Engine Braking?

[kiwi135]

How does it work . I mean everyone says that it is better than using your brakes (fuel economy wise i mean) obviously it would help with making them last longer but i just figured it works like this.

Change into a lower gear which = the engine is working at a higher rate = engine consuming more fuel

as opposed to putting into neutral and using the brakes

someone explain to me pleez

[rawr]

use both brakes and engine braking at the same time, less pressure on the brakes, does use a bit more fuel but wouldn't be much.

Do heel toe to gain extra jdm points

[aaronng]

How does it work . I mean everyone says that it is better than using your brakes (fuel economy wise i mean) obviously it would help with making them last longer but i just figured it works like this.

Change into a lower gear which = the engine is working at a higher rate = engine consuming more fuel

as opposed to putting into neutral and using the brakes

someone explain to me pleez

You need to blip the throttle when you are downshifting, otherwise you wear out the clutch and you defeat the purpose of engine braking, which is to reduce wear.

Also, your "Change into a lower gear which = the engine is working at a higher rate = engine consuming more fuel" is not true. The amount of fuel the engine consumes depends on the amount of air entering the combustion chamber. So if you are at a higher rev but you are not using the accelerator, then you use very little fuel, usually less than having the car in neutral because there is no momentum to keep the engine spinning and it has to open up the throttle a little bit to keep the engine at idle RPM.

[rawr]

aarong, give me your brains so I can rule the world

[kiwi135]

you need to blip the throttle when you are downshifting, otherwise you wear out the clutch and you defeat the purpose of engine braking, which is to reduce wear.

Also, your "change into a lower gear which = the engine is working at a higher rate = engine consuming more fuel" is not true. The amount of fuel the engine consumes depends on the amount of air entering the combustion chamber. So if you are at a higher rev but you are not using the accelerator, then you use very little fuel, usually less than having the car in neutral because there is no momentum to keep the engine spinning and it has to open up the throttle a little bit to keep the engine at idle rpm.

very helpful thanx aarong i can finally see the light .

[trism]

i woulda thought that was common sense, not some magical knowledge?

[kiwi135]

i woulda thought that was common sense, not some magical knowledge?

thats good to know!



























noooooooooooottttt

[aaronng]

aarong, give me your brains so I can rule the world

But if you take it, then I can't help people anymore.... :p

[dsp26]

You need to blip the throttle when you are downshifting, otherwise you wear out the clutch and you defeat the purpose of engine braking, which is to reduce wear.

Also, your "Change into a lower gear which = the engine is working at a higher rate = engine consuming more fuel" is not true. The amount of fuel the engine consumes depends on the amount of air entering the combustion chamber. So if you are at a higher rev but you are not using the accelerator, then you use very little fuel, usually less than having the car in neutral because there is no momentum to keep the engine spinning and it has to open up the throttle a little bit to keep the engine at idle RPM.

to simplify this for those who still don't understand... your throttlebody is still closed as if you were on idle coz your obviously not accelerating... hence acting as a 'brake' for intake air thus slowing the engine down.

[CRXer]

injectors actually turn off completely when closed throttle/rpm above about 950rpm for b18c as example.

if u slot into neutral,then motor goes to idle settings,obviously using fuel....

[dsp26]

^^wait really? so no fuel is being pumped into the cc's while i'm engine braking and the ecu knows to do this?

[CRXer]

yes ronny,
no fuel whatsoever,as long as your gearbox is maintaining engine speed above about 950rpm on the b18c.

fuel economy is what its all about....

[dsp26]

so kinda the same affect as me turning my engine off completely while car is moving then putting it back into ign and simply putting gas on except it's fuel and not ign?

[aaronng]

to simplify this for those who still don't understand... your throttlebody is still closed as if you were on idle coz your obviously not accelerating... hence acting as a 'brake' for intake air thus slowing the engine down.

If you are idling and the engine is not connected to the wheels (either neutral or clutch in), then the throttle is slightly open and air is being sucked in. That means fuel is being injected and used.

If the car is in gear and your foot is off the clutch and the vehicle is moving, the rotation of the wheels turn the gearbox which in turn keeps the engine turning. Not sure if the throttle body is totally closed (I am guess it would still be slightly open), but it would be smaller than if you were just idling.

[CRXer]

so kinda the same affect as me turning my engine off completely while car is moving then putting it back into ign and simply putting gas on except it's fuel and not ign?
not quite following what u r saying,but if u put the igntion back on,then there is still no fuel if the engine is above 950rpm & the throttle is completely closed.
not sure whether spark turns off,but there would be no real need to turn it off in any case,theres still no air/fuel to fire.
dont forget,most of the noise u hear from a motor is not explosions but air pumping & rapid air expansion etc.

aarong,throttle plate is completely closed when foot off accelerator,idle passages past the throttle plate take over instaed

[aaronng]

not quite following what u r saying,but if u put the igntion back on,then there is still no fuel if the engine is above 950rpm & the throttle is completely closed.
not sure whether spark turns off,but there would be no real need to turn it off in any case,theres still no air/fuel to fire.
dont forget,most of the noise u hear from a motor is not explosions but air pumping & rapid air expansion etc.

aarong,throttle plate is completely closed when foot off accelerator,idle passages past the throttle plate take over instaed

Ahh, ok, then that's different from my car, which controls the throttle on its own.

