Engine Brakes, what are they?

[Dragonfly]

If I understand you right you say that a diesel engine pumping and compressing air under lift off consumes less kinetic energy than petrol engine pumping nothing under closed throttle?

[piast9]

do trucks etc have the engine air throttled on the over-run power-off to help the brakes ?

Some trucks (especially heavy ones) have dedicated systems helping to slow down on the downhill called retarders. There are several types of them. It is possible to alter valve timing or use additional exhaust throttle to enhance the engine braking or it may be entirely separate system using hydrodynamic brakes or electrodynamic brakes on the driveshaft.

The use of engine - based retarders you may hear because they alter the noise produced by the engine when activated on the downhill.

[Psyclone]

do trucks etc have the engine air throttled on the over-run power-off to help the brakes ?

Some trucks (especially heavy ones) have dedicated systems helping to slow down on the downhill called retarders. There are several types of them. It is possible to alter valve timing or use additional exhaust throttle to enhance the engine braking or it may be entirely separate system using hydrodynamic brakes or electrodynamic brakes on the driveshaft.

The use of engine - based retarders you may hear because they alter the noise produced by the engine when activated on the downhill.

Light trucks generally use an exhaust brake while heavier trucks use a system within the engine to help with the stopping of the trucks. Cat uses there own brand now called a C-brake, has done since 2008. And if i remember correctly, it was either Cat or Cummins where there larger engines had more braking power than motive power. They all work as soon as you let off the throttle.

http://www.zafr.com/trucktcom/retarder.htm has some info

[cf181]

what is they effect of engine braking on traction while cornering as opposed to regular braking?

[NewtonMeter]

what is they effect of engine braking on traction while cornering as opposed to regular braking?

A dynamic difference would be that engine braking operates only on the driven wheels, whereas "regular" braking operates on all four (depending on the brake bias, ofcourse). But on an isolated wheel basis, I don't think the effect would be much different to regular braking.

I.e., you still have a friction circle and whether the longitudinal force component (in this case braking) acting on the wheel comes from engine braking or "regular" braking makes no difference and the effect on the remaining lateral force component will be the same.

But I might be wrong, feel free to correct.

[autogyro]

There is no 'pitch component on the vehicle under rear wheel engine braking.
Conventional front wheel braking creates a torque reaction in the hub carrier that assists pitch to nose down.
The rear brakes also create this torque reaction which mainly increases vehicle weight.

KERS 'brakeing' is difficult to balance with tyre grip/wear and rear stability because of this.
The current regulations on diffs are only a compromise that restrict development.

[olefud]

what is they effect of engine braking on traction while cornering as opposed to regular braking?

A dynamic difference would be that engine braking operates only on the driven wheels, whereas "regular" braking operates on all four (depending on the brake bias, ofcourse). But on an isolated wheel basis, I don't think the effect would be much different to regular braking.

I.e., you still have a friction circle and whether the longitudinal force component (in this case braking) acting on the wheel comes from engine braking or "regular" braking makes no difference and the effect on the remaining lateral force component will be the same.

But I might be wrong, feel free to correct.

I think you have it right. Front to rear brake bias is adjusted to compensate for engine braking. As a rule, the front brakes should lock before the rear.

[autogyro]

Engine braking varies depending on which gear is selected and the rpm of the engine, fueling, ignition etc.
Without a fully automatic gearbox/ 4W Kers or a driver skilled in the old art of balancing the car mechanicaly, the technology becomes a fred Karno attempt to balance a multitude of varying forces simply to make the car drivable.
Fortunately for the marketing gurus in F1, the God of aero always comes to their aid providing the ideal over bearing force to mask out the majority of the effects.
This is one reason why the technology of powertrains, braking and alternate energy application remain in the 19th century.

[Nando]

Regarding the turbo diesel, could the turbo have anything to do with it as well?
Seeing as it will continue to spin a little even after you let go of the throttle?

Interesting thread regarding engine braking and what causes it.

[ringo]

The compression ratio of an F1 engine is very high. It takes energy to compress the air and fuel mixture. Therefore there is a power loss with compressing before igniting the fuel mixture. Think of compressing a blocked bicycle pump.
When the engine is off throttle you may say, there is less air in the cylinders to compress, but there is still a resistance to compression from the little air present on compression stroke, and also the resistance to "stretching" of the air between the closed throttle plate and the pistions on the intake stroke.
Think of the intake stroke with a closed throttle as blocking a vacuum cleaner with you hand (throttle plate) and then notice the increased load on the motor.

