Hi All, writing a blurb to display with the Petter S-type made me think about the principles of operation of semi-diesel engines and made me wonder if semi-diesel, four-strokes were produced. The only semi-diesels I know of are two strokes like the S-type, Bolinder, Tangye M, Crossley VO and a Moteurs Ballot I've seen.
I can't see in principle why it wouldn't work, though there must be some problem, or surely they would have been widely marketed.
The term semi-diesel does not appear anywhere until after a certain company that flogged two-stroke engines used it in their advertising. The reasoning was that it ran on fuel oil similar to a diesel but used half the compression pressure and cost less than half the costs of a diesel to run per hp developed.
Once applied to their product it became a general term used to describe similar two-strokes such as the Bolinder etc.
The Akroyd principle is surface iginition with compression of air only in the cylinder followed by injection of fuel at peak cylinder pressure. It applies to two or four strokes.
..... I think .....
Cheers Paul
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Internal Fire, Museum of Power, Wales
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What was that design that ran on diesel with injectors but was spark ignition
4 stroke with a quite low compression ratio? Hesselman? Could that be described as a 4stroke semi-diesel?I'm not sure if it was lamp start or needed petrol to start. I know about Kelvin petrol start diesels- they are full diesels once started Robert.
I wonder if there really is a firm dividing line between 'semi-' & 'full-' diesel? I know that the semi-diesel uses surface ignition to overcome lower compression ratio, but there were plenty of engines needing glow plugs/papers/cartridges whatever, and I think I'm right in saying that some relied on a hot spot to aid combustion. Where, for instance, do the Field Marshall tractor engine, and the later Widdop 2-strokes, both of which used CAV-type injectors, fit in? The Widdop, AFAIK, was a straightforward development of earlier lamp-start versions. Comments?
Both diesels. Combustion is a result of the heat generated by compressing the air followed by injection when running normally. Anything that does not achieve ignition through the heat generated by compressing the air in the cylinder is not a Diesel. Assistance in starting such as wick etc does not count as it is not a permanent heat source.
A lot of wick start engines, such as the Ruston CSB engine in our roller, are started on half compression and need the wick to kick off before going onto full compression. The CSB will start immediately even when cold on full compression but you have to weigh at least 20 stone and be very brave to swing it or it will chuck you 20 ft backwards if you don't get it over compression!
The term semi-diesel is not really meaningful, an engine either uses the diesel principle or not, most "semi-diesels" run on the Akroyd principle or a development of - i.e surface ignition.
Agreed, but my understanding is that with some of these engines there is an area designed as a 'hot spot' to help combustion to start
*during normal running* - it could even be the end of the wick holder, for instance.
started on half compression and need the
immediately even when cold on full compression
will chuck you 20 ft backwards if you don't
In contrast to the Lister CS/JP and the early 'L' series Gardners, which used *increased* compression so as to start readily without wicks etc. I suppose when electric or air start was an expensive extra feature, there was a limit to the size of engine to which this method (higher comprression) could be applied.
principle or not, most "semi-diesels"
Except that the amount of surface ignition needed depends upon the load, with the Bolinder type of adjustable sprayer providing adjustment on that basis? Of course, one can always argue that we shouldn't call any of them 'diesels' anyway
I have a cross section of a Widdop combustion chamber and it doesn't show anything.
We have a number of engines here that require wicks and all of those are definitely for starting purposes only. In the case of the CSB we ran out of wicks a while back and tried heating the end of the wick holder to cherry, didn't protrude into the combustion chamber though so didn't work.
Certainly always thought the FM was a Diesel, guy down the road has 12 (yep) will enquire, got to know now.
Apparently one of the new Paxman designs runs such a high boost pressure that it will not start without the entire engine being pre-heated as you cannot achieve a high enough temperature with the cold engine and the turbos not running.
Paul
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Internal Fire, Museum of Power, Wales
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I know we have been through this before, but I don't think we reached a satisfactory conclusion and it is certainly something that I would like to know.
