But no one yet has mentioned the magic words -- motoring torque. It's hard
data to find, I don't have any that I can share for F1 or other racing
engines, sorry.
On Sat, 9 Feb 2002, Ruud van Gaal wrote:
> On Fri, 08 Feb 2002 14:20:01 -0600, rrevved <ed_b...@nope.com> wrote:
> >On Fri, 08 Feb 2002 14:25:57 GMT, r...@marketgraph.nl (Ruud van Gaal) wrote:
> ...
> >>Still, I'd think an F1 car will use a much lower inertia (less
> >>weighing flywheel) to be more responsive. Although ofcourse a higher
> >>inertia gives you a more stable accelerating/braking car.
> >Correct. And that's why engine braking is MUCH higher on any racing
> >car than a street car. Have you ever driven a purpose built racecar?
> Nope, sorry to say that I haven't.
> >In case you haven't, when you close the throttle into a corner,
> >you get much more natural (engine) braking than a street car.
> >A LOT more. It changes the handling of the car dramatically.
> Ok, so then it seems both the engine inertia is lower, AND the engine
> is fighting (giving negative torque) more than in a streetcar.
> >The reason is the lightened driveline components as well as the
> >generally lower gearing of the gearbox.
> The lower gearing is a secondary effect though and has nothing to do
> with the engine wanting to brake itself. The thing is I'm not looking
> for a net effect, since if some mysterious power was rotating the
> wheels, the engine wouldn't brake anyway, and you'd think engine
> braking torque was 0, which it isn't; it's just counterbalanced.
> So forget everything behind the engine and flywheel. Uptill there it's
> an autonomous system and should be viewed as such to look at 'engine
> braking'. Keeping the car in neutral or with the clutch applied, for
> example.
> > But forget the gearbox
> >for a second.
> Oh, yes, right. :)
> > That racecar will slow very quickly, without any
> >brakes whatsoever versus a street car. I'm talking about a
> >purpose-built race car here. F1/F3/Sprint Cars/Nascar/ CART, etc..
> >They all use lightened flywheels. They all will slow down much
> >quicker than a street car.
> Ok, then probably, with your experience, given a 0.74 coefficient for
> an F1 car, a street car might end up more like half of that (instead
> of being more).
> >As I said, just listen to the cars in the paddock. That should be
> >enough proof that the engines slow down very quickly when the
> >throttle is suddenly closed after being blipped. There are no
> >aerodynamics or torque or wings or anything else when they
> >do that. Just engine and flywheel.
> That's true.
> >>Did you drive it at LeMans back then? Would be way cool. :)
> >Graham Hill drove it at LeMans in 1956, I got it in about 1970
> >and raced SCCA with it. Chassis number 247.
> Must have been awesome. :)
> >I sold it in about 1975 because I'm stupid ..:)
> Ouch! ;-)
> >>I think that's more to do with the low INERTIA of the components,
> >>rather than the high braking coefficient (which just gives you the
> >>torque with which the engine is trying to slow down, not the actual
> >>speed at which this finally happens, since that is related to inertia
> >>as well).
> >Correct. Guess what causes inertia? You need an object
> >which has weight and a force which puts it in motion. The result
> >is a certain amount of inertia.
> Not at all; inertia is a fixed property of an object, just like mass.
> Nothing 'causes' inertia; it's there because the thing has
> molecules/mass.
> Like F=m*a, T=I*w where T=torque, I=inertia, w=angular acceleration.
> Mass is a constant property of an object, just like inertia (rather
> the inertia matrix).
> > If the object is lightweight, there
> >isn't as much inertia as with a heavy object.
> Right, but the inertia of the object remains fixed; it's the torque
> that changes (the engine pushing or pulling rotationally), and the
> angular acceleration is a result of that (w=T/I).
> >>F1 engines must be powerful to push the cars through the aero drag,
> >>even with high wing settings. So a massive engine braking contradicts
> >>that goal, since at high RPM the engine braking would really fight the
> >>engine from generating any torque to push the car forward through the
> >>air.
> >When the throttle is closed on any car, it isn't 'pushing'
> >anything.
> Surely the engine is pushing the drivetrain in the reverse direction
> of which it (the engine/drivetrain) is rotating in. Otherwise it
> wouldn't decelerate. So the engine is torqueing the drivetrain, no
> doubt in my mind.
> > Drag and inertia decide what happens, not power.
> Power is just a measurement of energy and is just a nice statistic,
> drag and inertia are inputs, yes.
> >>So I'd think striving for less engine braking would be beneficial
> >>for an F1 car; at high speeds you depend on the aero downforce and
> >>brakes to get good braking, instead of using the engine as a brake.
> >Nope. The purpose of low inertia driveline components
> >is for weight reduction, acceleration enhancement and braking
> >improvements. I'm surprised you don't know this.
> Didn't know about the braking improvements, but that surely does make
> sense. It seems though the downforce and brakes are most important,
> since the downforce generates implicitly the maximum longitudinal
> braking force you can apply to the car without locking the wheels. I'd
> say the brakes of an F1 car are powerful enough at any speed to lock
> the wheels, so in that sense, engine braking is of no importance for
> the total amount of braking you can apply to the tarmac; the limit is
> easily reached by the brakes alone, so engine braking just gets you
> there faster.
> But I may be completely off there, and F1 brakes may be impossible to
> lock at high speeds (but I don't think so).
> >With your theory, a racecar would be better off with
> >heavy driveline components, which of course is not the
> >way it is done.
> Hm, it's getting late. Have to rethink that. Indeed, less
> flywheel/engine inertia gives better deceleration. In my perspective,
> it's probably that the brakes can overcome the flywheel inertia as
> part of their job, so at a certain point you don't need a lighter
> flywheel.
> But I guess then maybe the wheel brakes aren't capable of locking the
> drivetrain at 300 km/h? Otherwise they could brake both the wheels and
> the engine, so the flywheel wanting to spin on wouldn't make that much
> of a difference.
> >> engineAcceleration=engineTorque/engineInertia
> >Ruud, guess what causes that inertia? One -major- thing is the
> >flywheel.
> Right, that and the engine parts themselves.
> >>Nope, they're completely separate. I define the engine braking
> >>coefficient as being a measure of compression braking, and any
> >>friction in the engine. I don't try to include any other implicit
> >>effects.
> >You should.
> >Lightened flywheel/driveline is a MAJOR performance
> >consideration in a racecar. They actually drive totally
> >differently.
> It's in there; my engine inertia includes both flywheel and engine
> parts themselves (they are summed). The rest of the drivetrain
> ofcourse also has inertia and can be tweaked in every which way.
> The inertia just doesn't come into play until there's a torque from
> the engine and you want to know how fast all the parts in the
> drivetrain are going to accelerate (which is done in 2 parts in my
> sim; preclutch and postclutch; they can rotate at different speeds,
> which I need because I have an analog clutch simulated).
> Interesting chat, haven't written this much stuff in a post for some
> time. :)
> Ruud van Gaal
> Free car sim: http://www.racer.nl/
> Pencil art : http://www.marketgraph.nl/gallery/
