hoped to answer this question myself on Friday at the science museum but the 8 year old young lady I was escorting wanted to play with the computers and lifts.

AJH

Reply to
sylva
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The (petrol) hit/miss engines which I know intimately, govern by holding the exhaust valve open when the revs are too high, this allows the engine to coast , without having to overcome compression, and without drawing in any fuel mixture. When the revs drop, the exhaust valve closes (and then is controlled by its cam again) and normal operation resumes.

I think most early gas engines had a gas admission valve which was governor operated to admit fuel or not, as determined by the revs. Whether they operated with the exhaust valve opened at this point I'm not sure, but someone here will know.

Regards, Arthur G

Reply to
Arthur Griffin & Jeni Stanton

exhaust valve open when the revs are too

and without drawing in any fuel mixture.

again) and normal operation resumes.

Thanks for that Arthur. What are the major drawbacks of hit and miss, because on the surface it appears to offer the advantages of a 100% volumetric efficiency like a diesel but using spark ignition?

AJH

Reply to
sylva

the exhaust valve open when the revs are too

compression, and without drawing in any fuel mixture.

its cam again) and normal operation resumes.

Reply to
Roland and Celia Craven

An old book I read put it in terms something like this:

Hit and Miss as a system of governing is a rather drastic and crude system of speed adjustment. A whole power stroke is lost in order to reduce speed, whereas with throttle governing small adjustments can be made to the amount of mixture fed to the cylinder, giving a much finer system of adjustment.

Regards, Arthur G

Reply to
Arthur Griffin & Jeni Stanton

It is strange that the USA continued with Hitt & Miss governing long after the rest of the world had gone to throttle (petrol) or fuel (diesel) governing.

In practice, most engines were probably not on the governor when working and a decent temperature was maintained.

Peter

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

formatting link

Reply to
Peter A Forbes

speed adjustment. A whole power stroke is

adjustments can be made to the amount of mixture

I'll grant the drastic and crude but its pragmatic, for pumping applications the change in speed is not a big problem once a sensible hysteresis is agreed between the hit and miss. Loss of a power stoke does not matter with fuels that can tolerate the lower cylinder temperature (i.e they have a low flash point) its no different to lowering the average power on a conventional engine by throttling. It still has the advantage of higher volumetric efficiency, leading to better thermodynamic performance and conversion than throttling. Offset against this are the increased cycles per power stroke, so higher losses overcoming engine loads, friction and pumping air in and out of the exhaust.

Anyone care to comment further?

Arthur I now realise from your reply that this is very similar to what Daimler Benz were planning on one of their V8 luxury cars to increase efficiency, except they planned to chop out a cylinder or two sequentially and by using the engine management computer.

AJH

Reply to
sylva

speed adjustment. A whole power stroke is

adjustments can be made to the amount of mixture

From making this statement earlier on:

"From what I understand early gas engines operated on an open throttle and controlled revs by either having a spark or not. How did the fuel enter the mixture and did it just exit unused if the spark did not occur?"

I'm curious as to how you arrived at the above post?

Tom

Reply to
Tom

If you run your engine at 3600rpm or even 6000rpm (ie a modern engine). I guess missing firing cycles would not make such a big impact on speed - especially if the governor was electronic hall effect to give very fine tolerance. Would this be the key to efficiency... a bit like Pulse Width Modulation?

It's certainly an interesting proposition

Adrenalin

Reply to
pete

Have I wrongly inferred something from what I have been told?

Until Arthur pointed out it was holding the exhaust valve open I had assumed fuel would be wasted. Once he pointed this out a number of points in the systems favour spring to mind. Then I remembered a conversation with an engineer seconded to Daimler benz from Ricardo when I collared him at a wedding reception four years ago.

As Roland commented there was a problem with paraffin fuel and my question related to an early gas engine I had been told of I again inferred this would be less of a problem with a fuel with a low flash point (town gas) running with spark ignition.

As I have said before, I am not a collector but interested in engines and their history.

I also have an interest the third world, principally clean cooking, my interest was raised, by an Australian poster to a discussion list, in hit and miss engines because they ran on producer gas in the past.

Did you have a special point in asking?

This group has provided me with valuable answers in the past. AJH

Reply to
sylva

I can see two cases, one in the multi cylinder engine where the cylinders are cut sequentially. Here the loss of temperature between cylinders in the water jacket would be minimal, as long as the "misfire" did not markedly affect vibration, as I said I had hear this was being developed by Daimler Benz.

The other would be in a large single cylinder with a large flywheel, as in the past, where accurate speed was not essential but power modulation at high efficiency was. An asynchronous generator springs to mind.

It's a funny thing synchronicity where a couple of coincidental remarks that are strongly related happen at the same time.

