Using a glow plug to get cylinder temp reading??

[crossbones]

Hello list. I am in need of some opinions, information and advice. Basically, I am asking IF this will work and by no means saying that it will work. What I am thinking of trying is to find the approximate peak cylinder temperature in a IDI diesel using the glow plug as a sensor to take the readings with. It sounds logical (at least to me) that if I take a new glow plug and take a ohm's resistance reading while using diesel fuel with the engine running at operating temperature and then take the reading while using say veggie oil as fuel under the same conditions that any difference in the readings could help in making any adjustments with timing, injector pressure, etc. to bring the engine back to similar efficiency as with using diesel fuel.

At this time this is a exercise in theory and I have no information on its practicality.

If this exercise has any merit, would someone be kind enough to offer any suggestions as how to take the resistance readings and extrapolate any information of value.

Thanks in advance,
crossbones

 

[CRG]

Too much thermal mass in a glow plug to find peak cylinder temps?

 

[MikeHalloran]

Given the turbulence in the chamber, I'd guess it could work. I don't think reading a handheld ohmmeter would do it, though. You'd have to measure the resistance dynamically, use a crank angle sensor to gate the measurement window, and sweep the crank angle through the combustion event (or, practically, across multiple combustion events) to capture the peak.

I dimly recall reading about using a spark plug to infer gas temperature in an SI engine; i.e. when it wasn't triggering the combustion event, some borderline magical apparatus applied a steady high voltage across the electrodes and measured the current carried by the ionized gas. I recall also words about using that signal to close the loop, but it may have been conjecture. [I hate 'technical' articles that include words like 'would' or 'should'.]



Mike Halloran
Pembroke Pines, FL, USA

 

[crossbones]

I did a test in real time and conditions just to see what I might find out. First I tried to get a ohms reading, but found that the meter fluctuation to be a little to much to deal with. Later thoughts about the fluctuation is that it may have been caused by the rapid rise and decline of temperature in the cylinders. (just a thought). Next, I tried to get readings in millivolts.....this is what I found.......

This is the criteria of the test I have done so far. IP timing and injector pressure set to factory static settings.

1. Truck is driven at operating temperature for 10 minutes

2. Truck is then driven over a mostly flat road at 2500 rpm's and a speed of 45 mph

3. A reading is taken while at this rpm and speed from the buss bar that connects the four glow plugs together...........using a Fluke digital multimeter I get a reading of
8.9 to 9.4 millivolts........readings are very responsive to accelerator and road grade......readings are very repeatable over the same road and conditions........

4. Truck is brought to a stop and allowed to idle at 1000 rpm's for 1 minute.......while running at this rpm a reading is taken..........5.6 to 5.7 millivolts.....very repeatable.....

I find it to be to ironic and to simple to be true that when I apply these millivolt readings to a thermoelectric voltage chart for a Type B thermocouple and get the following temperature relationships:

millivolts temperature in F
5.6 1980°
5.7 2000°
8.9 2550°
9.4 2630°

The temperature range of 2550° to 2630° is very close to the 2500° target temperature to maintain the NOx levels of this era engine from the very limited information that I can find at this time.

I am not saying this is conclusive information, but I do think at this time that it may be used as a reference point when making a change with fuel, IP timing or injector pressure. If this procedure is found to be repeatable, I can easily see that it could be used to help in making small injector pressure changes, valve lash settings,etc to each cylinder in a multi cylinder engine to get a smoother running engine and possible improvements with efficiency. I have some nice test equipment coming in the next few weeks that will tell me dynamic timing at any rpm and on all cylinders. Perhaps a better picture will develop with more information.

I welcome any comments that can further this exercise or to disprove it entirely. I am not looking for a way (at this time) to tell the exact peak cylinder temperature, but to just maybe find a relationship to have a base line to work from when making any changes while the engine is still in the truck.

thanks,
crossbones

 

[MikeHalloran]

Looks like you've got a usable parameter.

A few minutes with a hacksaw would divide the bus bar into segments, and you could have individual cylinder readings. Bridge the gaps with starter relays or bolts to warm up the plugs.




Mike Halloran
Pembroke Pines, FL, USA

 

[CCycle]

Interesting observation and thanks for the good description of how you got the measurement.
You do not mention what type of glow plug you are using. The older low speed type used on heavy equipment and ag equipment might be faster than the sheathed glow plug. At any rate, because of the thermal mass and the measuring conditions you at best are getting an average reading of all cylinders.
Type B thermocouple is Pt, 30% Rhodium vrs Pt, 6% Rhodium. It has very low voltage output. Your glow plugs are entirely different materials so the voltage to temperature relationship of the type B couple does not apply. Non the less they are probably acting like a thermocouple. They probably are a steel sleeve and a nichrome wire which would be similar to a type J couple (Iron vrs Constatan). Note that the voltage to temperature relationship of a J couple is about twelve times as great as a B couple. If this is the case, the temperatures you are measuring are a lot lower which would be consistent with average engine temperatures.
As for the usefulness of the reading, it probably would be an good indication of the overall temperature of the working parts of the engine.

 

[crossbones]

Hello. Sorry for the length of this post, I hope that the information will lead (at least for me) to a better understanding of how to test and use the glow plug as a practical test tool. I did a second test on a different truck and was amazed at the results. All of my (6) diesels are the Isuzu 2.2 IDI engines from the 1981-1987 era. The first test truck is turbo with 4x4 equipped with a tach (my best running truck). The second test was a N/A engine 2x2 without tach. Both test were traveling over the same road and conditions in 4th gear at 45 mph.

Buss bar test: 52.6-53 millivolts verses 8.9-9.4 reading on first truck

I almost stop here thinking there is nothing of value here, but then thinking why in the world would the reading be this different with both engines using the exact same glow system and the same ...new glow plugs......(the true manufacturer of these glow plugs is unknown because the brand name of the supplier is a speck buyer and put them in there box.

Next I removed the buss bar in order to test each glow plug:
I found two with reading in the 8-9 millivolt range, one at 28, and one at 50-52

I have never been satisfied with cold cranking on this engine so I started exchanging out the two with high reading with 7 new in the box glow plugs. All 7 new plugs tested the same ohm's at ambient temperature (same as the plugs when installed in the engine a while back). I found a total of 4 plugs that had high readings (two in the engine and two out of the new stock)

After finding 4 plugs that showed repeated reading in the 8-9 range, I then started noting reading for cylinders with exchanging these plugs into each cylinder.

I found consistent reading on #1 cylinder with 10.2-10.4 with all four plugs and 8-9 on the other three cylinders with all plugs..............

With burned fingers and this information, at this time, I think it reasonable to conclude that using a consistent reading glow plug(s) that one can isolate a cylinder that is in some way performing different from other cylinders........

These trucks glow controllers (factory literature) are set for a controlled max of 900°c and other literature saying that 850°c is the min to cold start a typical diesel engine, it seams reasonable that the scale of resistance change in a given glow plug would have a great bearing on its ability to reach its needed temperature in the time frame required.......
conclusion......I need a different suppler of glow plugs and a different way of testing them........

At this time I do not think that actual number of the reading are that important (unless someone could come up with scale using the variables that can be read) but instead use a percentage of change from cylinder to cylinder using a consistent reading glow plug and / or to perhaps weed out a glow plug that may not be performing to its max.

I am not "smart enough" to think that I am the first person to test this idea. Any information pro or con is welcomed.

thanks,
crossbones