Heavy Duty Diesel Trucks

Current EPA On-Road Measurement Capabilities: Heavy-Duty Diesel Trucks

Several years ago, in response to concerns about the utility of dynamometer data for airshed modeling, EPA's Office of Research and Development began construction of an on-road test facility to characterize the real-world emissions of heavy-duty trucks. The facility was designed to effectively demonstrate the full range of vehicle operation and to measure the emissions produced. Since it began operation, the facility has been continuously upgraded to incorporate state-of-the-art technology, testing three heavy-duty tractors along the way. Fully self-contained and relocateable, the facility has conducted testing in Georgia, West Virginia, Michigan, and Ottawa, in addition to three test sites in North Carolina.

General Description

Integrated into a class 8b tractor trailer, the facility is essentially two mobile laboratories that have been adapted to mobile operation. The main laboratory occupies the front 10 feet of a 45-foot enclosed trailer space. Everything in this space is designed for unattended operation. The second lab space is built into the sleeper area of the truck cab and includes an operator control console. The laboratories are supported by a power generator and an air compressor, both diesel-powered. Each lab space is climate controlled by its own air conditioner, each of which operates independently of the truck's climate controls.

The trailer layout is shown below. The main lab space contains all of the power distribution equipment, the compressed gases, and a shock-mounted instrument rack. In addition to the continuous emissions monitoring (CEM) system, the instrument rack holds the computerized data acquisition system (DAS), which monitors and records the responses of the CEM system and all of the other sensors located throughout the facility.

The secondary lab space has evolved, over the course of this test program, from a simple operator monitoring station to a fully integrated functional component of the facility. To adapt this area, the cab sleeper interior was completely dismantled and replaced with instrumentation and an operator seating position. From this position, the operator can use all of the cab-mounted test equipment, he can communicate verbally with the truck driver, and he can monitor and control much of the trailer-mounted equipment through his link to the DAS computer. The control center also provides an information display to the driver for those tests that require real-time driver feedback.

Equipment located outside of the lab space includes the support equipment (generator, compressor, fuel tank, etc.), the loading weights, and the various probes and sensors that feed information to the DAS. All modifications to the truck and trailer are designed to have little or no impact on the operating characteristics, including aerodynamics, rolling resistance, maneuverability, and driver visibility.

Current Capabilities

The facility CEM system measures O₂, CO₂, CO, NO_x, and THC using source level instrumentation (i.e., the sample is not diluted). Exhaust flow is calculated from the velocity head and static pressure measurements, then used to calculate emissions in grams/hour. These emissions can be converted to grams/mile or grams/brakehorsepower·hour using the speed sensor and the real-time horsepower measurement system. Other measurement capabilities that have been incorporated into the facility include the following.

Engine speed and operating temperatures
Vehicle g-force (combined effect of acceleration and grade)
On-demand particulate sample collection on selectable small filters
Exhaust plume sampling along the length of the trailer
Global Positioning System data

The facility includes 18 tons of removable weight which simulates the effects of truck payload on emissions. The weight is removable in 12 discrete increments, limited only by the necessity to distribute the load evenly within the cargo area. Typical test weights are 39000, 57000, and 75000 pounds GCW. It is expected that the facility's load variation capabilities, along with its ability to relocate to areas of specified roadway grades, will allow any test vehicle to experience a full range of operating conditions (limited only by its drive train capabilities).

In-Progress Capabilities

Facility personnel are currently evaluating several instruments (shown below) for incorporation into the facility. These instruments are in-house, undergoing bench testing, and should eventually occupy the lab space in the cab of the truck.

Scanning Mobility Particle Sizer (SMPS) — Provides a high resolution (128 bins) particle size distribution from a dilute sample using an electrostatic classifier and condensation nucleus counter
Electrostatic Low Pressure Impactor (ELPI) — Provides real time size distribution data (12 bins in the range of 32 nm to 10 microns) from a dilute sample using a multistage impactor
Polycyclic Aromatic Hydrocarbon (PAH) Monitor — Provides real-time measurement of PAH concentration (often linked to carcinogenicity) from a dilute sample
Aethalometer — Uses a continuously advanced filter medium (i.e., filter tape) and optical measurements to quantify the carbon black content of a gaseous sample

In general, the facility has been designed for adaptability from the start. Most upgrades and modifications have left room (i.e., data acquisition channels, signal connections in umbilicals, utility requirements, physical space) for future upgrades, which are always carried out by the same team of experts that built the facility. The facility has also accepted “temporary” upgrades for special purpose, short-term projects, ranging from chassis dynamometer testing to providing CEM system support to a separate, stationary facility.

Project Examples

Evaluation of Road Load Horsepower — Before the latest series of equipment upgrades, there was no direct measurement of horsepower (currently supplied by a drive shaft torque-meter and rotational speed sensor). In order to accurately quantify horsepower, the project team had to measure and combine the forces that contribute to on-road power demand: grade force, acceleration force, rolling resistance, and wind drag. Where grade force is a function of roadway gradient and vehicle weight and acceleration force is a function of mass and velocity change, the last two forces (rolling resistance and wind drag, collectively referred to as “road load”) must be measured experimentally and characterized mathematically. To accomplish this, the facility was relocated to New Bern, NC, where there is a suitably level and straight roadway to conduct coast-down testing. This testing was conducted at each of three test weights, covering the full range of operating conditions. Data analysis included finding iterative solutions to the road load force equation, validating test runs and discarding outlyers, and converting the results to a Force=f(Velocity) equation.
Dioxin Sampling — This special-purpose test program was carried out on each of the first two trucks. Using EPA stationary source reference method 23 (40CFR Part 60, Appendix A), a metered sample of the truck's exhaust was pulled through a glass probe and filter, then cooled and passed through a XAD-2 trap specifically prepared for collection of dioxin samples for ultra-sensitive analytical techniques. The challenges for this test program were threefold: designing a sampling system that would physically withstand the rigors of mobile operation, collecting enough sample in a single test to satisfy minimum detectability requirements, and modifying the metering equipment for remote operation and control (i.e. from the truck cab). In spite of these challenges and limited resources (time and budget), this test program was considered a success. The analytical data, combined with the sampling data, the CEM data, and the truck operational data, yield results in a variety of mass-based emissions units (g/hour, g/mile, g/kg fuel, g/grams/brakehorsepower·hour, etc.).
On-road Parametric Testing — This is the core capability of the facility: establishing repeatable combinations of load, grade, speed, and acceleration and measuring the emissions that occur. As has been done many times in dynamometer studies, the facility established the power-dependence of various emissions (particularly NO_x emissions) under real world conditions. For each of the first two trucks, the parametric test data compared reasonably well with subsequent dynamometer testing of the trucks' engines.
Dynamometer Cycle Testing — Using the level-grade test roadway and the driver-feedback interface, each of the trucks was driven through several chassis dynamometer cycles. Testing the cycles on-road removed any inaccuracies that may be introduced by improperly loaded dynamometers, while introducing some real-world variability. Emissions variability was generally on the order of ±10%.