Race Engine Technology issue 036 : FEBRUARY 2009
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FOCUS : EXHAUSTS Jack Kane looks at the state of the art in exhaust system technology John Judd: THE CHALLENGE OF TOURING CARS THE COMMUNICATIONS HUB OF THE RACING POWERTRAIN WORLD
AUSTRALIAN POWER Inside a V8 Supercar motor
EXHAUST
TECHNOLOGY Header state of the art
EDL K2000 The challenge of Super 2000
Sound Aand fury
ll too often the engine exhaust is an afterthought for the engine and chassis builders, yet its design and construction impacts significantly upon car performance. The exhaust system can be a vital tool for optimising the performance of the engine, through the way in which its design manipulates the pressure waves that can crucially assist cylinder filling and scavenging. On the other side of the coin, the exhaust system presents many challenges. It is a major loss-path for thermal energy; and it can be a car-packaging nightmare. The environment that a competition exhaust system, and particularly engine header pipes, must survive can only be described as a brutal combination of temperatures, stresses, corrosion and vibration. Contemporary exhaust technology can help reduce the
Figure One: The Harsh Environment FEBRUARY 2009 USA $20, UK £10, EUROPE e15
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increasing aperture. The instantaneous velocity of the exhaust gas flow at any point is determined by the pressure gradient and the cross-sectional area at that point. In the header, a smaller tube diameter will increase the velocity at a given RPM, which might enhance the pressure wave tuning (the second component, discussed below). However, if the diameter is too small, there will be flow losses and consequent pressure gradient increases, which can offset any tuning gains. So the selection of proper tubing diameters is an important part of the design. In the early part of the exhaust cycle, the pressure difference across
the valve is high, so the instantaneous gas particle velocity through the small exhaust valve aperture is very high. Sometime past mid-exhaust stroke, the majority of the exhaust gas has left the cylinder. At that time, the valve aperture area is quite large and the cylinder pressure is approaching atmospheric, which causes the instantaneous particle velocity across the valve to be much lower. It is at that phase of the exhaust cycle where the second component becomes important. To help with the explanation of the second component, Figure Two
shows traces of in-cylinder pressure (black), port pressure at the intake valve (light blue) and port pressure at the exhaust valve (red), taken from a simulation of a high BMEP engine operating near the optimum tuning point for both intake and exhaust. When the exhaust valve opens, the large pressure differential across
problems and help to maximize the potential gains of the system. I find it interesting, having spoken to several highly-placed and well recognized experts in this field, that while there is general agreement about what features cause improvements to happen, there are varying opinions about the reasons why those improvements occur.
EXHAUST BASICS The computation of what actually goes on during an exhaust cycle is a highly complex problem in compressible fluid flow, the details of which are explained in detail in several texts, my favourite being Professor Gordon Blair’s Design and Simulation of Four Stroke Engines. For the purposes of this article, the following overly-simplified explanation will serve to illustrate the principles. There are two separate components to the exhaust event. The first is the removal of exhaust gases from the cylinder, which occurs as a pulse of hot gas exiting the cylinder and flowing down the header primary tube. The second is the (much faster) travel of the pressure wave along the exhaust pipe caused by the pressure spike, which occurs when the exhaust valve opens, and the various reflections of that wave. Taking proper advantage of these pressure waves (component two) can produce dramatic improvements in clearing the cylinder (component one) and can strongly assist the inflow of fresh charge. Considering component one, when the exhaust valve first opens
in a four-stroke piston engine, the in-cylinder pressure is still well above atmospheric. In a naturally-aspirated spark ignition engine burning gasoline and operating at high BMEP, the pressure can be 7 bar or more, and the pressure in the exhaust port at the valve is somewhere near 1 bar (atmospheric). As the valve opens, the pressure differential across the rapidly-changing valve aperture (pressure ratio of approximately 7) starts exhaust gas flowing through the small-but-
