Call it an earthquake, call it Big Bang for truck makers, call it whatever you like, but it all started with the announcement of an 'unbinding Memorandum of Agreement' struck between Navistar and Caterpillar. The big news here is simple. Caterpillar is out of the on-highway business. We say big news but this is probably an exaggeration; big but very well-trailed news is probably more appropriate. We have known that something was going to happen with reference to Caterpillar and its on-highway truck engine business ever since the US truck manufacturing industry started to verticalise. The shift to this model leaves independent engine suppliers out in the cold; DTNA would rather sell DDC engines, VTNA would certainly prefer that its products are delivered c/w a Volvo driveline, and PACCAR is rather hoping that its customers will favour the MX unit after EPA 10.

Then up pops the news that DTNA has added Cummins to its engine list for EPA 10. No surprise here insofar as the US truck operator has always asked for a choice of engines. But in as much as the OEMs are desperate to verticalise, and to regain margins from suppliers, this does seem an odd move.

But the real devils exist within the detail. Cummins has proposed an EGR-based engine for EPA 10, and makes a very good business case for doing so. Offering an EGR solution is a move that is utterly, implacably at odds with Daimler's global mindset, which opines that SCR is the way ahead as we move towards a single, global emissions protocol.

This is not an issue of one technology type over another. SCR and EGR are merely different routes towards the same destination, which is legislative compliance. Neither solution can be judged fairly until both have been trialled in a real world, legally-compliant environment by those who are the ultimate arbiters of such things: namely the people who write checks for the same. But enthusiasm for SCR is now obviously on the wane. Let us recap for a moment.

DTNA, AB Volvo and PACCAR are all wedded firmly to a rapid, coherent rollout of SCR within North America post 2010. This is not merely a market-specific strategy, but one that lies at the core of all three companies' global strategic positions moving forwards. All three might have been expected to have come out of the blocks with an enthusiasm for SCR that verges on the evangelical. And yet two of the three - PACCAR remains typically reticent - are now offering the message that other technology types are available. BTW, if anyone still doubt that Cummins remains the big player here, the news that Navistar has added it to the International line up should dispel such thoughts, and should immediately get tongues wagging about the strength of the International - MAN relationship. But we digress.

US truck operators - as with their counterparts in Europe - are faced with an operational environment that is challenging at best. Confronted with two technology types, they are likely surely to opt for the tried and tested, which, in US terms, is EGR. This is not a case of one being better than the other, but of one being more familiar than the other. SCR is a set of letters, EGR is an engine type, and until that changes, then it seems reasonable to assume that it is towards the latter that buyers will be looking.

In order to underpin sales of SCR-equipped trucks, the SCR- supplying OEMs need to confront the vexed question of infrastructure. NO DEF in the tank means reduced torque, disgruntled customers and longer journey times. In a fragile operational environment, this is a risk, and truck buyers are risk averse.

But the message that is now being sent to the likely providers of a DEF infrastructure is one along the lines of 'If you build it, they may come'. The OEMs would very much like their customers to buy SCR-equipped trucks, but, just in case they don't feel like doing so, a competing technology is available. How will the potential infrastructure providers feel about managing this risk? Probably about as equivocal as the SCR-centric OEMs now appear to feel about their core technology. In 18 month's time, there needs to be an infrastructure in place for the supply of DEF throughout North America. The fact that both Volvo and Daimler are now offering an alternative technology type suggests that both OEMs feel that either there won't be, or that demand for such an infrastructure will be muted at best.

This seems like the death knell for SCR at EPA 10. But, at a more fundamental level, it also seems like the moment in which a huge shift takes place. For the past decade or two, the argument has been one of the EU model of truck manufacturing - vertical and internal - raining supreme. Captive is good.

No more. The structural margin improvements that might accrue from such a model now appear to have been placed on one side. Analysts will no doubt take note, and will rate accordingly. The comfortable accepted model that has defined the truck industry for the past few years is now a thing of history. Quite what will come in its place remains to be seen.