Running Side by Side with Diesel in Bus Fleet, CNG Emerges As ...

The Cleaner, Less Costly Choice

The Clean Air Act Amendments of 1990 and the Energy Policy Act of 1992 generated interest in cleaner burning, domestically produced alternative motor vehicle fuels.

In response to this legislation, the Central New York Regional Transit Authority (CENTRO) introduced eight natural gas-fueled buses into service in January 1993 in a project sponsored jointly by Niagara Mohawk Gas and CENTRO.

The three-year field test in Onondaga County, NY, was part of a larger New York State Alternate Fuels Consortium project. CENTRO's goals in introducing the natural gas buses into urban transit service were to dramatically reduce air pollution and lower operating costs using a domestic fuel. It also anticipated that the buses' natural gas engines potentially would not require overhaul for the life of the vehicles (12 to 15 years) because of the fuel's cleaner-burning qualities.

Methods

The three-year field test was designed to provide a direct comparison between eight "test" natural gas buses and eight "control" diesel buses used in transit service over the same routes. The study evaluated the cost of operating and maintaining the buses.

Both groups used the Cummins L-10 four-cycle engine; the natural gas buses used the L-IOG (the L-I0 engine equipped to burn natural gas) while the diesel buses used the standard L-lO, an engine noted for efficient operation on diesel fuel. This comparison was considered to be more accurate than comparing a control group of buses using the Detroit Diesel 6V92 engine because it is a two-cycle model. Additionally, future diesel engines used in urban transit service likely will be four-cycle because of air quality certification requirements.

Fuel consumption data for the the test group were recorded manually by CENTRO personnel during nightly refueling trips to Niagara Mohawk's Hiawatha Boulevard compressed natural gas (CNG fueling station. Fuel consumption data for the control group and maintenance cost data for both groups were recorded automatically by CENTRO's IBM main frame computer. The diesel buses operated on No.2 diesel, a fuel with higher energy content [138,000 British Thermal
Units (Btu)per gallon - higher heating value], than the No.1 diesel fuel (133,000 Btu per gallon - higher heating value) used by some transit properties in the consortium. The bus fuel efficiencies for this test were different than tests using Detroit Diesel 6V92 engines and/or No. 1 diesel fuel for their control fleets. Operating results for the project were issued in a series of 11 quarterly reports from January 1993 to October 1995 and a final report delivered in March 1996. Emissions comparisons of representative natural gas and diesel buses were performed for the New York Natural Gas Bus Consortium at the New York City Department of Environmental Protection Frost Street Vehicle Emission Test Laboratory.

The results were used to quantify emissions differences for the test and control buses and predict emissions differences for the entire CENTRO bus fleet. The study also evaluated operating costs for a natural gas fueling station used by the CENTRO bus fleet, excluding the cost of station construction.

Emissions Test Results

Figures 1, 2 and 3 compare carbon monoxide (CO), 'nitrogen oxide (NO) and particulate (soot) emissions for diesel and natural gas buses. The natural gas buses tested well in all three categories, producing: 0.431 grams per mile (gpm) of CO emissions, a 98 percent reduction compared to the 21.987 gpm of CO emissions for the diesel buses; 29.476 gpm of NOx emissions, a 38 percent reduction in comparison to the 47.399 gpm of NOx emissions for the diesel buses; 0.112 gpm of particulates, a 95 percent reduction in comparison to the 2.33 gpm of particulates produced by the diesel buses.

The Clean Air Act Amendments of 1990 provide an opportunity for fleets to capitalize on emissions savings, creating a market for technologies that reduce emissions compared to the technologies they displace. Once the value of emissions reductions is quantified, the quantity of emissions reductions theoretically can be sold in the commodities markets in the form of emissions credits.

Figure 4 provides a low and high projection for the value of "clean air credits" for CENTRO's planned fleet of natural gas buses. Emissions savings are predicted to be 16 tons of NOx and 21 tons of CO and particulates in CENTRO's first year of operating a 28-bus CNG fleet. In the year 2000, when CENTRO's CNG bus fleet is projected to grow to 127, emissions savings are predicted to be 72 tons of NOx and 95 tons of CO and particulates.

Potential revenue from emissions credit sales ranges from a low of $9,500 per year to a high of $154,000 per year in the first year of operating the 2S-bus fleet. In the sixth year of operation, with a 127-bus CNG fleet, projected emissions credit sales revenue ranges from a low of $43,000 per year to a high of $704,000 per year.

Operating Efficiency and Cost Comparison

For the three-year test period (Feb. 1, 1993, through Oct. 31, 1995), the gas buses averaged 47,473 Btu per mile (2.91 miles per equivalent gallon) compared to 35,476 Btu per mile (3.89 miles per gallon diesel) for the diesel buses.

