Morph wrote:High EGT tells you your engine is running lean. Your engine needs oil to keep cool and the fuel you are supplying it has insufficient oil in it to keep it lubricated sufficiently.. Low EGT tells you your engine is running rich. Here you have lots of oil, which cools the motor but also is not a good thing. You need to have a happy medium.
The common, simplified version is that a high EGT is due to a LEAN mixture and a low EGT is due to a RICH mixture. That's not always the case. There are other factors also that determine a high/low EGT reading. I'll try to explain as best as I can:
An EGT gauge measures the temperature in the area where the sensor is installed. The critical area that actually needs to be measured is INSIDE the combustion chamber, but that is not practically possible, therefore the sensor is installed outside of the combustion chamber and the reading obtained is only a representative reading of what is actually happening inside the combustion chamber.
Because the sensor is not installed in the combustion chamber, factors that are NOT related to carburetion or fuel/air mixture such as engine loading and the design of the exhaust can also affect the EGT reading.
ENGINE LOADING
Lets look at engine loading: What does it mean? In its simplest form its basically the following: A propeller with a fine pitch or one that is shorter than it's supposed to be, will produce a lightly loaded engine ie. the revs you will be able to get out of the engine will be much higher than normal and a propeller with a coarse pitch or one that is longer than it should be will have the result of the engine producing less full revs than what it should have.
We also have to look at one of Newton's Laws to explain this one. This law states, in part, that "energy cannot be destroyed"
The burning of a fuel/air mixture inside the combustion chamber converts chemical energy (fuel/air) into heat energy. The burning gases expand and some of it forces the piston back down. So, some heat energy has now been converted into mechanical energy. Not all the heat energy is used to move the piston back down and the unused heat energy, in the form of hot gases, are forced out of the exhaust port. Now add up all the energy used and it will total the amount of energy we had in the fuel/air mixture to start off with.
So what does all this mean? Well, if the engine is lightly loaded, then only a small portion of heat energy will be required to force the piston back down and turn the engine over and a larger portion of unused heat energy will be going out the exhaust port and right into the path of the EGT sensor that is situated just outside the exhaust port, with the result being a higher than normal EGT reading. If the same amount of fuel/air mixture is applied to a higher loaded engine, then more mechanical energy is required to turn the prop at the desired rpm thus more heat energy will be converted to mechanical energy when the piston moves back down and less heat energy will be going out the exhaust port. The heavier loaded engine will produce more power and it will have a lower EGT reading. The lightly loaded engine will have higher EGT's as explained, and if you try to ENRICHEN the fuel/air mixture, the EGT will go UP rather than down because more burnt fuel/air hot gas is going to go out the exhaust port, simply because it's not needed to turn the engine over.
EXHAUST SYSTEM
Let's look at the way the exhaust operates: In a two stroke engine, the usable power band and maximum power output is dependent on the way the exhaust is designed. There are positive and negative pressure pulses that travel back and forth inside the exhaust. A negative pulse generally pulls out the burnt gases from the combustion chamber and draws in a fresh charge of fuel/air. A positive pulse returns to the combustion chamber and, if timed correctly, can/will/should hold the fresh charge in the combustion chamber until the piston can cover the exhaust port and detonation will/should then take place in the combustion chamber.
No two stroke exhaust system can provide the optimum power that the engine needs over the entire RPM range and there will be RPM range/s where the timing of the reflected pressure pulses will be out and they will not be arriving back at the exhaust port at the correct time. For instance, a positive pulse will be pushing some hot gases back towards the exhaust port when it shouldn't be and when these hot gases arrive at the EGT sensor, the negative pulse may be pulling hot gas out of the combustion chamber, thus dramatically increasing the EGT reading because of the more than usually hot gases present AT THE LOCATION OF THE SENSOR, NOT IN THE COMBUSTION CHAMBER. Don't just take my word for it...go and test it yourself.
Here's how to test....have a good look at your EGT readings throughout the entire RPM range. You will notice that there are narrow ranges where the EGT is higher than normal. By just increasing or decreasing the RPM a little, you can avoid these hot regions. So, as can be seen from the way the exhaust operates, a higher EGT reading is possible and it had nothing to do with the fuel/air mixture but everything to do with the design and construction of the exhaust system.
A word of caution: These hot regions are normally in the middle somewhere of the RPM range and NOT at the top end. No amount of jetting and fiddling with your carbs are going to fix that. Just accept it. It's the way the exhaust is made, and try to avoid those RPM ranges.
If you have a plugged exhaust, the same thing may happen. Because the internal design of the exhaust system is altered by gunk/junk/carbon etc. you may experience a couple of hundred RPM drop after initially having a normal RPM, higher than normal fuel consumption and higher than normal EGT, which you won't be able to fix by enrichening the mixture.
If you have read carefully and paid good attention, the answer to a previous question regarding when to decoke your exhaust system, will be obvious. If not, make a deposit of $Z100 000 000.00 into my account and I will explain again :D :D
Duck Rogers
for example what would a high EGT mean or a low one?
