Air can't freely flow at all - crankcase depression is used in many racing engines, and can free up a lot of power - 30hp or more (depending on the engine of course, but 30hp on a 200hp engine is perfectly possible). Windage losses are massive.

Would there be any resistance at all in the crankcase? Its all open space pretty much and will always have the same volume of air in it...


The crank breather is usually to vent ignition blow past gasses that get past the rings back into the intake or atmosphere...

If there was any real pressure then ur dipstick would have to screw in or etc etc...

Yes, a lot of resistance. There isn't a net pressure (or shouldn't be), so the dipstick doesn't pop out, but you've got oily vapour flying around, a crank whirling around, air having to change direction... The greater the depression you can run, the more power you can liberate. And, like supercharging, you get more power by running in a partial vacuum than you lose generating that vacuum. It's one of the three main reasons of dry dumping an engine (less crank oil drag and an overall lower CoG being the other two).

Quote from tristancliffe :

Air can't freely flow at all - crankcase depression is used in many racing engines, and can free up a lot of power - 30hp or more (depending on the engine of course, but 30hp on a 200hp engine is perfectly possible). Windage losses are massive.

In USA (also at wiki), "windage" is a reference to drag losses due to the oil being moved around in the crankcase. I'm not sure if there's a special term used for the power losses due to air flow in the crankcase.

In some cars the PCV valve helps reduce pressure somewhat, since it connects the crankcase to the lower pressure air in the manifold, although this also draws oil mist into the intake, and at full throttle, the pressure in the manifold is at it's maximum. Some older race cars with straight exhause pipes (no back pressure), would take advantage of visocity effects and connect the crankcase to side ports on the exhaust pipes taking advantage of a type of venturi effect.

However, the most common method used for drag and some race cars is to use a conventional pump to evacuate the crankcase. The pump comsumes power, but the decrease in power loss through air movment in the crankcase is even more, so there's a net benefit.

Do a web search for "crankcase vacuum pump" and you'll find quite a few links.

Ah, So its more like aerodynamic resistance in the crackcase? With a vacuum pump it would thin the atmosphere in the crack and reduce resistance.... Altho the vacuum wont effect the mechanicals directly, it clears the working space around them?....

i would have thought allmost anyone into cars would have known this allready?

Quote :

In some cars the PCV valve helps reduce pressure somewhat, since it connects the crankcase to the lower pressure air in the manifold, although this also draws oil mist into the intake, and at full throttle, the pressure in the manifold is at it's maximum. Some older race cars with straight exhause pipes (no back pressure), would take advantage of visocity effects and connect the crankcase to side ports on the exhaust pipes taking advantage of a type of venturi effect.

However, the most common method used for drag and some race cars is to use a conventional pump to evacuate the crankcase. The pump comsumes power, but the decrease in power loss through air movment in the crankcase is even more, so there's a net benefit.

Do a web search for "crankcase vacuum pump" and you'll find quite a few links.

It's like you read what I wrote, and then thought you had thought it, so wrote it for my benefit.

Come on you big dunce, read my post and you'll see I just told you about releasing power via crankcase depression.

Quote from JeffR :

In some cars the PCV valve helps reduce pressure somewhat, since it connects the crankcase to the lower pressure air in the manifold, although this also draws oil mist into the intake, and at full throttle, the pressure in the manifold is at it's maximum. Some older race cars with straight exhause pipes (no back pressure), would take advantage of visocity effects and connect the crankcase to side ports on the exhaust pipes taking advantage of a type of venturi effect.

However, the most common method used for drag and some race cars is to use a conventional pump to evacuate the crankcase. The pump comsumes power, but the decrease in power loss through air movment in the crankcase is even more, so there's a net benefit.

Quote from tristancliffe :

It's like you read what I wrote, and then thought you had thought it, so wrote it for my benefit.

Feeling a bit "cranky" today? My post was intended for the others here, explaining the 3 most common methods used to evacuate the crankcase.

It's like that Blackadder scene, when Baldrick has a great idea and tells us of it. Then Blackadder, who apparently wasn't listening says "Wait, I've just had a great idea".

Quite why you took it on yourself to pointlessly 'inform' others when they hadn't asked and it wasn't on-topic is anyones guess, but if it makes you feel extra clever then so be it.

