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Race Engine Technology issue 052 : FEBRUARY 2011
FOCUS : EXHAUST SYSTEMS
Wayne Ward explains the crucial role played by exhaust systems in helping to maximise the performance of race engines
JON HILTON: Radical KERS for Le Mans 2011
THE COMMUNICATIONS HUB OF THE RACING POWERTRAIN WORLD LAMBORGHINI
V12 EXPOSE How a road masterpiece became a race winner
COMPETITION
EXHAUST Exploiting hot air for peak performance
CFD FOR RACE
ENGINES Can a computer-based analysis provide real answers?
FEBRUARY 2011 USA $20, UK £10, EUROPE e15 01
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Piping hot D
espite what some people in the race engine world may think, the subject of exhaust systems is a surprisingly interesting one. They have a critical bearing on the performance of the engine, where they play an extremely
important role in the engine’s gas exchange processes by controlling unsteady gas dynamics. In the early days of internal combustion engines, this
role was not clearly understood; exhaust systems were used simply as a duct to transfer the exhaust gas to a more convenient location before expelling it into the atmosphere. In the early 20th century, researchers such as Morse, Boden and Schecter (1) studied the strong effect that exhaust geometry has on engine performance. Today, it is accepted that the exhaust geometry must be optimised for a race engine to be effective. For highly optimised engine installations, as we might expect to fi nd in Formula One, the exhaust system design has been used not only to improve engine performance but also to infl uence vehicle performance, and to a surprising degree. Engineers today “stand on the shoulders of giants” – to borrow a phrase used by Isaac Newton, among others – and by “giants” I mean engineers of yesteryear, such as Hooker et al (2) working for Rolls-Royce in the UK and Pinkel and Voss (3) working for Vought-Sikorsky in the US, who in the 1940s discovered the extra thrust that could be developed for aircraft by properly directing exhaust fl ows.
In the late 1980s, Norton engineer Brian Crighton used the energy of the exhaust gas to draw more cooling air through the engine of the Wankel-powered Norton race bikes, using a principle similar to that of an ejector pump. More recently, Formula One has used blown diffusers to improve downforce at the rear of the car. The exhaust system, in terms of pure function,
is to remove the burnt gases to a more convenient location. However, its use as a performance development tool is considerable and it has a critical bearing on the shape of the torque curve as well as affecting peak power.
Let us briefl y consider a simple single-cylinder, four-stroke engine with a simple exhaust pipe. A pressure (compression) wave generated at exhaust valve opening travels the length of the exhaust pipe until it meets an expansion in area (that is, an open end, or a suddenly divergent section), at which point a rarefaction wave is refl ected back toward the exhaust valve. The easy point to remember here is that when a wave of any kind comes to the open end of a tube, it is refl ected back in the opposite sense – that is, a compression wave produces a refl ected rarefaction wave, and vice versa. When a wave hits a closed end of a pipe, however, it is refl ected back in the same sense, so a compression wave is refl ected back as a compression wave. The timing of the returning rarefaction wave is critical. If it arrives just before exhaust valve closing, it ‘sucks’ burnt charge out of the cylinder, aiding scavenging. If the rarefaction wave arrives at the exhaust valve at the wrong time though, it will be refl ected and may, if time permits, result in a compression wave arriving back at the exhaust valve just as it closes, pushing burnt charge back into the cylinder. When this happens, the content of the cylinder when both valves are shut contains a higher proportion of residuals. A further effect that also cuts performance is that cylinder fi lling is less effi cient in the case where the cylinder contains a greater mass of hot gas. When a wave encounters a tapered section
Fig. 1 – This novel ‘helical’ collector design is for a 4-1 exhaust system; 4-1 collectors are common in many forms of racing, from club-racing motorcycles to Formula One (Courtesy of BTB Exhausts)
rather than a simple opening, the effect is to broaden the range over which the tuning effect is experienced. This is the principle on which two-stroke expansion chambers operate,
although two-stroke engines are becoming less common in motorcycle racing, where they once dominated.
