One of the limiting factors for RPM is piston speed, which is not so
important in itself, but translates into acceleration with each engine
stroke, and thus is transformed into force. As an example I submit my 53
Austin Healey 100 with a stroke of 111.1 mm and a red line of 4800 RPM.
These forces can be reduced in only two ways, either by reducing the mass of
the reciprocating parts while strengthening the pistons, crank and
connecting rods or by reducing the distance traveled in the bore (shorter
stroke). Honda's ability to increase RPMs while also increasing stroke
indicates they have chosen the first option. Ah the wonders of modern
materials science!
Bill Lawrence
christian147@juno.com wrote:
> >torque = cylinder filling.
> not quite, torque is proportional to cylinder air trapping, how much air
> is in the cylinder when at IVC, thats not the same as filling.
>
> one of the great things about computer based finite amplitude wave
> simulators is that you get to really change just one thing and see what
> difference it makes in terms of torque and HP curve and pressures in
> intake, cylinder, exhaust and intake and exhaust relative mach indexes.
> >From that I can say that bore to stroke ratio has little effect on the
> torque curve, but when you build real engines and change stroke you
> almost always change the stroke to rod ratio, that has a very real effect
> on torque curve because it changes peak piston velocity and where it is
> reached, that changes peak port velocity and how the valves should be
> timed.
>
> as for the long stroke being a low speed motor, I point to the new K24
> motors from Honda (current Accords, CR-V and Element). 2.4 L 100mm
> stroke 86mm bore, not very square, but they rev as freely as the engines
> they replaced (more so in the case of the Accord).
>
> Chris Hall
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