You usually add a video of an arcade game to make your points, but I don't see one here?

So to sum up then.

Engine braking is:

• The (partial) vacuum in the Cylinders created by restricting the intake flow to a level that is unable to fully fill the cylinder during the induction phase.
  
• This (partial) vacuum is then acted upon by the (presumably near atmospheric) pressure on the other side of the cylinder inside the crankcase.
  
• The resultant force is in opposition to that coming from the transmission being caused by the momentum of the vehicle that wants to maintain its speed.
  
• The result being that the speed of the vehicle is reduced.
  

Did I understand it correctly?

so let me get this straight

compression of the air in the cylinder does not contribute to engine braking, because the compressed air will "bounce back", returning the energy?

ive always tough it brakes by forcing the engine to go on lower revs while gear is in and that way making the wheels to spin slower

Quote from batteryy :

ive always tough it brakes by forcing the engine to go on lower revs while gear is in and that way making the wheels to spin slower

Yeah, but the point of the thread is: what causes 'the engine to go on lower revs' once you let off the throttle? Why do you feel a pretty powerful force slowing the engine down (and through the gearbox and wheels, slowing the car too) that's powerful enough to lock the wheels in a RWD car, rather than the engine speed just coasting down slowly when the throttle is taken off?

I've never found a car where engine braking will lock the rear wheels - from 30hp 'machines' to several hundred hp machines. Engine braking just isn't that strong. Lifting the throttle, even at 19,000rpm won't lock the wheels in most cars (if most cars were capable of 19,000rpm!).

Locking the wheels due to downshifting is a different matter, and drivetrain inertia comes into it. In a car where the engine has very little inertia (compared to the wheels say) then downshifting from 6th to 1st would just cause the revs on the engine to rise very quickly, but (if inertia is low enough) wouldn't cause wheel locking.
Do the same thing, with the same drivetrain/engine but with the addition of a massive flywheel, and the wheels will lock - it's 'easier' for the wheels to go to engine speed than it is for the engine to go to wheel speed. Tyre grip obviously plays a part in that.

Engine braking has very little to do with the tendency for wheel locking.

Fair enough

I expect that, in an engine similar to F1 engines, that engine braking is much more of a problem. The massively over-square designs coupled with crazy rotational velocities must give tremendous engine braking compared to engines most of us will ever experience, hence I've read that just lifting off the throttle in an F1 car creates over 1g of braking (admittedly aero drag is likely to be a large part of this too), and the presence of the engine brake reduction in the BF1 indicates that in real life, it's easier to cope with when there's some electronic trickery to help.

Crashgate3 - I'm sure batteryy's post was semi-sarcastic.

I'm not sure exactly what the F1 systems were. I know they were, basically, automatic throttle applications, but were they purely on the lift off overrun, or was it when after a downshift when the wheels would want to lock due to the sudden difference in engine speed versus road speed?

I've never heard of an F1 car crashing because the wheels locked just from lifting (or if there is sudden electrical failure for example), but I have heard of them locking, like any other car, from downshifting too early/quickly/far, so I suspect it was to combat the latter - and personally I don't put that under the same type of Engine Braking subsection as caused by windage losses, friction and work done against throttle butterfly Engine Braking.

Quote from Bob Smith :

I've read that just lifting off the throttle in an F1 car creates over 1g of braking (admittedly aero drag is likely to be a large part of this too).

Virtually all of that 1 g is due to aero drag. F1 race cars have their ECU's and clutches programmed to limit the amount of engine braking. Racing motorcycles and some street bikes use "slipper" clutched to limit engine braking. The Corvette Z06 limits engine braking in a dumb way, keeping the the throttle by wire partially opened when you lift off, somewhat annoying in stop and go traffic since you're forced to used the clutch in some cases to get around this.

I have yet to see a real engine braking 'torque curve' and see how much there is. I have torque curves with also a 'losses' curve in there, the power loss when ON power, which I assume will be the engine braking minus the 'closed valve' effect.

In rFactor its quite funny, engine braking you could theoretically tweak by ear, how long it takes for the engine to rev down, but there is engine braking and inertia that do this.

I have seen a V8 engine with the inertia of a truck tire and the braking torque of a.. V8 engine.