Does that idle passage close if you are driving at speed with zero throttle?

[jks24]

good read this

[jords]

http://en.wikipedia.org/wiki/Engine_braking

FYI Read this.:thumbsup:

[Riviera]

yea i was gunna post that but meh, and it is the same process for all engines
in the diesels though u can get a massive compression skid out of them lol

[JohnL]

aaronng,
The throttle plate and IACV don't do anything out of the usual when the engine is on the overun. Overun fuel cut off isn't cotrolled using aiflow, though the IACV does remain momentarily open immediately after the throttle plate has been closed to (I think) assist in burning still unburnt fuel as it flows through the exhaust system (an emissions control measure).

My understanding of what happens is that the ECU detects that the throttle is at the closed position (using the TPS input) but also detects that engine rpm is above X, and using these inputs the ECU 'decides' that an overun condition must exist and as programmed cuts the injectors until either the throttle is opened again or the rpm reaches a set point somewhat above idle speed.

This feature lessens fuel consumptioon and emissions, and helps to prevent backfiring (which can damage the cat apart from anything else...).

The braking effect comes from internal engine friction (and to a much lesser degree from internal gearbox friction). Some of the braking effect comes from the compression of gas in the cylinders, but since the throttle plate is shut very little air should be able to flow into the cylinders from the intake side, so perhaps there is some reverse flowing gas coming in via the exhaust valve...?

[unity]

Can anyone explain why engine braking kicks in sometimes and doesn't at other times? Anytime I approach a decline I take my foot off the accelerator to let the car cruise. But the car will only sometimes engine brake, and it gets pretty annoying when it only engine brakes on very small declines.

[CRXer]

Ahh, ok, then that's different from my car, which controls the throttle on its own.

Does that idle passage close if you are driving at speed with zero throttle?
aaronng,wouldnt have a clue how your fancy dbw works,im too poor,my throttle still runs off a cable...
does the K24 dbw even have an idle valve? or does it do some fancy throttle plate work?

idle passages are always open,even if the idle valve was fully shut,the idle set screw is still bleeding,theres even a little bit of PCV at play. On EGR cars,theres a little bit of that too....

john,most of the effect is compression,theres always air available to compress,due to idle passages & overlap.

[string]

I don't think compression has much effect on the engine braking. Any energy that you lose to heating the air (which is at near vacuum) during the compression stroke you'll get back on the power stroke.

[aaronng]

Mine doesn't have an idle valve. Maybe that's why I can get better than manufacturer quoted consumption numbers....

[aaronng]

aaronng,
The throttle plate and IACV don't do anything out of the usual when the engine is on the overun. Overun fuel cut off isn't cotrolled using aiflow, though the IACV does remain momentarily open immediately after the throttle plate has been closed to (I think) assist in burning still unburnt fuel as it flows through the exhaust system (an emissions control measure).

My understanding of what happens is that the ECU detects that the throttle is at the closed position (using the TPS input) but also detects that engine rpm is above X, and using these inputs the ECU 'decides' that an overun condition must exist and as programmed cuts the injectors until either the throttle is opened again or the rpm reaches a set point somewhat above idle speed.

This feature lessens fuel consumptioon and emissions, and helps to prevent backfiring (which can damage the cat apart from anything else...).

The braking effect comes from internal engine friction (and to a much lesser degree from internal gearbox friction). Some of the braking effect comes from the compression of gas in the cylinders, but since the throttle plate is shut very little air should be able to flow into the cylinders from the intake side, so perhaps there is some reverse flowing gas coming in via the exhaust valve...?
Thanks for explaining. If there is still air going in and the injectors are cut off, wouldn't there be a split second of lean combustion when the left over injected fuel in the manifold enters the combustion chambers after the injectors are closed ?

[dsp26]

^^I was thinking that... coz that perfectly explains exhaust pop on throttle back off

i'm guessing this is something that can be confirmed by someone with a wideband setup?

[aaronng]

^^I was thinking that... coz that perfectly explains exhaust pop on throttle back off

i'm guessing this is something that can be confirmed by someone with a wideband setup?

Exhaust pop is caused by a removal of the resonator/muffler and/or a rich situation. Not a lean one.

[JohnL]

john,most of the effect is compression,theres always air available to compress,due to idle passages & overlap.

My point is that when the throttle plate is closed there is actually a very limited airflow through the induction system relative to the rpm at which the engine is turning. The flow through the bypass passages and past the slight gap around the circumference of the TP will be very restricted compared to what would be required to even remotely fill the cylinders.

So, overrun will create the highest vacuum (lowest pressure) conditions ever seen in the plenum / manifold. So, if on the overun the majority of airflow into the cylinders is coming from the induction side then there would be relatively little air available to be compressed by the rising pistons, and thus relatively little engine braking effect (at least from the compression of gas).

