TacomaJD
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FWIW, I enjoy reading your posts.Can you quote the statement that you should rarely use the engine for slowing?
Because on page 13 of the NC700 manual, under "Engine Braking" I'm seeing the following (emphasis mine):
"Engine braking helps slow your motorcycle down when you release the throttle. For further slowing action, downshift to a lower gear. Use engine braking with intermittent use of the brakes to reduce speed when descending long, steep slopes."
I am interpreting this to mean always engine brake, downshift for better effect, and make sure not to cook your brakes during prolonged braking periods.
I'm not sure the heating up/compressing of the air contributes significantly to the energy dissipation. Depending on valve timing, the pressure differential is going to be very small or even zero. Remember the same mass of air is decompressed to (approximately, based on valve timing) the same volume on the power(less) stroke, most of the energy put into the compression is regained during the expansion. The majority of engine braking is correctly noted above as coming from the pump force required to draw in the air from the manifold with a closed throttle, as well as the small component coming from friction in the drivetrain.
This is different from big trucks that have actual engine brakes ("jake brakes"), where the exhaust valves are opened before the power(less) stroke to vent the compressed air to atmosphere.
Assumptions:
Compression isentropic
Air represented by an ideal diatomic gas, therefore Cp/Cv = K = 1.4
Ambient air temperature is 293K or 20C
Intake air conservatively assumed to be ambient temperature. In reality, due to reduced pressure through closed throttle, intake temperature is below ambient.
Known:
V1 / V2 = 10.7 (Compression Ratio)
T2 / T1 = (V1 / V2)^(K-1)
T2 = 293 (10.7)^(0.4)
T2 = 756K or 483C
Considering the heat of combustion is generally 700C or greater, and usually above 1000C, you're reducing the thermal load on the cooling system.
And no no no no no, the engine doesn't have to absorb "the heat that would have gone into the brake pads from using brakes only." The brake heat is the dissipation of kinetic energy as thermal energy due to friction. No friction, no heat. Engine braking relies on pump force to transfer energy, not friction. Different processes, different energy conversions.