By C.J. Baker
|The exhaust gas temperature (EGT) pyrometer can be one of the most important gauges on a turbo-diesel-powered vehicle. It can warn the driver of situations that are potentially damaging to the engine, and it can also be used as a guide for optimizing fuel economy.|
Diesel engines are not indestructible. Aside from running a diesel without oil, or filling the fuel tank with gasoline instead of diesel fuel, few things will damage or kill a diesel engine faster than excessive exhaust gas temperature (EGT), yet strangely enough, no diesel pickups or motorhomes come equipped with a pyrometer as standard equipment to monitor EGT. During normal operation of such vehicles, EGTs usually stay within safe limits, but situations can occur where the EGT gets too high, doing serious engine damage without any warning to the driver. A pyrometer that displays a diesel's EGT can warn the driver of dangerous conditions before such damage occurs. That's probably why the Banks DynaFact pyrometer is one of the most popular items we sell, and why a DynaFact pyrometer is included as part of many of our power systems.
A pyrometer is a temperature gauge designed to measure high temperatures above those measurable with an ordinary thermometer. It consists of a temperature-sensing probe (thermocouple) that is placed in the area, or flow, to be measured. The probe is connected to a gauge, which is located a safe distance away from the high temperature source. On a diesel, this means the pyrometer sensing probe is mounted in the exhaust manifold or immediately after the turbine outlet of the turbocharger, and the gauge is mounted in the driver's compartment. The purpose is to measure and display EGT in degrees Fahrenheit (F.). Where the probe is positioned before the turbine section of the turbocharger, the EGT may also be called the turbine inlet temperature. As you would expect, EGT measured after the turbo is called turbine outlet temperature.
We might mention that some mechanics fear installing the pyrometer thermocouple in the exhaust manifold for fear the probe will break or burn off and blow into the turbocharger. Such a piece of foreign material entering the turbine would cause serious damage that could in turn break the compressor wheel of the turbo, sending broken pieces into the intake system of the engine where even more damage could occur. While the above scenario is scary, it is also unlikely. Today's quality pyrometers feature thermocouples that are sheathed in stainless steel to prevent just such an occurrence. It is exceedingly rare to find a diesel mechanic that can honestly say he's ever seen a thermocouple that has failed and fallen into the turbo on a diesel pickup or motorhome. It just doesn't happen with a good pyrometer.
Whether the pyrometer thermocouple is mounted before or after the turbine is usually a matter of finding a suitable mounting location, or of convenience. It should be noted that when the EGT is measured after the turbine, the turbine outlet temperature at full throttle or under a heavy load typically would be 200º to 300º F. lower than the EGT measured in the exhaust manifold. The temperature drop after the turbo indicates the amount of heat energy in the total exhaust gas flow that was used to drive the turbocharger. The temperature drop through the turbine is also related to the total flow and speed of the flow through the turbo. At part throttle, under light load, such as cruise conditions, the turbine outlet EGT may be as much as 500º F. lower than the turbine inlet temperature, but the total exhaust flow is much less than at full throttle. At high turbine speeds (under heavy load) the exhaust gases simply don't have time to give up as much heat energy as they speed through the turbine. This variance is why installation of the thermocouple in the exhaust manifold is considered more accurate. The EGTs discussed in the remainder of this article will all be turbine inlet temperatures.
At Banks, we recommend the installation of a quality pyrometer on any turbo-diesel vehicle. It's an inexpensive upgrade that allows the driver to keep his engine out of EGT trouble, and it can even be a guide to optimum fuel economy, but more on that later. For more information on the Banks pyrometer, see Banks DynaFact Gauges.
So why is EGT important? EGT is an indication of how hot the combustion process is in the cylinders, and the amount of "afterburning" that is occurring in the exhaust manifold. EGT is also directly related to the air/fuel ratio. The richer the air/fuel ratio in a diesel, the higher the EGT will be. Two things can create a rich mixture under heavy loads or at full throttle: the first is too much fuel, and the second is not enough air. That seems simple enough, but it's the second part, not enough air, that could get a stock, unmodified truck or motorhome in trouble. Anything that restricts intake airflow, or intake air density, limits the air mass that gets to the cylinders. Think of it as the amount of oxygen getting to the cylinders to support the combustion of fuel. This could include: a dirty or restrictive air cleaner, a partially blocked air intake, high outside air temperature, high altitude, restricted airflow to or through the radiator or intercooler, and high water temperature. The vehicle's water temperature gauge will provide a warning of a cooling system problem, but the other problems aren't likely to be noticed without a pyrometer unless the driver notices excessive exhaust smoke. A pyrometer also reacts more quickly than the water temperature gauge, so it allows the driver to spot a problem sooner and avoid engine damage. A restrictive exhaust system can also reduce the airflow through the engine, resulting in a rich condition. Any of the above conditions can result in excessive EGT if the vehicle is working hard, such as pulling a heavy load, running at sustained high speed, subjected to climbing a long grade, etc.
