Cooling fin design of an air cooled engine is critical to getting the heat out of the engine cylinder and for ensuring that there are no hot spots. Therefore, I’ve been looking at some of the work that people have done in this field and then collating some of the fundamental principles:
- Increasing the number of cooling fins increases the cylinder cooling.
- However, the cooling may decrease with an increased number of fins and reduced spacing at stationary to low speeds. This is due to the fact that the air does not flow well between the fins at low flow speeds.
- At low speeds the temperature on the leeward side increases and this results in cylinder bore distortion.
Air passes over cooling fins to remove the heat from the system.
- no radiator or pump and so lighter system
- no leakages
- can operate in cold climates
- less efficient
- needs a good airflow
When considering cooling fins we can relate those cooling an engine cylinder to those in a heatsink designed to cool electronics.
Heat Sink – A structure that is mechanically attached to a device that generates heat, in order to lower the overall thermal impedance between the point source of the heat within the device and its cooler surroundings.
The fundamental aspects to consider in heatsink design are:
- Surface area – you want to maximise the surface area as this is where the thermal transfer to the coolant medium takes place.
- Aerodynamics – the coolant medium needs to be able to easily flow around the heatsink
- Thermal transfer – heat flow within the heatsink needs to be good to get the heat out to the fins and so maximise the use of the large area.
- Contact – you need to maximise the contact with the part needing to be cooled and maximise the efficiency of the contact. This also needs to be maintained over the lifetime.
Number of Fins
Making the fins thicker will increase the heat transfer through the heatsink and to the fins, but will reduce the number of fins that can be fitted within a given package space and hence thereby reducing the surface area available for heat transfer. In addition, the thickness of each fin has to be sufficient to withstand any vibration that the part may experience in service.
The orientation of the heatsink will have an effect on it’s efficiency, especially when relying on natural air convection.
Standard computer fans can achieve a maximum heat transfer coefficient ~150Wm-2K-1.
A rough surface finish will increase the surface area. If a positive air flow is being used over the heat sink then a rough surface will lead to a thicker boundary layer and this will act as a barrier and reduce heat exchange.
Thermal cycling must be taken into account when designing any heat sink as it can result in fatigue of the part itself or fatigue of the joint between the part that needs cooling and the heat sink.
- A. Biermann, H. Ellerbrock, “The design of fins for air-cooled cylinders“, Nasa Tech Reports, Jan 01, 1941 – this is one of the definitive reports.
- O. Schey, V. Rollin, “The Effect of Baffles on the Temperature Distribution and Heat-transfer Coefficients of Finned Cylinders“, Nasa Tech Reports, Jan 01, 1936 – this is a great paper with images of the test setup and data, great reading material for optimising an air-cooled engine design
Engine Design – a number of short articles discussing aspects of engine design and in particular model engine design.