I fly a 2009 C182T with CAP. We record the "TACH" time from the Engine/System page where ENG HRS appear. On the steam gauge Cessnas I fly TACH time is read off of the tachometer and generally understood to record the time that the engine is run above approximately 1000 RPM which produces the requisite oil pressure. The POH for the 182T only describes that the HOBBS requires oil pressure of 20 PSI to run. However, I have not been able to find what sensor or stimulus runs the ENG HRS. Anyone know where this information can be found?
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Your understanding of how tach hours work is not correct, you are conflating some elements of Hobbs time and tach time with each other.
Tach "time" is not electrical, it is driven solely by the rotation of the engine. In my 182, the tach "odometer" is calibrated so that 1 "hour" on the meter means 144,000 rotations of the engine... That's 2,400 RPM * 60 minutes = 144,000 RPH. If the engine makes 72,000 rotations, the engine hours meter will read 0.5. I could be turning the prop by hand and take two full days to spin the prop 72,000 times, the tach will still read 0.5.
I often cruise at about 2,300 RPM, and thus after a one hour flight, my "tach time" will be 2,300/2,400 or 0.96. Fly for an hour at 2,000 RPM, and tach time will be 2,000/2,400 or 0.83.
So "engine hours" aren't really hours at all, but rather "equivalent hours if the engine was running at typical cruise RPM". It's most likely based on your POH's published cruise numbers from Section 5. It's intended to be an accurate reflection of engine use, and thus is the basis for engine maintenance intervals like TBO, SBs, and oil changes. The Hobbs is more oriented to an accurate reflection of aircraft block time, more useful for rental rates, etc.
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$\begingroup$ Are you sure it's calibrated 1/1 to redline RPM and not rated cruise RPM? $\endgroup$– John KOct 21, 2022 at 13:57
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1$\begingroup$ I suspect you are exactly right. I'll clarify the answer, thanks! $\endgroup$– Max ROct 21, 2022 at 14:23
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$\begingroup$ One reason I liked to cruise a bit oversquare, like 22"-24"/2100 rpm. Slows down the tach hours per hour in the air, reduces overall wear and tear, and you can't hurt a normally aspirated engine doing it as long as you aren't in a yellow arc or it's running rough. $\endgroup$– John KOct 21, 2022 at 15:13
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2$\begingroup$ 23 over 2100 at 6,000ft gets me 12.6 GPH, and works out to be my singular lowest cost per NM, it's definitely a sweet spot. I'm still traveling at 149 Kts so I'm giving up 6 Kts. I'm getting 96% of the speed in return for 87.5% of the engine wear and 91% of the fuel burn. $\endgroup$– Max ROct 21, 2022 at 15:32
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$\begingroup$ Great! Thank you for the explanation. I frequently hear people describe HOBBS time as run when electrical power is turned on and know this to be incorrect for the planes I fly at least. Can you reference any documentation that supports the tach time calculation? Thanks again! $\endgroup$– MikeGOct 22, 2022 at 13:13