J.A.W. wrote: ↑
Sat Aug 28, 2021 12:32 am
Tommy Cookers wrote: ↑
Tue Jun 14, 2016 11:56 am
...the Sabres smaller bore and much shorter stroke would seem ideal for poppet valves
(given the fuel quality revolution)
Sabre funding was maintained both to keep a sleeve-valve foot in the liquid-cooled door,
and a reserve engine for the Vulture.
As regards this view T-C, which you have reiterated in this thread, you might find this
period graph to be of some interest since it shows the Sabre's architecture did in fact
suit the volumetric efficacy of sleeve valves, versus big 2V hemi/4V pent-roof poppets...
(though I don't see what that has to do with the Sabre architecture) ....
the main 'American 2 valve poppet' engine then was the 1820 Wright Cyclone (made for 25+ years)
conveniently for this discussion it had the same bore:stroke ratio as the Bristol engines shewn
but more impressively the later Cyclones at takeoff ran at 2800 rpm - ie 16.3 m/sec or 3850 ft/sec mean piston speed
and the Cyclone supercharger efficiency was improved from 60% to 75% during WW2 - and more post-war
this improvement (as I said before) relieved the suggested problem of poppet valve breathing
over a few years the 1820 Cyclone doubled in power
logic says (though authors don't think of this) that historical improvements in strength ....
(allowing more piston acceleration/rpm but demanding relatively bigger ports/valves at some point)
imply that the ideal bore:stroke ratio would have increased over the years
ie that the old low bore:stroke ratio engines were right for their times
because (strength considerations say) rpm can increase in proportion to the square root of a stroke reduction ....
reduced cylinder volume (eg given bore:stroke ratio) increases breathing (relatively) of both sleeve-valve and poppets
reduced cylinder volume (eg given bore:stroke ratio) increases rpm so increased breathing is handy
these were the basis of Ricardo's and Heron's US Hyper program suggestions ie improving the volumetric 'efficiency'
ie displacement per unit time was raised by raising rpm - but unlike motor sport WW2 didn't have any engine rules
and increased bore:stroke ratios (as Napier) ie tend afaik to reduce the sleeve-valve's breathing (relatively)
(the port height reducing with stroke so port area doesn't rise with rpm rise)
though increased bore:stroke ratios (as Hyper) does increase the poppet-valve's breathing (relatively)
(so only 2 poppets were deemed necessary)
presumably cylinder size/bore:stroke reduction would allow poppet valve breathing to equal sleeve valve breathing
the Sabre was apparently intended for 4000 rpm but only got there via a convoluted and protracted route