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/ Categories: Archive, oil-system

Dry-sump engines - cooling the oil

oil-pumpsMany fundamental decisions have to be made when it comes to cooling an engine in the chassis. Do we have one radiator or perhaps two? Do we place them at the front of the car or at the side - or possibly, if more weight is required over the rear wheels, can we get adequate cooling by placing it at the rear?

To those assigned with cooling the engine and students of vehicle design, these decisions are no doubt all too familiar. But when it comes to the oil system, however, especially dry-sump systems, I doubt if many have even thought about it.

With hoses running from the engine to the scavenge pump, and from the scavenge pump to the oil tank as well as from the oil tank through the filter and back to the engine again, there would seem to be plenty of opportunity to stick in the odd cooler here or there. But when it comes to deciding the optimum place, what should we be thinking about and where should it go?

Let me reassure you right at the start, as far as I am concerned there is no right or wrong place. The important feature, however, is that the fluid should enter the engine at the required temperature and the pressure drop across the whole system should be wholly acceptable at the flow rate required.

Some ways may be slightly less efficient in terms of the size of the cooler required while saving on the hose lengths needed. Similarly, the positioning of the cooling matrix may be more critical from an aerodynamic viewpoint and, although important, size and weight may not be the overriding characteristics.

oil-pumps oil-cooler-on-the

In the end, there are basically two main approaches - running the scavenge oil through the cooler or to run the pressurised oil through the cooler. In the case of the former the oil, as a mixture of oil and air/combustion gas, will have a considerably greater volume; in the case of the latter the cooler will experience full gallery oil pressure, and will consequently need to be stronger and therefore heavier. Alongside this, with scavenge cooling the radiating matrix may need to be larger to cope with the reduced thermal efficiency, while that of the pressurised version will be smaller and more compact.

However, for the convenience of plumbing, it is often better to cool the scavenged oil. But depending on the temperature increase across the engine a hot oil is much easier to de-aerate than a cold oil, and so as well as all the above, the size of the oil tank may also be reduced if the oil is easily de-aerated. On balance, I therefore much prefer to mount the cooler in the pressurised line, for not only is the oil in a better condition and heat dissipation more efficient, the flow through the scavenge system will be less restricted.

In the end, the best method is the one that works, and while opinion is divided it is hard to get it totally wrong. The important thing, however, is to place the filter just prior to the oil going into the engine main oil gallery.

Fig. 1 - Oil cooler on the Gold Leaf Team Lotus, Lotus 49, when the oil was cooled using a separate system

Written by John Coxon

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