In a recent post I extolled the benefits of running even a large vehicle on biodiesel. I focused mostly on the CO2 emissions, since that is the big buzz word, and the commonly known greenhouse gas.

I failed to discuss other emissions from such an engine and fuel combination. I’d like to talk about that now.

First, the good news. For all emissions excepting NOx, biodiesel burns cleaner than petroleum diesel, and in many cases cleaner than gasoline. You can see exactly how much so in this summary from the biodiesel board, which uses the EPA’s own research as it’s source.

Now, the bad news. Burning biodiesel does not help mitigate it’s high NOx emissions. Actually, the research has yet to prove if biodiesel noticeably changes NOx emissions either for better or for worse as compared to petroleum diesel. However, the fact remains that NOx emissions for a diesel engine is drastically higher than that of a similar output gasoline engine.

Why? Well, I’m not a scientist, and I can’t present my own measurements and findings, but I can tell you is that my research has told me that NOx is more-or-less a byproduct of heat, and compression in the context of an internal combustion engine. Unfortunately, I can’t find any single resource to link you to that summarizes it elegantly and succinctly (except maybe this), so you’ll have to take me at my word, or do the research yourself.

With that said, it becomes readily apparent why diesel engines, regardless of the fuel they are consuming, would have a higher NOx emission. A diesel engine depends upon heat and compression to ignite the fuel, so it is by nature going to have higher NOx emissions. Using biodiesel should not significantly change this, since it’s still being ignited in the same way.

Now, not all is lost the Tier 2 Emissions standard that the EPA is imposing in 2009 addresses NOx emissions, and NOx is in-fact one of the primary reasons for the new standard. In order to meet these standards manufacturers are using all sorts of different strategies. Some of them are new and innovative like urea injection and “self cleaning” particulate filters. Others have been around for years in gasoline vehicles, like exhaust gas recirculation (EGR) and Catalytic Converters. While these new emissions standards and equipment do pose a bit of a challenge for those manufacturing performance and aftermarket equipment for these vehicles, overall I think it’s a step in the right direction. Besides, many companies already have performance tuners and exhaust systems for the 6.7L Cummins diesel, and the 6.4L Ford Power Stroke, both of which meet the stringent new Tier 2 emissions standards.