The Environmental Trade-Off: Diesel Emissions Standards vs. Fuel Consumption
There is great concern in the United States for more environmentally friendly motor vehicles. It
follows that this would lead to vehicles which not only burn fuel with decreased emissions, but
also can do so while achieving better fuel economy. It seems only logical that if burning fossil
fuels is such an environmental concern, that having vehicles which consume less fuel per mile
driven would be an important goal from an environmental stand point. However, in the world of
diesel emissions standards, fuel consumption and emissions have an inverse relationship.
In October of 1997, the Federal Environmental Protection Agency (EPA) became proactive in
developing a set of new emissions standards for heavy-duty diesel engines to be implemented in
2004. The main concern in developing these standards was to lower the Nitric Oxide and
Nitrogen Dioxide (NOx) emissions from heavy-duty on-highway diesel engines. The goal was
to have heavy-duty diesel NOx to approximately 2.0 g/bhp/hr1. This was implemented using
Exhaust Gas Recirculation Technology, known as EGR. This initially was a very effective
technology, getting the NOx emissions very close to the goal for most heavy-duty diesel engines.
The three primary manufactures of heavy-duty, on-highway diesel engines: Detroit Diesel,
Cummins, and Caterpillar, were generally able to comply with the standards. This was also done
without losing much in the way of vehicle performance and reliability.
The next step of standards were implemented in 2007 and sought to reduce the NOx emissions to
approximately 0.20 g/bhp/hr2. The 2007 emissions standards also implemented requirements for
the quality of diesel fuel. Namely, to decrease the sulfur content of diesel fuel itself. Important
to note: older diesels manufactured before the implementation of the EPA standards can still run
just as efficiently on the low sulfur content diesel. However, the newer engines will not function
properly on non-compliant high sulfur content fuels.
While the standards for 2004 and 2007 were generally attained by most manufactures of heavyduty diesel engines, the implementation was not met without a trade-off, namely, a dramatic
increase in fuel consumption. The author of this blog works for a small motor coach company.
The fleet of that company consists of Prevost H3-45 coaches. Currently a 2001 model, a 2005
model, and a 2007 model. It is important to note that the dimensions and Gross Vehicle Weight
Rating (GVWR) of all three of these coaches are nearly identical. The 2001 model, which was
manufactured and placed into service before the implementation of the EPA requirements
regularly achieves between 7 and 7.2 miles per gallon of fuel. (This may sound terrible, but it is
actually quite remarkable for a 48,000 pound vehicle with a 435 horsepower engine). To
compare, the 2005 vehicle, meeting the 2004 emissions standards gets an average of 6 miles per
gallon. When fuel mileage is only in the single digits, a difference of one mile per gallon is quite
significant. Further, the 2007 vehicle, which meets the applicable 2007 EPA standards, has
never seen fuel economy higher than 5.9 miles per gallon of fuel.
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http://www.dieselnet.com/standards/us/hd.php
Id.
Placing the obvious business concerns about fuel consumption aside, it seems ironic that the
environmental standards which require the vehicles to allegedly run cleaner, also cause the same
vehicles to burn much more fuel per mile driven. It seems that fuel consumption is, or at least
should be, an important environmental concern. This is a two-fold argument. Fossil fuel sources
are limited, and increasing consumption will impact the reserves in existence, not to mention
increase fuel prices. Additionally, because the EPA standards are measured in emission levels
per mile driven, the more fuel burned will likely lead to increased emissions overall, because a
greater quantity of fuel will be consumed in the same number of miles. Sadly, it seems that the
EPA standards are a double-edged sword. Hopefully, more research will be conducted and a
means of decreasing emissions without adversely impacting fuel economy will be achieved.