Exhaust piping

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History

Early last century, as piston engines came under the scrutiny of academics, it was noticed that if an engine was run at an RPM such that the firing frequency coincided with the natural frequency of the exhaust system, i.e. the 'note' of the exhaust pipe, a resonance was set up in the pipe. However, it was not until MIT published a paper on how this phenomenon could be exploited to increase the performance of an engine, that the tuning of exhaust systems really took off. Among the first to experiment with exhaust tuning were the motorcycle engine tuners. In the late 1940s and early 50s, they tried different length exhaust pipes on their single-cylinder Nortons etc to obtain maximum power and to shape the torque curve. There is some evidence that Mercedes-Benz were experimenting with exhaust systems on their pre-War GP cars, but there is no mention of this in any of the books on these cars and the technique was probably not fully understood. It was not until the 2.5l GP cars of the 1950's that we really see exhaust system tuning applied to Formula One racing engines.

Design

The objective of the engine designer is to create a negative pressure at the exhaust valve during the overlap period when both exhaust and intake valves are open. To do this he designs an exhaust system that resonates at a particular RPM, and uses the pressure waves reflected by the ends of the pipes to modify the time history of the pressure at the exhaust valve. By coupling two or more of the cylinders' exhaust primary pipes together, interaction between the pulses created by each cylinder modify the pressure characteristics at any given RPM. The ends of each primary pipe are brought together in a collector, such that their ends are close enough together to interact, and the tail pipe(s) form a secondary resonant system. At the same time, the designer will chose intake lengths to form another resonant system, which also interacts with the exhaust system. When two or more cylinder's exhaust pipes are coupled, the firing order of the engine becomes significant, and the firing order of V-10's are chosen as a compromise between exhaust tuning and the torsional dynamics of the crankshaft.

Exhaust pipe exiting upwards in the F2002The frequency of an exhaust pipe is set by its length. The shorter the pipe, the higher the frequency. As engine RPM has risen over the years, the length of the exhaust pipes for a given engine configuration has shortened drastically.

To accomplish an 'as much as possible' ideal exhaust, some very different factors have to be compared and calculated. Some of these all very important factors are the exhaust and intake geometry, valve timing, exhaust gas temperature, velocity, and RPM all affect the characteristics. Reminding these characteristics, an exhaust system can only be optimised for one certain RPM. The lenght of the exhaust pipe affects as well the maximum RPM and suppleness of the engine in lower RPM. The exhaust is therefore a compromise between engine power in lower of higher RPM.

Upper exhaust on EJ12That brings us to the aerodynamic part of the car. As soon as aerodynamicists knew that a diffuser speeded up the air passing under the car and generated downforce, they discovered that blowing exhaust gasses into the diffuser would affect the downforce significantly. However, another problem arose with diffuser blowing: as soon as the driver lifted the throttle, it would affect the flow of air in, after and immediately before the diffuser and thus also the downforce which was in conflict with the general FIA reglutions. Although, as it is proven that any exhaust affects the aerodynamics, forbidding the diffuser blowing would make all exhausts illegal. This effect on the aerodynamics will probably have been a reason for Ferrari to abandon the regular type of exhausts, and start blowing the exhaust air over the rear top of the car, before the air has passed the rear wing. Raising the exhaust pipes needed heat shielding of the aerodynamic parts of the car. Due to the hot air passing by the rear wing, it would become very fragile if it wasn't protected against such high air temperatures.

However, two further trends have led to Ferrari considering and then adopting an alternative exhaust arrangement. In the quest to move weight forward that has resulted from the width limitation on the rear tyres and the grooves in the treads which led to Bridgestone introducing a wider front tyre in 1998, the engine has moved forward relative to the rear of the bodywork, defined in the regulations by the rear axle centre line. At the same time the peak RPM of engines climbed relentlessly upward, now over 18,000rpm. Thus, while exhaust pipes needed to become shorter to stay in tune with the higher RPM, the exhaust pipes had to be longer to reach the trailing edge of the underbody. For Ferrari, the arrangement that provides for nearer optimum length exhaust pipes, by leading them the short distance from the engine to an exit port set into the top surface of the bodywork, is better than one that still blows into the base region around the trailing edge of the diffuser. For McLaren and their engine partner, Mercedes-Illmor, the latter layout provided them for a long time with the best compromise.

Radiator outlet

Another remarquable appearance was re-invented by McLaren. The 'chimney' or 'funnels' in front of the rear wheels do not put out engine exhaust gasses, but rather hot radiator air. Some other teams have copied this novelty in 2001. The principle behind this is actually very simple. On the one hand, the air passing through the radiators isn't specified by the throttle, but it depends on the car speed. Increasing the throttle makes the engine suck move air into it, and thus generates more exhaust gasses. Radiators are only provided by air flowing in, due to the movement of the car. So the effect of engine exhausts is just that little faster on downforce in acceleration then with radiator air. On the other hand, it is very interesting of blowing out this hot air as soon as possible, bacause it heats op the inside of the car. Keeping the air longer under the bonnet, will increase the engine temparature. We shall see in the future who will have made the best compromise.


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