Oil coolers

The design of oil-air coolers in motorsport has not changed drastically in the past 60 years, with the basic design harking back to World War II. As with so many technological developments, the generally available ‘modular’ style of plate and bar oil cooler owes its existence to aero engine development, in particular the legendary Merlin V12.

There are many similarities between race engine and aero engine development, with designers of both striving to find the most efficient and lightest possible engineering solutions. In the case of oil coolers, the increasing heat rejection characteristics of the Merlin over its predecessors meant a new cooler had to be designed that provided sufficient heat transfer in a compact and lightweight package. It was therefore logical that post-war racecar engineers would, for the very same reasons, adopt the design.

The design has been optimised over the years, but the same basic features have remained. The intention is to provide maximum contact area for the oil and air, while minimising any reduction in oil pressure. This is achieved by using what is known as a bar and plate construction. The fluid tubes are created by sandwiching two square bars between two flat plates, with the assembly then brazed together. These tubes are fixed between two end tanks through which the oil enters and exits, with the voids between the tubes taken up with closely packed fins which provide most of the cooling area.

This type of construction provides far better flow and heat rejection characteristics than simple round tube and fin coolers, which used to be the standard in automotive applications. These simply consisted of a round tube with fins attached, with the oil flowing from one end of the tube to another. Although cheap to produce, the large number of tube bends creates a significant pressure drop, and a much larger cooler is needed to obtain the same heat rejection as a bar and plate type cooler due to the smaller tube and fin surface area. There are a number of variations on the basic bar and plate cooler, including types that use seamless C-section tubes to increase burst pressure by eliminating seams.

Beyond the benefits of the basic construction, most high-quality bar and plate oil coolers have their efficiency further increased thanks to the inclusion of ‘turbulators’ within the fluid pipes. When fluid flows through a plain tube the fluid nearest the wall is subject to frictional drag, which has the effect of slowing down the fluid at the wall. This laminar boundary layer can significantly reduce the tube-side heat transfer coefficient and consequently the performance of the heat exchanger.

In an effort to promote turbulent flow, many cooler manufacturers fit shaped elements within the oil pipes. The shape and size of these elements can have a considerable impact on the pressure drop across a cooler, so their design is a trade-off between increased cooling and ensuring that an acceptable pressure drop is maintained.

For the same reasons, some oil cooler designs also use turbulators on the air side, with turbulent flow increasing the volume of air coming into contact with the cooling fins. In many racing applications though, where the smallest possible coolers are often specified to help keep frontal area (and thus drag) low, turbulent flow within the cooler is to be avoided at all costs.

Written by Lawrence Butcher