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Posted to uk.d-i-y
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The physics of cars - a question sequence.
On 08/04/2016 21:43, wrote:
On Friday, 8 April 2016 13:03:23 UTC+1, The Natural Philosopher wrote:
Engine RPM is not directly and irrevocably poroprtional to road speed
WE have a gearbox.
Yes, and in every gear the engine RPM is directly and irrevocably proportional
to road speed.
That's only true for a manual transmission. At the risk of being accused
by you know who of introducing another red herring, as soon as you
insert a torque converter between the engine and gearbox, your cosy
relationship goes out the window! So we ignore that, and assume that we
have a conventional manual transmission.
The speed/time graphs are exactly the same shape in every gear.
They're not, actually. They get considerably less steep in each
successive gear.
Gearbox output torque vs road speed graphs are essentially the same
shape in each gear but are, of course, scaled differently.
Acceleration is greatest at the
highest TORQUE regardless of which gear is selected.
This is where we go round in circles! It's true that peak acceleration
in each *gear* coincides with the engine's peak torque. But, in many
cases, you can obtain a *higher* acceleration at a given road speed by
using a different gear and running the engine at max power.
That acceleration will be lower than you would get in that 'different'
gear at peak engine torque but it would be at a higher road speed. For
maximum accelerative performance you need the maximum available
acceleration all the way up the speed range - so you need to run the
engine as close to max power as possible.
--
Cheers,
Roger
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