212 replies to this topic

[Hoax]

Hi!

Me and my pals are currently discussing F1 engine brake mapping and what is controlled with different map settings. One or two of my freinds are believing that F1 engines are using "compression release engine braking". With compression release engine braking I mean that the exhaust valves are opening when the piston is reaching the near top of the compression phase to let the compressed air escape from the cylinder. They believe that changing the setting change the timing of opening and closing the exhaust valves.

So, the questions:

Do F1 engines use "compression release engine braking"?

What do the engine brake map setting adjust more than the amount of fuel injected during throttle off?

[vvillium3]

I don't know if this is going to answer your question or not. But, F1 restricts variable valve timing.... I don't know much about engine breaking, but I am sure with clutch engagement and ignition timing, the resistive power of the motor on the wheels is quite flexable....

[J. Edlund]

Variable valve timign isn't used, that one is quite given.

Most likely they control how the drive by wire throttles work under braking situations.

[zac510]

With compression release engine braking I mean that the exhaust valves are opening when the piston is reaching the near top of the compression phase to let the compressed air escape from the cylinder.

I fail to see how this would slow the vehicle? It is the compression that makes the engine harder to turn and subsequently slows the vehicle.

[phantom II]

The cheat team used their TC for rear ABS for a while.

Originally posted by vvillium3
I don't know if this is going to answer your question or not. But, F1 restricts variable valve timing.... I don't know much about engine breaking, but I am sure with clutch engagement and ignition timing, the resistive power of the motor on the wheels is quite flexable....

[Ursus]

Originally posted by zac510



I fail to see how this would slow the vehicle? It is the compression that makes the engine harder to turn and subsequently slows the vehicle.

You still get the expansion of the compressed air that would cancel the compression. Engine braking comes from intake stroke with closed throttles and general losses IIRC.

[zac510]

True.
Then you can't open the exhaust valves ATDC because that's when the intake valves open!

[Ross Stonefeld]

Originally posted by phantom II
The cheat team used their TC for rear ABS for a while.

Everyone does this.

[Monstrobolaxa]

I know Ferrari a few years ago as a kind of traction control would cut-off 2 cilinders when acelerating, only when wheel spin was detected. (can't remember where I read it)!

So possibly ignition cutting might be a way to go....the compression fase of the engine cycle is one you shouldn't miss, with the compression alot of braking force is applied (if ignition is cut or even if fuel supply to the cilinders is cut).

[J. Edlund]

Originally posted by Monstrobolaxa
I know Ferrari a few years ago as a kind of traction control would cut-off 2 cilinders when acelerating, only when wheel spin was detected. (can't remember where I read it)!

So possibly ignition cutting might be a way to go....the compression fase of the engine cycle is one you shouldn't miss, with the compression alot of braking force is applied (if ignition is cut or even if fuel supply to the cilinders is cut).

Cut the ignition, use the throttles or advance the ignition is possible methods to control the engien output in the case of traction control.

During (full) engine braking you normally cut the ignition and/or fuel injection. If the throttles are opened up a bit under those conditions the engine braking will be reduced.

Compression doesn't cause any engine braking, engine braking is caused by friction and throttle losses.

[Monstrobolaxa]

Originally posted by J. Edlund


Cut the ignition, use the throttles or advance the ignition is possible methods to control the engien output in the case of traction control.

During (full) engine braking you normally cut the ignition and/or fuel injection. If the throttles are opened up a bit under those conditions the engine braking will be reduced.

Compression doesn't cause any engine braking, engine braking is caused by friction and throttle losses.

I kind of mixed everything up....I've been sleeping 4-5hours a day....because of alot of work.

what I actually ment was more or less what you said. Ferrari was cutting off 2 cylinders while braking to help...of course ignition cutting is a comun use in traction control!

Big cock-up...a very bad post from me! hehe

[Christiaan]

I seem to remember a thread on this forum discussing engine braking in F1. The feeling was that the engines and clutch system was too delicate for this because of risks of over-revving etc. That seemed to make sense to me especially considering the types of clutches used

[alpa]

Originally posted by Ursus


You still get the expansion of the compressed air that would cancel the compression. Engine braking comes from intake stroke with closed throttles and general losses IIRC.

Hi,

I was reading all the threads I missed. I like this forum, it makes me think of the old EFI-DIY list.

I'd like to understand why throttle losses would make more engine brake and compression work less brake ?
I've been believing throttle losses (throttle, valves, ...) and compression losses are the part of the
'pumping work' which is lost.
If trying to brake by compression the compressed air will push pistons back.
But if doing that by depression pistons will have nothing to compress.
Where is the difference ?

