Using the thermal properties of titanium to improve power and reduce aero drag

Monday, December 02, 2013

Tags :  advanced-materials

It sounds too good to be true doesn’t it – more power and a lower-drag car, simply by using a bit of titanium. It won’t make you more attractive to the opposite sex, but titanium can do something for your success. Titanium alloys are most often used for their combination of strength and low density, or for their temperature resistance. As a material for cyclically loaded fasteners, their combination of high strength and low stiffness is useful.

The property that makes titanium able to give a slight power increase is thermal conductivity, or rather its unusually low thermal conductivity. At around 6.7 W/mK (46.5 BTU-in/hr-ft²-°F) it is much lower than most of the alloys we would use in race engine construction. 

Material

Thermal conductivity, W/mK

% Thermal conductivity, cf 6061

Aluminium 6061

167

100

Steel 4340

44.5

26.6

Stainless steel 304

16.3

9.7

Inconel 625

9.8

5.9

Titanium

6.7

4

So, how does this translate to power? Well, the effect is subtle but the answer is simple. If we reduce the temperature of the inlet system, the incoming charge picks up less heat and therefore arrives in the combustion chamber at higher density. Higher density equates to more mass in a given volume, and more trapped mass leads to more output (assuming no loss of combustion efficiency).

One way to do this is to decrease the heat transfer along the inlet tracts. The hottest part is closest to the combustion chamber, and wherever there is a joint between inlet components, we have an opportunity to limit heat transfer. A gasket is a very simple way to achieve this, and many polymers have a thermal conductivity very much lower than titanium. However, the fasteners offer a heat path, and it is worth using titanium fasteners to reduce thermal conductivity across the joints in an inlet system. Where titanium threaded fasteners are prohibited (as in Formula One), titanium washers or bushes combined with any one of the allowed fastener materials will also make a useful difference. Having been involved in a project to reduce inlet temperatures a few years ago, the simulation results were encouraging.

So, how can we reduce aero drag? Well, if we move to the opposite side of the engine, we can use the same strategy to reduce heat transfer into the engine from the hot exhaust system. Having seen the results of attempts to develop a solid ceramic exhaust gasket for a race engine to achieve this effect, I’d recommend using something more conventional. Using a titanium fastener and/or washers to reduce the flow of heat into the heads necessarily reduces the amount of heat the engine needs to reject to the air via the water system. Therefore, if the engine water system is fully optimised, smaller radiators, smaller cooling ducts and less airflow is required.

It is unlikely that swapping a few fasteners is going to allow you to reduce radiator size to any noticeable extent. However, combining lots of small measures to reduce heat rejection from the engine can lead to a small but useful improvement.

Written by Wayne Ward

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Comments

Hello, I have two questions:

In this specific case you consider a naturally aspirated engine. how much power loss estimate would one get if he runs the engine system, say 20C higher than previously and what are the issues which can arise from that situation? Would a similar loss happen for a non-intercooled turbocharged engine running hotter? What is keeping race engines from running very high water pressures of 5-6 bar like the trail F1 has took before a limit on pressure was imposed to 3.75?