Timing adjustment methods - Page 5 - PeachParts Mercedes-Benz Forum

funola · #1 01-21-2011, 02:46 PM

Quote:

Originally Posted by leathermang

I would like to throw some carbon into this discussion...
one of the main reasons glowplugs burn out is a direct short produced by the build up of carbon.. thus the recommendation in the FSM about reaming the glowplug hole before putting a new one in..
What would be the result in these measurements of the voltage going to these plugs if one were shorted out ?..

AND isn't it more important the amount of CURRENT going to the plugs ?

Carbon build-up is a thermal issue. It cannot electricly short out a glow plug! Isn't that obvious just looking at the glow plug electrode?

Stretch · #2 01-23-2011, 10:21 AM

I like the idea that you could use an existing part of the car in a way that wasn't originally intended - I can not think of another automotive part that serves two different purposes like this might - so if it were possible to use a glow plug to measure temperature pulses I think that would be great. Though my gut feeling would be that a dedicated transducer that was designed to do the job would do it better.

Even though a glow plug is not intended to make measurements:- For it to do this the output from a glow plug need not have a correct magnitude (corresponding to a temperature) but it would have to be able to react quickly enough to a change in temperature. In electronics this is referred to as rise time. See:-

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

The rise time of a glow plug could be measured. As long as it is faster (and hopefully way faster) than the rate of ignition then it could be considered to be a transducer that is capable of measuring the pulses of heat caused at ignition.

To my mind that seems to be step one - ascertain whether a glow plug could in principle measure temperature changes at the rate of ignition.

Step two would be to ascertain when the point of ignition occurs in the cylinder and when the heat has propagated to the position of the thermocouple. So in effect a measurement of the "speed of heat" between the point (or points) of ignition in the cylinder and the glow plug that is positioned in the pre-chamber. I reckon this would be a complicated transfer function to measure.

I think step 2 is of vital importance as if you can not ascertain that the pulses that you can measure with your transducer occur at the time of ignition, then you are not going to be able to reliably use this information to estimate the timing.

In addition to these two problems there would undoubtedly be signal conditioning hurdles to overcome. But I like the idea - no matter how whacky it seems - mainly because it feels like you could almost get something for nothing!

If I can get hold of some decent instrumentation I'll start my own high school science experiment.

ForcedInduction · #3 01-23-2011, 12:02 PM

Quote:

Originally Posted by Army

I like the idea that you could use an existing part of the car in a way that wasn't originally intended

That can be applied to anything, though that doesn't mean its always smart to do so.

Stretch · #4 01-23-2011, 12:40 PM

Quote:

Originally Posted by ForcedInduction

That can be applied to anything, though that doesn't mean its always smart to do so.

Oh I completely agree - it could be a load of old hogwash - but if I can get my grubby paws on some instrumentation then I'm curious enough to see if I can make any sense out of it.

leathermang · #5 01-21-2011, 02:56 PM

LOL,
Carbon as a bridge wetted by fuel can not transmit electricity ?

I do understand that carbon is a thermal issue...

but I am pretty sure the FSM also describes it as a ' ground fault' problem with regards to glowplugs...

funola · #6 01-21-2011, 03:01 PM

Quote:

Originally Posted by leathermang

LOL,
Carbon as a bridge wetted by fuel can not transmit electricity ?

I do understand that carbon is a thermal issue...

but I am pretty sure the FSM also describes it as a ' ground fault' problem with regards to glowplugs...

The resistance heating element is encapsulated by a sheath and potted in a high temp compound (magnesium dioxide?). Carbon and fuel cannot get past the sheath unless it's reptured! LOL

leathermang · #7 01-21-2011, 03:08 PM

Quote:

Originally Posted by funola

The resistance heating element is encapsulated by a sheath and potted in a high temp compound (magnesium dioxide?). Carbon and fuel cannot get past the sheath unless it's reptured! LOL

I can see that you are going to force me to search my paper manuals manually.. the only downside to paper...

paddo · #8 01-21-2011, 09:20 PM

I wonder if it's one of those things where an experienced mechanic can adjust the IP timing just by feel/ear - maybe even finer than the recommended methods?

leathermang · #9 01-21-2011, 10:29 PM

Quote:

Originally Posted by paddo

I wonder if it's one of those things where an experienced mechanic can adjust the IP timing just by feel/ear - maybe even finer than the recommended methods?

No.

leathermang · #10 01-23-2011, 01:52 PM

Army, before you spend too much time on that deal... have you visualized what you would actually be measuring and if that would be of any value ? Even if the rise time was very fast..
You do not have a static heat source which the glowplug is in contact with... out of the total 720 crank rotation only a split second is the hot air in contact with the end of the glowplug hot. The amount of heat it will be exposed to will thus be affected by the rpm in addition to all the other stuff....and I can not see how even if everything were in your favor with the physics of the ' thermocouple' your readings would be anything other than a ' dulled' average of what was happening... I think it is more likely you will be measuring convection derived head temperature more than combustion temperature.. or at the very least an unkown ratio of the two..... which itself will probably a ' moving target' as the engine heats up...

Stretch · #11 01-23-2011, 02:52 PM

Quote:

Originally Posted by leathermang

Army, before you spend too much time on that deal... have you visualized what you would actually be measuring and if that would be of any value ? Even if the rise time was very fast..
You do not have a static heat source which the glowplug is in contact with... out of the total 720 crank rotation only a split second is the hot air in contact with the end of the glowplug hot. The amount of heat it will be exposed to will thus be affected by the rpm in addition to all the other stuff....and I can not see how even if everything were in your favor with the physics of the ' thermocouple' your readings would be anything other than a ' dulled' average of what was happening... I think it is more likely you will be measuring convection derived head temperature more than combustion temperature.. or at the very least an unkown ratio of the two..... which itself will probably a ' moving target' as the engine heats up...