In my mods I set engine braking to slowly increase from idle rpm to a % of the engine's peak torque applied at the peak power RPM. While subjective, most my cars use between 20% and 25% so if an engine has 300Nm at 4000rpm and 350hp at 5500, and I set engine braking at 20%, I have 20%*300Nm = 60Nm engine braking at 5500rpm..

I doubt engine braking can be severe enough to be the sole cause of lift off oversteer or rear end lockup. Inertia also plays a role when downshifting, if the engine has to catch up a few thousand rpm in a short time.

Its an interesting topic, but who has real 'engine braking dyno' data?

Quote from JeffR :

Virtually all of that 1 g is due to aero drag. F1 race cars have their ECU's and clutches programmed to limit the amount of engine braking.

Not any more, as Engine Brake Reduction was banned. Some morons who can't read think that Engine Braking was banned, which is quite amusing coming from so called 'fans'.

Quote from JeffR :

Racing motorcycles and some street bikes use "slipper" clutched to limit engine braking.

But they only actually slip after downshifts, not just when you close the throttle.

Quote from JeffR :

The Corvette Z06 limits engine braking in a dumb way, keeping the the throttle by wire partially opened when you lift off

Just like in F1? How else do you reduce engine braking via an ECU if it's work done against a closed throttle?

If anyone can show me a car crashing or locking it's wheels purely due to closing the throttle then I'd love to see it. I suspect, even if there is one or two, they'll be rare and of particularly extreme vehicles. If lack of evidence is ever a proof of something, then I think lack of evidence will prove that engine braking isn't strong enough to lock wheels.

And of course engine braking goes through the gearing so in higher gears there is even less of it noticed at the wheels.

A bit off topic, but I think the original post was answered. As mentioned before do a web search for "crankcase vacuum pump" for more info.

Quote from JeffR :

Virtually all of that 1 g is due to aero drag. F1 race cars have their ECU's and clutches programmed to limit the amount of engine braking.

Quote from tristancliffe :

Not any more, as Engine Brake Reduction was banned.

The seamless shifter logic is still there though. The removal of traction control and brake reduction have cost about 3/10 to 4/10ths of a second per lap. The brake reduction was compensated for with a more forward bias on the braking. It's a bit tricky because the downforce has a rearward bias to make the cars safe (less oversteery) at high speed.

Quote :

Racing motorcycles and some street bikes use "slipper" clutched to limit engine braking.

Quote :

But they only actually slip after downshifts, not just when you close the throttle.

The clutches limit braking torque, and it's true that closign the throttle won't generate this kind of torque.

Quote :

The Corvette Z06 limits engine braking in a dumb way, keeping the the throttle by wire partially opened when you lift off, somewhat annoying in stop and go traffic since you're forced to used the clutch in some cases to get around this.

Just like in F1? How else do you reduce engine braking via an ECU if it's work done against a closed throttle?[/quote]In the case of F1 cars, the ECU also controlls the clutch, and that aspect is still there as part of the seamless shift transmissions.

Better stated is that the Z06 has a poor implemenation, as it holds the virtual throttle open even at very slow speeds. 1st gear redlines at 61 mph, but you can just feed in a bit of throttle at around 10mph to 20mph, and lift off, but the car continues to run at that speed, requiring you fight the virtual throttle with the brakes and/or the clutch in stop and go traffic.

What's worse is that the ECU is smart enough to also recognize engine braking tire slippage, (compares rear tire speeds to front tires speeds), which should be the main trigger for engine braking reduction. There's also a torque sensor in the drive train, but it's only used to cut fuel if excessive torque is detected. Also stability control, which involves individual wheel braking. The low speed aspect of this was just dumb. Aftermarket programming of the ECU can fix these issues, but then some aspects of your warranty might be voided.

Regarding the torque sensor and cutting fuel, the so called traction control only retards spark, instead of cutting fuel, very ineffective, so the only noticable difference between traction control on and off, is with it on, you see an indicator display while your tires spin, but there's is little or no difference in the actual tire spin.

Quote :

If anyone can show me a car crashing or locking it's wheels purely due to closing the throttle then I'd love to see it.

Not in straight line, but there are some cars that have plenty of lift throttle oversteer. The higher powered Caterhams will do this. My guess is the Z06 with the ECU re-programmed will also do this, although I doubt the Z06 would recover as quickly as the Caterhams do.

