Engine Braking?

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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

[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

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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.....

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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

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...