On the other hand, considering that the atmospheric end of the exhaust system is never closed by any type of valve as the induction system can be, if there is a substantial reversion of gas flow coming into the cylinder through the open exhaust valve, then it may be that most of the gas that is compressed in the cylinders to create the engine braking effectb is actualy entering the cylinders via the exhaust valve..?

I know from personal experience that an engine with a higher CR does create a stronger engine braking effect than a similar engine with a lower CR. A lessening of engine braking effect is also evident with engines that have lost substantial compression due to wear.

An interesting experiment to see if we can get a seat of the pants feel for any difference in engine braking effect relative to just closing the throttle, might be to switch off the engine while the throttle is still wide open (beware steering locks...).

[JohnL]

Thanks for explaining. If there is still air going in and the injectors are cut off, wouldn't there be a split second of lean combustion when the left over injected fuel in the manifold enters the combustion chambers after the injectors are closed ?

Quite possibly, but I can't see it causing any problem...

[JohnL]

Exhaust pop is caused by a removal of the resonator/muffler and/or a rich situation. Not a lean one.

Also, not only must unburnt fuel exist, so too must sufficient oxygen and heat. So, there may be an external backfire when the hot gasses (and hot unburnt fuel content) reaches the end of the pipe and finds oxygen. This tends to be more likely to create a backfire with a shorter exhaust because the temp is higher when the hot unburnt fuel vapour reaches oxygen at the end of the pipe.

An internal backfire (i.e. inside the exhaust system) might occur if the system has a leak (e.g. manifold gasket leak) and can 'suck' oxygen into the pipe at the 'hot end'. (A hole on the exhaust system - especially nearer the exhaust valve may allow air to be 'sucked' into the pipe due to the positive and negative pressure waves within the exhaust system).

Any backfiring is of course much less likely if the injectors are cut on the overun, which must be why backfiring is much less common in these days of ubiquitous fuel injection than it was in the days when FI was uncommon or non-existant (carbies of course not cutting fuel on the overun).

[CRXer]

My point is that when the throttle plate is closed there is actually a very limited airflow through the induction system relative to the rpm at which the engine is turning. The flow through the bypass passages and past the slight gap around the circumference of the TP will be very restricted compared to what would be required to even remotely fill the cylinders.

So, overrun will create the highest vacuum (lowest pressure) conditions ever seen in the plenum / manifold. So, if on the overun the majority of airflow into the cylinders is coming from the induction side then there would be relatively little air available to be compressed by the rising pistons, and thus relatively little engine braking effect (at least from the compression of gas).

On the other hand, considering that the atmospheric end of the exhaust system is never closed by any type of valve as the induction system can be, if there is a substantial reversion of gas flow coming into the cylinder through the open exhaust valve, then it may be that most of the gas that is compressed in the cylinders to create the engine braking effectb is actualy entering the cylinders via the exhaust valve..?

I know from personal experience that an engine with a higher CR does create a stronger engine braking effect than a similar engine with a lower CR. A lessening of engine braking effect is also evident with engines that have lost substantial compression due to wear.

An interesting experiment to see if we can get a seat of the pants feel for any difference in engine braking effect relative to just closing the throttle, might be to switch off the engine while the throttle is still wide open (beware steering locks...).

yeh string is right,allthough obviously u wont get it all back on the power stroke,u'll get enough to negate the compression effect.
for a normal car engine,u'd prob have to go back one stroke & most of the effect would be intake trying to pull open a vacuum....

a steering lock will only pop when u pull out the key,so lets give it a go.......
somebody????.......my car is in a million little pieces atm......

aaronng,u prob drive more efficiently then your average man,i could highly imagine u getting better than the factory is willing to quote in any case ;)
also like john said,the lean condition wouldnt matter,for u now are not trying to force a piston against the drivetrain anymore.

[CRXer]

^^I was thinking that... coz that perfectly explains exhaust pop on throttle back off

i'm guessing this is something that can be confirmed by someone with a wideband setup?
WB just goes instantly out of range,most dropping out at 20-22 AF

[aaronng]

aaronng,u prob drive more efficiently then your average man,i could highly imagine u getting better than the factory is willing to quote in any case ;)
also like john said,the lean condition wouldnt matter,for u now are not trying to force a piston against the drivetrain anymore.

I WOT to redline in 2nd gear at least twice a day though. I drive it hard enough that I have one of the worst vibrating engine of the Euros. Difference is I don't use more than 1/4 throttle and 2500rpm until the oil temp reaches 70 ºC, so my engine's piston ring seal should still be pretty good that the efficiency has not reduced, eventhough it has been to a few track days and has 85000km. The rest of the car on the other hand.... feels very very loose, rattly and knocky.

[CRXer]

see,your always thinking about it,average joe doesnt think at all.
what percentage of your driving do u think that coupla runs to redline makes up?
very little i presume...

[JohnL]

yeh string is right,allthough obviously u wont get it all back on the power stroke,u'll get enough to negate the compression effect.