The engine braking effect is can then be said to be the retardation of the crank shaft when the throttle is closed due to the combination of friction between moving parts, and resistance to compression of the air (doing work on the air). We call this friction power.

And here's a break down of why friction power decelerates the car when you come off throttle:

Brake horsepower (the power at the flywheel) = indicated power (power produced by combustion) - Friction power ( caused by friction of the moving parts, the resistance from compressing the air)

Now when you go off throttle, you take out the fuel injection and ignition, so the indicated power variable goes to zero.


So Brake Horse power = 0 - Friction power

Bp= -Fp.

The negative is not negative power, but it indicates the direction of power flow across a system boundary. The engine is no longer producing power to over come a load; it's no longer putting power out of the sytem. It is now behaving like a load, it is drawing power from outside into the system!

that is the simplest mathematical/ thermodynamic explanation of it. The engine is now a load. The momentum of the car will now go through the drive train and act to overcome the friction. the flow of power is now reversed. This engine in nothing more than a power absorber; a overly complicated air compressor and like any air compressor it needs to be powered. Conveniently it will take this power from the car's momentum.

The same applies to KERS charging, and also alternator charging. The teams may use these retarding effects to have an input on the car's dynamics, since they can slow the car without the use of the brakes and that's a good thing, since the car's stability is not upset drastically.

[ringo]

in fact if it were possible for the driver to press the throttle while on the brakes and then the fuel management would cut fuel to the engine. it may be very interesting the improvements in braking. If the rules did not stipulate that the throttle had to be controlled by the driver's foot at all times, there would be no need to step on the throttle pedal.
Any how i think engine braking strategies really play into the life of the engine, fuel saving and also vehicle dynamics.
And we can't forget the elephant in the room; the braking effect of all that drag force on the car.

[Nando]

Also the gearing plays a role in how severe the engine braking is.

Nice post +1

[Tommy Cookers]

in fact if it were possible for the driver to press the throttle while on the brakes and then the fuel management would cut fuel to the engine. it may be very interesting the improvements in braking. If the rules did not stipulate that the throttle had to be controlled by the driver's foot at all times, there would be no need to step on the throttle pedal.
Any how i think engine braking strategies really play into the life of the engine, fuel saving and also vehicle dynamics.
And we can't forget the elephant in the room; the braking effect of all that drag force on the car.

the above seems to be a contradiction of the preceeding post, as it suggests that more throttle opening (fuel off) will increase engine braking, it will not
a major part of SI engine braking is always the suction load on the piston trying to suck air through a nearly closed throttle, feeding off the vehicles momentum
the effect was easy to demonstrate until vehicle design changes about 25 years ago, and was well known
eg it was easier to push-start a motorcycle with the throttle well open while pushing up to speed
a light aircraft will glide better engine-off if the everything is cut off and the throttle opened
80s cars could be modded by replacing the throttle microswitch (that told the injection that the driver was coasting) with a latching equivalent that would continue the fuel cutoff when the throttle was re-opened (once), this hugely improved coasting at speed

the compression losses are less, because there is re-expansion ?

[autogyro]

Current KERS is pretty inefficient for harvesting.
It uses a generator bolted to the front of the crankshaft.
On deceleration the energy is absorbed by the engine, which is wasted energy and excess wear.
The layshaft gearbox also wastes energy as heat under powertrain braking.
It is only after all this waste and wear that the generator harvests.

Engine,or properly described 'powertrain' braking, upsets car balance far more than balanced wheel braking.
Wheel brakes use the torque reaction in the hub carrier to input energy into the chassis which can be used to assist balance.
Engine braking on the rear only can only unbalance the car unless the car is going in an absolutely strait line where it becomes a tyre grip problem because of less control than with wheel braking.
Four wheeled electric braking for energy harvesting would be the best balanced braking possible.
Of course there would need to be a method to disipate the excess energy.

[Tommy Cookers]

Current KERS is pretty inefficient

Four wheeled electric braking for energy harvesting would be the best balanced braking possible.
Of course there would need to be a method to disipate the excess energy.

KERS in F1 is limited to only about 5% of the friction braking power
this is roughly the same as the engine braking

to do better we would need 4 wheel KERS and proper millisecond-by-millisecond management of the electrical loading of the wheels (this is in effect partial traction control and ABS behaviour)
the rulemakers are avoiding this, presumably because the teams want them to
agreed, storage is the real factor limiting the extent of KERS
(but more KERS becomes irresistible able if the F1 fuel rate is reduced far enough)