Metro Magazine

AUG 2014

Magazine serving the bus and rail transit & motorcoach operations since 1904

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A7 AUGUST 2014 BUS OPS s proximately 135 to 140 scf of natural gas. • Diesel engines are more fuel effcient than CNG since they operate at higher compression ratios, thus sizing calcula- tions should add 7% to 15% to CNG fuel consumption to account for this reduced effciency. • CNG vehicles emit approximately 20% to 25% less CO2 than gasoline and diesel vehicles. Determine Station Type There are three styles of CNG stations in wide use today. These are time-fll — also called slow-fll — cascaded fast-fll and buffer fast-fll. 1. Time-Fill Station In a time-fll station, gas is frst dried to reduce the moisture. Then, after compression, CNG is dispensed through a mani- fold to all vehicles simultaneously. This system is simple and can be the most cost-effective method because the compres- sor is often smaller than required for fast-fll, and there is no storage or cascading required. There may be manpower sav- ings as drivers connect to the manifold and walk away, with no time lost in fueling. Time-fll has several limitations. It is applicable only when feet vehicles return to a common facility for several hours, preferably overnight. Also, while it is possible to accurately measure fuel dispensed to each vehicle, it is generally cost prohibitive and complicated. Time-fll stations are well suited to trash collection vehicles, school buses and other vehicles that domicile overnight in a home base. 2. Cascade Fast-Fill Station One answer to the problems of a time-fll station is a cascaded fast-fll station. The primary difference between time-fll and cascade fast- fll is the use of a group of high-pressure vessels divided into banks (usually three), which are automatically "sequenced" or "cascaded" to fll a vehicle. The cascade fast-fll provides higher fows than the com- pressor alone, so vehicles fuel in a similar time to liquid-fu- eled vehicles. Individual metering is easy, and fast-fll accom- modates feets that do not stay in a yard. Capital cost can be a deterrent from using fast-fll. Note that as storage is depleted (below 60% to 70% of stored volume), fll times increase, particularly with marginal or undersized compressors. Cascade stations are versatile and can fuel a wide variety of vehicle and feet types. 3. Buffer Fast-Fill Station For applications where sustained fast fueling of high fuel use vehicles is required, buffer fast-fll may be the best option. The primary difference between buffer and cascade fast-fll is that with a buffer system, vehicles are flled directly from large compressors rather from the storage. The buffer storage is used to allow the compressors to keep pumping even when no vehicles are fueling — this gas then supplements compres- sor fow when vehicle fueling resumes. The buffer fast-fll provides high fow rates because large compressors are used, so even large vehicles fuel in a similar time to liquid-fueled vehicles. Individual metering is easy, and flls do not slow down after fueling several vehicles as is com- mon with cascade systems. Capital cost of buffer stations is high because of the large equipment but they are well suited to transit bus operations. Understand Station Components The following is a brief overview of station equipment: 1. Gas dryers Gas dryers are needed to reduce the moisture content of the gas . Usually, station inlet drying is preferred over high-pressure drying as it is safe, reliable and effective. To reduce capital and maintenance costs, a single tower dryer with a manual regeneration system can be used in many small to medium-sized CNG stations. 2. Gas Compressors Compressors are the heart of the station. Many styles and manufacturers exist, but all are reciprocating compressors — that is, they contain pistons that run in cylinders. Variations are due to differences in the heritage of the machine. The "package" — equipment on the skid supporting the compressor — can be more critical than the compressor block design. Some block manufacturers have several "packagers" with varying quality and capabilities. Don't assume that one compressor package is equal to the next because they share a common block manufacturer. In machines with 50 hp or less, the lowest -cost compres- sors tend to be converted air compressors. This usually limits suction pressure to 5 lbs. to 15 lbs. per square inch gauge (PSIG) and most are lubricated design. Above 50 hp, industrial and gas patch adaptations become more common. These machines are more complex than con- verted air compressors but tend to be more rugged. At least one compressor manufacturer markets a "nonlu- bricated" compressor, meaning the cylinders receive no lubri- cation. This can simplify the machine and possibly eliminate oil carryover to the vehicle. However, there are drawbacks to nonlube compressors, including potentially lower life of inter- nal wear items in the compressor, including rings and piston rod seals, and a need to run compressors slower and cooler. The "NGV1" nozzle shown is designed to fast fll small- to medium- sized vehicles including this shuttle bus. Time fll dispensers use a simpler version of the "NGV1" nozzle.

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