[Slipstream-devel] New engine model

[Dimitris Papavasiliou]

Hello,

The engine model has been changed in the last commit to more closely
resemble the actual setup of most motorcycles.  Up to now the model
had a lumped design which grouped the gearbox and clutch into one
joint which was at least suspicious in terms of realism but seemed to
work and was fast (There was one point where it certainly wasn't
realistic and that was that the clutch had to transmit the resulting
torque after speed reduction in the gearbox).  The current model has a
separate joint for the clutch which connects the flywheel to the
gearbox mainshaft and a proper gearing joint that applies the forces
between the engaged gears.  The only difference between this setup and
a typical real-world one is that the primary reduction is lumped with
the gears so that there's only one gearing.  This was done in order to
make the simulation faster but has the consequence that the engine
needs to rotate in the opposite direction compared to a conventional
setup.  This theoretically makes a difference in the gyroscopic torque
that is applied to the chassis but whether that is noticeable or not
is questionable.

There's an extra parameter now in the engine page which is the peak
torque that can be transferred through the clutch before it slips.
This can be set to a little above the maximum torque the engine can
produce.  A theoretical figure for that can be found in the torque
diagram in the engine page (it's the peak of the green curve) but this
doesn't take inertial losses into account so the actual figure would
be a little lower.  This can be found by starting slipstream with
-Oexamine='math.length(parts.engine.torques[1])' and then plotting the
resulting file to find the actual maximum torque developed at the
crankshaft.  Anyway the theoretical figure will do fine.

The vehicle handling seems a bit funny now with the new engine but I
can't place how exactly.  Maybe it was some other change I made and
forgot about.  Feedback is welcome.

D.