All about CNG buses
Natural Gas Vehicles
Increasing numbers of transit agencies across the world are making the choice to convert their fleets to use compressed natural gas (CNG), and even more are considering it.
Though these alternate fuel vehicles (AFV) now account for a considerable chunk of all new vehicle orders, it becomes difficult for fleet operators to fairly evaluate the potential benefits of an alternative fuel program when they are confronted with misinformation or poor comparisons based on false assumptions.
Let us look at the misconceptions that have made their way into anecdotal stories, media reports, and even some poorly researched white papers and feasibility studies conducted at the time the AFV industry started over fifteen years ago.
Myth # 1
CNG buses emit same amount of particulate matter (PM) as diesel buses
CNG buses emit virtually no visible PM or black soot at the tailpipe. In chassis dynamometer testing conducted at a West Virginia University, CNG buses consistently emit dramatically less PM than diesel buses. For instance, testing of commercial buses in Boulder, Colorado, on the central business district driving cycle demonstrated a 97% PM reduction and a 58% reduction in oxides of Nitrogen with buses running on CNG rather than diesel.
Note that the trace amount of PM associated with CNG is generally attributed to crankcase lubricating oil consumption, not the fuel.
Myth # 2
CNG buses emit more ultra-fine particulates than diesel buses
Although CNG buses emit over 90% less particulate mass than conventional diesels, the particulates that are emitted are smaller in size. However, early studies that have compared CNG and diesel particulate matter size have reached contradictory conculusions. Techniques are being developed to better measure PM size distribution and understand how that distribution is affected by test procedures.
Myth # 3
CNG buses create more greenhouse gases than diesel buses because they emit more methane, which is a greenhouse gas that is 20 times stronger than CO2
Natural gas has inherently lower carbon dioxide emissions compared to diesel. Considering the total fuel cycle of both diesel and CNG including the emissions created during fuel production, CNG buses appear to have total greenhouse gas emissions created during fuel production, CNG buses appear to have total greenhouse gas emissions that are very similar to, if not slightly better than, diesel buses, despite emitting higher levels of methane.
Myth # 4
CNG buses are much more expensive than diesel buses
CNG buses cost about $25,000 to $50,000 more than a conventional diesel bus, but CNG fuel usually costs less than diesel fuel. At 25 cents per gallon savings, the typical CNG bus could pay for itself in just a little more than three years. And greater savings in fuels cost can result in even quicker paybacks.
The incremental cost of diesel-hybrid buses is estimated to be about twice that of CNG buses, but payback analysis will be based on improved fuel economy alone since there will be no per-gallon cost savings with the continued use of diesel fuel.
Myth # 5
Advanced emission control technology for diesel engines will make CNG buses unnecessary
Diesel emissions from new buses have improved dramatically in the past twenty five years, with most of the gains resulting from improved engine design. In the future, it is almost certain that diesel engines will have to rely on emission control devices to further reduce emissions. However, the durability of these emissions control devices in use on diesel and diesel-hybrid buses, depends greatly on fuel quality and is still to be determined. Should these devices degrade as the buses age and accumulate mileage, future emissions will be much higher . . . similar to those of the high emitters observed within the ageing gasoline vehicle fleet on the road today. Also, diesel buses emit toxins (such as benzene and 1,3-butadiene) that CNG buses do not. Based on the testing available today, it appears that CNG buses will always have PM and toxic emission benefits over diesel buses because natural gas combustion inherently produces lower levels of these emissions.
(See AFV Emissions)
Myth # 6
Availability of ultra-low sulfur diesel fuel and diesel hybrid-electric bus systems will negate the emissions benefits of CNG buses
Testing of prototype hybrid buses using advanced emission control technologies and experimental ultra-low sulfur diesel fuel has shown particulate matter emissions levels close to those of natural gas engines, which need only a minimal amount of exhaust emission control. However NOx emissions from these buses were higher than those from CNG buses. In addition, the special diesel fuel required if these emission control systems are to function properly on diesel engines is not expected to be commercially available at competitive prices. Further, even with the use of ultra-low sulfur diesel fuel, natural gas emission control devices will most likely be less expensive and longer lived than those for diesel engines simply because there is inherently less NOx and PM to control with natural gas and because natural gas has fewer contaminants that degrade these devices.
(See AFV Emissions)
Myth # 7
CNG bus fuel tanks are prone to explode
The technology for making CNG tanks is well known and mature. In the event of a vehicle collision, CNG fuel tanks are much stronger and safer than either diesel or gasoline fuel tanks, The few instances of CNG tank failures were studied carefully, and the problems, mostly involving support strap failure or tank abrasion during normal operation, have been re-mediated.
Myth # 8
CNG buses are unsafe
CNG buses have some different safety concerns than diesel fuel buses, but overall, there is no evidence that CNG buses pose any greater risk of fire or explosion than diesel buses, Natural gas buses have on-board gas detectors and other safety equipment specially designed to ensure safe operation.