I would imagine that in an ideal world without losses when IVC == EVO there would not be breaking, if IVC < abs(EVO) (say IVC=+20, EVO=-60) there would be more compression than expansion, so that would make braking.

[EfiOz]

The only way to get any engine braking is by advancing ignition a loooonnnnggg way on deceleration. Braking forces purely from compression are quite small as their is very little cylnder pressure to begin with due to the closed throttles.

[alpa]

Originally posted by EfiOz
The only way to get any engine braking is by advancing ignition a loooonnnnggg way on deceleration. Braking forces purely from compression are quite small as their is very little cylnder pressure to begin with due to the closed throttles.

I would avoid such advance as all the energy usually used to push pistons would be converted into heat and melt pistons and heads. At least this is my vision.
When talking about compression braking I mean compressing the air coming from say a BIG idle valve or another throttle bypass.

[Oho]

Originally posted by EfiOz
The only way to get any engine braking is by advancing ignition a loooonnnnggg way on deceleration. Braking forces purely from compression are quite small as their is very little cylnder pressure to begin with due to the closed throttles.

Well now I recall reding from another thread on this very forum that the major contributor to engine braking in Otto engine is vacuum on the intake side (manifold, plenum what sthe difference) with throttles closed and for this very reason Diesel engines sport very weak engine braking and thus are more or less frequently ecquiped with exhaust brakes.

[alpa]

Originally posted by Oho


Well now I recall reding from another thread on this very forum that the major contributor to engine braking in Otto engine is vacuum on the intake side (manifold, plenum what sthe difference) with throttles closed and for this very reason Diesel engines sport very weak engine braking and thus are more or less frequently ecquiped with exhaust brakes.

Living in a country (France) where more than 50% of cars are (turbo) diesels I can garantee you that diesels have HUGE engine break. It's more or less like an electric vehicle to drive: you press it accelerates, you release it slows down. And diesels don't have throttle for the moment.
When asking my question I have not thought at all about diesels .

[Greg Locock]

At the risk of being accused of being an empiricist (not an insult) why not try it in your own car. Put it in gear and decelerate (with the ignition off). try opening and closing the throttle. Does it make any difference? Not on my car.

[EfiOz]

Diesels also havecompression ratios greater than 17:1 and therefore generate a lot more cylinder pressure in an over run condition than a equivalent petrol engine.

It's still only part of the engine braking forces.

Hurting an engine on deceleration due to lean mixtures or high advance is a myth and/or a bad internet theory. We've run high comp engines to 50deg and no fuel on decel with no long term effects. Again, the destructive potential inside the cylinder is quite low due to the very low pressures involved (maybe 20-30kpa initially).

[NTSOS]

At the risk of being accused of being an empiricist (not an insult) why not try it in your own car. Put it in gear and decelerate (with the ignition off). try opening and closing the throttle. Does it make any difference? Not on my car.

Oh Greg......didn't you say that your motor has the rot and might not be the best choice for a test vehicle?

John

[soubriquet]

Originally posted by Oho
...Diesel engines sport very weak engine braking and thus are more or less frequently ecquiped with exhaust brakes.

Diesels have much better engine braking than the petrol equivalents. Trucks are equipped with exhaust brakes because these reduce wear on the brakes in general use. Also, trucks use drum brakes and need to be driven conservatively in hilly terrain. It is possible to fade them to nothing with a single application. Believe me, I've done it.

[McGuire]

Since a diesel engine has no throttle, under decel it produces no virtually engine braking in the commonly understood sense. A four stroke SI engine produces some engine braking because it has a throttle. When the throttle is closed on decel, the engine becomes in effect a vacuum pump.

Visualize a plastic drugstore syringe like this one:


Let's imagine the piston-plunger is around 25mm (one inch) in diameter while the inlet/delivery orifice on the front is say 10mm in diameter. Little physical effort will be required to draw back on the plunger because air is drawn in at nearly the same rate the plunger is pulled back. Now imagine the orifice is tiny, only .5mm in diameter. More physical effort will be required to pull back on the plunger, because the air must be pulled in through a tiny orifice.

In a nutshell, that is engine braking. It is very easy to instrument a car's engine to measure the relevant pressures. For example, a typical engine might produce 18" hg of manifold vacuum at idle, and near 0" hg at WOT. However, on closed-throttle decel the manifold vacuum may rise to 23" or 25". Obviously, that additional vacuum represents pumping work performed by the engine which is not offset by any power produced. In other words, engine braking.

