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| What was done to on-highway trucks from 2007 to 2010 is what is happening to off-highway engines from 2008 to 2015 (2014 for <750hp engines) as far as allowable emissions are concerned. I've studied it a little and would say the biggest difference is in the way the engines are tested for emissions compliance between on and off road engines. Off-road uses ISO 8178 test standard. They test at different engine loads at 2 different engine speeds---first is the RPM stated for rated engine power, second is RPM at peak engine torque. They weight each load rating/RPM test, giving more weight to the higher loads at rated engine speed.
On-highway is different, and likely much harder to achieve due to their test requirements. They run a test basically monitors actual usage across a large variety of situation (traffic driving, open road, etc...) The one thing to note is emissions are measured by grams per bhp-hr. The nature of use between a tractor and an on-highway truck would favor the tractor. A truck is very likely rarely using 90-100% engine load where a tractor will. The more power needed, the more air/fuel is pushed into the cylinder, the hotter the cylinder burn, the less particulate matter is created by the engine, and the more NOx is created. DEF can get rid of NOx, not so much PM.
In a tractor with SCR/DEF, the engine is ran at maximum performance using HPCR, turbo, etc... to create a very efficient, hot, clean burn. This reduces the PM to a very low rate, but creates more NOx. The DEF reacts with NOx to create nitrogen and water.
In a tractor with EGR, the engine doesn't get as hot or clean of a burn because of the non-oxygenated gasses being recirculated through the engine. This lowers NOx emissions, but creates elevated PM--and the need for a PM filter. With a highway truck not able to run at higher engine loads, I doubt they can get a hot enough or clean enough burn to meet PM emissions and at the same time, the EGR alone can't handle the NOx emissions when they do run at a high engine load (pulling up hill, starting off).
One other thing to note that I found interesting. An EGR engine requires extra cooling capacity due to the need to cool the EGR gasses, not because of the actual engine operation. The engine actually has a cooler combustion temp. | |
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Pickups without EGR?
