Wayne Ward explains the crucial role played by exhaust systems in helping to maximise the performance of race engines Piping hotMEDIA. espite what some people in the race engine world may is to remove the burnt gases to a more convenient location. However, think, the subject of exhaust systems is a surprisingly its use as a performance development tool is considerable and it has interesting one. They have a critical bearing on the a critical bearing on the shape of the torque curve as well as affecting performance of the engine, where they play an extremely POWERpeak power. Dimportant role in the engine’s gas exchange processes by Let us briefly consider a simple single-cylinder, four-stroke controlling unsteady gas dynamics. engine with a simple exhaust pipe. A pressure (compression) In the early days of internal combustion engines, this wave generated at exhaust valve opening travels the length role was not clearly understood; exhaust systems were of the exhaust pipe until it meets an expansion in area used simply as a duct to transfer the exhaust gas to a (that is,ONLINE an open end, or a suddenly divergent section), more convenient location before expelling it into the at which point a rarefaction wave is reflected atmosphere. In the early 20th century, researchers suchHIGH back toward the exhaust valve. The easy point to as Morse, Boden and Schecter (1) studied the strong REPUBLISHEDremember here is that when a wave of any kind effect that exhaust geometry has on engine performance. ORcomes to the open end of a tube, it is reflected back Today, it is accepted that the exhaust geometry must be in the opposite sense – that is, a compression wave optimised for a race engine to be effective. produces a reflected rarefaction wave, and vice versa. For highly optimised engine installations,OF as we might BE When a wave hits a closed end of a pipe, however, it is expect to find in Formula One, the exhaust system design reflected back in the same sense, so a compression wave is has been used not only to improve engine performance but reflected back as a compression wave. also to influence vehicle performance, and to a surprising The timing of the returning rarefaction wave is critical. degree. Engineers today “stand on the shoulders of If it arrives just before exhaust valve closing, it ‘sucks’ giants” – to borrow a phrase used by Isaac Newton,TO burnt charge out of the cylinder, aiding scavenging. among others – and by “giants” I mean engineers PRINT If the rarefaction wave arrives at the exhaust valve of yesteryear, such as Hooker et al (2) working for at the wrong time though, it will be reflected and Rolls-Royce in the UK and Pinkel and Voss (3) may, if time permits, result in a compression wave working for Vought-Sikorsky in the US, who in arriving back at the exhaust valve just as it closes, the 1940s discovered the extra thrust that could IN pushing burnt charge back into the cylinder. be developed for aircraft by properlyNOT directing When this happens, the content of the cylinder exhaust flows. when both valves are shut contains a higher In the late 1980s, Norton engineer Brian proportion of residuals. A further effect that also Crighton used the energy of the exhaust gas to cuts performance is that cylinder filling is less draw more cooling air through the engine of efficient in the case where the cylinder contains thePROPERTY Wankel-powered Norton race bikes, using a greater mass of hot gas. a principle similar to that of an ejector pump. When a wave encounters a tapered section More recently, Formula One has used blown rather than a simple opening, the effect is diffusers to improve downforce at the rear of Fig. 1 – This novel ‘helical’ collector design is for a 4-1 to broaden the range over which the tuning the car. exhaust system; 4-1 collectors are common in many effect is experienced. This is the principle on forms of racing, from club-racing motorcycles to Formula The exhaust system, in terms of pure function, One (Courtesy of BTB Exhausts) which two-stroke expansion chambers operate, 36 FOCUS : EXHAUST SYSTEMS Fig. 2 – Top Fuel engines open their exhaust valves when the cylinder pressure is about 200 bar (Courtesy of Ron Lewis/Al-Anabi Racing) MEDIA. POWER although two-stroke engines are becoming less common in motorcycle Fig. 3 – This close-up of a Cosworth Formula One engine shows the racing, where they once dominated. close proximity of the exhaust to In a more sophisticated exhaust system, for example on a four- ONLINEthe cam cover, coil beam and airbox (Courtesy of Cosworth) cylinder engine, the situation with pressure waves is far more complex. When a compression wave arrives at a four-into-oneHIGH collector, it has an effect on all the other cylinders. The superposition REPUBLISHED of all the waves acting in the system has to be considered, and is the OR reason why the different arrangement of pipes into a given