Thus, the overall fuel economy of the natural gas buses was about 25 percent less than the diesel buses for the test period. This difference in fuel economy between the test and control buses can be attributed to differences in overall bus weight, engine design and drivetrain rear· end ratios.

Fuel cost is another important category to consider when comparing CNG and diesel buses. A five-year comparison of diesel and CNG prices shows the average price was 20 percent less for CNG than diesel. Figure 5 presents the fuel and operating cost comparison for CENTRO's proposed fleet of 28 natural gas buses based on five-year average fuel costs.

Overall savings from natural gas operations are projected to be $30,332 per year compared to diesel, including addi tional operating costs of $1,521 per yea for the 28-bus fleet and $31,853 per yea savings on heat for CENTRO's bus garage and headquarters building.

CENTRO plans to replace its entire fleet with natural

gas buses over a 10 year period. Figure 6 shows overall savings for natural gas operations: $25,812 per year compared to diesel with a fleet of 159 natural gas buses, including additional operating costs of $6,041 per year and $31,853 per year savings on heat for CENTRO's bus garage and headquarters.

Maintenance Issues

Evaluation of maintenance costs for the natural gas and diesel buses over the three-year test period shows maintenance costs to be nearly equal on a per-mile basis. However, in the first year of the test, several maintenance issues specific to the natural gas buses were identified. These issues included failure of temperature-relief devices (TRDs), solenoid valves and shut-off valves in the fuel supply system of the gas buses, problems with high-voltage wires and spark plugs in the ignition system and a sensitive mechanical governor.

Problems with the solenoid valves, shut-off valves and mechanical governor were resolved in the first two years of the project, while the TRD and ignition system issues were not satisfactorily resolved until 1995.

Resolution of maintenance issues specific to the gas buses required close cooperation with the bus manufacturer, the engine manufacturer, the gas valve manufacturer and the Gas Research Institute - and in many instances required redesign and field testing of parts and systems.  The cost to resolve these issues was regarded as a non-recurring development cost of the new technology, covered by manufacturer's warranty with the assistance of project staff and management and the support of research and development personnel.

More Factors Favoring CNG

The test group of CNG buses averaged 13 parts per million (ppm) total wear metals in the drain oil over an 18-month period from June 1994 through December 1995 versus 53 ppm total wear metals for the diesel buses. Therefore, the total wear metals in the drain oil of the gas buses was four times (400 percent) lower than the total wear metals in the drain oil of the diesel buses.

Each natural gas bus weighs 3,840 pounds, 13.7 percent more than its diesel counterpart, meaning wear on the brakes and bus suspension system would likely be greater for the natural gas buses than the diesel buses. Surprisingly, however, expenditures on brakes and suspension system repairs for the period January 1994 through October 1995 actually were lower for the CNG buses than the diesel buses both in total dollar amount and percentage of total maintenance expenditures.

Conclusions

Natural gas buses provide dramatic reductions in carbon monoxide, nitrogen oxides and particulate emissions compared to their diesel counterparts, although methane emissions increase.

"You can put a white handkerchief over the tailpipe of a natural gas bus and pull it away clean," says John Clare, CENTRO's vice president of administration.

Testing, including the "white handkerchief test", underscores the emissions differences between diesel and natural gas buses. In the future, lower emissions from natural gas buses may provide an economic benefit to entities operating gas buses because of the potential sale of "clean air credits" in the open market. Operating buses (and buildings) on natural gas will produce a net savings in operational costs for CENTRO. The operating cost for a fleet of CNG buses is very close to diesel when projected over five years. In CENTRO'S case, conversion to natural gas should result in lower fuel costs because the cost to heat CENTRO's bus garage will drop with large-volume gas purchases. Maintenance costs for the two groups are approximately equal, as are the costs to repair brakes and suspension systems. Over the 12-year life of an urban bus, however, the maintenance costs of the natural gas buses may be lower than diesel because of the lower rate of engine wear (400 percent lower total wear metals in the drain oil) for natural gas engines compared to diesel engines.

CENTRO plans to buy 15 new natural gas-powered buses based on the findings of this study. It also plans to build a $4.5 million CNG fueling station near its headquarters by the summer of 1997 and hopes to buy 99 more natural gas buses by 1999, according to Clare, making 80 percent of its fleet natural gas-powered. In addition to CENTRO's fleet, the new station could fuel vehicles owned by neighboring companies such as United Parcel Service and Federal Express, according to Joseph Calabrese, CENTRO's executive director.

Judson Brown served as project manager for the three-year study (1992 - 1995) of compressed natural gas buses for CENTRO and Niagara Mohawk Gas in Syracuse, New York.  The article is reprinted with permission for the purpose of stimulating discussion on low emission vehicles and sustainable transportation alternatives in 2010.