[alpa]

Originally posted by McGuire
Since a diesel engine has no throttle, under decel it produces no virtually engine braking in the commonly understood sense. A four stroke SI engine produces some engine braking because it has a throttle. When the throttle is closed on decel, the engine becomes in effect a vacuum pump.

Visualize a plastic drugstore syringe like this one:


Let's imagine the piston-plunger is around 25mm (one inch) in diameter while the inlet/delivery orifice on the front is say 10mm in diameter. Little physical effort will be required to draw back on the plunger because air is drawn in at nearly the same rate the plunger is pulled back. Now imagine the orifice is tiny, only .5mm in diameter. More physical effort will be required to pull back on the plunger, because the air must be pulled in through a tiny orifice.

I still don't understand why I would need more forces to pull a compressed closed syringe than to push a closed syringe filled with air ?
And why diesel engines have so much engine break ?

[EfiOz]

No throttle?!?

[alpa]

Originally posted by EfiOz

Hurting an engine on deceleration due to lean mixtures or high advance is a myth and/or a bad internet theory. We've run high comp engines to 50deg and no fuel on decel with no long term effects. Again, the destructive potential inside the cylinder is quite low due to the very low pressures involved (maybe 20-30kpa initially).

I thought you've been suggesting to advance ignition with a well filled cylinder.

You say "to 50deg and no fuel on decel". I don't see any interest to alter ignition without fuel. Or do you mean you let enter few mixture and advance the ignition ?

[alpa]

Originally posted by EfiOz
No throttle?!?

Diesels don't have throttle, they are always in a lean condition. That's why they produce so much particles.
They are going to have throtlle to lower the particle level.

[NTSOS]

I still don't understand why I would need more forces to pull a compressed closed syringe than to push a closed syringe filled with air ? And why diesel engines have so much engine break ?

With a diesel, think air spring effect, the energy required to compress the air is mostly recovered and returned to the crankshaft as the piston moves back down the cylinder.

A jake brake circumvents the spring effect by opening the exhaust valve near TDC and explosively releasing the compression pressure to atmosphere. Without a jake brake, there would be very little engine braking....except for internal engine friction and parasitic losses generated by various accessory drives.

Juan

[alpa]

Originally posted by NTSOS


With a diesel, think air spring effect, the energy required to compress the air is mostly recovered and returned to the crankshaft as the piston moves back down the cylinder.

And what's difference with the spring effect of pumping an empty cylinder ? Why this energy would not be recovered ?

A jake brake circumvents the spring effect by opening the exhaust valve near TDC and explosively releasing the compression pressure to atmosphere. Without a jake brake, there would be very little engine braking....except for internal engine friction and parasitic losses generated by various accessory drives.

Juan

Correct me if I'm wrong, according to you diesel's engine brake is due to higher friction and accessory losses. Because I've never seen "jake brake" systems on regular cars.
So when I'm driving a 150HP 300Nm 2L 4cyl turbo diesel car I have more brake than driving a 2L v6 petrol only because I have more losses on a diesel. Sounds strange. How do these diesels do to reach 300mkm mileages with so much frictions ?

[soubriquet]

We are in a situation where the theory is fine and dandy, and dancing with the fairies.

I've been driving diesels since 1966. I learnt to drive on a Massey Ferguson 35 with a diesel engine. I've spent 13 years being paid to drive diesel 4wd Toyotas and Nissans across the deserts of Australia. I spent 5 years driving 32 tonne general haulage artics across Europe. To claim that diesels have less engine braking than petrol engines is barking lunacy

[crono33]

not being an engineer, i am an empiricist :-)

i can assure you that opening up the throttle will allow an engine with an almost dead battery to turn and start (done it countless times on countless motorbikes)

according to theory, it shouldnt work, and in fact opening the throttle should make it harder to turn the engine due to more charge to compress.

i cannot supply any explanation :-)

Originally posted by Greg Locock
At the risk of being accused of being an empiricist (not an insult) why not try it in your own car. Put it in gear and decelerate (with the ignition off). try opening and closing the throttle. Does it make any difference? Not on my car.

[McGuire]

Originally posted by soubriquet
We are in a situation where the theory is fine and dandy, and dancing with the fairies.

I've been driving diesels since 1966. I learnt to drive on a Massey Ferguson 35 with a diesel engine. I've spent 13 years being paid to drive diesel 4wd Toyotas and Nissans across the deserts of Australia. I spent 5 years driving 32 tonne general haulage artics across Europe. To claim that diesels have less engine braking than petrol engines is barking lunacy

No offense, but you are simply uninformed about the precise meaning of a common industry term: "engine braking." To wit: the decelerative force on the vehicle on rolldown resultant to pumping work performed by the engine. Since a diesel has no throttle, it performs no pumping work on decel and thus there is no "engine braking" as such -- unless an exhaust brake or Jake brake is installed. That's what these devices are for eh. On a four-stroke SI engine they would serve no useful function. Teats on a mule etc.

