Is engine braking really necessary?

[Tommy Cookers]

I was thinking that these days it wouldn't be difficult to eliminate engine drag on the over-run (by opening the throttles, fuel off (although the rules might not allow it).
Regarding the diff mode, isn't this driver-selected corner-by-corner and one might have thought that an open diff mode would help corner entry on tight corners ?

[hardingfv32]

You don't want to completely eliminate the engine drag torque (by rev matching, blipping etc) because there will be nothing engaging the diff locking to stabilize the car.

Is this physically correct? If they are using a hydraulic actuated diff clutch system, the two axles should have a resistive (better term?) connection to each other at all times regard less of the engine's connection.

Remember how there was the special Benetton front axle brake system.

Brian

[autogyro]

Diff locking or slip control is only needed if engine braking (overrun) is used or of course when power is applied to the powertrain in corners.
If it were not for energy harvesting, a modern F1 car would probably achieve a faster lap time with an open diff, no engine braking and only application of power in a strait line.
The trade offs would be interesting to calculate.

The reason is the same as for all other mechanical developments that have been strangled from use in the last few decades.
Downforce levels that are way to high and totaly artificial compared to conventional race car thinking.

[gato azul]

Engine braking does not add heat load to the brake rotors. This can be significant with a heavy chassis and steel rotors, which may be subject to fade.

What I was talking about has nothing to do with using the engine braking to physically slow the car. Im talking about using the drag torque to apply locking to the diff and stabilise the car. I believe this is the primary use of engine drag torque these days.

You don't want to completely eliminate the engine drag torque (by rev matching, blipping etc) because there will be nothing engaging the diff locking to stabilise the car.

Tim

This would [only] be the case, with an mechanical locking diff using "ramps" on teh power and coast side [ZF type diff].
Not sure if F1 uses these type of diff today. With an hydraulic "clutch pack" powered by a hydraulic system, and ECU/Computer controlled, you could achieve any amount of locking in any phase of the corner, independ of engine drag/drive torque.

IMO it is not "strictly nevessary", as for the OP question, the car would drive/rcae without it, but as other allready said, it can have it's benefits ( and draw backs).

It provides a means to alter/control/change brake bias (brake torque/force distribution) during the braking phase and from corner to corner.
For a high downforce race car [but not only there], this can be an advantage.
With other (reglement related) limiations for systems to do the same thing, it may was just a means to an end.

As others coined the phrase, " a form of rear axle stability control", or rear axle ABS.
I'm pretty sure, today they (F1) use mainly KERS for a similar purpose.

In more conventional forms of racing (manual gearbox) with RWD cars, it can be used to help the cornering in some cases, by using the last downshift (if more the one gear needs to shifted down), to "unsettle" the rear a bit, to help turn in.

On a side note.
I know more then one race/championship winnig driver(s), who when racing FWD Touring cars, clutched during downshift, to make sure that he does not
get any unsettling effects from downshifts (no engine braking wanted).
Thee guys where usualy the once who where the latest of the late brakers, and any form of aditional braking torque, had the potential to make the wheel lock, which in an FWD car, means U/S and losing the line.
With 19" rims,big brakes & relatively small tyres, there was no shortage in stoping
power from the brake package, so you did not need the engine braking to gain overall improvements in braking performance.

It's on of the "horses for courses" things, different people will do it differently, depending on the type of car/bike and racing & perhaps personal preferences/driving styles. I don't think you can come up with an cut&dry yes/no
answer to the OP question.

[Tim.Wright]

Diff locking or slip control is only needed if engine braking (overrun) is used or of course when power is applied to the powertrain in corners.

Not entirely true. Even if you don't have engine braking, you still want to provide a stabilising yaw moment on the rear axle under brakes. Using the coast locking on the diff is just handy way to do it.

This would [only] be the case, with an mechanical locking diff using "ramps" on teh power and coast side [ZF type diff].
Not sure if F1 uses these type of diff today. With an hydraulic "clutch pack" powered by a hydraulic system, and ECU/Computer controlled, you could achieve any amount of locking in any phase of the corner, independ of engine drag/drive torque.

I need to refresh my memory, I was under the impression that the diffs were electrically adjustable by the driver, but were not fully active. So still effectvely a normal locking diff but with static adjustments made hydraulically.

Does anyone have any more detail on this?

Tim

[autogyro]

I think teams are allowed to adjust diff action through a set number of states in an attempt to balance and control rear axle energy harvesting with the rear brakes.
I believe fully automaticaly controlled limited slip action is banned because of the traction control risk.

The FIA's obsession with banning traction limiting while allowing the automation of almost every other driver skill became more than just amuseing a long while ago.
It has not helped with KERS development as can be seen by the raft of failures.

The problem today is the risk of braking traction when energy harvesting is applied under hard braking.

Having the motor/generator on the crankshaft doesnt help. (there are much better ways to harvest the energy).

IMO with high downforce (if no KERS were fitted) an LSD for cornering or lateral control would not be needed and could reduce performance due to the torque loss at the diff which is always increased by their use.

I could give you a hydraulic diff I designed when Gordon Murray fitted his rear axle brake pivot device to his F1 cars.

It simply applied controlled slipping retardation to the inside wheel on corners to increase turn in, stability and cornering speed (without extra brake wear).
Banned of course (as were Gordon's brakes, helped disallusion him to F1) and anyway that was before the rediculous levels of downforce came in, where such ideas would not be nearly as worthwhile (like most things mechanical now).

[Tommy Cookers]

Freewheels were quite common on road cars (lastly on 2 strokes), they allowed skill-free clutchless downchanges on the over-run (but not engine braking).