We've already mentioned that excessive EGT can cause engine damage or turbocharger damage, but let's get more specific. Which parts will fail first is a matter of the design and materials used in the various parts of the turbo-diesel, but usually it starts with the turbocharger. Under sustained excessive EGT, the square corners at the outer ends of the vanes, where the material is thinnest on the turbine wheel, can become incandescent and then melt, resulting in a rounding off of the square corners. If you or your mechanic finds this indication before anything more serious happens, consider yourself very lucky, because shortly after the tips melt, the turbine wheel goes out of balance and wipes out the turbocharger bearings, which may or may not result in shaft failure and destruction of the turbine and compressor wheels. Excessive EGT can also erode or crack the turbine housing. In extreme cases, high EGT can drive the turbocharger into an overspeed condition that exceeds the designed operating speed due to the additional heat energy. When this happens, either the turbine wheel or the compressor wheel may burst. If the turbo doesn't go first, excessive EGT, if sustained, will damage the pistons. Such damage can include piston deformation, melting, burning, holes, cracking, etc. This damage is cumulative, so if you slightly burn a piston top, the engine may continue to run without problems, but the next time you run excessive EGT more damage may be done, and so on, until failure occurs. Piston failure can be catastrophic -- that means very expensive. At a minimum, an engine overhaul will be required, and that too is expensive. Excessive EGT can also cause exhaust manifold and cylinder head cracking. Exhaust valves can fail from high EGT as well. Among the first engine parts to suffer damage will be those made of aluminum since aluminum has a lower softening and melting temperature than steel or cast iron. Diesel pistons are aluminum, and a growing number of diesels also use aluminum cylinder heads.
We mentioned earlier that excessive EGTs are due to a rich air/fuel mixture, which can be caused by too much fuel. Too much fuel is typically the result of modifying a turbo-diesel for more power. Not all diesels are modified for speed or maximum pulling power; some diesels are modified for better towing and passing performance. There are many products on the market that claim to increase diesel power, but almost all of them increase fuel delivery at full power with little regard for EGT. It is superior engineering, extensive testing, and calibrated fuel management that set the Banks power systems apart. Banks systems, from Git-Kits through the top-of-the-line PowerPacks, are designed and built to avoid excessive EGT. Banks Power systems are engineered to give the best value in power and reliability.
So the big question is, what constitutes excessive EGT? If everything is working properly, 1250º to 1300º F. is a safe turbine inlet temperature, even for sustained running, mile after mile. Above 1300º F. things can start to get edgy. Remember, excessive EGT damage is cumulative. Over 1400º F., you're usually gambling against a stacked deck and it's only a matter of time until you lose. The higher the EGT, the shorter that time will be.
There are some exceptions to the above EGT limits if the driver is willing to trade off some risk of engine damage for brief spurts of maximum power or performance, such as for a quick burst of acceleration, a drag racing contest, or even a truck pull event. To fulfill that need, Banks has developed the Big Hoss line and the Six-Gun Diesel Tuner line, with the optional Speed-Loader. These racing products allow operation at EGTs above 1300 degrees, but unlike the makers of competitive diesel tuner boxes who place no limits on excessive EGT, Banks still builds in limits with the Speed-Loader, and adjustable EGT stops for the 7.3 L Ford PS Six-Gun system. The Six-Gun and Speed-Loader products are not intended for use on motorhomes or pickups towing trailers. For the owner of a diesel motorhome or a pickup used to pull a trailer, it's always better to play it safe and adhere to the 1300° limit, and Banks power systems for these vehicles deliver such safe performance.
As we pointed out earlier, high EGTs are the result of too much fuel for the available air. If you see EGTs climbing over 1300º F., the fastest way to reduce the amount of fuel going to the engine is to back off the accelerator pedal. Another possible solution is to downshift if your speed permits it. For example, while the engine might be capable of producing enough power to pull the load in fifth gear at high EGTs, running in fourth gear at lower EGTs is definitely easier on the engine as long as the engine's RPM red line is not exceeded.
Excessively high EGTs mean over-fueling, so "driving by the pyrometer" to keep EGTs in the safe zone can actually improve fuel economy. Some drivers swear by this procedure. This is true even when the EGT are below the danger point. Of course, driving by the pyrometer can be a nuisance, and it takes away from the driver's full attention to the road. Still other driver's aren't technically astute or don't fully understand the dynamics of what we've just discussed. In addition, some drivers just don't want to be bothered. That brings us back to Banks power system engineering. All the Banks systems (excluding Banks racing products, see Racing the Diesel elsewhere on this site) are engineered to first improve the airflow capability of the engine. By increasing the airflow of the diesel, then fuel can be added in a precisely calibrated manner to increase power while maintaining an acceptable air/fuel ratio that doesn't create excessive EGT. All power systems for '94-04 Ford Power Strokes, '94-04 Dodge/Cummins 5.9L turbo-diesels, and '93-02 Cummins 5.9L and 8.3L motorhome turbo-diesels feature the Banks OttoMind fuel calibration system to correctly add fuel to match increased airflow in such a manner as to keep peak EGT below 1300º F. Every power system has its own specifically calibrated OttoMind for the power level of the system on that particular vehicle. The calibration is engineered to coincide with the engine manufacturer's EGT recommendation and Banks' extensive testing. Power systems for the '98-04 Dodge/Cummins 24-valve 5.9L pickups, and '98-02 Cummins 5.9L ISB and 8.3L ISC motorhomes get the added benefit of Banks exclusive TLC2 (temperature limiting control), which is built into the OttoMind. TLC2 monitors the EGT and automatically reduces the amount of fuel added by the OttoMind to keep the EGT from exceeding 1300º F. The TLC2 feature is not available for the Ford Power Strokes since it is not compatible with the Ford computer electronics.
So far, we've been talking about peak sustained EGTs at full power or under a heavy load, and certainly EGT needs to be kept within limits for engine and turbocharger reliability. At all other times, the EGT of a turbo-diesel will be lower, usually below 1000º F., and sometimes much lower. Such low EGTs pose no threat. In fact, the lower the EGT for a given speed and load, the more efficiently the engine is running. Most owners will note a reduction in EGT at cruising speeds after installing a Banks power system on their turbo-diesels, and that's good news.