AJH

Reply to
sylva

Only that you have made a blanket statement about H & M governing that incorporates two different principles, one where the gas supply is interrupted and the other where the exhaust valve is left open and no subsequent vacuum results. Both have totally different consequences on the further inductance of a fuel mixture. The first has a scavenging effect on the cylinder while in the idle strokes, whereas in the case of the second, no scavenging takes place. So on the next firing stroke one has a fully scavenged cylinder, while the other has to deal with residual products of combustion.. As for higher volumetric efficiency, as one system generally used automatic inlet valves.....

Tom

Reply to
Tom

Reply to
Roland and Celia Craven

Not my field anymore, but I know there are now several production car engines which have electronically controlled valve operation. This is really an area for comment by Eric Brain into whose field of expertise we have strayed ;o))

I have no hesitation in putting my hand up and asking Pete to describe what Pulse Width Modulation and "Hall effect" are..........

Regards,

Kim Siddorn,

Reply to
J K Siddorn

Imagine a stream of pulses each 10sec apart and lasting 1sec, or another stream with pulses 5sec apart lasting 5sec, finally 1s apart lasting 9s.

The average energy in the first is 10%, the second 50% and the last

90%. By adjusting or modulating the width of each pulse you have variable average energy.

This an effect that a magnetic field has on certain semiconductors. The actual effect might be linear but the normal electronic device you can buy is a switch. Above a certain level of magnetic flux the, solid state, switch operates.

Try google...

Reply to
Dave Liquorice

What electric cookers have done for 50 years. If you want to vary the output of something, and it's too difficult to turn it down to "half power", then you can instead switch it fully on and off in cycles. Long on and short off gives you 3/4 power, short on and long off gives you 1/4 power. Do it quickly enough and the switching becomes invisible.

A "simmerstat" on a cooker is a little electrically heated bimetallic strip, working a big powerful contact. Effectively it's an electro-thermal oscillator. By adjusting the duty cycle or "mark/space" of the switching times (turn the knob) you vary the effective power to the ring.

Solid state ultra-reliable magnetically triggered switching devices. They're noted for being very cheap to buy as ready-made chips, and insanely expensive when mounted into the distributors of Volvos ! Twenty odd years ago, the very poshest of keyboards had a Hall effect switch under each key - no spring, it used the same trigger magnet to pull the keycaps back up. Very light action, but still gave that "IBM like" satisfying "falling through the floor" action when you pressed a key even a little, not like the mushy sponges we typoe on today.

The physics is cute. Take a cube of a semiconductor and stick electrodes on four sides. Now apply a magnetic field across the other two. If you apply a voltage to two opposing faces you'll have charge carriers flitting from one side to the other, but the magnetic field makes them drift sideways while they do it. As a result, a small voltage shows up across the other two faces.

Reply to
Andy Dingley

"Andy Dingley" wrote (snip):-

Also more efficient - most devices, engines included, run less efficiently at half (or whatever) power.

Like a bigger version of a flasher unit (for trafficators)

Reply to
Nick H

Ok first I assume you are using the american term for petrol when you refer to gas? So on the modern engine the petrol injectors stop. However the Daimler Benz engine probably uses a mixture of throttling and cutting the injectors. Let's not dwell on this modern implementation as it is probably quite complicated.

The fault I saw with this was that although the exhaust would allow atmospheric pressure in the cylinder the inertia of the gas leaving the exhaust would still form a small depression and as the carburetor or gas mixer was not throttled some fuel would still go in via the inlet valve and be wasted.

Agreed the air still gets pumped from inlet to exhaust, an this is a full volume of air each cycle. This would actually cool the engine quite quickly, a bad point.

I cannot see this as by implication of having valves this is a four stroke cycle. Once the final power stroke occurs the piston has swept the cylinder contents into the exhaust. It then stays open and exhaust is sucked back in (notwithstanding my comment above that the inlet would still see some depression). This continues for a number of cycles until the governor re enables the exhaust cam. The first firing cycle then will be one whole revolution after the exhaust valve shuts under cam control. So the purging of the system will be the same as if the normal cycle had taken place, or have I missed something?

Yes the drag on these would do nothing for volumetric efficiency but presumably full mechanical opening and closing of valves must have featured in hit and miss engines, I'll have to make a point of seeing one working one day.

I had always assumed that hit and miss was just a kludge in very early engines, having their workings explained makes me realise they were a sensible solution to special requirements, just like the surface ignition engines (semi diesels) I was enlightened here about.

AJH

Reply to
sylva

Very happy to start again Tom, I have given my view now what do you find in error?

AJH

Reply to
sylva

Try desisting with assumptions and start again...

Tom

Reply to
Tom

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