I agree that it is a complicated situation - I think the transfer function between ignition and the point of measurement inside the pre-chamber makes it particularly difficult. It would be easier if the transducer was positioned in the combustion chamber - but hey that's the way the car has been made. I'm going to see if I can borrow some equipment - I have a cunning plan!

whunter · #12 01-24-2011, 04:28 AM

Quote:

Originally Posted by leathermang

Army, before you spend too much time on that deal... have you visualized what you would actually be measuring and if that would be of any value ? Even if the rise time was very fast..
You do not have a static heat source which the glow plug is in contact with... out of the total 720 crank rotation only a split second is the hot air in contact with the end of the glow plug hot. The amount of heat it will be exposed to will thus be affected by the rpm in addition to all the other stuff....and I can not see how even if everything were in your favor with the physics of the ' thermocouple' your readings would be anything other than a ' dulled' average of what was happening... I think it is more likely you will be measuring convection derived head temperature more than combustion temperature.. or at the very least an unknown ratio of the two..... which itself will probably a ' moving target' as the engine heats up...

As a theoretical exercise I will comment on the idea:

* The glow plug is a HIGH temperature heating element, NOT a thermocouple.
* Glow plug failure is common = false data.
* Minimum operating piston cylinder ignition temperature per MB is 850° F.
* Each cylinder is a totally separate environment.
* On an OM617 the math breaks down like this: 850 RPM ÷ 60 seconds = 212.5 power strokes ÷ 5 cylinders = 42.5 power strokes per cylinder = roughly 0.708 second of combustion data per cycle + immediately followed by exhaust (cooling), intake (cooling).

* Please review the reaction time of thermocouples in the following data, and note that it would be considered an insulated unit..

Quote:

http://www.w-dhave.inet.co.th/Thermocouple.html

6.2 Thermocouple Response Times

The response time for a thermocouple is usually defined as the time taken for the thermal voltage (output) to reach 63% of maximum for the step change temperature in question.
It is dependent on several parameters including the thermocouple dimension, construction, tip configuration and the nature of the medium in which the sensor is located.
If the thermocouple is plunged into a medium with a high thermal capacity and heat transfer is rapid, the effective response time will be practically the same as for the thermocouple itself (the intrinsic response time).
However, if the thermal properties of the medium are poor (e.g. still air) the response time can be 100 times greater.
http://www.w-dhave.inet.co.th/Thermocouple.html

http://hypertextbook.com/facts/2005/EileenTang.shtml

leathermang · #13 01-24-2011, 07:10 PM

Quote:

Originally Posted by whunter

As a theoretical exercise I will comment on the idea:

* Each cylinder is a totally separate environment.

* On an OM617 the math breaks down like this: 850 RPM ÷ 60 seconds = 212.5 power strokes ÷ 5 cylinders = 42.5 power strokes per cylinder = roughly 0.708 second of combustion data per cycle + immediately followed by exhaust (cooling), intake (cooling).

I hate to bring this up since Whunter Agreed with me...and was correct in the direction he was headed...
but I do not believe that the ' word math problem' was set up correctly... and thus came up with the wrong number as a result...
Anyone else see a/the discrepancy ? I left the part about 'each cylinder' in because keeping that in mind can help simplify the setup.

I believe the answer to be: 1.0 / = power strokes time (combustion access time) per second per cylinder at 850 rpm.
I used the inverse of one second to figure the access time.

850 divided by 2 is the number of four stroke cycles per minute per cylinder... with one power stroke in each.

425 power strokes per cylinder per minute divided by 60 is power strokes per second equals 7.08333333 power strokes per second.
or 1 / 7.08333333 =

IE, seven combustions per second into one second.

OR one second divided by seven combustions..

0.141176471 seconds as the access time. ( seconds of combustion data per cycle )...
IE,
One Tenth of a second sensing access by this non thermocouple ( rounded off )...
I could be wrong....But it would be the first time ....

whunter · #14 01-24-2011, 08:22 PM

Quote:

Originally Posted by leathermang

I hate to bring this up since Whunter Agreed with me...and was correct in the direction he was headed...
but I do not believe that the ' word math problem' was set up correctly... and thus came up with the wrong number as a result...
Anyone else see a/the discrepancy ? I left the part about 'each cylinder' in because keeping that in mind can help simplify the setup.

I believe the answer to be: 1.0 / = power strokes time (combustion access time) per second per cylinder at 850 rpm.
I used the inverse of one second to figure the access time.

850 divided by 2 is the number of four stroke cycles per minute per cylinder... with one power stroke in each.

425 power strokes per cylinder per minute divided by 60 is power strokes per second equals 7.08333333 power strokes per second.
or 1 / 7.08333333 =

IE, seven combustions per second into one second.

OR one second divided by seven combustions..

0.141176471 seconds as the access time. ( seconds of combustion data per cycle )...
IE,
One Tenth of a second sensing access by this non thermocouple ( rounded off )...
I could be wrong....But it would be the first time ....

No doubt you are correct.
I am not a mathmagician..

0.708 or 0.141176471 are both far beyond thermocouple accuracy.

.

casioqv · #15 01-24-2011, 08:29 PM

This thread is hilarious.

I prefer to use a dowsing rod for setting my injection timing. If you hold the rod in one hand while adjusting timing with the other, it will twitch when it's just right