The aircraft Exhaust Gas Temperature instrument (EGT) uses a probe installed into the exhaust stack of the aircraft engine. With a multi-probe system, with probes installed just downstream of each cylinder's exhaust port, the pilot can compare each cylinder's exhaust temperature. The question one usually asks is why each cylinder has a different EGT.
When the multi-probe EGT was first introduced many mechanics discouraged their use. As soon as the pilot noticed that each EGT was different, this up-till-then perfectly running engine had something wrong with it. The mechanic would then be asked to fix the engine. Since the engine was operating normally (as the designer intended) there was nothing to fix. Possibly the engine could be re-designed to equalize the EGTs but this is a design issue and not a maintenance issue.
Its easy to assume that if all the EGTs are the same, then each cylinder is producing the same output; fuel/air mixtures are the same, power output is the same. We should have a smoother running engine with less stress because each cylinder is contributing an equal share. Is this a correct assumption?
Assumption 1. If EGTs are the same the fuel/air mixtures are the same.
False. As you lean the engine the richer cylinders increase their EGT more than the leaner cylinders. As the EGT gets closer to peak, the temperature rise gets smaller. EGT's on rich cylinders catch up to the lean cylinders and the temperature difference between cylinder's lessons.
The fuel/air mixture differences between individual cylinders doesn't reduce. Its just that the leaner cylinders respond less to a mixture change than the richer cylinders. Past peak EGT the situation reverses. Lean cylinders respond faster and opposite to cylinders still rich of peak. Rich and lean cylinders may have the same EGT but one is lean of peak and the other is rich of peak. Equal EGT's don't mean equal fuel/air mixtures.
Assumption 2. If EGT's are the same - power output is the same.
False. If the fuel/air mixtures aren't the same between cylinders then power output is not the same.
Normally, a fuel injected engine has each nozzle flowing precisely the same amount of fuel. Any difference in EGT's are the result of differences in the cylinder's ability to pump air. It would be best to equalize air flow as this is what is causing EGT differences and power differences between cylinders. With each cylinder receiving the same amount of fuel and each cylinder receiving the same amount of air, then EGT's, power will be the same. How might we equalize air flow among cylinders?
We could tune the intake or exhaust system, or we could have each lobe on the camshaft slightly different (tuned, if you will). The affect of using camshaft lobe tuning is to modify intake and exhaust valve lift and duration to equalize air flow. Intake and exhaust system tuning is commonly used. I've never heard of anyone doing camshaft lobe tuning. In fact, I just thought of it a few days ago. In either case the engineering is costly. Instead, some companies have taken the easier approach and thrown out equal fuel flow and are now adjusting fuel flow to match air flow differences.
What happens if we change our fuel injection nozzles to flow at different rates, but at rates that will equalize the fuel/air mixture between cylinders? Now at last we would have the same fuel/air mixture in each cylinder and we have equal EGT's. But we still have a problem.
Some cylinders get more fuel and more air so produce more power. Even though fuel/air mixture ratio is the same, the bulk amount of fuel and air is not the same. Because some cylinders receive more bulk fuel/air mixture than other cylinders, power ouput is different between cylinders.
EGT's are the same, power output is not. We have improved the ability to lean the engine and to have an OVERALL leaner mixture because the fuel/air mixture between cylinders is closer. This allows improved leaning and engine fuel economy. The Gami website has more information on gami injectors for turbocharged engines.
