Quote:
Originally Posted by hat_man
Hello everyone,
I know I have seen this term used here before, but I'm not sure how it relates to FE, how to calculate it, or how to use that calculation to help me improve. Can anyone give me a tutorial? If you need any specific information about what vehicle, it would be Oh Deer in my signature. I cruise at just a hair over 2k RPM at 55 mph.
Thank you for any help.
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During transient loads between idle and WOT, the percentage of heat energy supplied, and the net power extracted varies with rpm.
Exhaust, Oil, & Miscellaneous heat rejects in lock step with rpm.
Cooling system follows it's own path.
Hydrodynamic shearing losses in the oil for sliding and rotating components increase with rpm.
Pumping losses fall with rpm, at minimum at WOT.
An old metric was to operate an engine at an 80% load factor to achieve the highest operating efficiency ( 40% thermal efficiency ) which tended to occur where torque peaks.
Piston speed has to do with hydrodynamic losses created as the reciprocating piston rings ride the cylinders. The 'speed' has to do with their averaged surface feet per minute travelled. Drag varies with the square of the velocity. Power to overcome the drag varies with the cube of the velocity. Just like in air with the car's body.
As an air pump, if you double the engine's rpm, you essentially double the gross horsepower, but at the cost of geometric increases in internal losses, so your 'net' horsepower falls off geometrically as it's trying to increase.
The rpm for the engine's rated torque is still probably a decent metric for determining a 'cruise' rpm.
Turbocharging artificially increases the apparent displacement of an engine, allowing really 'flat' torque curves, and at very low rpm. A 'two-fer.'
Electric motors are a 'three-fer.' All available torque is available at all rpms, from zero on up.