You are only talking about how much the vehicle slows when you remove your foot from the accelerator pedal, which is neither here nor there at the moment.

[NTSOS]

Correct me if I'm wrong, according to you diesel's engine brake is due to higher friction and accessory losses.

Ok......diesel's engine brake is due primarily to jake brake.

John

[soubriquet]

No offense taken. I am no engineer. I am uninformed. I am simply the driver.

Statement. Letting off the throttle with a diesel gives a much greater braking effect than with a petrol engine.

I don't want to pick a fight with people I really respect, but here the theory doesn't match reality.

[phantom II]

em.pir.i.cism - quackery. Webster.;)

Originally posted by crono33
not being an engineer, i am an empiricist :-)
i cannot supply any explanation :-)

[alpa]

Originally posted by crono33
not being an engineer, i am an empiricist :-)

i can assure you that opening up the throttle will allow an engine with an almost dead battery to turn and start (done it countless times on countless motorbikes)

according to theory, it shouldnt work, and in fact opening the throttle should make it harder to turn the engine due to more charge to compress.

i cannot supply any explanation :-)

My measures on injection engines show that while cranking the intake pressure is very near to the ambient one. Which means the air flow is below the closed throttle max flow. So the only thing you change by opening the throttle is to enrich the mixture (if it does anything with the particular engine management).

[alpa]

Originally posted by soubriquet
No offense taken. I am no engineer. I am uninformed. I am simply the driver.

Statement. Letting off the throttle with a diesel gives a much greater braking effect than with a petrol engine.

I don't want to pick a fight with people I really respect, but here the theory doesn't match reality.

Exact. I suspect most of you have never driven diesel engines. Ask anyone driving in Europe about the brake and he/she'll tell you how diesels brake. Now the theory should be adjusted.

I still wonder why compression energy is recovered and not the one spent to create vacuum. I'd like to understand what's the fundamental physical difference between compressing air and creating depression.

[NTSOS]

Since Diesel engines are superior to throttled engine as it relates to engine braking, I suppose it's possible that Jacobs Vehicle Sytem's entire business is based on sheer lunacy and a fignewton of their imagination. :yawn:

John

[WPT]

Must consider pressure on both sides of the piston, no? WPT

[NTSOS]

I do not doubt your experiences with diesel engine braking. I'm wondering if some of the above mentioned vehicles use some type of exhaust back pressure assisted braking. It would be not be noisy like a jakebrake and it could be overlooked.





Exhaust Back Pressure Brake

John

[kNt]

Originally posted by alpa


I still wonder why compression energy is recovered and not the one spent to create vacuum. I'd like to understand what's the fundamental physical difference between compressing air and creating depression.

It's just two different things within Thermodynamics or you could se it as two quite similar processes (they would be the same if they were reversible) with different initial values, the one would start at a vacuum and the other at ~ambient pressure (depending on turbo / suction etc.).



Most Diesels in Cars are Turbodiesels anyway wehere the principle is a bit different I think

[EfiOz]

Originally posted by alpa


Diesels don't have throttle, they are always in a lean condition. That's why they produce so much particles.
They are going to have throtlle to lower the particle level.

I musta been looking at somehting else all those years.........

[EfiOz]

Originally posted by alpa


I thought you've been suggesting to advance ignition with a well filled cylinder.

You say "to 50deg and no fuel on decel". I don't see any interest to alter ignition without fuel. Or do you mean you let enter few mixture and advance the ignition ?

That's right. While a spark ignition engine (with a throttle!) is decelrating, the only way to gain engine braking is by running a lot of advance and a little fuel. The cylinder pressure (and therefore load) is so small that the flame front has a braking effect on the piston.

Otherwise, CR has little to do with as you've not got enough gas to compress.

[Greg Locock]

OK, how about this as a really horrible estimate of the pumping losses. open throttle, no fuel, SI engine

VE is say 90%. Losses in the exhaust side roughly same as intake. So overall we 'lose' about 20% of the enegry in the ntake charge of air. That's about 3 psi (I told you this was horrible). that is 20000 Pa

Say we have a 4 litre engine at 3000 rpm. it pumps 4*3000/2/60=100 litres of air per second, or 0.1 m^3 s-1

So the power is 20000*0.1, or 2 kW, or 3 hp. 3 hp of braking is bugger all.

