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Banking on the

25th January 2007
Page 24
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Scania claims no after-treatment for its XPI engine. Ivan Stjernqvist talks to Scania's head of Engine Development Jonas Hofstedt.

Jonas Hofstedt does not try to hide his enthusiasm for Scania's latest engine. "We achieve Euro-S with a clear margin and fuel economy is unaffected. And we manage without after-treatment at all power levels," he says. "Effective breathing with electronically controlled variable-geometry turbo and common-rail multi-injection with 2,400 bar is the secret behind the XPI engine's promising results."

Throughout our interview and exclusive walk around the engine laboratory at Scania's Technical Centre in SOdertalje, Hofstedt is unable to disguise his pride in the new engine platform,which will succeed all Scania's present engines from the small five-cylinder ninelitre, through the six-cylinder 12-litre 'volume engine' to the legendary 16-litre V8.

-Our strategy has been clear.Hofstedt says. "After-treatment is undesirable as long as sufficient improvement in the combustion can be obtained. However, we don't rule out the need to combine an optimised combustion with, for instance, an SCR catalyst, an oxidizing catalyst or a particulate trap."

XPI stands for Extra High Pressure Injection and refers to the new common-rail injection, developed and produced in co-operation with Cummins. It is the first time Scania has used common-rail injection technology.

The present engine range uses electronic unit injection, either from Bosch (PDE) or Cummins-Scania (HPI).The change in injection equipment has demanded some modifications to engine design but not to engine size. Were there other reasons for the decision to build a completely new engine?

"Currently our engines are working with a combustion pressure of approximately 180 bar. There will be an increase with the new engine." says Hofstedt, "not so much because of the emissions but because of demands for higher efficiency in the form of fuel economy.

"We will increase swept volume a little in all engines how much, and whether it will he in the stroke or bore, we aren't ready to reveal.

-In any case, with the new engine design we're preparing for increased demand well into the future.

"The new XPI engine generation is cast and machined partly in Sodertalje, where we've just converted our old coal-fired ovens into a modern electro-foundry. and partly in Sao Paulo. Brazil, where the five-cylinder block is cast and machined. We've had success with trials of CG I casting techniques, but have gone back to ordinary casting because it has been shown to be good enough.

"Otherwise, the basic engine design and construction are unchanged, so that the machining can be carried out on the current engine line."

Separate cylinder heads Scania is the only engine manufacturer to retain the principle of one cylinder head for each cylinder, so has standardisation gone too far? Daf, Iveco. MAN, Volvo and DaimlerChrysler, for instance, use a large cylinder head as a reinforcing element, integrating the intake manifold into the cylinder head.

-This question has been turned over many times in this building,says Hofstedt. "There are arguments for and against a full-length cylinder head, but with the camshaft location we've chosen, we are free to choose.

"We have neither leakage nor strength problems in the cylinder block and when all advantages and disadvantages were totalled up, the choice was quite easy. For service reasons, our marine engine department is extremely pleased that we decided to go for separate cylinder heads.

"High injection pressure at all engine speeds is paramount for keeping particulate formation to a minimum, so we went for common-rail injection — which we'd already been developing with the American CumminsScania company," Hofstedt explains as we make our way through the corridors and test XPI engines are being tested everywhere, and there are no restrictions on what we are shown or can ask about.

For example, we see what happens when a small pilot injection is added 600 before top dead centre, which a technician then manipulates.The effect is immediate —with full pilot,engine noise goes down immediately.

"It's one of the main reasons we've chosen common-rail," says Hofstedt. -We can juggle with up to five injection pulses to each combustion, and we achieve fantastic results. With the pilot injection we could eliminate virtually every engine noise, but that would reduce efficiency, and good fuel economy has the highest priority in this building.

"Engine noise is only dampened enough to meet the rules.

"The common-rail high-pressure pump has two cylinders (three on the Vs), and the pressure is managed lightning fast, so that we do not work with higher pressure than necessary — but 2,400bar can always be supplied immediately." As well as the 'rail', with pressure pipes for each individual cylinder head, there is also the big variable-geometry turbocharger (VGT). The similarity to American EPA 2007 truck engin es with high EGR is striking.The compact turbo has a big control unit in the middle which indicates that it is a 'sliding-vane turbo', as used by Holset.

"The VGT has a key role in the gas exchange by facilitating a high EGR rate," says Hofstedt. -It enables the flow of exhaust gases to be redirected to the intake side without losing energy.The air density has to be high, and this is obtained via efficient charge-cooling.

"At the same time, the increased EGR amount has to be cooled as it's moved across to the inlet side.

"All this puts big demands on the turbo, which with fast and precise electronic management controls the VGT's boost pressure. The VGT has the benefit that engine brake power will increase consequentially."

The increased EGR also puts new demands on the engine's cooling system, as it has to dissipate much more heat.This will have been addressed by the time the new XPI engine generation is installed in the whole range.

Product launch

Scania has long said that the XPI engine generation will come to the market in good time for Euro-5 and the first sightings are expected to be at Amsterdam's RAI Show in October, starting with a 12-litre unit.

The XPI engines will pass directly to Euro-5 certification, which in relation to Euro-4 will mean a still lower NOx level (2.0g/kWh) and an unchanged PM level (0.02g/kWh).

Scania reaches the low NOx content by means of high EGR, while the low PM content is achieved with an injection pressure of 2,400bar combined with multi-injection common-rail. There is no oxidising catalyst or PM cat to clean up, nor any form of active or passive PM trap—only a new patented silencer, intended to contribute to better fuel economy and possibly recover "the good, old Scania engine sound".

-Our first test drives confirm that assumption, but there still remains another series of adjustments and field tests." Hofstedt concludes. •


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