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More fuel burned, less cooling needed?

Home Forums TR3OC Members Forum More fuel burned, less cooling needed?

This topic contains 5 replies, has 2 voices, and was last updated by  james herbert 3 months ago.

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  • #7164

    John Allaway
    Participant

    This isn’t important, just idle pondering, but does anyone happen to know how come the big bore kits for Triple offered by some manufacturers / retailers, have far fewer cooling fins than standard?

    This applies to several (across the British bike range, not just triples).  The most pronounced example must be the Hyde kit, which has only 4 continuous fins, each of which is considerably thicker than the fins on a standard cylinder block.

    Given that these engines don;t exactly have generous cooling (especially for the middle cylinder) to begin with, I struggle to understand how its possible to get away with increasing the capacity while removing precious surface area.

    Any thoughts anyone?

  • #7169

    james herbert
    Participant

    Cooling considerations. Let us only bother with the case of full power, most fuel burnt in least time – one quarter of it is usefully converted to thrust, the rest divides approximately half each to exhaust gas and discarded via engine metal and the oil cooler. The fire starts when the piston is very near TDC and is released when the useful 25% has been extracted.  During that extraction the burning gas cooled as its volume, but not weight, increased.  At TDC heat loss from gas is mostly to cylinder head and piston crown, but toward mid stroke before the exhaust valve opens heat is also flowing to cylinder wall.  The cylinder head and piston crown surface is 12% larger in a 71mm bore than 67mm one, the cylinder wall area increase is 22% larger if the stroke increased to 82mm. It is unlikely that power rises in direct proportion to capacity because the peak power rpm drops; assume 68 as opposed to the factory 58hp.  So 17% extra power and concommitent heat losses.  Radiant heat is proportional to the 4th power of temperature,(4th power means square of square, so double temperature radiates 16 times as much heat) or more usefully to lose an extra 17% the temperature only has to rise 4%.  I suggest the centre of piston crowns are much more critical than the nearly irrelevent fins.  The wind blast is better to left and right of outer cylinders than behind the centre and the Hyde block has fins which connect centre cylinder to that wind, also provide stiffness.  A proper air cooled engine has baffles to make air scour hot metal, see Deutz, Vespa or Pratt & Whitney.  I climbed Kirkstone Pass two up with a tailwind equal to my progress, very hot iron 500 single, oil smoke from rocker leaks but no melted piston.

  • #7171

    John Allaway
    Participant

    Interesting James, thanks.  There was me thinking the explanation was probably that the manufacturers couldn’t manage to make barrels with fine finning similar to the originals and hoped they’d get away with the best they could do, and were fortunately right!

  • #7174

    james herbert
    Participant

    Triumph, like most bike makers except BSA, had neither foundry nor forge, it bought metal parts formed to shapes it had drawn, costs vary with complication. Drilling, tapping, facing, polishing done in-house. The same applies to aftermarket go faster parts, though these may well be machined by an outside workshop too.

     

    An enthusiastic marque believer asks which magneto should be on a 1932 XYZ350 is shocked to be told “Whichever had not refused credit to XYZ that month.”. The guarantee was 13 weeks parts only, and not for proprietry things like chain.

     

    Cooling fins often placed to please draughtsmen eyes.  NB huge cut-outs for pushrods making hot cylinders D or pear shaped.  Harold Willis did measured experiments on Velo KTT, it needed those deep fins for 28hp to keep below the temperature he decided.  As I demonstrated an all iron cylinder and head coped without airflow but burning oil spill smoke drifted gently forward past my left knee.  Bikes are not really air cooled, they rely on radiation.

     

    You can learn much from study of aircraft engines which had to work continuously hard and really were air cooled after real measured experiment.  A few still in commercial use 80 years after designed.  Dependable Engines.

  • #7177

    John Allaway
    Participant

    I’ve seen various air cooled radial aircraft engines, or bits of them. The finning is incredibly fine – almost impossible to imagine how they managed to cast such fine fins and complicated shapes.  Surely surface area is a genuinely important factor re. cooling of air cooled cylinders and heads?  In which case, fewer, coarser fins ought to be less effective than more numerous and finer, all other variables being similar?

  • #7188

    james herbert
    Participant

    Before the First World War few petrol engines would sustain full power for more than a minute, once exhaust valves glowed dull red the undoped fuel would pre-ignite. That was ok for a boat or car, just use a big engine at low rpm. A Rolls Royce Silver Ghost was 7 litres, for crossing France at 45-50 mph. Ford T 3litres for 30mph. M Bleriot needed at least 20 of the 25 for 45 minutes. So he had holes round cylinder toward bdc, these released burning gas before the exhaust valve and thus saved it from heat. The inlet mixture was not prone to escape as it was not above ambient air pressure. The Gnome Monosoupape rotary was very ingenious: had a single ohv exhaust symetrical in centre of head, best cooled seat possible, then after the hot gas had cleared it stayed open and fresh cool air swept in. Other air became rich petrol mixture and entered via valve in piston crown. On top of the cooled ex valve there was no vibrating reciprocation. Cylinders turned a pure circle, pistons and rods slightly less pure but on a different centre. The flimsy airframe was grateful.

     

    War provided real money, logical analysis and logic. Every one of British military aeroplanes in 1914 had French engines. Many never lasted 10 hours. Daimler, Maybach, Austro-Daimler, BMW made water cooled 6 or V12s. The RAE, as it came to be, Farnborough tested to see if, and how, air cooling would be effective. Yes, but on the proviso that: valves were ohv, radial in hemispherical head which must be aluminium. Every one of those features essential. At Shuttleworth we can see the huge daddy of all fully radial Rudge heads, ex RAE circa 1919.  AJS used iron but their 350 beat the 500s in 1921 simply by doing as RAE said.

     

    The Army needed a tank engine, Ricardo made that and noticed the varying qualities of petrol from Borneo, Pennsylvania etc. Antiknock additives followed so hot valves no longer caused pre-ignition.

     

    Ignore the small inline ones, all major air cooled aero engines had individual cylinders and eventually sheet metal guides, or baffles, to make thin freezing air scour the surfaces.  At take off in tropics, or altitude, there was often water injection because not enough air, also instead of spit and hope oiling there were floods of it to cool the sodium filled valve stems.

     

    Nobody can believe the rear of a Trident’s centre pot is air cooled.  I don’t believe the usual British single with worthless finning where pushrods go was wise.  Now Vincent… Copied the Farnborough layout, as Armstrong Siddeley, Wright and PW.

     

    In 1957 BOAC demoted the Stratocruiser from the Atlantic, sent one to Nigeria to wow VIPs but it failed to survive taking off, fresh engines and dawn needed for trip home but the Merlins of Argonaut coped daily.

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