The New Year promises to bring great changes to the world of diesel power in the United States!
The elimination of all but 15 PPM (parts per million) of sulphur in on-highway diesel fuel by October will be followed by the implementation of stringent diesel emission standards that will apply to 2007 model year vehicles.
As these emission regulations are poorly understood, we will attempt to clarify them.
We will discuss only the primary troublemakers that occur at the diesel tailpipe. These are NOx (oxides of nitrogen), PM (particulate matter), and NMHC (non-methane hydrocarbons).
NOx results from the interaction of nitrogen and oxygen in the air when subjected to high temperatures that occur in the combustion chamber of a diesel engine, especially under load.
PM consists primarily of small particles of inorganic matter or other byproducts of combustion. Sulphur is a primary ingredient of PM.
NMHC is unburned hydrocarbons, essentially fuel that did not burn in the engine. Did you ask why they are described as “non-methane” hydrocarbons? This is because methane, although a potent greenhouse gas, is not regulated by the EPA. Manufacturers must concern themselves only with the hydrocarbons in the tailpipe that are not methane! In a diesel engine, methane can account for up to 2 percent of the HC’s in the exhaust. By the way, the new standards apply to all fuel types, be it gasoline, diesel, or alternatives. However, this article is limited to a discussion of diesel emissions.
When considering diesel emission regulation we need to think about three classifications of vehicles. The first is the Light Duty Vehicle Class, which includes cars and light trucks up to 8500 lbs. gross vehicle weight. The second group is a new category known as Medium Duty Passenger Vehicles (MDPV), which has been enacted to cover the class of large SUVs and passenger vans that weigh in between 8500 lbs and 10,000 pounds gross vehicle weight. The final classification is the Medium and Heavy Duty segment. This includes all heavier trucks and buses up to and including large semi-tractors, buses, straight trucks, etc. We also have to consider the Federal Tier Two Bin system which classifies a vehicle’s overall emissions performance by bin. There are now ten bins but there will only be eight after 2008. A zero emission vehicle would fall into bin 1. The dirtiest vehicle allowed would be a bin 10 (bin 8 after 2008). Most vehicles should fall into bin 5.
Emissions are measured by weight. The unit of measurement used is the weight, in grams, produced by the engine per mile of driving and divided by the brake horsepower of the engine. This provides a measurement of emissions products in grams per brake horsepower per mile. The measurements are taken over a preset drive cycle, designed to duplicate all modes of engine operation. The cycle is known as the FTP 75 (Federal Test Procedure). This drive cycle lasts about thirty minutes and is implemented in an EPA certified test cell using a computer controlled FTP program, dynamometer, and exhaust measuring equipment to certify the results of each test. To measure PM, for example, a filter is weighed on an EPA certified laboratory grade scale. The filter is then inserted into the exhaust stream of the test engine so that the exhaust must flow through it. The FTP 75 procedure is run. The filter is then removed and weighed again. The increased weight of the filter will be calculated. This will be the weight of PM produced during the test procedure. This figure is then divided by the brake horsepower of the engine and the miles run during the FTP to determine the weight, in grams, per brake horsepower/mile.
Now to try to state simply what is a very complicated set of standards I will have to make some approximations for the light and MDPV Vehicles. Medium and Heavy Duty standards are more straightforward. So, in a nutshell, here are the emission standards for diesel vehicles (“g” will denote “grams/bhp/mile”): At the present time, medium and heavy duty diesels must not exceed 2.5 g NOx or .1 g PM. Light duty levels are slightly higher. Now here are the standards for 2007 and beyond:
NOx: For all light duty, including MDPVs, the average manufacturer’s fleet NOx standard will be 0.07 g. Through 2008 this can vary from 0g to 0.9g, but after 2008 it cannot exceed 0.2g for individual models. Medium and heavy duty truck engines must meet a corporate average of 1.2 g. To meet this standard for 2007, most manufacturers will apply a 50/50 rule. Half of their production will exceed the 1.2g standard, up to a max. of 2.5 g, while the other half of their production will produce less than 1.2 g. This will produce an effective corporate average at the 1.2 g standard. However, by 2010 all engines must meet a NOx standard of 0.2 g. Isn’t that simple? Wait, it gets better. Remember those NMHCs discussed earlier? Well, the NOx values given here must also include all the NMHC produced by the vehicle as well! However, since diesels are very efficient; NMHC levels are very low and are not a significant issue for diesel emission standards.
PM: Light duty must produce .02 g/bhp/mile. To meet bin 5 status, a light duty vehicle will need to emit 0.01 g/bhp/mile! Some MDPVs will be able to produce up to .12g, but the average for MDPVs will be in the 0.06 to 0.08 g/ range.
Medium and heavy duty PM regulations are straightforward. They must emit no more than 0.01 g from 2007 onward.
The diesel engine industry can and will meet these regulations. To do so we will see a number of new after treatment devices and engine strategies all based on the use of ultra low sulphur diesel (ULSD). The drawbacks to ULSD are its reduced energy density, about 1 percent or 1300 btu/gal less than today’s diesel fuel, and its higher cost due to the additional steps needed to remove the sulphur. (As of December 2005, ULSD is selling for around $4.30/gal.) Also some aftertreatment systems, such as particulate filters to eliminate particulates, require additional fuel to burn out the trapped particulates and regenerate the filter. Specific technical information about these systems will be forthcoming. The NAFTC eNews has already discussed some of these technologies!