Flicking through a few of those 'modern power generators' type books last night, suggested that the hot bulb Akroyd type engine injects fuel into the bulb during the inlet stroke where it vapourises. The following compression stroke forces air into the bulb forming a combustible mixture with the oil vapour, which is then ignited by a combination of compression and the heat from the bulb. Diesels and semi-diesels inject fuel near the end of the compression stroke, combustion taking place immediately. The only material diference between the two appears to be that in the full diesel the heat generated by compression alone is sufficient to initiate combustion, whereas the lower compression ratio of the semi-diesel requires the assistance of a deliberately poorly cooled part of the combustion chamber to ignite the charge.
Now, 'book learning' is a wonderful thing but I'm sure we must have people on this NG who actually own such engines and can confirm at what point in the cycle fuel is injected.
A Petter S sprays its fuel about 15deg BTDC but a Tangye M and Ogle spray a little after BDC. I suspect we are the unintended victims of the incorrect historical perspective that gives Rudolf Diesel a pre-eminent place whilst largely ignoring the work of many others especially Herbert Akroyd Stuart. The diesel name having become the market leader I guess other makers jumped on the bandwagon to gain credibility and sales. Can anyone pin down the first occurrence of the term semi-diesel? just my 2p Roland
Seems to make sense if the compression ratio is lowered to make use of full boost from a turbocharger. At cranking speeds the turbo has no effect, so the temperature is not high enough to fire the diesel. Once running the turbo is spooled up (and hence recycling energy for pumping the air in from the exhaust) and the volumetric efficiency exceeds 100% so the compression pressure rises to a working pressure.
How do they get the engine up to temperature?
Finally how long is it likely to take me to reach you from Barry, near Cardiff?
My big Campbell is a four stroke Semi-Diesel, lamp start, surface ignition engine or hot-bulb, (which I prefer) engine. It runs on diesel although I haven't got as far as trying yet. I do know that the Hornsby I used to own would run on diesel, kerrosene or parafin but preferred kerro as the diesel was that bit too heavy. After asking around I was told this was because of the compression ratio and also when comparing the sprayer from the Hornsby and the S-type the spray produced was like comapring a firemans hose to a bathroom shower. So the level of assistance required by the hot-bulb for purposes of achieving igniton is very variable depending on the level of sophistication of the design. Also the Hornsby didn't have any form of ignition tube as is the case with most later engines. Also incidentally in S-type literature it claims that if you use the cartridges to start the end of the holder after starting performs the function of the ignition tube. In my experience the cartridges cannot be relied on like a blowlamp and the old boatmen told me that in anything smaller than the
18/21 they had a tendancy to block the exhaust port with spent cartridge. Finally I have also been lead to believe that the deviding line between semi and full diesel is around a compression ratio of 14:1. Over that and you are only introducing heat to help start the engine.
Oh and Widdop produced both semi and full in tandem. Lovely quality engines.
Although the Field Marshall probably does scrape into the 'full diesel' category, I suspect that something such as the end of the paper holder does help with the ignition. My experience was that if you stall the engine, you have to be *very* quick restarting it not to need to start from scratch with another paper. They do have a cartridge start option, but the cartridge holder is a long way from the cylinder & must be connected by a lengthy port of some sort.
The big 2-stroke marine Crossleys, on the other hand, have only a marginal need for the wicks for starting & will happily restart without for some considerable time after stopping (just as well for a direct coupled direct reversing propulsion engine )
But where have they all gone :-( I've always fancied one, had to turn down an EMX2 in about 1966, too big for the boat & more money than we had at the time :-(
Absolutely, still means the use of external heat to start it though - and it is still a "D"iesel.
Cardiff to here is a straight run down the M4 and then 20 miles on A roads, hour and a quarter to hour and a half depending on traffic and the weight of your right boot.
Paul
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Internal Fire, Museum of Power, Wales
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I wasn't implying any different, the advantage has to be in the heat recovery from expanding exhaust gas through the turbine to actually blow the piston down, the next stage is the turbo compound which I believe is on some scania lorries (the nomad having lost out to fan turbo props), notwithstanding the turbo alternator caterpillar are proposing.
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