Steve:
Thanks for your explanation. I think all of us could use an objective set of criteria for determining when we are under loaded, or maybe more useful- criteria that would indicate than an engine is properly loaded. So let me propose some, taken from your article and your posting above:
1. Coolant temperature at 160 F or above.
2. Oil temperature at 180 F or above.
3. Exhaust turbo only has a light dry soot coating (we can't easily see injector tips or ring carbon, but turbo soot should be a decent indicator).
4. Oil analysis shows less than 0.5% fuel content.
I believe that these criteria will tell you if you are loaded decently. On my Yanmar 370, I meet all of these criteria with the possible exception of oil temperature while running at about 1,600 rpm and producing roughly 60 hp with occasional high power runs to blow out the accumulated soot.
David
The only criteria I would add to this is exhaust gas temp above 450-500F. An exhaust gas probe would tell you this, but that's not something most folks have or will add. Alternatively you could shoot the dry portion of the exhaust elbow immediately after the elbow with an IR pyrometer for a rough reading on EG temp.
The coolant should always reach and ultimately run at the thermostat's rating. If it's more than a few degrees lower while under load something is wrong.
The turbo soot accumulation is pretty subjective, one man's build up is another's patina. And, unless it's disassembled, you can only see the intake turbine on the turbo, not the portion that operates in the exhaust gas stream, and that's where soot, and un-burned fuel would accumulate.
Additionally, fuel dilution is dependent on the lube time, the number of hours on the oil. Some engine manufacturers limit the absolute fuel dilution to 5%, however, if that occurs after 70 hrs then that's problematic (and the oil would need to be changed), while if it occurs after 200 hours then it's far less of an issue. I have clients running displacement trawlers, powered with common rail engines, using synthetic oil, doing oil changes at 450 hrs (this is within the manufacturer's spec's provided you use syn oil), practicing the periodic run up, with no appreciable fuel dilution or other contamination showing up in the oil analysis other than a slowly falling TBN number, which is normal, the threshold for which is 2.5.