F1 rules now force the drivers to 'play racers', by going through all the gears sequentially in a fake display of skill ?

Road cars have been stuffed with very cheap microprocessor-based control systems for 25 years now; shouldn't engine braking be left to the historics ?

[autogyro]

Road cars have been stuffed with very cheap microprocessor-based control systems for 25 years now; shouldn't engine braking be left to the historics ?

Depends how much of your car you want to control yourself and how much you want someone else to do it for you.

[alelanza]

To address the thread's subject. It's not so much that it's necessary but rather, how could you get 'rid' of it? with the current regulations and engine designs? short of going into neutral which will then make for a jerky destabilizing transition to power-on.

What i'm not 100% clear on is on how they achieve the various engine braking levels that the driver chooses, i guess it can be as simple as idle speed setting, but other than that with the current engines spark advance is the only other thing i can think of that could affect this.

Rear wheel braking is balanced with KERS harvesting to prevent rear wheel lock up.

And brake balance. And i'm not sure if you're implying that the energy harvesting dynamically adjusts to prevent lockup? because it does not. Both harvest rate and engine brake levels are non automatic as per the rules, the driver has to select their intensity and then live with it through the corner.

Because all items that effect potential rear wheel lock up are computer controlled

They aren't as of 2008 and onwards

If it were not for energy harvesting, a modern F1 car would probably achieve a faster lap time with an open diff, no engine braking and only application of power in a strait line..

Nope, F1 cars from 08 and 10 didn't have KERS nor EBS and still had entry, mid and exit of corner diff settings, with only mid-corner being somewhat close to an open diff.
And, how would you power out of a corner with an open diff? that's one notion i wasn't expecting to find in a racing related forum

[autogyro]

Nope, F1 cars from 08 and 10 didn't have KERS nor EBS and still had entry, mid and exit of corner diff settings, with only mid-corner being somewhat close to an open diff.
And, how would you power out of a corner with an open diff? that's one notion i wasn't expecting to find in a racing related forum

Of course but open diffs have been used in the past with success.
Mainly when proper tyres were used.
Gordan Murrays brake controlled system was also projected for an open diff.
The (diff) action was taken up by controlled braking, traction limiting).
Limited slip diffs either have high torque loss or if lockers they are fierce in actuation. The technology is ancient.(forced to be kept in use by regulations)
It is a simple matter with modern electronics to control the differential traction via the brakes to achieve the ideal for any traction limit requirement.
Unfortunately, instead the regulations force the designers to juggle old fashioned technology that is supposed to give the spectator a modern spectacle.
The diff slipper settings you mention are essential with high downforce, because there is insufficient 'feel' for the driver to apply power in any other way without braking traction.
Of course, without high downforce, the speed in corners would be lower but the skill level would be far higher.

[Tommy Cookers]

Good points, well made !

Be aware that Autogyro's posts are out of sequence, this time making me look a fool.

IMO there's really 2 different situations

High speed, high downforce, straight line high 'g' braking with energy recovery

Lower speed, low downforce, lower 'g' trail braking and corner entry, followed by corner exit at high tractive thrust

The diff behaviour is unimportant to the first, but critical to the second, and must be varied throughout

IMO engine braking is not needed for either situation, except by arbitrary rules
(KERS/'son of KERS' clearly has the potential to be controlled to act as ABS)

I have already suggested 2 ways of eliminating engine braking, hoping for a response

Keep at it,chaps !

[marcush.]

how much engine braking torque is available with close to zero flywheel mass ?
Renault is tailoring fuelmaps in overrun to manage tyre wear by the looks of things so they try to reduce engine braking?

[xxChrisxx]

Engine braking magnitude isn't affected by the flywheel mass. Engine braking is caused by the engine acting as an air pump, so it's directly proportional to how much air you are pumping.

No flywheel mass will mean that the engine will slow down (in revs) more quickly because it has less inertia.

[Tommy Cookers]

Engine braking is more proportional to how much air you are trying to pump, but not pumping as the throttles are closed

So, fuel-off/ throttle opening is one woy of controlling or eliminating engine braking. The revs would tend to bleed down to a useful level for the transition to cornering and corner exit. Air is still being pumped, but with minimal losses, so none of the maximal pumping losses that we call engine braking.

[gato azul]

Engine braking is more proportional to how much air you are trying to pump, but not pumping as the throttles are closed

So, fuel-off/ throttle opening is one woy of controlling or eliminating engine braking.

Not sure, you can "eliminate" engine braking this way [unless you can alter your valve timing, or have a other means to "decompress" the cylinder, which you don't in F1 by the rules].

If you overrun with WOT, you will compress air inside the cylinder during the compression stroke, which requires engergy/power.
If you overrun with closed throttle, you will try to pull a vacuum on your intake stroke, which also requires energy/power to do so, but won't compress air during the compression stroke.

One may has higher power/engery consumption then the other, depending on engine specification, and things like blow by etc., but I don't think, that you can "eliminate" engine braking totally this way.
You may can change it's magnitude a bit, and the timing intake vs. compression stroke.

But unless, you can alter your valve timing during this phase, you will either have a period, where you try to pull a vacuum or compress a amount of air inside the cylinder.

The easiest way to fully eliminate it, would be to "de-clutch" during braking/coast down, as in the FWD touring car example, provided earlier on.
Or as in the 2-stroke car example, by using a "free wheel" mechanism.

But the reason you do this in 2-Stroke car engines, is that you will lack lubrication, if no fuel (oil mixture) get's sucked into the engine. (overrun with closed throttle), which could/would lead to engine failures, during extented periods of "engine braking".
Long downhill passages on a mountain road for example.