(See Natural Gas – Safety)
Myth # 9
CNG bus maintenance garages and refueling facilities are more dangerous than diesel facilities
There is no evidence to support this claim. Both natural gas and diesel fuels are flammable – that is why they are useful as fuels. Each requires handlers to use safety and fire protection equipment designed specifically for that particular type of fuel. however, diesel bus facilities typically store much larger quantities of fuel on site than CNG facilities. Ground soil contamination from leaking diesel tanks is another concern that CNG facilities do not face.
Myth # 10
Natual gas vapors are toxic to breathe
Natural gas vapors are odorless and nontoxic to breathe. The familiar natural gas smell is actually an odor added to the gas as a safety feature. Gasoline and diesel vapors, however do not contain toxins that are dangerous to ingest or breather. Any fuel vapor that builds up in an enclosed space can cause asphyxiation.
(See Natural Gas – Safety)
Myth # 11
Hybrid technology will make CNG buses obsolete
CNG buses provide greater emission benefits today than diesel-hybrid buses using ultra-low sulfur diesel fuel, at much lower cost (as a result of loser incremental bus prices and the lower price of natural gas). CNG engines can be used in hybrid applications as well as diesel engines, with similar reductions in fuel consumption and even loser emissions.
(See AFV Emissions)
Myth # 12
CNG engines are too expensive to use in hybrid buses
Although CNG tanks are more expensive than diesel fuel tanks, fewer of them are needed in a hybrid application. In addition, the emission controls necessary to meet future emission standards are likely to cost less for CNG than for diesel engines. And CNG is less expensive than the ultra low sulfur diesel fuel required for advanced diesel engine emission control devices. If we examine their costs on a life-cycle basis, it is likely that CNG hybrid buses will be competitive in cost with diesel-hybrid buses.
Myth # 13
Diesel buses are easier to maintain than CNG buses and require no special training for maintenance staff
Mechanics must be highly skilled to maintain either type of buses. The mechanics must be familiar with the properties of each fuel type and must be properly trained on each system. Advanced technology diesel propulsion systems – such as hybrid-electric-diesel systems and the exhaust emission control systems required for achieving significant emissions reductions with diesel engines – are much more complex than standard diesel power trains. These advanced technology systems will require additional special training, tools, and test equipment beyond that required for regular diesel or CNG bus mechanics.
(See AFV Emissions)
Myth # 14
Maintenance costs are much higher for CNG than for conventional diesel or hybrid-electric diesel buses
CNG engines are spark ignited, and thus require periodic ignition maintenance on spark plugs, wires. Diesel engines do not use spark plugs, so this type of maintenance is unnecessary. Diesel-hybrid buses must also contend with periodic replacement of electric battery systems, which have significant costs and waste disposal requirements, Because of CNG’s cleaner combustion process, contamination of engine lubricating oil is greatly reduced compared to diesel. As a result, some engine manufacturers have essentially doubled the recommended oil change interval for CNG engines. Transit properties that monitor their oil quality through independent analysis substantiate this performance.
Fewer oil changes result in savings in the cost of bulk oil, filters, hazardous waste disposal, and labor. This cleaner internal operation results in less engine wear. Some transit agencies have reported CNG engines with no signs of needing expensive mid-life rebuilds.
Clean-up in the shop, engine compartment, and bus parking areas is also less because CNG use does not result in heavy PM deposits
Myth # 15
CNG buses are so much heavier than diesel or hybrid-electric diesel buses that tires, brakes, and other systems will wear out much quicker
CNG buses do weigh more than conventional diesel buses, but less than hybrid-electric diesel buses. This is due to the extra weight of the CNG fuel tanks. However, CNG tanks made of carbon fiber composite materials are standard equipment on modern buses and are considerably lighter than the metal tanks used on earlier buses. The extra weight of hybrid-electric diesel buses is due to the battery packs. Additional wear and tear for either of these heavier bus systems, however, has not borne out in actual practice. The tires, braking systems, and other equipment are all designed and sized to accommodate the load rating of the entire bus and passenger payload. Modern engine-transmission speed retarder systems on conventional diesel and CNG buses and regenerative braking systems on the hybrid-electirc buses provide deceleration assistance, Transit operators report that buses assigned to routes with poor road conditions, steep grades, and drivers with aggressive braking habits manifest the greatest wear patterns, regardless of bus fuel type.
Sound bytes comparing natural gas buses and diesel buses are often over simplified and may be misleading – dig in a study the details!
Here are a few of the highlights.
Current natural gas buses offer emissions benefits in comparison to diesel buses and even diesel-hybrid buses. Natural gas buses do cost more to buy than conventional diesel buses, but less than advanced technology diesel-hybrid buses. Some CNG bus fleets have documented operating costs equal to or lower than diesel buses. Because fuel costs can make a BIG difference, make sure your analysis reflects current market conditions and up-to-date fuel prices.
It takes a very conscientious effort to implement a clean bus program. Every decision must be based on factual information and not sensational headlines or industry folklore.
Information gathered from Fact Sheets on Clean Cities, by National Renewable Energy Laboratory of US Dept of Energy
Alternative Fuels Data Center at www,afdc,doe.gov