In an air-cooled engine, we think it is important to monitor all cylinder head temperatures (CHT's) and exhaust gas temperatures (EGT's), not just one. Many pilots and builders spend large amounts of their hard earned savings to buy the aircraft of their dreams. However from that point onwards, many seem to want to save on everything else. The "accessories" needed to register engine functions are often reduced to the bare minimum in an attempt to keep their total expenditure as low as possible. The closer their aircraft project gets to the completion stage, the more an irrational urge to save takes over, often overriding safety concerns and common sense.
Why the need to register all four CHT's instead of buying only one sensor for the "hottest" cylinder?
Our answer; "How do you know that the "hottest" cylinder is always going to be the hottest in all flying conditions?" and "What happens if one of the other three cylinders develops a problem?"
Unless someone has exactly the same aircraft, engine and cooling configuration as you have, and they have tested the aircraft and its' engine installation, accurately measuring and recording ALL CHT's under many flying conditions, you can't be sure which cylinder will be your hottest. Often one or two CHT's will be very similar and the hottest cylinder might vary depending on the flying and atmospheric conditions. Also with age and use, the sensors' accuracy will worsen, connectors and electrical conductors will become worn, and the reading will no longer be reliable.
Excessive CHT can cause the cylinder head or its components to fail, either in the same flight or sometime afterwards. What the actual level of "excessive" is depends on the material used and tolerances the manufacturer implements. ULPower will decide what the official critical level is. Don't presume that large safety factors have been used as an excuse to go over the published limits..... Every time you exceed the limits, you are seriously shortening the life of the affected component. A couple of minutes of excessive T° could be equal to a 100 hours of normal operation.
It can never be ruled out that an engine will not malfunction, and it usually happens in the air. If one cylinder starts to run hot, and you only have one CHT sensor on your engine, you've only a 25% chance (for a 4 cylinder engine, 16.6% chance for a 6) that you will detect the problem before you start to hear or feel that something is wrong with your power plant. If you monitor all CHT's this advance warning may well give you the time to land safely with the aircraft still under power and only minor damage to the engine.
The cost for this early warning system: a one time "insurance cost" of 3 extra CHT sensors and learning what "normal" CHT's are for your aircraft !
Monitoring EGT is also important because it gives a good indication of how each cylinder is working. The EGT spread (the difference between the highest and the lowest T°) should be as small as possible because this indicates that each cylinder is getting the same quantity of fresh air and the fuel/air mixtures are identical, which in turn indicates that the individual cylinders are supplying equal amounts of power to the crank shaft.
If one EGT reading becomes substantially higher or lower, this is an indication that something is wrong. A higher than normal EGT could mean less fuel (leaner and hotter) mixture is being supplied - which could be caused by a partially blocked fuel injector. Perhaps there is a leak in the induction manifold and extra air is being drawn into this cylinder making its' mixture leaner and hotter. Lower than normal EGT's are indicative of richer than normal fuel mixtures. This could be caused by a fuel injector which does not close completely in between cycles, or a blockage in the inlet manifold causing less air to be drawn into the cylinder. One EGT sensor will usually not tell you very much because it only measures the average EGT of all the cylinders. In a four cylinder engine using only one EGT sensor, a 50° drop from normal could indicate that one cylinder (which one?) is 200° lower than normal, or that two cylinders (which two?) are an average of 100° lower than normal.
In short, monitoring all EGT's allows the pilot to have almost instantaneous feedback if the power produced by each cylinder varies significantly. EGT monitoring is another cheap but reliable early warning system.
Monitoring your engine's operating temperatures and pressures is vital. Make sure the battery is always in good shape, keep an eye on the voltage and monitor current drain and charging for abnormal conditions. The lives of fellow passengers, yourself and people on the ground depend to a great extent on your continual vigilance when you fly powered aircraft. Learn what is "normal" for your engine, and watch out for warning signs. Small changes in the temperatures and pressures of your engine, could have serious implications if you don't notice them in time, or choose to ignore them.
A few hundred Dollars/Euros for extra sensors on top of the total price of your aircraft is peanuts when compared to the potential cost of losing your aircraft, serious injury to yourself and/or fellow passengers, or having to pay the ultimate price......... Be sensible; if you don't think you can afford 3 extra sensors to measure the other CHT's and 3/4 sensors to measure EGT's, then you certainly can't afford to fly your aircraft in the future either. You probably shouldn't have bought your aircraft at all !!
Check the accuracy of your sensors and instruments regularly to make sure the readings are correct. Replace or repair defective sensors and instruments before your next flight.
Don't save on safety!