the valve causes a rapid rise (a “spike”) in the port pressure behind the valve. The second component is the result of the effects of the pressure “spike” which occurs at EVO, shown by the peak in the red line in Figure Two, just after EVO. That pressure spike, or pressure wave, moves down the pipe at the sum of the local sonic velocity plus the particle velocity of the gas flow. Whenever the pressure wave encounters a change in cross-sectional area of the pipe, a reflected pressure wave is generated, which travels in the opposite direction. If the change in area is increasing (a step, a branch, a collector, or the atmosphere), the sense of the reflected pressure wave (compression or expansion) is inverted. If the change in area is decreasing (the end of another port having a closed valve, or a turbocharger nozzle, for example), the sense of the reflected wave is not inverted. The amplitude of the reflected wave is primarily determined by the proportionate change in cross-sectional area (area ratio), but the amplitude is diminished in any case. For purposes of approximation, the particle velocity can be ignored because its effect is essentially self-cancelling during the round-trip of the wave. However, highly- accurate simulations must take it into account. These waves are sometimes called finite amplitude waves, because their amplitude is such that they generate significant particle velocities, unlike sound waves, where the pressure amplitude is so small that the resulting particle velocity can be assumed to be zero. In the case of the currently-flowing header primary, the EVO- initiated positive pressure (compression) wave is reflected back as a negative pressure (expansion) wave. If the arrival of the reflected negative pressure wave back at the exhaust valve can be arranged to occur during the latter part of the exhaust cycle, the resulting lower pressure in the port will enhance the removal of exhaust gas from the cylinder, and will reduce the pressure in the cylinder so that when the
Figure Two: Pressures, On Tune (from Engine Analyzer Pro)
intake valve opens, the low pressure in the cylinder begins moving fresh charge into the cylinder while the piston is slowing to a stop at TDC.
Note in Figure Two, how the cylinder pressure (black) and exhaust port (red) pressures go strongly negative from approximately mid- exhaust stroke to TDC. Note also how the second-order reflected positive pressure wave in the intake tract (light blue) reaches the back of the intake valve just before IVO, and works together with properly- timed exhaust negative pressures to begin moving fresh charge into the cylinder.
If, on the other hand, the negative exhaust pressure wave arrives at a non-optimal time, its effects can be detrimental to the clearing of the cylinder and ingestion of fresh charge. A reflected positive wave during overlap (from a turbocharger nozzle, for example) can push a large amount of exhaust gas back into the cylinder and the intake system. Figure Three shows the same three pressure traces when the engine is operating well above the intake and exhaust tuning points. In addition to reduced breathing efficiency, note the additional pumping losses from the higher cylinder pressure in the latter portion of the exhaust cycle, caused in part by the late arrival of the reflected negative exhaust pulse. The timing of the arrival of the negative wave at the back (port) side of the exhaust valve is determined by the engine RPM, the speed of sound in the pipe and the distance from the valve to the relevant change in area. Those three factors will cause the exhaust tuning to come in and out of tune over the engine operating speed range. Sophisticated designs can produce systems having more than one tuning point. The most significant example of exhaust pulse tuning is dramatically demonstrated by the operation of crankcase-scavenged, piston-ported two-stroke engines. At the relevant tuning distance from the exhaust valves, the primary tubes from two or more cylinders are often joined together into a larger collector tube, which provides the area increase to generate the reflected waves described above. Using a four-into-one system as an example, the four primary tubes
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036 contents
• upFRoNt: GRID We discuss the arrival of a new Ford NASCAR Cup V8, changes in Formula One, in Prototypes, in Sprint Cars and more
• DoSSIeR:
WaLKINSHaW RacING v8 The biggest form of racing down under is V8 Supercars, we look at the GM/Holden contender
• FocuS: pIStoN pINS Call them ‘gudgeon pins’ or ‘wrist pins’; they play a vital role in the race engine
• eXpo: pMWe & pRI Two major industry trade shows wrapped up 2008
• MotoRcYcLe: BMW As World Superbike racing anticipated the arrival of the Munich marque, Neil Spalding enjoyed a sneak winter preview…
• FocuS: eXHauStS Jack Kane considers the technology of a hot and bulky yet all too often overlooked key to race engine performance
• DoSSIeR: eDL K2000 EDL doesn’t only do pure race engines. We look at the production-based Super 2000 I4
• SpecIaL INveStIGatIoN:
vaLve SpRINGS (pt 2 oF 4) Professor Blair and his learned associates continue their detailed and revealing investigation of coil spring behaviour
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ISSUE 036 race engine TECHNOLOGY FEBRUARY 2009
WR HOLDEN V8 SUPERCAR • EDL K2000 • PISTON PINS • EXHAUSTS • VALVE SPRINGS (2) • BMW WSB • PMWE • PRI
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