Feeling the heat
Some of us have the regular chance to see an engine running hard on the dyno, and in many cases it is visually obvious that there is a lot of heat being radiated from the exhausts. There are many video clips and pictures on the internet showing exhaust systems glowing bright red, especially on very high-speed engines such as found in Formula One. Whilst it is clear that there is a great deal of heat being generated, what to do with this excess of energy is not clear.
The management of this heat under the engine cover or bonnet (hood) is a concern in many applications, most especially where turbocharging is employed. Whilst no longer used in Formula One (where power outputs approaching 1000 hp per litre were suggested in qualifying trim), turbocharging is still popular in many branches of motor sport such as rally and sports cars. Before we concern ourselves with heat management, we should consider what the problems are with having a red-hot exhaust.
A problem which can often be seen where the exhaust system is too close to the bodywork, we can see blistered paintwork and sometimes holes burnt into the bodywork. In a famous case in 2008, Kimi Raikkonen’s Ferrari exhaust failed during a race, resulting in the contents of the airbox being revealed to the television-viewing public around the world; I’m sure that these pictures would have been looked at in detail by their rival engine suppliers. Whilst due to exhaust gases rather than radiation from hot metal, this shows the damage that can be done by excess heat. Radiated heat from exhausts can lead to increased coolant and lubricant temperatures which would lead to the requirement for coolers of increased dimensions and, where cars are aerodynamically optimised, increased drag. The lower viscosity due to the oil being hotter is generally held to give an increase in performance due to lower frictional losses, but this is often more than offset by the aerodynamic loss.
Excessive temperatures under the engine cover due to exhaust heat often leads to failure of electronic components, or mechanical failure of adjacent components.
If aluminium components become overheated their strength can be decreased with an attendant loss of fatigue life, thus requiring early replacement or re-engineering.
Where the inlet system is subjected to this radiated heat, it can lead to increased charge temperatures and consequent loss of performance, and this can lead to very serious losses of performance in turbocharged engines.
So, where we have to deal with a hot exhaust, what can be done to eliminate or mitigate the effects of heat? There is the option of providing cooling to the affected components or areas of the car, but this can be complicated if there is more than one component to provide cooling for and can also have an aerodynamic penalty. The second option is to try to contain the heat within the exhaust so that it cannot radiate as effectively. There are a few options for this, including wrapping the individual exhaust headers in an insulating cloth which is available from many motor sport suppliers, creating a ‘bag’ or a ‘blanket’ from special insulating materials, or having the pipes coated to lower the thermal conductivity. All of these options have the aim of lower surface temperatures. As temperature (absolute temperature) has a fourth-order effect on radiative heat transfer, any lowering of surface temperature has a very significant effect. A 10% lowering of absolute temperature (approximately 100 degrees C for an initial temperature of 750 C) gives a 46% reduction in radiating power output. The remaining strategy for dealing with the heat is to shield the parts concerned so that radiated heat is absorbed by an intermediate part. Such heat shields are very effective but need to be at such a distance from the exhaust so as not to touch in service.
Written by Wayne Ward.