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http://www.foxvalleykart.com/print/prt_egt.htmlEXHAUST GAS TEMPERATURE DE-MYSTIFIED
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By Kurt Huber and John Copeland More and more serious racers are taking an alternative approach to judging the condition and performance of their engines. By using exhaust gas temperature they have added a powerful diagnostic and tuning tool to their arsenal of racetrack weapons. Exhaust Gas Temperature (EGT) measurement has been a fact of life in other forms of motorsports for years, yet it's use in karting has been relatively limited until recently. Here's the skinny.
If combustion was a perfect process, the exhaust gas from an engine would contain only nitrogen, water vapor, and carbon dioxide. But in the real world it also contains carbon monoxide, hydrogen, unburned fuel, other hydrocarbons, plus traces of aldehydes, alcohols, ketones, phenols, acids, nitrogen oxides, carbon, and lots of other stuff. And that's assuming that we're starting with conventional, legal fuel. There has been lots of things written about illegal additives and how dangerous they can be. Please think before you or anybody you know add anything extra to your fuel. Think about the price you or your friends or family might pay, health-wise, just because someone is looking for an edge. But this article is not about what fuel is composed of, but rather about the temperature of the combustion products and how their measurement can be an even bigger edge.
In the temperature measurement industry their are two basic types of measurement devices. The first is called resistance temperature detection or RTD. This type of device is basically a very fine wire encased in a container, or bulb. As the temperature of the bulb changes, the electrical resistance of the wire changes. By passing a small current through this wire and measuring the resistance, the temperature can be determined. This is the method used by your Digatron cylinder head temperature gauge. And as any of you knows who've used a CHT GAUGE for any length of time, these CHT sensors, while accurate, are relatively delicate. Rough service (like on a kart) is not generally recommended. They also have a temperature limit that makes them unsuitable for use measuring EGT.
The other major means of temperature measurement is the thermocouple. The thermocouple is a unique device. There are several different types of thermocouples, using different materials for different temperature ranges, but they all operate by the same basic means. A thermocouple consists of two wires, of different materials, welded or fused together. For the temperature range we are most interested in, the type K thermocouple is most suitable with a maximum temperature of 1900 degrees Fahrenheit. In a type K device one wire is an alloy called CHROMELŪ*, and the other an alloy called ALUMELŪ*. A small portion of each wire is exposed and the two are welded or fused together. That assembly is encased in an electrically insulated sheath and the other ends of the wires are connected to a very sensitive voltmeter. Now here's where the thermocouple differs from the RTD. When the fused end of the thermocouple wire is heated, it generates it's own current. It's only a matter of millivolts (that's one one-thousandth of a volt), but the voltage generated is an accurate indicator of the temperature of the end of the thermocouple. A real bonus for motorsports is that these thermocouples are remarkably sturdy and reliable. With no delicate parts to break, unless you exceed their maximum temperature, they're pretty hard to damage. In fact, every gas- fired furnace and water heater uses one to tell the gas valve that the pilot flame is lit.
The thermocouple probe is carefully fitted into the exhaust system, relatively close to the engine. For maximum accuracy you want the tip of the thermocouple to be centered in the exhaust gas stream as it comes out of the engine. But there is considerable debate about how close to the engine it needs to be. Digatron's information advises mounting the probe between 3 and 4 inches from the piston face. But many snowmobile racers routinely set their EGT pickups as much as 8 inches from the exhaust port. In fact, it really doesn't matter exactly where the probe is mounted, although the closer to the exhaust port, the less the ambient air temperature will cool the header and affect the readings. One word of caution, however. Comparing EGT readings between engines or karts whose EGT probes are not mounted exactly the same distance from the piston will get you in trouble. If you use EGT, mount the pickups in all your headers at the same length. Otherwise you might just misinterpret the readings.
On the subject of mounting the EGT probe, there is some concern among 4 cycle racers about disrupting the gas flow in the relatively small diameter headers that are most common on 4 stroke engines. Introducing the probe, with a diameter of about .125 inches, about 1/2 inch into a 1 inch diameter header will consume about .0625 sq. inches of header cross section. That's about 8% of the total area. To test just what effect this might have on the absolute flow, we checked it on the flow bench. Our testing revealed that fitting a Digatron EGT probe into a .990 diameter header reduced the flow by __%. This is approximately the same flow as using a .960 header. You may want to factor that into your pipe selection if you're going to use EGT on your 4 cycle. You'll also want to factor in the value of knowing what your air/fuel ratio is doing versus whatever minor loss is exhaust flow there might be.
There are lots of myths and questions surrounding EGT and it's use. Some folks figure that, if you have a Cylinder Head Temperature gauge (CHT), you already have all the information you need, and that EGT is redundant. While you can certainly get by on just head temp, CHT and EGT each tell you slightly different things, and using them together tells you some things that neither one could tell you alone. EGT has some advantages because of it's basic construction and it's mounting location. A thermocouple responds very quickly. Because the CHT sensor has to respond to the temperature on the outside of the head, it cannot respond to changes in combustion temperature as fast as the EGT probe that is directly in the exhaust gas stream. Secondly, the EGT probe is not exposed to the outside air, it is not affected by changes in outside temperature. By comparison, since the CHT is measuring the temperature of the cylinder head casting itself, and since the cylinder head is one of the engine's primary means of shedding heat to the air, the cooler the air, the cooler the CHT reading and vice versa. For quick, consistent temp readings, EGT is definitely worth a look.
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