Diesel Vehicles Crankcase Emissions Fact Sheet

Summary

Crankcase emissions are oily aerosols escaping around piston rings in reciprocating engines. For currently used diesel school buses, independent studies have found no measurable negative impact on cabin air quality during normal bus operations. Federal and California rules for 2007, covering emissions controls in heavy-duty diesel vehicles (such as school buses), will include control of crankcase emissions. Manufacturers will make changes in new diesel engines such as closing the crankcase, which will reduce odor and produce a cleaner engine compartment. Current vehicles can be retrofitted to control crankcase emissions; however, most retrofits focus on reductions in exhaust pipe emissions (for example, through the use of particulate filters) since this achieves a more substantial emissions reduction. A recent University of California study showed that the use of an after treatment device to eliminate crankcase emissions had very little if any effect on school bus cabin air quality.

Research Findings

Crankcase emissions (commonly known as “blow-by”) are oily aerosols escaping around piston rings in reciprocating engines. Diesel engines in current heavy-duty diesel vehicles, such as school buses, are designed to vent the crankcase to prevent excessive pressure buildup. A study recently conducted by the Clean Air Task Force (CATF), in Atlanta, GA, raised questions about crankcase emissions and cabin air quality in school buses. CATF observed that cabin fine particulate matter (PM2.5) increased when the front of the bus faced into the wind, but found no effect on other measures such as ultra-fine particle numbers or CO concentrations. CATF attributed some of this particulate to crankcase emissions.

Independent research by others indicates that crankcase emissions have little impact on cabin air quality. These studies found that cabin air quality is the same as air quality around the roadway. In other words, the levels of particulate matter inside a school bus are similar to those found inside any other vehicle traveling on the same roads.

Independent researchers conducted two studies for International and found that the levels of fine particulate matter inside a school bus were at or below background levels or levels in vehicles traveling ahead of the test bus.

In the first study, researchers evaluated cabin air quality for particulate elemental carbon using two analytical methods in three school buses tested on a rural automotive test track (Borak et al. 2003). The buses were run separately 4–6 hours with windows closed driving the 4-mile course at 15–20 miles per hour with two 1 minute stops per lap to simulate pick-up and drop off of students. The average levels of particulate elemental carbon inside the buses did not exceed background levels in the air surrounding the test track.

In the second study, researchers measured fine particulate matter mass (PM2.5) in a school bus traveling on a typical bus route in Los Angeles during normal school commuting hours (EnSight 2003). For comparison, PM2.5 was measured in lead vehicle that preceded the bus. On the average the PM2.5 levels were higher in the lead vehicle than the school bus.

Other studies support these findings. In Fairfax, Virginia, twelve school buses were tested on their normal routes (O’Neill 2001). There were no noted elevations in respirable particulate matter (PM10) or in particulate matter elemental or organic carbon.

Emission Controls

A model developed by the U. S. Environmental Protection Agency showed that crankcase emissions for diesel engines were a very small part of overall vehicle emissions: an estimated 2% of exhaust hydrocarbon (HC) emissions, 0.2% of exhaust CO emissions and 0.05% of exhaust nitrogen oxides (NOx) emissions.

However, Federal and California air quality rules scheduled for full implementation in 2007 will address every relevant heavy-duty diesel engine emissions issue, including control of crankcase emissions.

Diesel engine manufacturers will design appropriate controls; the crankcase can be closed, for example, reducing odor and producing a cleaner engine compartment, without degrading engine performance.

Current vehicles can be retrofitted to control crankcase emissions; however, most retrofits focus on reductions in exhaust pipe emissions (for example, through the use of particulate filters) since this achieves a more substantial emissions reduction.

Recently, an after treatment device to eliminate crankcase emissions was evaluated by the University of California at Riverside (Fitz 2003). Measurements were taken for PAH, particle count, CO, and VOC levels inside the bus. Under unrealistic conditions that maximized the impact of crankcase emissions on cabin air quality, the device reduced total particulate counts in the school bus cabin by only 4% in one scenario and increased particle counts by 3% in another. Each of the other three measurements of cabin air quality demonstrated similar equivocal trends. This suggests that under normal operating conditions, the use of an aftertreatment device to control crankcase emissions would have little impact on bus cabin air quality.