In a more sophisticated exhaust system, for example on a four-
cylinder engine, the situation with pressure waves is far more complex. When a compression wave arrives at a four-into-one collector, it has an effect on all the other cylinders. The superposition of all the waves acting in the system has to be considered, and is the reason why the different arrangement of pipes into a given collector affects performance. It also explains why linking pipes in different confi gurations, such as four-two-one or four-one has an effect 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 considering the propagation, refl ection and superposition of pressure waves. Vorum (4) proposes in his papers that the engine cylinder and exhaust pipe act as a Helmholtz resonator during the period where the valve is open.
Harsh environment The harshest aspect of the working environment for an exhaust system is what is on the inside, as in the extremely hot and high-pressure exhaust gases. In certain cases, namely turbocharged applications, engineers may actually want to encourage combustion within the exhaust system. In Top Fuel engines [Fig. 2], the cylinder pressure when the exhaust
valve opens is commonly higher than the peak cylinder pressure in a turbocharged engine of about 200 bar. Car packaging constraints often mean there is little cooling airfl ow to the exhaust, especially in enclosed cars such as single-seaters and endurance racers (Le Mans, ALMS). Certainly Formula One is a special case [Fig. 3], where tight packaging constraints and the attendant lack of cooling conspire with
Fig. 3 – This close-up of a Cosworth Formula One engine shows the
close proximity of the exhaust to the cam cover, coil beam and airbox (Courtesy of Cosworth)
very high mass fl ow rates, and most of the exhaust system glows red- hot under full-throttle conditions. Radiated exhaust heat can cause problems for other components – electronics in particular. The materials used for exhaust manufacture range from the very mundane to some very exotic and correspondingly expensive alloys. In these times of fi nancial restraint in particular, people are often looking for the material that will do the job at minimum cost. In many cases, mild steel will be the material of choice. It has
the advantage of being very widely available at low cost, and is not subject to the price fl uctuations that can affect more highly alloyed materials, especially those containing elements traded as commodities. The now common alloy surcharges that refl ect current market prices for some elements are one reason why mild steel has a safe future for exhaust manufacture.
Mild-steel exhausts do need attention though if they are not to corrode badly. Basic surface treatments such as painting can be very effective, but must be re-applied periodically. Other, harder wearing coatings can be used but at greater expense. The next step up from mild steel in terms of cost are the austenitic
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Fig. 2 – Top Fuel engines open their exhaust valves when the cylinder pressure is about 200 bar (Courtesy of Ron Lewis/Al-Anabi Racing)
052 contents • IN CONVERSATION:
SONNY LEONARD Anne Proffit talks to the man whose name has become synonymous with big-block engines
• DOSSIER: LAMBORGHINI V12 GT1
Ian Bamsey discovers how Breuer Technical Development created a winning GT engine from Lamborghini’s almost half- century-old V12
• MOTORCYCLE:
MOTO3 PACKAGING How can you get a 250 cc four-stroke single into a 125 two-stroke package? Neil Spalding finds out
• FOCUS: EXHAUST SYSTEMS Wayne Ward explains the crucial role played by exhaust systems in helping to maximise the performance of race engines
• MUSINGS: FORD AND
MOUNTUNE AT LE MANS As Ford re-enters the Prototype spotlight, Ian Bamsey considers Mountune’s Duratec-based LM P1 and LM P2 engines
• FOCUS: COMPUTATIONAL
FLUID DYNAMICS With CFD finding growing use in race engine design, Wayne Ward and Phil Virr give an insight into the technology
• EXPO: IMIS & PRI 2010 Ian Bamsey reports on the IMIS and PRI, where the global racing industry congregated in December
www.highpowermedia.com
ISSUE 052 race engine TECHNOLOGY FEBRUARY 2011
Lamborghini V12 GT1 • Moto3 packaging • Exhaust systems Focus • Ford and Mountune at Le Mans • CFD Focus • IMIS & PRI 2010
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