Quote from tristancliffe :

If anyone can show me a car crashing or locking it's wheels purely due to closing the throttle then I'd love to see it. I suspect, even if there is one or two, they'll be rare and of particularly extreme vehicles.

Maybe related:

Quote :

In the return run he set 202 mi/h, but just past the time trap the car suddenly swerved, probably because Bible released the gas pedal too quickly. The machine crashed into the dunes about one hundred feet past the timing trap and rolled, coming to a rest two-hundred feet farther.

Source.

Quote from Niels Heusinkveld :

Its an interesting topic, but who has real 'engine braking dyno' data?

I think I took this screenshot about a year ago from a PDF I found online: http://www.vehicle-analyser.com/engine_friction_torque.png
Unfortunately I didn't keep a record of the source.

Quote from JeffR :

The seamless shifter logic is still there though. The removal of traction control and brake reduction have cost about 3/10 to 4/10ths of a second per lap. The brake reduction was compensated for with a more forward bias on the braking. It's a bit tricky because the downforce has a rearward bias to make the cars safe (less oversteery) at high speed.

The seamless shift has little to do with this, and neither do the banning of traction control or 'brake reduction' (which isn't brake reduction at all).
And to think that F1 cars have a rear biased aero map for 'safety' reasons...

Quote from JeffR :

The clutches limit braking torque, and it's true that closign the throttle won't generate this kind of torque.

Exactly.

Quote from JeffR :

In the case of F1 cars, the ECU also controlls the clutch, and that aspect is still there as part of the seamless shift transmissions.

I doubt the clutch is used as part of the seamless shift (which is upshifts only), and nor is the clutch used on downshifts, as F1 clutches are pretty fragile things, and wouldn't last being abused by constant slipping at every gear change.

Quote from JeffR :

Better stated is that the Z06 has a poor implemenation, as it holds the virtual throttle open even at very slow speeds. 1st gear redlines at 61 mph, but you can just feed in a bit of throttle at around 10mph to 20mph, and lift off, but the car continues to run at that speed, requiring you fight the virtual throttle with the brakes and/or the clutch in stop and go traffic.

You mean the idle control kicks in? My car drives along quite happily at 10mph (ish) with zero throttle too. That's no engine brake reduction really. At higher speeds then holding the throttle slightly open is a good thing if reducing engine braking is what you're after.

Quote from JeffR :

What's worse is that the ECU is smart enough to also recognize engine braking tire slippage, (compares rear tire speeds to front tires speeds), which should be the main trigger for engine braking reduction. There's also a torque sensor in the drive train, but it's only used to cut fuel if excessive torque is detected. Also stability control, which involves individual wheel braking. The low speed aspect of this was just dumb. Aftermarket programming of the ECU can fix these issues, but then some aspects of your warranty might be voided.

Not relevant to the discussion I don't think.

Quote from JeffR :

Regarding the torque sensor and cutting fuel, the so called traction control only retards spark, instead of cutting fuel, very ineffective, so the only noticable difference between traction control on and off, is with it on, you see an indicator display while your tires spin, but there's is little or no difference in the actual tire spin.

Retarding the ignition is almost certainly kinder on the engine and drivetrain than cutting fuel or sparks. But as nobody needs traction control (it comes as standard with the human body) then who really cares.

Quote from JeffR :

Not in straight line, but there are some cars that have plenty of lift throttle oversteer. The higher powered Caterhams will do this. My guess is the Z06 with the ECU re-programmed will also do this, although I doubt the Z06 would recover as quickly as the Caterhams do.

Don't confuse wheel locking due to engine braking with the load transfer (and consequent suspension movement, CoG movement etc) causing oversteer. You'd probably find (as I have ) that dipping the clutch mid corner (so no engine braking at all) would cause a very similar level of oversteer.

Quote from Bob Smith :

Maybe related:

Source.

Nice that we have to go back to 1929 to find a possible example of engine braking causing wheel lockup, and it wasn't even on a solid surface which would make lock up easier anyway!! I wonder if anyone can find an example from the last 10 years on a hard surface? Because I could show hundreds of examples of early downshifts causing accidents.

Quote from DragonCommando :

<massive post>

engines brake when you dont care of them - 1up.