I can see that on the down stroke the compressed charge will 'give back' at least some of the energy 'stored' in the compressed charge, but minus the energy converted by the compression into heat. It seems to me that this must ultimately be what happens to the kinetic energy of the moving mass, i.e. it's converted to heat through the vector of gas compression, thus creating the engine braking effect.


for a normal car engine,u'd prob have to go back one stroke & most of the effect would be intake trying to pull open a vacuum....

Which would have a limitation in that even with 100% efficiency the work is actualy done not by creating a vacuum, but by the underside of the piston 'pushing' against atmospheric pressure, which has a nominal value of only 14.7psi (give or take altitude variation and crankcase fluctuations, which won't be much).

So, even with an unrealistically high efficiency the work done creating at best a partial vacuum is not going to be the same as compressing in cylinder gas to pressures much greater than atmospheric...

-------------
It also just occurred to me that cutting the fuel injection on the overun should add to engine longevity since the bore walls aren't being rinsed with unburnt fuel (washing off the bore oil film), and less oil degradation due to less unburnt fuel finding it's way into the oil (i.e. less oil dilution).

[aaronng]

see,your always thinking about it,average joe doesnt think at all.
what percentage of your driving do u think that coupla runs to redline makes up?
very little i presume...

Little in daily driving, but of course I have trackdays to satisfy my cravings. :p

[string]

I can see that on the down stroke the compressed charge will 'give back' at least some of the energy 'stored' in the compressed charge, but minus the energy converted by the compression into heat. It seems to me that this must ultimately be what happens to the kinetic energy of the moving mass, i.e. it's converted to heat through the vector of gas compression, thus creating the engine braking effect.

You can only lose energy if something else gains energy. The only way the charge can lose heat energy is if it's temperature is greater than the surrounding cylinder walls/piston/head - otherwise the charge will gain temperature from the cooling system and give more energy back on the power stroke than you lost through compressing it!



Which would have a limitation in that even with 100% efficiency the work is actualy done not by creating a vacuum, but by the underside of the piston 'pushing' against atmospheric pressure, which has a nominal value of only 14.7psi (give or take altitude variation and crankcase fluctuations, which won't be much).

A quick fiddle with excel:


Pressure 8 psi
=0.055 N/mm^2
Area 81 mm
=5153.0 mm^2
Stroke 89 mm
Force 284.30 N
Work 25.30 J per cylinder per 2 revolutions

Cyl 4
Rate 3000 rpm
Power 2530.29 Watts




So, even with an unrealistically high efficiency the work done creating at best a partial vacuum is not going to be the same as compressing in cylinder gas to pressures much greater than atmospheric...

If we were compressing a reasonable quantity of air then I'd agree. It would heat up substantially and you'd lose lots of energy to the cooling system. Compressing cold vacuum is a different story...

[aaronng]

You can only lose energy if something else gains energy. The only way the charge can lose heat energy is if it's temperature is greater than the surrounding cylinder walls/piston/head - otherwise the charge will gain temperature from the cooling system and give more energy back on the power stroke than you lost through compressing it!

It's because there are other factors other than compression and expansion which cause engine braking. The engine is a mass of rotating and moving frictional surfaces being kept apart by a viscous fluid. That would provide the bulk of the engine braking, which is why you get more deceleration when engine braking at high rpm than at low rpm if you were in the same gear and at a low enough speed to avoid the effects of wind resistance. I doubt you will gain more energy back on the power stroke because when the air enters the cylinder it is heated up and is no longer at ambient engine bay temperature. So the heated air is compressed and can be similar to the cylinder wall temperature after compression. Even if you gain energy on the expansion that energy is still insufficient to overcome the resistance of the moving parts and the energy lost as heat each time the pistons change movement direction from up to down and back to up.

In high compression engines, the temperature heats up enough to lose some heat through the block.

[string]

It's because there are other factors other than compression and expansion which cause engine braking. The engine is a mass of rotating and moving frictional surfaces being kept apart by a viscous fluid. That would provide the bulk of the engine braking, which is why you get more deceleration when engine braking at high rpm than at low rpm if you were in the same gear and at a low enough speed to avoid the effects of wind resistance.

This is not in dispute. Any effect that relies on the work does by the moving piston or moving surfaces on the rotating assembly will have losses proportional to the engine RPM.



I doubt you will gain more energy back on the power stroke because when the air enters the cylinder it is heated up and is no longer at ambient engine bay temperature. So the heated air is compressed and can be similar to the cylinder wall temperature after compression.

The process of generating a vacuum in the cylinders greatly cools the charge - the temperature of the charge inside the cylinder at the beginning of the compression stroke will be much lower than the engine bay air. As a side point, consider the duration of time that the charge spends at it's most compressed (and highest temperature) state compared to the rest of the stroke.



Even if you gain energy on the expansion that energy is still insufficient to overcome the resistance of the moving parts and the energy lost as heat each time the pistons change movement direction from up to down and back to up.

A strawman. This isn't my argument.

[aaronng]

The process of generating a vacuum in the cylinders greatly cools the charge - the temperature of the charge inside the cylinder at the beginning of the compression stroke will be much lower than the engine bay air. As a side point, consider the duration of time that the charge spends at it's most compressed (and highest temperature) state compared to the rest of the stroke.

