Tommy Cookers wrote:even at 15000 rpm these engines will be over 85% mechanically efficient and 90% at 10500
being of small displacement and without the extreme b:s ratio and consequent frictional area of the current engines
(aircraft piston engines were at least 91% mechanically (friction) efficient at normal quite high power settings, as their
supercharging power is counted seperately and pumping 'losses' are similarly zero or favourable, this is a valid comparison)
so the increase in friction with rpm is balanced by the reduction in supercharging work (boost) with rpm
the real issue is the fall of in-cylinder thermodynamic conditions with falling boost as rpm exceeds 10500
this suggests minimal rpm range over 10500 (you don't design to need continuous knock sensor retard at 10500)
loss of in-cylinder efficiency over 10500 can be reduced by controlling turbine recovery/load for significant -delta P,even dilution
more importantly this will reduce pressure loss in blowdown and so increase efficiency outside the cylinder
so under these fixed-fuelling rules recovery can apparently be increased more than any corresponding decrease in crankshaft power
ie combined power will not fall over 10500, it might well rise
but I still think they won't run any greater rpm over 10500 than is forced by the gear rules
the gearbox is there to be used
btw I suggested these engines would have one exhaust manifold slightly longer than the other to give evenly-spaced delivery of the exhaust 'pulses' to the turbine, the recent photos seem to confirm this ??
the above old post bumped in response to ex's question (there's some good posts around P286)
the friction loss of 9% is drawn from 91% mech efficiency stated for the (UK) Puma engine in 1932, and more usefully
from (otherwise unpublished) data in C Fayette Taylor (of Allison tests basically of friction vs mean piston speed)
these aircraft engines have (minimal) piston contact areas and bearing areas not dissimilar to 2014
the 7% for 2014 was my guess based on anti-friction coatings and fluids
the 2014 engine running at 11500 rpm has a mean piston speed of about 3600 fpm (rather low for a modern race engine)
Allison showed frictional mep of 20 psi at 2800 fpm (extrapolatable to 26 psi at 3600 fpm) in valves-off motoring tests
at an imep of 450 psi there would be 11 psi added for mep-dependent friction at 3600 fpm
so total frictional mep is predicted to be 37 psi in our 2014 engine at 11500 rpm, about 9% (using traditional materials)
based on valves-on unthrottled motoring (fmep 37 psi at 3600 fpm) we get 48 psi, about 11% (using traditional materials)
and clearly the predicted increase in frictional mep between 10500 and 15000 rpm is roughly proportionate to rpm
conventionally engine power increases with rpm, the above is consistent with the concept of 'friction varying with the square of rpm'
but 2014 will have mep falling with rpm over 10500, so predicted friction rises more slowly with rpm
the above is consistent with a Coulomb term, an rpm-dependent (inertia) term, and mep-dependent term of frictional loss
the post reminds me IMO that running 10500-15000 means much running at a sub-optimal CR (or sub-optimally in other ways)
designing for 10500-12300 running or 13000-15000 doesn't
(ok we might design for 10800-12600 running vs a design for 12600-14500 running, to allow some margin for non-ideal gear ratios)
agreed, other inferences could be drawn from the 2014 rules
eg as previously posted, sub-optimal CR would leave more exhaust pressure energy for recovery (at high exhaust pressure)
