Engine Braking?

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Originally Posted by JohnL

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.

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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!

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Originally Posted by JohnL

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).

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A quick fiddle with excel:

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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

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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...

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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...

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Originally Posted by string

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!

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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.

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Originally Posted by aaronng

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.

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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.

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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.

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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.

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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.

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A strawman. This isn't my argument.

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Originally Posted by string

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.

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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.

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.

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.

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Originally Posted by string

You can only lose energy if something else gains energy.

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Like the water jacket (see speculation below).

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Originally Posted by string

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!

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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).

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Originally Posted by string

A quick fiddle with excel:

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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…

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Originally Posted by string

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...

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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.

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Originally Posted by aaronng

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,

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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?

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Originally Posted by aaronng

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.

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

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Originally Posted by aaronng

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.

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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).

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Originally Posted by aaronng

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.

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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...

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Originally Posted by aaronng

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

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I tend to think so...

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Originally Posted by string

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

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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...

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Originally Posted by string

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

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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.

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Originally Posted by aaronng

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.

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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).

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Originally Posted by string

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

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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...

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

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....