Where is that vacuum coming from? During the intake cycle, the inlet valves are open, drawing air in. Since the valves are open and air is being supplied into the combustion chamber as the piston moves downwards, there won't be enough vacuum to cool the charge.

Once the air gets into the hot combustion chamber, it heats up and it is no longer cooler than the engine bay air.

[string]

The vacuum arises from the piston trying to suck 450cc of air through a tiny hole. Hook up a boost gauge to your engine and watch what happens when you take your foot off the throttle - it goes well below atmospheric pressure.

The ideal gas law tells us that the same volume of air at a lower pressure will be cooler.

[JohnL]

OK, engine braking experiment completed, though not in a particularly rigorous fashion. Due to time and budgetary constraints the experiment was conducted relying solely upon my seatofthepantsomemeter.

I ran the tests a number of times within varying rpm ranges in different gears and in differing conditions, including; coasting down with the ignition on / throttle closed, and coasting down with ignition off and throttle wide open (as well as while rapidly opening / closing / opening the throttle).

Result; I couldn’t perceive any difference in engine braking effect between:

1) Just closing the throttle as per normal (with or without the engine shut down)

and

2) Shutting the engine off with a wide open throttle

This isn’t to say that there is zero difference between these two modes, it may just be that the seatofthepantsomemeter isn’t sensitive enough to detect it (which would however suggest a very small difference). Also, with ‘2’ there is a very loud induction roar resembling that of a Kenworth nearing a truck stop selling cheap pornography.

So, assuming that at least some of the braking effect is actually coming from gas compression, it doesn’t seem that it matters whether the throttle is open or closed. In this case I can only assume that with a closed throttle an adequate quantity of gas can get into the cylinder via the exhaust valve. This would be possible due to valve overlap and the exhaust valve still being open for some degree of crank rotation past TDC near the beginning of the induction stroke.


You can only lose energy if something else gains energy.

Like the water jacket (see speculation below).


The only way the charge can lose heat energy is if it's temperature is greater than the surrounding cylinder walls/piston/head - otherwise the charge will gain temperature from the cooling system and give more energy back on the power stroke than you lost through compressing it!

Assuming an adequately high initial cylinder pressure (i.e. not too far below atmospheric), it does seem possible that the heat generated in the compressed charge could exceed the water jacket temperature (which isn’t all that high relative to the temps likely to be generated by compression in a cylinder, i.e. high enough to require the fuel to have a high auto-ignition point in order to prevent detonation).

The trouble is that we (at least I) don’t know what the pre compression cylinder pressure might more or less be, so can’t even guess at the pressure and temperature rises. It does seem to me that if the overrun pre compression psi is significant then most of the compressible gas (at higher rpm) must be coming into the cylinder via the still open exhaust valve, since the inlet side is so restricted with the throttle plate closed.

Thinking on this, it doesn’t seem likely to me that the amount of gas entering the cylinder is actually likely to be enough to see particularly high compression pressures. It seems to be more and more likely that at least a significant proportion of the braking effect must be coming (as you suggest) from the underside of the piston working against crankcase pressure as a partial vacuum is created in the cylinder, manifold and plenum.

As this happens and the expanding charge in the cylinder loses pressure, it must also lose temperature (in theory until a perfect vacuum is created which by definition has no temperature, at all), and this heat energy must go somewhere.

The only ‘somewhere’ I can see appears to be into the water jacket. In this case it seems we are using kinetic energy to create a drop in temperature by moving energy from one place to another (as seems always the case when temperature is ‘artificially’ lessened, e.g. refrigeration etc).


A quick fiddle with excel:


That's a bit scary, I mean using spreadsheets, almost cheating…

(I must learn to use Excel one day…)

Not meaning to be unduly critical, but I wonder as to your assumptions? The end number for your hypothetical 4 cylinder engine (“2530.29 Watts”) seems not much more energy generated (‘absorbed’) within the engine than would be required to run a vacuum cleaner, which seems not enough to account for the braking affect as felt in a rather heavy vehicle. Still if it’s only part of the manner in which the kinetic energy is dissipated…


If we were compressing a reasonable quantity of air then I'd agree. It would heat up substantially and you'd lose lots of energy to the cooling system. Compressing cold vacuum is a different story...

Well, if I wanted to get really anal I'd point out that if you compress a vacuum you end up with… a vacuum.

At any rate, I’m not sure now where the majority of the engine braking effect is coming from (I used to think it was simple…). I suspect it must be a combination of compression, creating a vacuum, and the background pumping / frictional losses inherent in operating the engine.

[JohnL]

The engine is a mass of rotating and moving frictional surfaces being kept apart by a viscous fluid. That would provide the bulk of the engine braking,

Experiment; compare engine braking effect using a 20W oil as opposed to a 50W oil. If the fluid drag (friction between oil molecules, generating heat as 'waste' energy) is responsible for the majority of the braking effect then a big change in oil viscosity should have a significant affect?


which is why you get more deceleration when engine braking at high rpm than at low rpm if you were in the same gear and at a low enough speed to avoid the effects of wind resistance.

