Saturday, 20 April 2013
Daf, Paccar link for CGI 11-litre
Daf and Paccar have joined forces to manufacture a new 11-litre diesel engine with a compacted graphite iron (CGI) cylinder block and head.
Production of the new engine begins this autumn at Daf’s plant in Eindhoven, the Netherlands where it will be built alongside the long-serving Daf 9.2-litre engine and the 12.9-litre MX-13 engine.
Just how long the 9.2-litre engine will remain in production remains to be seen; certainly, the arrival of the MX-11 will provide Daf with a golden opportunity to remove its ‘old timer’, cut costs and upsize its products where appropriate.
North American versions of Daf’s MX-13 have been built since 2010 at the new and advanced plant of Daf’s parent, Paccar Inc., in Columbus, Mississippi.
According to Ron Borsboom, Daf Trucks product development director, interviewed in the April issue of Commercial Vehicle Engineer, the new engine’s cylinder heads and blocks are cast in Brazil and shipped to Columbus where they are machined before being sent to Eindhoven for the final manufacturing stages.
Although not disclosing the vendor by name on this occasion, the cylinder blocks and heads in CGI material are most likely sourced from Tupy SA in Joinville, Brazil, the leading foundry for the supply of this specialised type of grey iron.
The new components will prove a welcome volume addition to the foundry with Daf fitting the new Euro 6 diesel engines into its popular CF tractive unit, seen recently at the commercial vehicle exhibition in Birmingham, UK.
According to Commercial Vehicle Engineer, engineers at Daf Trucks’ base in Eindhoven and at Paccar’s head office are not the first to conclude that a swept volume of slightly less than 11 litres is ideal for European truck and bus engines of outputs in the 300 to 450bhp range.
Volvo has the 10.84-litre D11 – used also in the group’s Renault trucks as well as Volvo trucks, buses and marine applications. MAN has the 10.5-litre D20 (which appears in a slightly different guise as the MaxxForce 11 engine from Navistar in North America), and Daimler’s 10.7-litre OM470 which went into production last year.
Use of higher-strength CGI material for heavy components such as the block and head have allowed Daf engineers to shave a noteworthy 180kg from the MX-11 compared with the MX-13 engine. This is more than enough to compensate for the kerb weight added for Euro 6 exhaust after-treatment equipment.
It is one of the reasons, according to Commercial Vehicle Engineer , that the MX-11 is being offered in the XF tractor unit as well as the CF. It is thought as many as 25 per cent of XF operators will choose the MX-11 in preference to the heavier weight MX-13.
Although the MX-11 is a lighter engine, it remains, at 1,077kg, heavier than the 990kg claimed for the Mercedes OM470 engine, said to be the lightest Euro 6 truck diesel engine in its class.
Further input of North American experience (in addition to block and head machining) can be found in a Cummins Emissions Solutions exhaust after-treatment system comprising SCR (selective catalytic reduction), DPF (diesel particulate filter) and DOC (diesel oxidation catalyst) to meet Euro 6 emission limits which come into force on 1 January 2014.
Meanwhile, Paccar’s state-of-the-art manufacturing plant in Columbus has been charged with machining the MX-11 CGI components because it can bring the latest machining processes to bear for this specialised material.
Somewhat more difficult to machine than conventional grey irons, some process engineers have been challenged by the slower machining times and special cutting tools need for this material.
While at the moment this brings on-costs for European assembled engines, the longer term bonus will come when the MX-11 is assembled in North America for use in the region and elsewhere.
However, such drawbacks associated with machining tough CGI, if there are any, are more than outweighed by the improved performance which CGI can bring for diesel engineers.
The high brake mean effective pressures (bmep) demanded of an engine of this size at up to 440bhp is a central consideration in the design of both the cylinder head and the cylinder block being made of higher tensile strength CGI, long-favoured by Daf. Indeed, it is hard to imagine Daf and Paccar taking a backward step to conventional grey iron.
Peak cylinder pressures, while not being disclosed at this stage, are higher in the MX-11 than in any Daf engine thus far – perhaps up to 250bar according to Commercial Vehicle Engineer – and may explain which design life of this engine is 1.2 million km compared with 1.6 million km for the MX-13.
Considerable design effort has been directed at making the key components as stiff as possible – in this respect the head and block have been assisted by the choice of CGI – as this material assists with engine noise reduction.
While the MX-11 shares almost the same fuel injection, air management and after-treatment technology with the MX-13, there are some notable differences quite apart from cylinder head and block. These include pistons, crankshaft and the twin overhead camshaft configuration. There is also an inlet manifold forming an integral part of the head.
For fuelling, Daf continues to use Delphi’s common rail system; the exception being the pump generating fuel pressure in the rail is driven by a short camshaft mounted low in the block. Maximum fuel injection pressure is 2,500bar.
At the heart of the air management system is the latest sliding-nozzle variable geometry turbocharger from Cummins Turbo Technologies. This matches with cooled exhaust gas recirculation, a back-pressure valve linked to the exhaust after-treatment system and the Jacobs engine brake.
Exhaust temperature is an important element in keeping after-treatment working at the optimum level. The shift to a smaller engine assists in this respect because it is easier to maintain exhaust temperatures high with a small engine working hard than with a larger engine that is less challenged.
“Just as with the 12.9-litre MX-13 engine, the aim is not only to achieve the right exhaust gas composition, but also the right temperature in the soot filter,” said Borsboom. “The basic principle is to have as much passive regeneration of the soot filter as possible by getting the engine to create the ideal circumstances for this to happen.”
He added: “That is why the exhaust manifold and key parts of the exhaust system have been encapsulated. If, despite this the temperature of the exhaust gases drops too low due to a specific vehicle application, the engine will switch to active regeneration. A seventh injector, placed behind the turbo and in front of the soot filter has been added to the engine for this purpose. It can inject fuel into an oxidation catalytic converter in the exhaust to generate the required amount of heat. The SCR had also been designed to maintain optimum temperature. It can, through a smart coating, achieve maximum performance across a wide range of temperatures, which means the engine can function optimally and in the most efficient way.”
Commercial Vehicle Engineer concludes: ‘Meanwhile, it should not be forgotten that of the 50,000 or so diesel engines produced annually at Eindhoven, at present about 10 per cent go to third-party (non-Paccar customers), mainly bus and coach manufacturers such as VDL. The MX-11 would seem, on the face of it, to have huge potential to boost those third-party sales figures greatly, though no horizontal version has been developed and it seems there are no plans to do so. ∎