X
items
Your shopping cart is empty.
Product Qty Amount
From:
Until:

Archive
[email protected]
/ Categories: Archive, exhausts

Feeling the heat (3)

exhaustsIn previous articles, we have looked at why we might choose to insulate the exhaust system of a racing vehicle, the various methods by which we might insulate the exhaust and what the consequences of doing so might be.

In light of this, we discussed exhaust coatings with a British company, one of whose specialities is the application of thermal barrier coatings to exhaust systems. Zircotec, who are based in the middle of an area rich in motorsport expertise, have a number of exhaust coatings which are made available on a sub-contract processing basis. They claim to be able to reduce convective exhaust heat transfer rates to the surrounding area by up to 30% with their premium coatings and as much as 25% using their lower-cost processes. I have seen independent test data from a source which proves the voracity of these claims with some very exhaustive testing (no pun intended). The report that I have seen regarding these coatings was conducted using an engine from a high-output premium road car in controlled conditions and using varied simulated wind speeds to mimic the effect of airflow over the outer surface of the exhausts. One important point that is apparent from the results of the tests is that the increase in gas temperature due to the thermal barrier coatings is very small and of such a small magnitude on the scale of absolute temperature that I seriously doubt that any retuning would be necessary in terms of exhaust dimensions. There is no data contained in the report concerning heat transfer by radiation, which would be a significant effect in a racing engine, especially if it is supercharged or turbocharged.

exhausts Zircotec-Process

The coatings offered by this company are not paints, but are the result of a plasma-spraying process. Plasma is produced by the dissociation and ionisation of a gas, generally by means of an electric field. A plasma gun injects gas into a strong electric field and the result is that a plasma flame is emitted from the front of the gun. It is into this plasma flame that the powdered media is injected. The temperatures inside the plasma flame are sufficiently high to melt even ceramics. Owing to these high temperatures, there are very few materials which cannot be spray coated, providing of course that it can be supplied in a suitable form, this being a very finely divided powder. Whilst the temperature may be several thousand degrees, the mass of the sprayed particles relative to the mass of the comparatively much cooler substrate means that there is little heating of the substrate and little chance of damage being caused. Whilst we are unlikely to find an aluminium exhaust system, plasma coatings can be applied to low-melting point materials without physical damage and I have seen ceramic coatings applied by similar methods to aluminium parts for use in race applications. Certainly, when we talk of exhaust system materials such as steel, stainless steels, titanium or super alloys, there will be no damage caused by thermal spray coatings. The accompanying picture shows a coating being applied using the plasma spraying technique as described.

Fig. 1 - Plasma application process.

Written by Wayne Ward

Previous Article Exhaust Manufacture: Future directions
Next Article Feeling the Heat (4)