Now we can see why I couldn't detect any difference when I tried the experiment, my car only has a 2 litre engine, so Iwas trying to detect 1.5 hp of additinal braking.

Or the thermodynamics could be screwy. I thought of this while riding my pushbike to work, so I blame the lack of oxygen.

If we scale this up to an F1 car, 3 litres at 15000 rpm, we'd get 12 hp. The brakes themselves are capable of something like 1000 hp, so I'm pretty convinced you'd do it for balance and to avoid transients rather than some huge net advantage in braking.

[imaginesix]

Originally posted by Greg Locock
Or the thermodynamics could be screwy. I thought of this while riding my pushbike to work, so I blame the lack of oxygen.

The author and subject become one...

[desmo]

I've noticed I think more about powerplant efficiency when I'm the powerplant on a bicycle. I get significantly better mileage than my GF in the same car, same route and I think it's mostly attributable to that.

[alpa]

I made a test these morning (10minutes ago).
While driving in 4th gear I release the accelerator -> the car starts slowing down.
Then I cut off the engine -> no change
Then I press accelerator WOT -> no change
Would I have a diesel engine ? The car run on petrol for the last 250mkm.

Test conditions:
car: petrol Alfa Romeo 75TS (1988)
engine: rebuilt, in very good condition, 2L 4 cyl, Bosch EFI, Exhast Opening Advance:60 , Intake Opening ****** = 90 (in idle due to VVT)

[alpa]

Originally posted by NTSOS
I do not doubt your experiences with diesel engine braking. I'm wondering if some of the above mentioned vehicles use some type of exhaust back pressure assisted braking. It would be not be noisy like a jakebrake and it could be overlooked.

Exhaust Back Pressure Brake

John

I have workshop manuals of several 1.9L diesel cars. None of them has any additional breaking system.

What's funny in this story is that when I'm reading North American forums in english or french the people is convinced diesels don't have engine brake (but how many of them have ever driven diesels ?).
When reading french forums (most of people drive diesels here) the fact diesels have more engine brake is obvious. Would physic laws be different in France ?
Note that most of passenger car diesels are 1.9 and 2L, some of them are 2.4L, not 5 or 10L.

[McGuire]

Originally posted by soubriquet
No offense taken. I am no engineer. I am uninformed. I am simply the driver.

Statement. Letting off the throttle with a diesel gives a much greater braking effect than with a petrol engine.

I don't want to pick a fight with people I really respect, but here the theory doesn't match reality.

LOL there is nothing wrong with the "theory." You are simply confusing two different properties:

1) engine braking
2) the total rolldown rate of the vehicle

And obviously 1) is a component of 2).

A diesel produces no engine braking because it has no throttle and thus no net pumping work on decel. Unlike a gasoline engine, which has a throtle. Jake brakes, exhaust brakes and similar devices are used to provide engine braking on diesels.

And let me assure you that without such a device, a diesel engine does NOT offer more engine braking than a gasoline engine. LOL the diesel truck industry would be very surprised to hear that one.

[McGuire]

Originally posted by alpa


I have workshop manuals of several 1.9L diesel cars. None of them has any additional breaking system.

What's funny in this story is that when I'm reading North American forums in english or french the people is convinced diesels don't have engine brake (but how many of them have ever driven diesels ?).
When reading french forums (most of people drive diesels here) the fact diesels have more engine brake is obvious. Would physic laws be different in France ?
Note that most of passenger car diesels are 1.9 and 2L, some of them are 2.4L, not 5 or 10L.

LOL an under-two liter vehicle has no need for an exhaust brake. Unless you are going to tow a house trailer with it or something.

Many cars in this size/weight category have no engine braking at all, regardless of the engine used. If equipped with an automatic transmission the top gears will often be overrruning, or what used to be known in the USA as "free-wheeling." This was an actual feature on cars in the mid-30's.

Back to modern times... gas/electric hybrids use what could be called a "reverse Jake brake." Here the valve timing is interrupted to cancel engine braking, to give the regenerative braking system more work to do.


This thread has grown very silly.

[McGuire]

Originally posted by crono33
not being an engineer, i am an empiricist :-)

i can assure you that opening up the throttle will allow an engine with an almost dead battery to turn and start (done it countless times on countless motorbikes)

according to theory, it shouldnt work, and in fact opening the throttle should make it harder to turn the engine due to more charge to compress.

i cannot supply any explanation :-)

No, you are absolutely correct and there is nothing anti-theoretical about it. People who say otherwise have got their theories scrambled. If the engine is rotating and the throttle is closed, the pressure in the intake manifold will be less than atmospheric. Of that you can be sure.