Due to overheating problems on my air-cooled Honda NBC110 during long uphill climbs, I have decided to install an oil cooler. This task is complicated by the lack of accessible oilways in the cylinder head (as are present on some models), necessitating the use of an external oil pump.
I have settled on an HEP-02A electric fuel pump, with a claimed flow rate of 80-120l/hour.
This is a reciprocating solenoid type pump which uses either a diaphram or small piston to move fluid.
Heat Transfer Capacity
Taking a nominal pump capacity of 90l/min, we can calculate heat transfer capacity as a function of heat exchanger temperature delta. This requires us to know the specific heat of the engine oil, which we take from Santos, J. C. O. (2006) to be a conservative 2.5J/g/K. Oil density is also assumed to be 0.8kg/l. For the sake of simplicity the temperature relationships are dropped, noting that they would result in a slightly higher heat transfer coefficient due to a higher average temperature in the radiator, and longer flow time due to slowing of the fluid as it cools.
The heat transfer capacity thus is as follows:
$$Heat\ transfer\ capacity\ (Oil) = 90l/hour \div 3600s/hour \cdot 0.8kg/l \cdot 2.5J/g/K \cdot ΔT = 50W/K$$
We will thus have 1kW of heat transfer for every 20 degrees Celsius of temperature delta in the oil cooler.
Airflow Requirements
Because the radiator used is the high surface area split-fin type from an air conditioner evaporator, mass airflow rate should give a reasonable estimation of heat rejection, without needing to calculate the exact heat transfer coefficient.
Using an isobaric specific heat of 1.006kJ/kg/K, we can find the flow rate required as a function of the air temperature delta across the radiator.
$$Volumetric\ heat\ capacity\ of\ air = 1.006kJ/kg/K \cdot 1.225kg/m3 = 1.2324kJ/m3/K$$
At a relative air speed of 10m/s (36km/h), the heat rejection of the air thus is:
$$Heat\ rejection\ rate = 1.2324kJ/m3/K \cdot 10m/s = 12.324kW/m2/K = 0.12324kW/dm2/K$$
George, N. J., Obianwu, V. I., Akpan, A. E., & Obot, I. B. (2010). Lubricating and cooling capacities of different SAE 20W – 50 engine oil samples using specific heat capacity and cooling rate.
Santos, J. C. O. (2006). Specific Heat capacity Of some Mineral, Synthetic And Semi-Synthetic Automotive Lubricant Oils After Themal Degradation. India: Chemical Technology An Indian Jurnal.
https://wiki.anton-paar.com/nz-en/engine-oil/
https://www.engineeringtoolbox.com/air-specific-heat-capacity-d_705.html
