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Diesel Tuning Fundamentals: Exhaust Gas Temperature

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Exhaust Gas Temperature

05.08

00:00 - While we've discussed air fuel ratio and its effect on the performance of a diesel engine, it's still relatively uncommon in the aftermarket tuning industry to use or rely on air fuel ratio measurement when making tuning changes.
00:14 Instead the exhaust gas temperature is a much more common parameter to measure and use to guide tuning changes.
00:21 Measuring the exhaust gas temperature in itself is problematic, due to the harsh environment the sensor needs to operate in, and the high temperatures it will be exposed to.
00:30 For example it's possible to see sustained temperature in the range of 800 to 900 degrees centigrade.
00:37 In order to measure these temperatures a thermocouple is used.
00:41 This is a sensor which uses a junction between two dissimilar metals which form electrical junctions at differing temperatures.
00:50 The thermocouple produces a temperature dependent voltage which can be used to infer temperature.
00:56 The challenge with a thermocouple is that the voltages produced are very small, and to convert them to something useful, the thermocouple also requires a voltage amplifier.
01:07 This takes the output of the thermocouple and typically scales it between zero and five volts so that it can be read and displayed by an ECU, a logger, or a gauge.
01:18 The next problem we have is where to mount the EGT sensor as this can have a big impact on the temperature that the sensor will read.
01:26 What we really want is the sensor to read the temperature as the exhaust exits the exhaust ports and this can be achieved by mounting the sensor perhaps 30 to 50 millimetres from the head flange of the exhaust manifold.
01:39 The reality is that most people will rely on a single EGT sensor, rather than one for each cylinder, and in this case the ideal location is in the collector pre turbocharger.
01:51 This will allow the sensor to sample the overall temperature of the combined exhaust gas as it enters the turbo.
01:58 The problem with any location pre turbo is that it requires complete disassembly of the turbo and exhaust manifold, in order to drill and tap a thread into your desired location.
02:11 While some people do choose to install the sensor in place, this is going to end up with metal swarf and shavings going through your turbo, so I certainly don't recommend it.
02:21 Since removing the manifold is a time consuming process, it's common to fit the sensor into the downpipe after the turbocharger instead.
02:29 This is a quicker and easier location to install the sensor, but what we will see is a significant temperature drop across the turbine wheel of the turbo.
02:40 Depending on the engine, turbo size, and boost pressure, the temperature post turbo could be 150 degrees or more lower than what we would measure before the turbo.
02:51 Understandably this can be very misleading and even dangerous if you're relying on this information to aid your tuning and ensure reliability.
03:00 So the next obvious question once we have our EGT sensor fitted in a suitable location is what is a safe maximum EGT value? This is where it gets tricky because there isn't a single value that we can use on every application and expect reliability.
03:18 The reason for this is that the exhaust gas temperature only tells us part of the story since we're measuring the result of the combustion process, rather than what's actually happening inside the cylinder.
03:30 For example large injector pulse widths and very advanced injector timing could expose the piston to dangerous temperatures, however since most of the energy is used during the combustion process, the measured exhaust gas temperature may not actually seem dangerously high.
03:46 Likewise a large injector pulse width which continues to inject fuel late in the engine cycle may result in higher exhaust gas temperatures, as the combustion continues well into the power stroke.
03:57 Yet this may actually be less dangerous to the piston.
04:01 We also need to consider the EGT alongside how the engine is being used.
04:06 A high EGT may be perfectly safe for a few seconds, however if it's sustained for longer periods it may still cause damage.
04:15 With all of this in mind, EGT is typically used as a guide and is helpful to monitor as you build experience with a particular engine and learn what is reliable from that particular platform.
04:28 For some approximate ballpark values however it's generally accepted that peak exhaust gas temperature values between 650 and 750 degrees C, are safe for sustained high load operation such as towing.
04:42 In the drag racing world, exhaust temperatures as high as 950 to 1050 degrees centigrade may be experienced for short periods, but this is likely to take its toll on engine reliability.