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Smaller engines and increased turbo boost put the ‘knock’ back into motoring


Engine knock is becoming more prevalent, says Bernie Rosenthal, CEO Reaction Design, as manufacturers downsize engines and increase turbo boost. The problem is further exacerbated with the increasing use of blended bio fuels and the emergence of dual fuel engines that run on a combination of diesel and natural gas. Best known for its work in modelling fuels


and combustion simulation with the Model Fuels Consortium (MFC), Reaction Design has used this know-how to intelligently reduce the master models’ 4,500 or so species down to 400-500 for the purposes of its knock predictor – “…and even that is 4-5 times more than what you could handle in a common computational fluid dynamics (CFD) programme,” points out Rosenthal “That’s a real differentiator for Forte handling that number of species in roughly the same time frame as other tools handle fifty or so. It’s really the ability to understand which sub-component of the fuel is affecting which phenomena and what is going on [in the combustion chamber].”


Pressure points Engine knock is all about having ignition where it wasn’t predicted and where it isn’t needed, says Rosenthal, and “that turns out to be a combination of pressure and spontaneous combustion, due to pressure and the fuel-air mixture auto-igniting in a position where it’s not helping the engine. “All the traditional CFD approaches from


other businesses relied on chemistry solver technology that was either quite antiquated or required them to really use simplified chemistry


CFD knock simulation


Flame front


16 14 12 10 8 6 4 2 0


Calculate knock intensity -20


-10 0 10 Crank angle (°ATDC)


Temperature


2500 2125 1750 1375 1000


Onset of knock


Spark plug


models to be able to get a turn-around time from the simulation. One of the key enablers for us was that we brought in Chemkin-Pro technology, which is a very advanced numerical solver programme, and some really good fuel models to the party.” For Rosenthal, traditional knock control is


“a little bit of a guessing game”, as sensors tend to react to the phenomena, rather than predicting and then preventing the knock. “There’s always a trade-off as you set up the knock control to prevent it. But, if you don’t have a good handle on where that is or why it’s happening, then you could be setting an excessive or too narrow a margin.” There was, recalls Rosenthal, a major


hurdle to overcome, as he explains: “There’s an 6 5 20 30 40


0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6


Virtual pressure sensors


7 8 1


42 3


area here that is non-intuitive, from a physics standpoint, called the ‘negative temperature coefficient region’ that occurs. As temperature increases, ignition time decreases, except there’s an area where it blends out, even though it inserts a longer delay. If you’re not accurately predicting this zone of pressure, then you’re unable to understand why that secondary flame front happens. This was one of the pieces that made it hard to do this until now – not only understanding what was going on, but really the modelling behind it.” Once that had been understood, Reaction


Design then developed a series of virtual pressure sensors, eight in all, while using digital signal processing techniques as a high path


Volvo raises the pressure on diesels


Volvo’s next-generation diesel engines will mark a significant step forward in fuel economy, performance and emissions, thanks to the adoption of combustion pressure sensors and the highest injection pressures so far seen on a production passenger vehicle. Applied to Volvo’s new-generation VEA


engine family, due to be revealed this autumn, the so-called i-ART diesel technology raises the injection pressure to 2,500 bar and adds a cylinder-by-cylinder pressure sensing system that allows extremely precise monitoring of each of the multiple injection pulses on every


piston stroke. “It’s the second step in the diesel revolution,” said Derek Crabb, VP of powertrain engineering at Volvo. “It is a breakthrough comparable to when we invented the lambda sensor and the catalytic converter in 1976.”


Extra dimension Speaking to Automotive Design, Crabb explained that the new developments gave engineers an extra dimension of control over the engine’s combustion processes. “The more you can get control over the key parameter, which on diesels is the injection,


the more it helps you get a better balance between fuel economy, emissions and performance. It gives you a lot more freedom and a much better chance of getting through future emissions standards.” The Denso common rail injector system


runs at an unprecedented 2,500 bar, which allows fuel to be injected faster, more precisely and with a better spray pattern. This, in turn, gives more complete combustion, reducing the loading on the after-treatment system and possibly allowing a simpler system to be fitted.


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www.automotivedesign.eu.com


May/June 2013


Pressure (MPa)


Band pass filtered pressure (MPa)


Knock Modeling in IC Eng ni es with FORTÉ


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