Suction at tail pipe - Page 2 - PeachParts Mercedes-Benz Forum

blackestate · #16 04-29-2009, 12:45 PM

Had this happen on a 7.3 and it was a dropped valve.
I would pull the valve cover and take a look see if you have a broken pring soe something causing the valve to not seat.

leathermang · #17 04-29-2009, 01:05 PM

Quote:

Originally Posted by blackestate

Had this happen on a 7.3 and it was a dropped valve.
I would pull the valve cover and take a look see if you have a broken pring soe something causing the valve to not seat.

Thank You.

JonL's statement " A little suction like that probably isn't abnormal. " is not correct. Detecting a suction at the tailpipe is an indication of something wrong. I have seen it listed in old books as a diagnostic step.

CarlG · #18 04-29-2009, 04:54 PM

hehe. Sorry... I'm with you on this one. Perhaps there is technically some sort of suction form exhaust valves staying open a degree or two after the piston has reached the top of the cylinder, but such suction would be SO insignificant that you would definitely not be able to detect it with your hand.

If that was my car, the next thing I'd do (assuming I'd been provided with no other information than I have) is take off the air cleaner and put my hand over the intake manifold. If there's suction at the exhaust, I wouldn't be surprised if there was suction and compression at the intake manifold. If there is, you could have either a very serious timing issue, a broken cam shaft or timing chain, or something else completely unpredictable, like a mouse in the engine. Somehow I doubt this is a dropped valve. If only one valve dropped, in theory, the engine should start, and then run like crap with LOTS of knocking and pinging from that valve.

leathermang · #19 04-29-2009, 07:04 PM

CarlG, What do you mean " sorry "... I am not some black sheep you need to be ashamed of agreeing with....
LOL

I do think we should define some parameters when trying to discuss this whole thing...
Lets try to distinguish between a ' lack of power stroke' on a cylinder.... as could happen with a lack of fuel to a single cylinder...
and suction which would mean some way that the piston going down in the bore pulls air from the wrong place... the exhaust pipe.. as compared to the intake.
It would be easy to confuse them when down there at the exhaust pipe smelling those wonderful fumes...
Lets define the test equipment which might distinguish between those two types of symptoms....
If a playing card were held near the tip of the exhaust pipe the gas would push it away from the tip... and if a pulse was missed it would snap towards the pipe end....
Lets assume the card has its fulcrum at the top and only gravity to bring it back to vertical if gas pressure is reduced... I am using the lightweight card for an example because a heavier object ,having been pushed off vertical, would then have stored kinetic energy sufficient to cause it to drop back further than vertical... thus giving the false impression that it was pulled back towards the pipe due to suction... as compared to just a missed positive pulse of exhaust gas.
This is also the case if this item is hand held... the reaction just like someone let go of a rope in a tug of war.
So the ideal test instrument is very light, placed very close to the end of the exhaust pipe , nonflammable (LOL), hinged close to the top of the horizontal extension of the top of the exhaust pipe and hopefully dampened slightly in its reaction to the lack of backward pressure....
This would distinguish between a lack of positive pressure out of the exhaust...and actual suction caused by some deficiency in the one way function usually provided by the valve action in an engine.

JonL · #20 04-29-2009, 09:44 PM

Guys, I do not know if there will actually be a detectable suction pulse at the tailpipe during cranking, but it is for certain possible. Try to picture what happens during cranking with no firing.