Could not a higher engine braking effect at higher rpm equally be attributable to a greater quantity of gas per second being compressed / decompressed?


I doubt you will gain more energy back on the power stroke because when the air enters the cylinder it is heated up and is no longer at ambient engine bay temperature. So the heated air is compressed and can be similar to the cylinder wall temperature after compression.

I'd be surprised if ithe compressed charge weren't hotter than the cylinder wall, but I have no data. I seriously doubt that the compressed charge will gain heat from the cylinder walls, I suspect it will lose heat to the cylinder wall (though it will initially gain some wall heat in the early stages prior to and just after compression begins, as well as having picked up some heat from the manifold on the way in and as it passes the inlet valve).


Even if you gain energy on the expansion that energy is still insufficient to overcome the resistance of the moving parts and the energy lost as heat each time the pistons change movement direction from up to down and back to up.

The inherant friction created within the various oil films will create a braking effect, but the reciprocating motions are a net zero gain / loss. As the reciprocating mass (e.g. a piston) is accelerated it 'consumes' X energy, but as it decelerates it 'gives back' X energy...


In high compression engines, the temperature heats up enough to lose some heat through the block.

I tend to think so...

[JohnL]

The process of generating a vacuum in the cylinders greatly cools the charge

It will take energy to create the (partial) vacuum and thus to cause the loss of temperature. The temp loss of the charge as it decompresses is a secondary effect, I can't see that it's giving up this energy into the crankshaft...


the temperature of the charge inside the cylinder at the beginning of the compression stroke will be much lower than the engine bay air.

Perhaps, as a result of the pressure drop that causes the air to move into the cylinder, but consider that the manifolds etc won't be cold.

[JohnL]

Where is that vacuum coming from? During the intake cycle, the inlet valves are open, drawing air in. Since the valves are open and air is being supplied into the combustion chamber as the piston moves downwards, there won't be enough vacuum to cool the charge.

The vacuum exists because the atmospheric openings into the induction system are largely closed off, so no (or little) air is being supplied to the cylinder via the induction system (I suspect that more may enter by the back door, i.e. via the exhaust valve).

On the overrun the entire inlet system is subjected to a fairly strong vacuum so when the inlet valve opens there is not enough manifold pressure to 'push' any air into the cylinder (keeping in mind that in reality there is no such thing as suction).

[JohnL]

The ideal gas law tells us that the same volume of air at a lower pressure will be cooler.

But, (unless I'm very much mistaken) while temperature and pressure are related they are not entirely dependant upon each other. Having cooled as the pressure drops, that lower pressure gas can be heated up again with the addition of more heat...

[yourfather]

nice discussion we are having here.

but a new set of brake pads is like what no more than $400.

a new engine if you **** it up by over-revving = ~$????

just use the brake pads

[CRXer]

new set of brake pads down a long winding slope will happen quick with no engine braking.

so where does that leave us with the suction(oops i said it) on the valves during power?
or did i miss that bit.

suction exists john,u mean no such thing as neg pressure(till someone pokes his head in the nearest black hole to find out)
suction happened to me last night......its all taken from your point of reference....

[string]

John,

Regarding my usage of the word vacuum - I must apologise if I'm not anal enough, but my meaning is generally "less than atmospheric pressure". I generally attempt to maintain strict definitions but I am a Gen Y so I have to follow the stereotype sometimes.

I agree that the majority of the losses will be from the various frictions. My example with excel gives you a rough idea - 2.5kW is in no way enough to slow down a car. My posts have simply been to express my ideas about the power losses through the compression event.

When I've had a vacuum gauge attached to my car, cruising down big hill in top gear results in the gauge nearly hitting the maximum negative measure (absolute vacuum on this particular dial). Assuming that the gauge is inaccurate and badly calibrated, let's say that the actual manifold pressure is around 1.6psi above vacuum. Idle pressure is not too far above this so I don't see it as a stretch of the imagination. Once compressed by the cylinder in my example 9.2:1 CR engine the end result is [an unexplainably coincidental] 1 atmosphere - the same pressure it was as it was before entering the throttlebody. There are a few avenues for it to gain heat energy before entering the cylinder to be compressed but I think that exceeding the temperature of the water jacket is a stretch. Your experiment does not falsify my reasonings thus far which is always a good thing for an argument :)

Regarding excel: It is a wonderful tool. I'm a numbers and maths guy at the core and have been since I learned to program in primary school. Roughly banging out a few equations gives you a real insight into what's going on. The numbers don't even need to be close to accurate - the truth lies in the order of magnitude of the result. I could have any of my guessed factors out by a factor of 5, or 0.2 and the result is still meaningful since it shows an approximate relationship between the input variables and the output, which through text discussion alone, gives you no quantifiable and predictible connection, only conceptual. Approximations are only a problem if you can't also see a relationship between input error and output error. I felt at peace with the approximations I've made, a distinct contrast to some of the things I was taught in most electrical circuits university subjects which usually left me shocked thinking "how does the world function with engineers approximating like this". Thank the gods of engineering for safety factors :lol:

