Oil coolers - the Laminova
Engine oil is rather like that famous brand of lager that is said to refresh the parts other beers cannot reach, for in cooling the piston undercrown, valvetrain and bearings it is clearly performing a function in those areas inaccessible to the engine coolant. As in the case of the engine cooling system, this heat eventually has to be distributed into the passing air surrounding the vehicle.
For many years, and where natural flow of air around the sump was insufficient, the most common intermediary was an oil-air heat exchanger. Made from a series of pressed or stacked plates, or a system of flattened tubes and fins, these were highly efficient but suffered from packaging problems, often requiring long hoses or special ducting to a position on the body of the vehicle. Mounted near the front or sides, these coolers are also vulnerable to accident damage.
A more elegant solution to cooling the oil is to use an oil-water method, one of which is the Laminova laminar flow oil cooler. It consists of two concentric aluminium extrusions, one of which forms the outer housing into which the engine oil is introduced. The second, a far more complex affair, can best be described as a thick-walled tube, on the inside of which are a number of fully enclosed channels running through the tube wall parallel to the axis of the tube. On the external face of this tube is a series of very fine, 3 mm-high radial fins, 0.2 mm thick and 0.3 mm apart. A quick look at the diagram might give a clearer idea.
As the oil flows across and around the outside of this tube, the manufacturers claim that normally the flow would become turbulent and significantly increase the pressure drop in this part of the oil circuit. However, by introducing channels into the finning running the length of the extrusion, this is somehow avoided and the flow around it remains laminar. Compared with other designs, the pressure drop throughout the oil circuit is very low, while the heat transfer across the boundary between the oil on the outside and the engine coolant on the inside is very high.
As a general rule, turbulent flow in a cooler produces excellent heat transfer but high flow losses, while if the flow remains laminar heat transfer isn't generally quite so good but the flow losses are at a minimum. In many traditional designs, these parameters have to be balanced to give the optimum solution, but in the case of the Laminova, the huge surface area more than makes up for the loss in performance over turbulent flow designs. To optimise the heat transfer as well as selecting the correct size of unit, optional internal cores are available to give the best coolant flow characteristics at minimum pressure loss.
So is this a better design than the traditional plate or tube-and-fin design? Typical of an engineer, the response is: it depends. To use an oil-into-water cooler, the main engine cooling circuit needs to have sufficient extra capacity and therefore a slightly larger cooling area than is otherwise necessary. On the plus side, however ,the heat exchange between the oil and water is likely to be much better, and the design a much more robust affair. Positioned much closer to the engine, accident damage is less likely, and even in the case of engine failure, modular construction allows the unit to be dismantled and easily cleaned before re-use. Also from an engineering viewpoint, since hose runs will be much shorter, the pressure drop in the oil circuit should be at a minimum; and since the engine coolant warms up quicker, at start-up the oil will warm up much quicker, giving better engine protection during this phase.
In refreshing the parts that other fluids cannot reach, the Laminova oil cooler may not be the answer to all applications but it must surely be one worth looking at.
Fig. 1 - Unique core of the cooler
Fig. 2 - Internal core options
Written by John Coxon