EGT Logging revisit
 

I found the previous thread talking shortly about adding EGT logging and probes to Megasquirt. As I understand MSnSE does nothing with this data except log it.

Since that thread a few months ago has anyone further done so? Is it even worth it?

I've got a greddy EGT gauge I'm going to use just because I have it so I'll at least be able to monitor my temps with that.

http://megasquirt.sourceforge.net/ex...gt-circuit.jpg

That plus extra wiring and hassle almost seems like more trouble than its worth?

I struggle to see the value in logging EGT, especially considering the extra work involved. I find my EGT gauge data interesting, but of little actual value.

Talk to Reverant. He logs 4 EGT probes, one for each cylinder.

yeah, but he's using an innovate datalogger.

I know i just figure that someone using information from four EGT probes, even with the LM datalogger assembly might relate to some useful experiences with the EGT logging ability.

I don't really see the point, will your EGTs not be of appropriate level if your timing and fuel is tuned for optimal horsepower considering at that point you'll have optimum burn?
I really don't know myself.

The folks I know doing it tune first fuel with AFR's then watch EGT's and tune till the EGT's are getting out of the safe range so they know how much advance to run. Obviously no one wants to feed their pistons/valves to the turbo.

And I'd love to just be able to 'look at it later'.

Do you know what that chip does? Can you run two EGT's? I don't have two, but I'm curious. :-)

I'm surprised you feel that way.

knowing of the EGT's are too high when at WOT will allow you to advance the timing and stop burning you valves up and prematurely killing your engine.

IMO knowing the EGT's not quite on a level of knowing the AFR's but not far off. it's certanly an invaluable tuning tool.

(that said I've still to build my circuit...... )

car was tuned on a dyno
my egt probe only goes to 1600.
it's cool, but not that useful.

yeah I suppose it's more useful if you're road tuning.

Ben, when you say "only goes to" do you mean you run higher egt than 1600?

not while cruising, but under boost 1500F+ isn't hard to do.
cruising on the highway at 80mph indicated is around 1400F.

yeah - my buddie's diesel is supposed to burn the turbo at 1650* and he runs 1610, with a guage that only reads to 1650. Yipes! To me I'd want a little head room there, I just can't trust something right at the top of it's range.

I've heard Miatas are good to around 2k? Probably wrong though. I see around 1550-1600 on-track.

Good answers. Sounds like its a Go for building the circuit.

Pieced together the parts order from digikey and that little IC is pretty expensive for what it is... 20 bux!

No clue what it does, and don't even remember how thermistors even work.. I think it goes on voltage but I could be wrong? Unfortunately I'm not a circuit designer, just a builder. :(

Does anyone know what optimal EGT ranges are? I know there are a lot of variables, but just a rough guess? Is 2000 the total limit before destruction? Whats optimal with a good tune? Part of the problem is I lack theory on relevance of EGT to the tune itself. I understand that high egts are a bad thing, but was always told a high EGT is a result of a lean mixture under heavy load?

Curious.. What kind of load is that on the engine and whats your boost reading at an 80mph cruise reading 1400?

It's a thermocouple, not a thermister - there's a temperature dependant voltage for any two disimilar metals specially joined (welded). It's VERY small currents, and so you need sensitive equipment to measure it. Technically, you want to run the same wire type from the sensor all the way to the readout, good readout equipment will have the same metal all througout it - the type of metal depends on the thermocouple type - most often in automotive use you find "K-type" wires.

As far as temps, I'm not sure! I've heard so many different numbers I'm always afraid to say. I think 1800 is on the high side, I wouldn't go to 2k unless you verify it's ok! Once fuel is right, move timing till you get optimal temps, that much I know. :-)

(What's weird: For GOOD thermocouple reading, you typically need a second one, attached to a thermister. They are easier to characterize, and are used to correct the meter. I'm not sure if this IC you have has one integrated (but I doubt it, I've never seen it before) but it could lead to errors. Real, lab-grade thermocouples are stuffed full of more thermocouples to ensure they read their channels correctly.

I bought an EGT gauge as an "oh shit" indicator.