The engine is cranking at perhaps 300 RPM. Well less than half idle speed. Let's follow one cylinder, starting at TDC on the intake stroke. The intake valve is already open a little as the piston starts down. Atmospheric pressure pushes air into the cylinder to fill the volume of the descending piston. There is a pressure drop through the air cleaner, intake manifold, and intake valve, so the cylinder pressure never actually reaches atmospheric pressure during the intake stroke. The intake valve is still open at BDC, and it is still open for some degrees as the piston starts up on the compression stroke. During normal operation, this improves volumetric efficiency. At cranking speed, however, the air velocity is so low that soon after the piston starts upward it pushes some of the air in the cylinder back out into the intake manifold. When the intake valve closes, there is less than the theoretical maximum amount of air in the cylinder, and it is at lower than atmospheric pressure. The piston now compresses the air until it reaches TDC. The air gets hot as it compresses, and some of the heat of compression is lost to the cold cylinder head, piston, and cylinder walls. This lowers the compression pressure from the theoretical maximum adiabatic compression pressure. Also, since the piston is moving relatively slowly, there is more time for some compression leakage past the piston rings that are not yet at operating temperature. Now the piston starts down on the power stroke, only there is no power because the engine isn't firing. Some heat loss from the gas continues during the expansion stroke, further reducing the pressure from the theoretical. As the piston approaches BDC, the exhaust valve opens. Because of all the losses thus far -- the inlet restrictions, the loss of trapped mass because of the delayed intake port closure, the heat loss, and the cylinder leakage -- the cylinder pressure is LOWER than atmospheric pressure and a negative pressure pulse is generated in the exhaust system. As the piston continues downward, the pressure becomes even lower, until finally the piston is at BDC and reverses, starting upwards. Some degrees after BDC, the upward movement of the piston finally raises the cylinder pressure above atmospheric, but because of the low cranking speed and low temperature of the "exhaust" gas, the volumetric flow rate is far, far lower than during normal operation. Since the exhaust system is sized to handle much more flow, the restriction of the exhaust under these conditions is small, and very little pressure is generated in the cylinder to expel the air. Now the piston reaches TDC on the exhaust stroke and the pressure in the cylinder approaches very closely that of the exhaust manifold, which is only very slightly above atmospheric pressure. Then the piston starts downwards on the intake stroke, but because of the overlap built into the cam, the exhaust valve remains open for some degrees while the piston is descending, and the atmospheric pressure at the tailpipe acts on the whole column of air in the exhaust to push air back into the cylinder. As the intake valve opens the pressure balances between the intake manifold and exhaust manifold and both contribute to filling the cylinder. Finally, the intake valve opens more than the exhaust, and ultimately the exhaust valve closes and the cycle repeats.

So you can see that the exhaust pressure during cranking fluctuates around atmospheric -- sometimes above, sometimes below. The magnitude of the fluctuations is probably very small, and the influence of the number of cylinders and firing order and exhaust system design must be considered. There are also acoustic phenomena at play, where the pressure fluctuations can have resonances that amplify the effect, although this is probably a very small effect at cranking speed.

Don't say "it can't happen." It can, and it might.

el sea · #21 04-29-2009, 10:17 PM

Marine diesel owners are advised to not crank for long duration when trying to start their engines. The reason, the engine will suck water back up the exhaust and hydro the cylinders. This is when the cylinder is filled with water and since you cannot compress water you usually damage a valve, rod, head gasket, but always damage something. So an exhaust can pull a vacuum......

leathermang · #22 04-29-2009, 11:36 PM

JonL,
Most of that description is very close to right...
except for this sentence...

"Since the exhaust system is sized to handle much more flow, the restriction of the exhaust under these conditions is small, and very little pressure is generated in the cylinder to expel the air."

The piston constitutes a Positive Displacement Pump... so the pressure generated is way above what is required to move that air out of the bore.

Your description is close enough to accept UNTIL the exhaust tubes from the other cylinders are connected together... at that point and back to the end of the exhaust .... particularly at the cranking speed you choose to use for your example... and dealing with one or two mufflers before reaching the examination point on the exhaust... how do you expect the physics you described to ' act on that entire column of air' ? You just don't have the sucking power from the slight amount that diesel engines overlap the valve opening...
HOWEVER, if you had a breach in the valve sealing when the piston starts down on the power stroke... you would have a much much larger potential effect on that column of air...
On certain functions you cite inertia or scavenging.. but then ignore it where it does not help your argument.
Why did you not address my fine testing equipment design ...and the need to differentiate between loss of pressure and actual reverse flow of air in the exhaust system all the way to the end of the exhaust ?
We do not know for sure that what we are discussing is actually what the OP is experiencing...

JonL · #23 04-30-2009, 09:01 AM

Quote:

Originally Posted by leathermang

JonL,
Most of that description is very close to right...

Sorry, I don't feel you are qualified to make this judgement.

Quote:

except for this sentence...

"Since the exhaust system is sized to handle much more flow, the restriction of the exhaust under these conditions is small, and very little pressure is generated in the cylinder to expel the air."

The piston constitutes a Positive Displacement Pump... so the pressure generated is way above what is required to move that air out of the bore.

The maximum pressure generated by ANY pump cannot exceed the backpressure imposed by the restriction it is pumping against. At any given moment during the exhaust stroke the pressure in the cylinder equals the pressure in the exhaust manifold plus the restriction of the exhaust valve. There will be a very small added effect of inertia -- the force required to accelerate the air.

