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LPG Bus
The liquefied petroleum gas (LPG) bus uses mostly propane gas for combustion.
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LPG is produced in the extraction of heavier liquids from natural gas, and as a by-product in petroleum refining. Presently, LPG supply exceeds the demand in most petroleum-refining countries, so the price is low compared to other hydrocarbons. Depending on the locale, however, the additional costs of storing and transporting LPG may more than offset this advantage.

Propane, or liquefied petroleum gas (LPG), is relatively widely used in smaller vehicles, but so far comparatively rarely in full-sized transit buses - in contrast to CNG buses, which are used more frequently in this category. Engine technology for LPG vehicles is very similar to that of natural gas vehicles.

Power, acceleration, payload, and cruise speed are similar, but propane buses are less fuel efficient than diesel buses, and hence their driving range is somewhat lower than for comparable vehicles ( w2 ) ( 2 ).

As with CNG buses, LPG engines, which are also spark-ignited, can be stoichiometric or lean burn, the latter being the more promising option ( 2 ).

LPG is slightly heavier than air which means that LPG-gas coming out from a leakage may store up in spaces where the gas becomes highly explosive.

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"Propane bus engines generally have lower emissions than counterpart diesel engines, although generally not as low as natural gas or methanol engines. According to an official of the Propane Vehicle Council, the simple molecular structure of propane eliminates particulate matter. In addition, experimental propane buses operated at a California-based transit agency underwent tests that indicated very low nitrogen oxide emissions" ( 2 ).

LPG buses used in Vienna are reported to achieve emission values that are below the Euro III standards. According to ( w3 ), "completely soot-free combustion ensures low-noise operation and a longer service life".

In 1997, an LPG bus engine with "low emissions values" was reported on by ( 3 ). It had NOx emissions of less than 3.7g/bhp-hr and particulate emissions of 0.075 g/bhp-hr. According to a more recent report by the California Energy Commission ( 4 ), two commercially available, dedicated LPG engines have been certified to California's heavy-duty optional low-NOx credit standards (see table below) ( 4 ).

Dedicated medium heavy-duty LPG engines certified to low NOx standards (model year 2000):

Manufacturer

NOx (g/bhp-hr)

PM (g/bhp-hr)

IMPCO

0.8

n/a

Cummins

2.3

0.01

Engine specifications of the Cummins B5.9 engine can be found in ( 5 ).
Like all gas vehicles, LPG buses can benefit from the advantages of gaseous fuels, i.e. the near-absence of sulfur and a low carbon-hydrogen ratio, which tends to cause relatively low CO2 emissions. However, this latter advantage is (mostly) offset by the currently low energy efficiency since they are spark ignition engines that are throttled and have lower compression ratios than diesel engines.

It should be noted that these emission values do not necessarily justify a choice in favor of LPG buses. The relevance of the relative emission savings must be carefully analyzed in the light of the individual situation. Thus for instance, the total emission reductions achieved through a particle filter on a conventional diesel bus may be perfectly sufficient to alleviate a high PM level problem. Other factors, such as cost effectiveness, may then be decisive regarding the choice of system.

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The main experience with LPG exists with smaller buses rather than full sized ones. Hence there is relatively little available information on the technical reliability of LPG buses in day-to-day operation. According to ( 6 ), "performance and drivability of an LPG vehicle can equal that of a conventionally fueled vehicle."

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Information on financial implications of LPG usage is not consistent and appears to be contradictory. For example, the Alternative Fuels Data Center ( w2 ) makes the following statements:

  • Propane costs in fleets typically range from 5% to 30% less than those of gasoline.

  • Fueling station cost is similar to, or lower than, that for a comparably sized gasoline dispensing system.

  • Service and diagnostic equipment would probably be required if access to commercial propane vehicle maintenance facilities is not available.

In contrast to this, without being specific, the US General Accounting Office ( 2 ), states that "the use of propane as a fuel in transit bus fleets brings with it high operating and capital costs".

