Features - Technical
APRIL 29, 2000
BY PETER WRIGHT
It is ironical that one of the techniques used for tuning a high performance piston engine should be based on a technology discovered thousands of years ago, and yet it was not widely applied to racing engines until just over 50 years ago. Exactly when man discovered that blowing across the end of a hollow tube produced a defined note, and that varying the geometry of that tube could change the note, is not known. However, the oldest playable instrument so far discovered is a Chinese flute, made from the hollow wing bone of a crane and carbon dated as having been made around 9000 years ago. In the following millennia, man has learnt to exploit this phenomenon and invented a wide range of musical instruments by which the player can use a variety of techniques to change the geometry of the tube or tubes, to achieve a full range of notes.
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 Ô50s, 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 Formula1 racing engines.
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.
If this seems complex and difficult to analyse, it is, and for years exhaust and intake systems were the subject of a great deal of experimenting and testing on dynamometers. Exhaust and intake geometry, valve timing, exhaust gas temperature and velocity, and RPM all affect the characteristics, and any system is only optimised at one RPM. Torque curve shape is very sensitive to these effects, and is inevitable a compromise between maximum power and driveability. The computer has brought about the ability to model this complex dynamic system, and explore the effects of all the parameters that affect it. 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. Variable length intakes also became popular for NA engines, to broaden the torque curve (pioneered, I believe, by Mazda, on their Le Mans winning rotary engines). Perhaps the greatest exploiter of exhaust tuning are the 2-stroke engines. With their port timing such that there is considerable overlap between the intake port and exhaust port being open, they absolutely depend on exhaust tuning to maintain a high volumetric efficiency. A look at any high-performance 2-stroke exhaust system will confirm this.
The natural frequency of a pipe is set by its length, everything else being equal; 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. In the 1960's, the ability to cross couple exhausts on the two banks of the 1.5l V-8's, with their two-plane crankshafts, became impossible and led to reversed cylinder heads, with the exhaust system between the heads, resembling a bunch of snakes. Cosworth's DFV used a single plane crankshaft, and was exhaust tuned as two 4-cylinder, in-line engines and so did not require cross-coupling of the banks.
Turbo-charged engines are not critical to exhaust tuning once the exhaust gases have passed through the turbocharger. It was during the turbo-era that aerodynamicists discovered that using the exhaust gas flow to blow the diffuser of the flat-bottomed cars, increased the flow under the car and so increased downforce. Of course, when the driver lifted the throttle, the flow reduced and the downforce reduced. From this time, exhaust system design has been a compromise between aerodynamics and power, and the two departments have strenuously researched the affects in order to make their individual cases for priority.
When diffuser blowing first came in, the exhaust was arranged to blow tangentially along the surface of the inclined diffuser. The high velocity gases entrained air, energising the thick boundary layer, and effectively powered the diffuser as it drew air under the car. The fact that the throttles controlled the gas flow would appear to contravene the regulation that prohibits any part of the car that affects aerodynamic performance from being moveable. However, wherever the exhaust exited it would affect the aerodynamics, so diffuser blowing has escaped the regulation on the basis that if it was banned, all exhausts would be illegal.
Wind tunnel models incorporate small, ejector-type, air driven pumps that simulate the air flow into the ram intake and augment it as the exhaust flow. The effect of throttle-open and -closed can be tested and the changes in downforce and, more importantly the centre of pressure, can be measured. In recent years, the effect of the exhaust on the centre of pressure, and its variation with throttle opening have led to the true, blown diffuser being abandoned, and exhausts exits moved to blowing over the top of the lower body rear edge. In this position, they probably did more to increase the radiator air exit flow than influence the underbody flow.
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 provides them with the best compromise.
The two arrangements have other implications, for instance the Ferrari exhausts raise the mass of the tailpipe and part of the primary pipes above the lower position possible with the McLaren exhausts, and require heat shielding for the bodywork and top suspension links, particularly when they are manufactured in CFRP. The heat from the exhausts also plays upon parts of the rear wing structure that are normally spared, and must be protected or reinforced. However, it is the differences in aerodynamics that are the hardest to evaluate, particularly in understanding what Ferrari may have lost in removing the high energy gas stream from the underbody area.
The tendency recently is to exit more and more of the hot radiator air in front of or around the rear wheels, rather than leading it to the rear of the car and exhausting it into the underbody area - the McLaren "funnels-vents" that have appeared this year are an example of trying to manage this radiator air relative to the rear wing. It is possible that the benefit of low exhausts has diminished and tipped the balance in favour of the high ones. They inevitably impinge on the rear wing airflow, and getting that right is crucial. Adrian Newey and his aerodynamics team have obviously found a combination of exhaust outlet position and diffuser that gives them a benefit in either downforce or the change in characteristics with throttle opening that they want.
Maybe to understand why the two cars that are far ahead of the rest of the field differ in this respect, when so much else about the cars is visibly similar, one must stand back and look at the character of the two teams involved. Ferrari has always been engine dominated, though the series of non-Italian designers employed by them over the years - Dr Harvey Postlethwaite, John Barnard, and now the formidable team of Ross Brawn and Rory Byrne - have managed to shift the emphasis to chassis and aerodynamics. At McLaren, as with most English teams, it is the chassis and aerodynamics that dominate and the engine supply partner must provide a power unit that fits into the overall concept. Adrian Newey has brought the highest state-of-the-art aerodynamics to McLaren, and one would expect Illmor to follow his requirements. There cannot be much in it, and most of the other teams have now experimented with the top-exit exhausts, but not all have adopted them: two excellent handling cars, the Williams-BMW and the Minardi-Ford, being notable exceptions; neither car using an engine that revs as high as the Ferrari or Mercedes. It would not surprise me to see several teams revert to low exhaust in coming years, although as RPM rises still higher, the engines needs may finally dictate in favour of Ferrari's solution. We shall see.