Engine experts say choosing between the two technologies might be a moot point by 2014, when engine manufacturers must face a final round of emissions reductions. Tier 4 Final calls for near-zero levels of NOx and PM.

“The major change for Tier 4 Final is a further reduction in NOx, which may require all manufacturers of engines below 750 hp to use both SCR and cooled EGR with DPFs,” says David Combs, senior sales manager with MTU Detroit Diesel, which sells Mercedes engines for off-highway use.

Tractor makers agree that both technologies will likely be required to meet final regulations. However, all of them report that they are fervently pursuing other technologies that may reduce the burden of having to use both solutions.

“We’ve got three more years before we have to make a final decision on precisely what technology we will use,” says Case IH’s Tom Dean. “And there are new technologies coming out all the time that can help us achieve what we need to, but we’re confident the final solution will include key elements of the SCR approach we’ve introduced now for 2011.”