In ( 3 ) it is reported that "LPG buses have been used in Vienna, Austria, for 30 years. From their experience fuel efficiency is lower than with diesel buses, and the initial purchase price is 10% - 15% higher for LPG buses. Maintenance costs are slightly higher and catalytic converters must be replaced every 2 years. Maintenance facilities must be modified to increase the ventilation. These additional costs are offset by much lower fuel prices and significant reductions in emissions.

According to the Vienna Climate Protection Programme ( w3 ), fuel costs per kilometer of liquefied-gas-driven vehicles are 50% below those of conventional fuels due to mineral oil tax exemption, and thus the cost of LPG-propelled buses is amortized within about 3 years".

The International Energy Agency (IEA) provides the following comparison of current costs:

Category

Bus Cost(thousands of US$)

Other Costs

New diesel bus produced in developing countries by international bus companies that meets Euro II

100-150

Some retraining costs and possibly higher spare parts costs

Standard OECD Euro II diesel bus*

180-350


Diesel with advanced emissions controls

5-10 more than comparable diesel bus

If low sulfur diesel, up to w1 cents per liter higher fuel cost (for small imported batches)

CNG, LPG buses

25-50 more than comparable diesel bus (less in developing countries)

Fuel infrastructure costs could be up to several million US$ per city

Source: IEA data* Note that this range of prices includes transit buses in both Europe and North America. Buses in Europe are generally less expensive than in North America, with the prices in Europe for non-articulated buses generally below US$ 275 000.

In summary, it is clear that LPG buses cost more than conventional and even advanced diesel buses. In addition, fuel efficiency is poorer with LPG and driving range is less. Countries and cities that have wanted the lower emissions from these LPG vehicles, therefore, have tended to reduce or eliminate the LPG fuel tax to make the fuel economically competitive. It should be noted that propane costs can vary widely by country and within country.

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While LPG is the most widely used alternative fuel worldwide, its use in heavy-duty vehicles such as transit buses is relatively limited.

In 1999, the US General Accounting Office ( 2 ) stated that "the extensive use of propane in larger transit buses is currently hindered by the lack of a suitable commercially manufactured engine. Warranted commercially manufactured propane engines are available for buses up to 30 feet long. While propane engine technology is currently available, it has not been transferred to larger engines, although the potential exists ( 2 ).

EIA has estimated that 152 full-sized propane transit buses were in service in the United States in 1999. According to a Propane Vehicle Council official, convincing manufacturers to make the investment that would move propane technology to a 350- to 400-horsepower engine is the biggest impediment to increasing the penetration of propane into the transit bus market ( 2 ).

The Cummins B5.9 engine has been developed for medium heavy-duty purposes, and can be used in large pickups; small school buses; vehicles operated by transit properties including shuttle buses; step vans; delivery trucks; and port vehicles such as yard hostlers. The LPG version of the B5.9 engine (195 hp) is available throughout North America, and there are significant numbers in revenue service" ( 4 ) (see ( 5 ) for information on engine specifications of the Cummins B5.9).

LPG buses have been used in Vienna, Austria, for 30 years ( 4 ), and with about 500 LPG buses, the city is operating the world's largest bus fleet running on liquid petroleum gas (LPG) ( w3 ).

There is strong competition from the natural gas industry regarding the heavy-duty sector (see ( w4 ) for more information on this.)

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LPG's major disadvantage is the limited supply, which would rule out any large-scale conversion to LPG fuel. Additional obstacles to the widespread use of LPG buses in Latin American cities are likely to be the incomplete infrastructure (gas pipelines, refueling gas stations), the demand for additional training and knowledge about handling, inspection and maintenance, higher costs compared to diesel buses and lower driving ranges. Further challenges come from competition with natural gas buses as well as the development of advanced diesel buses equipped with particle filters and catalytic converters using low sulfur diesel fuel, which compete with gas buses as they, too, become cleaner ( 2 ).

See Also
New buses - LPG bus
General topics
Cleaner vehicles - Buses
Alternative fuels and energy sources - Liquified Petroleum Gas (LPG)

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