Engine Brakes, what are they?

[Phil]

I always hear about how engine brakes are used with braking and KERS harvesting, but how does it come in to play? For example, if there is a KERS failure the driver worries about how the car will respond to braking, with engine brakes being used. I'm thinking about Jenson Button in Abu Dhabi in 2011 for reference. Thanks guys!

This question reminded me of a frightning event that took place when I got a Lotus Exige to test-drive over a long weekend from my dealer. I was driving down a nice piece of road with some nice S corners and as I was coming down with a bit of speed, I downshifted from 3rd into 2nd gear as I was braking (I didn't over-rev!) and let the clutch engage far too quick without blipping the throttle or attempting any form of heel and toe.

The result was that the rears braked that much, that I lost all grip and nearly put the car into the ditch while in the corner. The Exige has a weight distribution of aprox. 40/60, so once the rear comes, it comes quick. Very frightning and I am so happy I was somehow able to retain control of the car. That could have turned out quite bad indeed.

[langwadt]

I always hear about how engine brakes are used with braking and KERS harvesting, but how does it come in to play? For example, if there is a KERS failure the driver worries about how the car will respond to braking, with engine brakes being used. I'm thinking about Jenson Button in Abu Dhabi in 2011 for reference. Thanks guys!

This question reminded me of a frightning event that took place when I got a Lotus Exige to test-drive over a long weekend from my dealer. I was driving down a nice piece of road with some nice S corners and as I was coming down with a bit of speed, I downshifted from 3rd into 2nd gear as I was braking (I didn't over-rev!) and let the clutch engage far too quick without blipping the throttle or attempting any form of heel and toe.

The result was that the rears braked that much, that I lost all grip and nearly put the car into the ditch while in the corner. The Exige has a weight distribution of aprox. 40/60, so once the rear comes, it comes quick. Very frightning and I am so happy I was somehow able to retain control of the car. That could have turned out quite bad indeed.

the two-wheel guys use a "slipper clutch", not sure if they did in the two-stroke day they don't have much engine braking

I believe the first Saab road car with four-stroke engine had freewheeling so it behaved like the old two-stroke

are there no limits to how much off trottle push they can do? kinda defeats the idea of kers if it isn't used for breaking but only to counter act (intensionally added) engine push

[Tommy Cookers]

the limit of push is (now) zero output torque
'engine braking' torque (ie zero or near-zero fuelling) is about -20% of maximum on-power torque
(and there wouldn't be much exhaust blowing with zero fuelling)

IIRC the 2 stroke freewheel was to avoid uneven running ('4 stroking') on the overrun ?
it allowed clutchless gearshifts on the overrun
also some 4 stroke cars had freewheels (for economy)
eg Rover till about 1963

[Dragonfly]

A two stroke engine in braking mode will suffer excessive wear since it is lubricated by oil in the fuel.
The famous "Trabant" and the larger "Wartburg" from the former GDR had free wheeling system ('freilauf' if I am not mistaken) for that reason.

[autogyro]

My Austin Wetminster A95 had a Borg Warner overdrive with free wheel in overdrive.

[olefud]

A two stroke engine in braking mode will suffer excessive wear since it is lubricated by oil in the fuel.
The famous "Trabant" and the larger "Wartburg" from the former GDR had free wheeling system ('freilauf' if I am not mistaken) for that reason.

As did the SAAB 93.

[autogyro]

Of course the teams could increase the 'gap' between gearshifts during deceleration when operating their 'seamless' gearboxes.
If they did so rapidly with very close shifting up to and including corner entry.
It would allow the 'blip' used to match the input and output speeds of their so called 'seamless' stepped layshaft 'sic' gearboxes to be increased both in time and rpm level, so as to give a large amount of exhaust gas flow but with zero or minus torque input.
The engine would in fact be disengaged for even longer during gear shifts than it already is.

Clever yea?

Not realy just a rules avoidance for the sake of aero.
Proves my no such thing as seamless gearbox argument however.

[autogyro]

Mind you it doesnt help kinetic energy recovery if they continue to use a motor generator on the front of the crankshaft.
Perhaps they will get there in the end.

[langwadt]

Of course the teams could increase the 'gap' between gearshifts during deceleration when operating their 'seamless' gearboxes.
If they did so rapidly with very close shifting up to and including corner entry.
It would allow the 'blip' used to match the input and output speeds of their so called 'seamless' stepped layshaft 'sic' gearboxes to be increased both in time and rpm level, so as to give a large amount of exhaust gas flow but with zero or minus torque input.
The engine would in fact be disengaged for even longer during gear shifts than it already is.

Clever yea?

Not realy just a rules avoidance for the sake of aero.
Proves my no such thing as seamless gearbox argument however.

I believe the rules state that a gear change must be completed in less that 200ms

[Tommy Cookers]

it seems that (petrol) road hybrids are now recovering KE via generator action when the driver has lifted (but not braked)
much of our driving time is spent this way, ie in the 'engine braking' condition
but now natural engine braking can be replaced by the synthetic engine braking effect of recovery by generator action
(presumably natural engine braking is reduced by automatically opening the engine's throttle, and zero fuelling of course)

to me this seems for the road much more useful than recovery under driver braking
(which is of little value or intrusive and legally dubious ?)

[autogyro]

it seems that (petrol) road hybrids are now recovering KE via generator action when the driver has lifted (but not braked)
much of our driving time is spent this way, ie in the 'engine braking' condition
but now natural engine braking can be replaced by the synthetic engine braking effect of recovery by generator action
(presumably natural engine braking is reduced by automatically opening the engine's throttle, and zero fuelling of course)

to me this seems for the road much more useful than recovery under driver braking
(which is of little value or intrusive and legally dubious ?)

Kinetic energy recovery is through the program that controls the electric motor on all modern EVs.
It is not dependent on driver foot braking although this is included in the systems.
Emergency braking (hard braking) still relies on mechanical wheel brakes.
It is possible of course to eliminate the need for any brakes other than the electric motor/generator's.
Just not yet developed fully or cleared for safety use.

Truth is that harvesting energy from braking is only any use for stop start motoring like local truck/van delivery or buses etc.
You save more energy coasting off throttle on ordinary A to B journeys.
It makes far more sense in the racing applications, with the hard (energy wasting) braking used.