Quote:

Your description is close enough to accept UNTIL the exhaust tubes from the other cylinders are connected together... at that point and back to the end of the exhaust .... particularly at the cranking speed you choose to use for your example... and dealing with one or two mufflers before reaching the examination point on the exhaust... how do you expect the physics you described to ' act on that entire column of air' ? You just don't have the sucking power from the slight amount that diesel engines overlap the valve opening...

I expect the physics to act the way physics always act. At low velocity, the "spring rate" of the air has very little effect. What happens at one end of the pipe will happen at the other end of the pipe almost instantaneously. The piston is not moving fast enough at cranking speed (I chose cranking speed because that is the speed at which this entire thread is relevant -- read the original post) for the mufflers to have any effect as "surge tanks" or whatever you are visualizing. As I said, the net effect at the tailpipe must also consider the number of cylinders, the firing order, and the exhaust system design. I did not attempt to explore these interactions in my description. They may well partially cancel out the fluctuations at the tailpipe, and they also may amplify the fluctuations.
The "sucking power" isn't a function of the overlap in that the inlet suction acts upon the exhaust as it does in a gasoline engine. I think you have misread my post 11. Post 11 is a response to your belief than my explanation is based upon hot cams in gasoline engines. Post 11 describes briefly why a diesel operates differently from a gasoline engine with respect to valve events - at normal operating speed. As we both well know, there is no appreciable intake manifold vacuum on a diesel engine. The "sucking power" I have attempted to describe comes from the opening of the exhaust valve while the piston is on the downward stroke. This happens both at the beginning of the intake stroke (during the overlap period) and at the end of the exhaust stroke (during what is known as blowdown). I have attempted to explain why at cranking speed the pressure in the cylinder when the exhaust valve opens may well be below atmospheric pressure, and will certainly be below atmospheric pressure by the time the piston reaches BDC. Thus there are TWO negative exhaust pressure pulses generated by each cylinder during a full four stroke cycle. Depending on the number of cylinders and exhaust system design, these negative pulses from different cylinders may overlap at times and have an additive effect, although I have not attempted to calculate if they in fact do so for a five cylinder MB engine.

Quote:

HOWEVER, if you had a breach in the valve sealing when the piston starts down on the power stroke... you would have a much much larger potential effect on that column of air...

An improperly functioning valve train could easily make large suction events in the exhaust, however the particular example you cite would create a positive pulse. During cranking, the pressure in the cylinder is highest just before the piston starts down on the power stroke.

Quote:

On certain functions you cite inertia or scavenging.. but then ignore it where it does not help your argument.

I only invoked inertia and scavenging to explain why camshafts are designed with valve events that do not occur neatly at TDC and BDC, and why there is an overlap period. These design features are intended to have effects at normal operating speeds, NOT at cranking speed. I have consistently said that inertia and scavenging effects are negligible at cranking speed.

Quote:

Why did you not address my fine testing equipment design ...and the need to differentiate between loss of pressure and actual reverse flow of air in the exhaust system all the way to the end of the exhaust ?
We do not know for sure that what we are discussing is actually what the OP is experiencing...

I think it is a fine idea. I'd like to do the test on a known good engine as well by holding the stop lever while cranking.

The point of my first post on this thread was simply to attempt to keep the OP from chasing big problems when little ones are more likely at fault. The OP gave very little information about what has been looked at thus far. You should also note that in my first post I suggested that something stuck in his intake would be a possible cause for a no start condition AND higher than normal suction pulses in the exhaust.

moon161 · #24 04-30-2009, 09:19 AM

Stop wasting time reading all of the arguments and take the valve cover off.

leathermang · #25 04-30-2009, 09:45 AM

Quote:

Originally Posted by JonL

Sorry, I don't feel you are qualified to make this judgement.

You contradict your own theory of the physics multiple times within each post...
You place a lot of power on the small overlap of the valves at really unimportant times in the stroke... but ignore how a malfunction of the separation of the intake/ exhaust functions on a MAJOR movement of the piston ( the exhaust valve open on the downward power stroke ) would affect the situation.
Your impression of the effect 10 feet away down the exhaust pipe and after going through a muffler of a low volume low pressure gas completely ignores the definition of a gas.

CarlG · #26 04-30-2009, 07:19 PM

Guys, there's an easy way to settle this. You both own diesel cars. Why not go out, disable your fuel system somehow, get someone to crank the engine over, and see for yourself if there is suction at the tailpipe. I'd do it myself if I wasn't currently rebuilding my transmission.