New emissions standards bring new questions. A farmer contacted me recently asking why tractors that use the selective catalytic reduction approach (with diesel exhaust fluid - or DEF) consume the fluid at a rate of 5% of fuel used which requires a DEF tank refill after every other fuel fill. He's been told that for his pickup - which has a 5 gallon DEF tank - he only needs to fill it at the normal oil change interval.
Great question. I turned to some folks at Agco to answer the question since they announced their use of DEF first for their Tier 3 engines and are moving ahead with that strategy as with interim Tier 4 machines and beyond.
Through an e-mail exchange, John Rogers, senior marketing specialist, Challenger tractors, offered some insight. He notes that for their Tier 3 emissions standards the DEF is consumed at a 3% rate, which is based on a heavy duty cycle and can actually be lower, which can give farmers as much as a 2.5 to 3 to 1 fill ratio.
Rogers does note, that for ag equipment you're talking about a much different duty cycle than with an on-road pickup. Tractors are heavy pieces of machinery and constantly under load, which means they're going to generate more oxides of nitrogen, requiring more DEF.
For the higher emissions standards that means more DEF into the system, and while Rogers didn't say what their interim Tier 4 standard was - manufacturers have been saying in the 3% to 5% range. He did say that the U.S. Environmental Protection Agency wants a 2 to 1 DEF to fuel ratio for interim Tier 4 engines, although this isn't required in the European Union.
Finally, Rogers did note that on-highway pickup diesels are not getting the mileage - in DEF - they were told they would get. Some of that information is anecdotal coming from Agco dealers with the newer pickups. Perhaps dealers, like farmers, are really making those heavy duty pickups work harder. The dealers I know are definitely pushing their vehicles hard to keep up with faster-moving farmers.
One interesting fact from Rogers is that he's found that farmers aren't waiting for that DEF tank to empty. He says growers with those systems area incorporating the fill of the DEF tank with their fuel fill to avoid running out of DEF.
As new emissions standards come on line, we'll all be learning about these practices in the real world. Thanks to the reader for the question and send along more questions when you have them to email@example.com.