Last week we installed a probe to measure intake air temp, and this time we'll install an exhaust gas temp (EGT) probe. The reason that we are using K-type thermocouples for temperature sensing now becomes clear - no other design of probe will happily cope with the very high temperatures developed by exhaust gases. How high? - try a constant 550 degrees C in normal driving and as much as 900 degrees C at high loads!
Exhaust Gas Temps
As the engine works harder, the temp of the exhaust gases increase. In a naturally aspirated car, the EGT at full load might be 700 - 750 degrees C if the mixtures are correctly rich. If the air/fuel ratio is overlay lean, this temp will dramatically increase to perhaps 850 or 900 degrees. In a turbo car, pre-turbo temps of 900 degrees are common at high loads, with post-turbo temps being 800 degrees or more. So as you can see, turbo cars run EGTs much closer to danger. Some turbos are rated at 900 degrees continuous, but - in the real world - if you measure anything over 800 degrees C after the turbo you're starting to push it a bit. However, as with all gauges, the worth of an EGT is in spotting sudden changes away from the norm - they could be indicative of a blocked injector or some other problem with the engine fuel, boost or ignition timing.
Sourcing the Probe
The probe that is used is another K-type thermocouple, housed in a high temperature stainless steel sheath. The selected probe must be designed to measure exhaust gas temperature - it's one of the highest temperatures commonly sensed. Many K-type thermocouples - even supposedly high temperature ones - max out at about 600 degrees C, which is too low in car exhaust applications.
There are three areas where cheaper probes prove inadequate - in the ability of the sheath to withstand the temp without melting off (!), in the temp capability of the insulation material used inside the sheath, and in the insulation used on the wires coming from the probe. The latter means that exhaust gas temp sensing thermocouples always use a stainless steel braided cable.
While all of this looks like it might make it really difficult to find a suitable thermocouple, that isn't the case. Simply approach any major industrial distributor of thermocouples, state that you want to measure exhaust gas temps on a high performance car engine, and they'll be able to provide you with a suitable design of probe. However, this is one area where buying a cheapy (eg bead-type design sold with multimeters) ain't gonna work.
Industrial Pyrometers in Adelaide, Australia, supplied the thermocouple that we used, which came with two metres of braided cable. It cost A$50.
Installing the Probe
The probe can be installed in a number of locations, but in the exhaust manifold close to the head is best. However, as with last week's installation story on the intake air probe, placing the EGT probe close to the valves isn't always very easy. If the engine is not turbocharged, the exhaust manifold can be drilled and tapped while remaining in its installed position. However, doing this on a turbo engine will result in the deposition of a whole lot of cast iron fragments into the turbine!
Often it is simplest to drill a hole in the thin-walled exhaust tube and then have a brass fitting brazed to the pipe, with the thermocouple screwing into that. In cars using long primary extractors, the decision then needs to be made as to whether the sensor will detect the temp of the exhaust gases coming from every cylinder (ie be placed after the collector), or just from the one cylinder (ie be placed on a runner close to the head). In race applications, it's common for four or six or eight thermocouples to be used - one on each primary runner. This approach can be taken (and later we'll show you how you can switch the thermocouples through to just the one display - it's harder than it first looks!) but this is probably overkill in a road car.
So, the further that the probe is placed from the head, the less accurately its reading will reflect the actual temperature of the exhaust gases - but the easier it is to install the probe! Our recommendations are as follows:
- Non turbo car, short cast iron exhaust manifold - at the first section of thin-walled exhaust pipe.
- Non turbo car, long runner cast iron manifold - in the first section of cast manifold where all exhaust output will be monitored.
- Turbo car - preferably in front of the turbo, but most commonly on the thin-walled exhaust pipe immediately after the turbo.
There are two reasons that it's best on a turbo car to have the probe before the turbo:
- An owner who installs an EGT probe in a turbo car normally expects to be working the engine hard - so good accuracy is important,
- and there is a major temperature drop (100+ degrees C) across the turbo as energy is extracted from the gases by the turbine.
However, having said that, here we'll install the probe after the turbo!
In the guinea pig Audi S4, there is a screw-in blanking plug installed in the cast-iron dump immediately off the back of the turbo. This cast-iron extension also mounts the oxygen sensor. Installing the EGT probe in this case was as easy as unscrewing the plug, and installing a threaded brass adaptor with the same thread that would also take the locking gland of the thermocouple. That's the short story, but here's the longer version.
I unscrewed the plug (using some penetrating lubricant to help loosen it) and then found a bolt that would screw into the tapped hole. With that bolt in hand, I rummaged through my collection of nuts until I found a nut that had the same thread as the bolt. So, I ended up with a sample nut with the same thread as the tapped hole in the manifold. Armed with this and the compression gland used to mount the thermocouple (see last week's story for more on this) I went along to a shop that sells brass pneumatic and hydraulic adaptors. An adaptor was found that had the right thread at one end to screw into my sample nut (and so into the Audi manifold) and at the other end had a thread that would take the compression gland nut with its internal collets.
There was a slight recess in the cast iron around the original blanking plug that I filled with a judiciously-chosen washer, with the brass adaptor then screwed home (but not too tightly - it's easy to strip the thread). The probe was then inserted far enough that the tip would just be in the gas flow (no point in making a big obstruction) and then the collet was tightened. Done!
Now it's very unlikely that in your car there will already be a drilled and tapped hole to take the probe, but the same general process will occur even if you need to have a fitting (eg a nut) brazed to the exhaust pipe first.
Next week - Displaying the Temperatures
Contacts:
Industrial Pyrometers
+ 618 8352 3688
Industrial Thermocouple Supplies
+613 93505400
TempScreen: Part 1 - Installing the Intake Air Temp Probe
TempScreen: Part 3 - Displaying the Temperatures
TempScreen: Part 4 - Building a Custom Temperature Display
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