And finally regarding the existence of the temperature of vacuum. The best example is the cosmic background radiation. Inside our observable universe at least, there is no such thing as zero energy space - every volume contains a torrent of photons whizzing around. Add up the energy of the photons and you get a temperature. An irrelevant point when talking about the temperatures of the compression event though so I'll leave it at that and call it a night :D

[CRXer]

throttle closed coast,MAP reads about 100mBar less than idle (according to who knows how accurate a MAP sensor is at that pressure) if that helps u geniuses work this out,
ie idle bout 250mBar,throttle closed coast bout 150mBar

[string]

suction exists john,u mean no such thing as neg pressure(till someone pokes his head in the nearest black hole to find out)
suction happened to me last night......its all taken from your point of reference....

Suction only exists as a higher level description of what's going on. When you suck with your mouth (or lungs), you are really just increasing the volume of the fluid (air in the mouth cavity) which must decrease it's pressure. When connected to a higher pressure supply, you get a flow of fluid attempting to equalise the pressures (nature's attempt to reach the lowest energy state in the system).

When the restriction between the high pressure supply (atmosphere) and the expanding chamber (lungs) is low, the increase in chamber volume results in immediate equilibrium as the air easily overcomes the passage from out to in. If you close your lips but leave a tiny hole it is extroadinarily difficult to breath - you are allowing a lesser flow of air to create the equilibruim state - either the rate of airflow must greatly reduce or you must generate a much lower pressure inside the lungs (in order to create a greater pressure difference between out an in to persuade the air to rush in at the original rate). Option 2 is difficult since the internal pressure of your organs will try to crush the low pressure you are attempting to create. You have now imagined what it's like for a piston with a closed throttle body - it doesn't have the choice to breath slowly though, so you end up with a chamber full of very low pressure air with the piston strongly opposing the pressure in the crankcase. The forces involved in this are nothing compared to an actual combustion event which goes to show just how strong steel is.

Cliffnotes: A combustion chamber is much stronger than a set of human lungs - at least when it comes to sucking air through a tight restriction :)

[JohnL]

so where does that leave us with the suction(oops i said it) on the valves during power?
or did i miss that bit.

suction exists john,u mean no such thing as neg pressure(till someone pokes his head in the nearest black hole to find out)
suction happened to me last night......its all taken from your point of reference....

I shouldn't have mentioned it. Saying that suction is merely a convenient term for something that doesn't really exist is always opening a can of semantically argumentative worms.

I know that 'suction' certainly seems to exist, and it is a useful shorthand concept. My point is merely that nothing ever gets 'sucked' into anything, it's always 'pushed' in by higher pressure. Unless prevented from doing so, in it's movement toward an entropic state energy (in this case pressure) always tries to move from an area of higher energy density to an area of lower density, but the area of lower density doesn't 'pull' the energy toward it.

[CRXer]

chill john,im just stirring,lol,i know what your saying....im just saying it depends where u choose to take your point of reference from.

would be most ideal if they abolished conventional current flow as well,that certainly confuses the shit outta me sometimes when it really comes time to breaking it right down.....

[string]

would be most ideal if they abolished conventional current flow as well,that certainly confuses the shit outta me sometimes when it really comes time to breaking it right down.....

You're probably overthinking it. When dealing with circuits you rarely need to deal with the charge carrier itself (electron) so it's negative charge doesn't matter - all the building blocks can be modeled based on positive charge flow in the direciton of conventional current. It's identical to consider the charge carrier as positive moving in the opposite direction which is exactly what conventional current is.

[CRXer]

your exactly right string & exactly my point.
point of reference sometimes confuses the issue at hand,be easier to set the standards a bit tighter so everyone is talking the same language.
not having a solid background in electronics,i do get caught overthinking it,especially when it comes to getting in & out of semiconductors & then tryin to refer back to something that is drawn up in conventional current.
if i had more experience & could just take it more for granted,this prob wouldnt happen...

oh god...i can hear the topic nazis coming now.....

[string]

I agree that semiconductors is silly business. Quantum physics at a mathematical level isn't joyous - I'd rather take for granted the final equations.

[JohnL]

John,

Regarding my usage of the word vacuum - I must apologise if I'm not anal enough, but my meaning is generally "less than atmospheric pressure".

String,
My apologies for being excessively pedantic. I understood what you meant, just yanked the chain lightly…


I generally attempt to maintain strict definitions but I am a Gen Y so I have to follow the stereotype sometimes.

So do I, I’ve been in to many discussions that turn out to be at crossed purposes due to careless terminology (others or mine…). I'm a tail end baby boomer (if that's an excuse for anything...).


I agree that the majority of the losses will be from the various frictions. My example with excel gives you a rough idea - 2.5kW is in no way enough to slow down a car. My posts have simply been to express my ideas about the power losses through the compression event.

This discussion is forcing me to examine my previous assumption that the great majority of the engine braking effect was being caused by compression of the gasses in the cylinder. It's an easy assumption to make, especially when the alternative term for this effect is 'compression braking'.

