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Practical Diesel Tuning: Exhaust Gas Temperature

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Exhaust Gas Temperature

05.11

00:00 - When it comes to tuning a diesel engine, we need to be able to keep a careful eye on the key operating aspects of the engine in order to ensure our tuning changes remain safe.
00:07 In order to do this, there are a variety of sensors that we need to consider adding to our engine so we can monitor the engine properly and we'll cover them here.
00:15 The majority of diesel tuning is achieved by reflashing the factory ECUs and usually it's not possible to integrate additional sensors into the ECU directly.
00:23 Instead we have to bring additional sensors into our scanner or datalogger package so we can view this additional data alongside the normal information we're logging from the ECU.
00:33 This makes the additional data much more useful as we can then read it in context with what the engine and ECU were doing at the time.
00:40 On the other hand, if you're tuning a diesel engine on an aftermarket standalone ECU, then it's almost certainly going to be possible to bring the additional sensors directly into your ECU where that data can be viewable directly on the tuning software package making integration nice and simple.
00:54 In this instance, it's usually possible to add some safeguards around some of these additional sensors to protect your engine from damage.
01:00 The first and perhaps most important sensor is an exhaust gas temperature sensor.
01:04 I will point out that many diesel engines now have EGT sensors fitted from the factory which makes our lives much easier but if your engine doesn't fall into this category, keep watching.
01:15 As its name suggests, the EGT sensor allows us to monitor the exhaust gas temperature and gives us some indication of the combustion temperature which is really what we're most interested in.
01:24 Now EGT alone doesn't tell us everything we need to know.
01:27 In the diesel tuning world, EGT is a good indicator of the state of your tune and the health of your engine.
01:32 An EGT sensor is a type of thermocouple which is placed in the exhaust gas flow.
01:36 The tip of the thermocouple consists of a junction between two dissimilar metals which generates a voltage relative to the temperature its exposed to.
01:43 The problem with the thermocouple is that the voltages produced are very small and to convert them to something useful, the thermocouple requires an amplifier.
01:51 This takes the output of the thermocouple and scales it between zero and five volts so that it can be read and displayed by the ECU, logger or gauge.
01:59 This isn't really a huge concern as typically you'll be fitting an EGT kit which contains the sensor, amplifier, and some way of displaying that data to us.
02:08 The bigger concern we have here is where to mount the EGT sensor as this can have a big impact on the temperature that the sensor will read.
02:15 What we really want to know is the temperature of the exhaust, the exhaust port and this can be achieved by monitoring the sensor near the head flange of the exhaust manifold.
02:25 The reality is that most people will rely on a single EGT sensor rather than one for each cylinder.
02:31 And in this case the ideal location is the collector pre turbocharger.
02:35 This will allow the sensor to sample the temperature of the combined exhaust gases as they collect right before they go into the turbocharger.
02:42 The problem with any location pre turbo is that it can make installation difficult and it's an extremely harsh environment.
02:49 Most OEMs place the EGT sensor post turbo because they last longer and can be equally effective at providing data so long as they have modelled the temperature delta across the turbocharger into the calibration.
03:00 Best case scenario, if you're going to add an EGT sensor pre turbo, you should disassemble the turbocharger or collector and drill and tap separate to the exhaust system so you can clean out any metal debris that might get in there.
03:13 Due to the difficulty of mounting the sensor pre turbocharger, I would advocate you can do post turbocharger if it's easier.
03:19 In this location however we will see a significantly lower temperature compared to what we would measure pre turbo.
03:25 And this limits the usefulness of the EGT data without proper knowledge of the temperature delta across the turbine wheel.
03:32 Depending on the engine, turbo size, boost pressure and load, this temperature delta across the turbine can be 150°C up to 200°C or more.
03:41 Understandably this can be misleading data and can be even dangerous if you're relying on it without accounting for this delta.
03:48 However if you know the delta and expect the delta, and set your limits appropriately, post turbo sensor can be just as effective as one mounted pre turbo.
03:56 Once you have the EGT sensor fitted to the exhaust, the next obvious question is what is a safe EGT? Unfortunately this is a little tricky to answer with a specific value because it will depend on how long the engine is under sustained high load operation.
04:10 For example, for a sustained running EGT in the range of 700°C or 1300°F, is generally accepted to be safe by the OEMs.
04:20 Once you head north of 760°C or 1400°F, most OEMs acknowledge that you're getting into the region where you're potentially risking damage to the engine, turbine, piston tops et cetera.
04:33 High EGT doesn't instantly indicate that your engine is going to be destroyed.
04:37 I would call EGT cumulative.
04:40 What I mean here is that if you're using a diesel engine for brief periods of time under high load operations which is drag racing, or spirited drive on the open road, high EGTs can be tolerated.
04:51 It's not uncommon for drag racing engines or tractor pull engines to see EGTs as high as 900°C or 1650°F plus.
04:58 These engines do have a limited life expectancy, they get rebuilt.