collector affects performance. It also explains why linking pipes in different configurations, such as four-two-one or four-oneOF has an effectBE on top- end performance and the character of the torque curve. [Fig. 1 shows a novel four-one collector.] This is not to say though that exhaust tuning is a simple matter of very high mass flow rates, and most of the exhaust system glows red- considering the propagation, reflection and superposition of pressure hot under full-throttle conditions. Radiated exhaust heat can cause waves. Vorum (4) proposes in his papers that theTO engine cylinder and problems for other components – electronics in particular. exhaust pipe act as a Helmholtz resonator during the period wherePRINT the The materials used for exhaust manufacture range from the very valve is open. mundane to some very exotic and correspondingly expensive alloys. In these times of financial restraint in particular, people are often Harsh environment looking for the material that will do the job at minimum cost. The harshest aspect of the working environment for anIN exhaust system In many cases, mild steel will be the material of choice. It has is what is on the inside, as in theNOT extremely hot and high-pressure the advantage of being very widely available at low cost, and is not exhaust gases. In certain cases, namely turbocharged applications, subject to the price fluctuations that can affect more highly alloyed engineers may actually want to encourage combustion within the materials, especially those containing elements traded as commodities. exhaust system. The now common alloy surcharges that reflect current market prices In Top Fuel engines [Fig. 2], the cylinder pressure when the exhaust for some elements are one reason why mild steel has a safe future for valvePROPERTY opens is commonly higher than the peak cylinder pressure in exhaust manufacture. a turbocharged engine of about 200 bar. Car packaging constraints Mild-steel exhausts do need attention though if they are not to often mean there is little cooling airflow to the exhaust, especially in corrode badly. Basic surface treatments such as painting can be very enclosed cars such as single-seaters and endurance racers (Le Mans, effective, but must be re-applied periodically. Other, harder wearing ALMS). Certainly Formula One is a special case [Fig. 3], where tight coatings can be used but at greater expense. packaging constraints and the attendant lack of cooling conspire with The next step up from mild steel in terms of cost are the austenitic t 37 FOCUS : EXHAUST SYSTEMS Fig. 4 – This stainless steel system is used on an choice for many forms of racing, including NASCAR. historic sportscar, but stainless remains a popular choice, even in NASCAR (Courtesy of Burns Stainless) Fig. 4 shows a stainless system on an historic sportscar. Titanium is a popular material for exhausts and is used especially widely in motorcycle racing [Figs. 5 and 6], where the mass of the system is well supported. Titanium’s great advantage compared with the other structural exhaust materials is its low density. A typical engineering titanium alloy, Ti-6Al-4V has a density of 4.4 g/cm3, where steels are commonly about 7.8 g/cm3 and many other alloys more dense thanMEDIA. this. Many exhaust companies use low- alloy titanium materials, but one company that specialises in titanium exhausts admits to using more highly alloyed and specially developed ‘alpha’ titanium materials, which include elements such as silicon, aluminium, niobium and iron. Titanium, however, has a couple of disadvantages. The first is that it can become brittle with use, owing to oxidation at high temperature, Fig. 5 – This titanium exhaust system is and can exhibit cracks after a relatively short time. The second from a motorcycle racing application. Bike problem is one of time-dependent strain, or creep. This well-known exhausts are well supported, so titanium finds wide appeal from road bikes to phenomenon is most often demonstrated by measuring strain due to MotoGP. Note link pipes connecting headers a constant load over time, and is particularly associated with high- (Courtesy of Yoshimura) POWERtemperature operation of components. Exotic materials Superalloys (see below) are often developed for high strength/ stiffness at elevated temperatures as well as resistance to creep. For an unsupported exhaust system,ONLINE such as we might find in Formula One – where exhausts are commonly fastened only to the cylinder head – HIGH titanium is seen as a non-starter owing to creep, especially given the REPUBLISHEDvery high temperature exhaust. Some companies continue to look at titanium materialsOR that are more capable at temperature for exhaust use, but cost is prohibitive (we should remember that titanium is quite stainless steel grades, and again these are widely available in tube happy as an exhaust valve material in many cases).