That sounds familiar. My brothers company builds and calibrates lab grade testing equipment and they have used Fluke single channel thermocouple and meter for sale.. I commented that I would likely have bought it if the unit had two inputs. Was interested in intercooler in, and intercooler out temps, though actually I only really need the inlet because I have the MAT sensor.. Thats besides the point. He explained in laymen how they work.. was still a little bit over my head. :) Only thing I do remember him saying is what you mentioned about same wire from end to meter for best results.

I don't think this is really rocket science, really.. just need a "close enough" recording.

haha. I don't need anything more to scare me, this project has more than enough things!

Yeah - as long as you're ok being 1.5 margin-of-errors away from the edge. If your pistons melt at 1600, and your guage is only accurate to +/- 50*, then you can't go beyond an indicated 1550. And since it's +/-50*, you might actually be running 1500* but still back off to be safe.

But yeah - there's a point where it's not worth worrying about. I wonder how hard it is to build a corrected circuit, or maybe that chip already does it.

You could switch back and forth. At work we have an old reader with two inputs, it's pretty nice. Also, if you're in a narrow-ish range and you don't care about accuracy so much, thermisters are easy to read (just a resistance).

I dunno about you, but my car has a OEM temp sensor in it. I could probably move it to the other side of the turbo.

I guess I would search on thermocouple multiplexers. If that's easy, one readout could read them all, and you could litter your motor with them. Put one in your passanger seat to see if you're impressing any female passengers, or getting male ones to wet themselves.

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in the past, I've used thermos to calibrate my temp guage and similar stuff, they are so small you can slip them under most hoses. I have a single channel readout from some old job that closed down. Some of the better fluke meters will also read thermocouples.

what does an escort GT have to do with megasquirt? LOL

As Abe said, thermocouples have nothing to do with thermistors. Thermocouples are based on the Seebeck effect, where joints of discrete, different metals create a voltage differential between the two metals, on what is called "the hot junction" (where the two metals join and the temperature is measured). The voltage is very small - 0V for 0*C (32*F) and just 4mV for 100*C (212*F), non linear. Typical metals used are Iron-Constantan for type J and Chromel-Alumel for type K. Type J can only read up to 750*C; type K can go up to 1250*C.

The purpose of the AD595CQ chip is to AMPLIFY this voltage, to a measurable magnitude - about 10mV per 1*C and it does so linearly: 0C = 0V, 1200*C = 12V (2192F). You then use a simple 1/2 voltage divider (10K/10K) to scale this to 0C=0V, 1200*C=6V, which means that 1000*C=5V (remember that it is linear).

I have found that most modern gauges lack Cold Junction Compensation. The cold junction is where the two metals join on your circuit (yes, they do join internally on the circuit). The voltage measured at the ends of the thermocouple wire is not only dependant of the temperature of the probe (hot junction) but also of the temperature at the circuit end of the wires (ie where you connect the wires to the circuit) - the cold junction.

For a reliable measurement there MUST be Cold Junction Compensation. If your gauge does NOT include CJC, when your cabin is still cold in the winter (say 5C/40*F) and then you turn on the heater and after 5 mins, the temperature of the gauge (which is very near the heater core and hoses) reaches 50-60*C, you are experiencing a drift error of up to that temperature differential. With an AD595CQ, if you make sure that the temperature of the cold junction (where you solder your wires or connectors on the PCB) is the same as the temperature of the AD595CQ chip, you are golden.[1][2] Just make sure that you solder/connect the wires as near to the AD595 chip as possible and NEVER use any different wires/connectors to extend the thermocouple wire. Only use special type K wires and connectors.

Jim

[1] There is a drift error on the AD595CQ chip as well, as the CJ temperature (ie PCB temperature) drifts from the ideal 25*C: +/- 0.6*C output error per 25*C of difference, which is really insignificant for our type of applications.

[2] The AD595CQ also uses an internal ice-point (0*C) reference for proper measurement which is calibrated during the manufacturing stages and this one of the reasons why the AD595CQ is pretty expensive for what it is.