If it’s correct that compression is only responsible for a relatively small % of the engine braking effect, then it does seem problematic that engines with poor compression typically appear to exhibit less engine braking effect, though this could be associated with loss of ring tension or ring sticking (i.e. possible root causes of the compression loss) and thus less friction as the rings slide up and down.

It’s also problematic that engines with a higher CR also appear to have significantly greater engine braking effect, but I can’t rule out placebo…

I’m still thinking that in cylinder compression is most probably responsible for a substantial portion of engine braking effect (perhaps still the majority…?), just not almost all of it as I was previously thinking.



Assuming that the gauge is inaccurate and badly calibrated, let's say that the actual manifold pressure is around 1.6psi above vacuum. Idle pressure is not too far above this so I don't see it as a stretch of the imagination. Once compressed by the cylinder in my example 9.2:1 CR engine the end result is [an unexplainably coincidental] 1 atmosphere – the same pressure it was as it was before entering the throttlebody.

Spooky, but quite explainable as coincidental (unless one happens to be into cosmic conspiracy theories…).

Keep in mind that manifold pressure is not in cylinder pressure, and that the manifold is directly connected to at least three other cylinders that are all scavenging the manifold at differing but overlapping moments.



There are a few avenues for it to gain heat energy before entering the cylinder to be compressed but I think that exceeding the temperature of the water jacket is a stretch. Your experiment does not falsify my reasonings thus far which is always a good thing for an argument :)

I think you’re assuming that the only avenue for gasses to enter the cylinder is via the induction system (which is very highly scavenged and thus at very low psi because it’s choked off at it’s entrance). If it were then your in cylinder pressure numbers are probably valid, but I strongly suspect that a substantial quantity of gas (or at least a significant quantity) is entering the cylinder by way of reversion through the exhaust valve (the exhaust system still being open to atmosphere).

If I’m correct then the pre compression cylinder pressure may well be substantially higher than your 1.6psi induction manifold pressure, and as a result so too will the fully compressed cylinder pressure.

I seriously doubt your fully compressed figure of near (only) 1 Atmosphere. Just doing a compression test will show much higher in cylinder pressures than this, even with the throttle closed. Admittedly this is only at cranking speed, and the volumetric efficiency (no matter which way gas may be entering the cylinder) may be somewhat different at higher rpm (such as your 3000rpm assumption), but I doubt it’s hugely different…


Regarding excel: It is a wonderful tool. I'm a numbers and maths guy at the core and have been since I learned to program in primary school.

In primary school one of my teachers practiced mathematics aversion therapy upon the boys (but not the girls…) by inflicting physical pain when we got a maths solution wrong. Not a good start, as a result I like cryptic crosswords, but not Sudoku…


………. Approximations are only a problem if you can't also see a relationship between input error and output error. I felt at peace with the approximations I've made,

I still suspect the assumptions, as I’ve attempted to explain above.



a distinct contrast to some of the things I was taught in most electrical circuits university subjects which usually left me shocked thinking "how does the world function with engineers approximating like this". Thank the gods of engineering for safety factors :lol:

Yes, it’s often; “based on W, X and Y assumptions calculate that a unit needs to be Z strong, then make it Z x 2…”


And finally regarding the existence of the temperature of vacuum. The best example is the cosmic background radiation. Inside our observable universe at least, there is no such thing as zero energy space - every volume contains a torrent of photons whizzing around. Add up the energy of the photons and you get a temperature. An irrelevant point when talking about the temperatures of the compression event though so I'll leave it at that and call it a night :D

Ah, but energy isn’t necessarily heat, and “a torrent of photons whizzing around” are photons of radiation, not heat per se, each photon being an exciter that creates heat only when it has a material upon which to act?

Isn’t it more a case of; provide a material within a vacuum upon which the radiation can act, and the material will be warmed as a result of being acted upon by the radiation? Otherwise we just have energy passing through space. There needs to be a ‘something’ for there to be heat (i.e. atoms oscillating more vigourolsy), i.e. radiation per se isn’t heat, but it is an agent that can create heat in a material…

This is getting a tad off topic…

[JohnL]

When the restriction between the high pressure supply (atmosphere) and the expanding chamber (lungs) is low, the increase in chamber volume results in immediate equilibrium as the air easily overcomes the passage from out to in.

"Immediate" of course means 'rapid', not 'instantaneous'. If it were truly 'immediate' then internal combustion engines would produce the same torque regardless of rpm because the cylinder would always completely fill to atmospheric pressure whether at 1000rpm or 10,000rpm...

More pedantry, sorry...

[JohnL]

chill john,im just stirring,lol,i know what your saying....im just saying it depends where u choose to take your point of reference from.

would be most ideal if they abolished conventional current flow as well,that certainly confuses the shit outta me sometimes when it really comes time to breaking it right down.....

The simplest appearing things often turn out to be hugely complicated when you really chase that rabbit deep down the hole...

'Roll / weight transfer theory' is an excellent example, but I also find anything to do with electronics mystifying...