Le Mans exhaust systems
Race engine exhausts are expected to do a tough job without suffering failure, a job they generally do very well. The results of a broken pipe can be very noticeable for two main reasons - loss of performance and a marked increase in the amount of noise generated, especially on silenced systems where the breakage is upstream of the silencer. With a premium placed on both reliability and system mass, many race exhausts therefore use titanium, stainless steel and Inconel materials.
Perhaps the greatest challenge in terms of mileage are the endurance races, of which there can be no more famous example than Le Mans, the Mecca for exponents of sportscar racing for many decades. Exhaust systems are expected to last for the practice and qualifying sessions, as well as the race itself. This can easily be in excess of 4000 km, during which there is little opportunity (or hopefully zero opportunity during the race) to inspect the exhaust system. Sportscar racing has embraced the move toward the higher specification materials with the use of Inconel. Titanium has little chance in endurance race exhausts, although lightly stressed parts could conceivably be used with success.
The cars are expected to be silenced and to come within strict guidelines on noise, and over the course of a 24-hour race, silencer degradation can be serious, with loss of silencer packing material being a problem where the traditional perforated tube silencer is used. With the marshals and technical officials perhaps forcing a car to come in for repairs if an exhaust system becomes too noisy, there is a real incentive to ensure the reliability of the system from the point of view of the headers and the silencers.
Turbocharged engines have an advantage over naturally aspirated engines, as the turbine takes a lot of energy from the exhaust flow, so they generally require less silencing. However, with high mass flow for a given engine size, the exhausts used on a turbocharged engine have to cope with much more heat and therefore higher temperatures.
As anyone who has seen the diesels in action - love them or hate them, they are very fast - will attest to their very low noise emissions. Part of this is due to the requirement for exhaust after-treatment, where the exhaust gas has to pass through filters in order to burn off particulates. Again, these filters have some noise attenuation, and the exhaust flow arriving at the filter will have had some energy removed by the turbocharger.
The issue of heat at Le Mans is a serious consideration for the race team and the engineers who design the cars. The race is often hot, and the engines in the prototype cars are fully covered, with no louvres or chimneys in the engine cover to aid cooling. Radiant heat from hot exhausts and the general heating effect due to convection under the engine cover can lead to reliability problems for other components, especially electronics. Unless sufficiently protected from heat, soldered connections can fail due to the solder being melted. This is just another challenge for the exhaust designer, who needs to keep heat away from critical electrical components.
Fig. 1 - Le Mans presents a particular challenge in terms of exhaust durability, silencing and protection of adjacent components from exhaust heat
Written by Wayne Ward