lpg n ngv

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LIQUEFIED PETROLEUM GAS

(LPG)

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What Is LPG ?

Liquefied Petroleum Gas (LPG) is of ten incorrectly identified as propane.

In f act, LPG is a mixture of petroleum and natural

gases that exist in a liquid state at ambient temperatures when under moderate pressures (lessthan 1.5 MPa or 200 psi).

The common interchanging of the two terms isexplained by the f act that in the U.S. and Canada LPGconsists primarily of propane (see Table 3).

In many European countries, however, the propane content in LPG can be as low as 50% or less.

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Table 3. Composition of LPG (U.S. HD-5 standard)

Propane 85% min. by liquid volume

Propylene 5% max. by liquid volume

Butane & heavier HC 2.5% max. by liquid volume

Sulfur 120 ppm max. by weight

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The major sources of commercial LPG are natural gas processing and petroleumrefining.

Raw natural gas of ten contains excess propane and butanes which must be removed to prevent their condensation in high-pressure pipelines. In petroleum

refining, LPG is collected during distillation, from lighter compounds dissolved in the crude oil, as well as generated in the "cracking" of heavy hydrocarbons.

Therefore, LPG can be considered a by-product and its exact composition andproperties will vary greatly with the source.

LPG provides about 8% more energy per unit weight (LHV = 19,757 BTU/lbm) than gasoline.

Theoretically, vehicle operation with LPG should be more efficient than with

gasoline, i.e., the vehicle should attain better specific f uel consumption andimproved mileage.

However, this will only happen if the engine design is optimized for LPG f uel. If a gasoline engine is converted to operate on LPG this increased efficiency will not be realized due to the lower density of LPG compared to gasoline and also its slightly higher oxygen demand (LPG stoichiometric A/F = 15.8).

The lighter density f uel displaces air in the intake manifold, and thus, less air per

cycle is induced to the cylinders. This translates to a decreased volumetric efficiency and a loss of power compared to the original gasoline rating of the engine.

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LPG COMPONENTS

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The advantages of LPG include

Because LPG vaporizes when released from the tank and is not water soluble, LPG does not pollute underground water sources.

Power, acceleration, payload and cruise speed are comparable tothose of an equivalent vehicle f ueled on gasoline. Propane has a high octane rating of 104, in-between Compressed Natural Gas(CNG) (130) and regular unleaded gasoline (87).

Ref ueling a propane vehicle is similar to filling a gas grill tank; the time it takes is comparable with that needed to fill a CNG, gasoline or diesel f uel tank.

Its high octane rating enables it to mix better with air and to burn more completely than does gasoline, generating less carbon. With less carbon buildup, spark plugs of ten last longer and oil changesare needed less frequently.

Because it burns in the engine in the gaseous phase, propane results in less corrosion and engine wear than does gasoline.

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The drawbacks of LPG include:

In cold conditions, below 32 degrees Fahrenheit, starting could be a problem because of the low vapor pressure of propane at lowtemperatures.

One gallon of LPG contains less energy than a gallon of gasoline. The driving range of a propane vehicle is about 14 percent lowerthan a comparable gasoline-powered vehicle.

LPG is generally higher priced than other f uel alternatives such asCNG and gasoline.

There are over 4,000 LPG ref ueling sites in the US, more than all of the other alternative f uels combined. Most of these stations, however, are not readily available to consumers on a 24/7 basis. This is one of the reasons why most on-road applications are bi-f uel vehicles, which burn LPG and gasoline

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NATURAL GAS VEHICLES

(NGV)

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Natural-gas Vehicle Design

Natural-gas vehicles use the same basic principles asgasoline-powered vehicles.

In other words, the f uel (natural gas in this case) is

mixed with air in the cylinder of a four-stroke engine and then ignited by a spark plug to move a piston upand down.

Although there are some diff erences between natural gas and gasoline in terms of f lammability and ignition temperatures (see chart below), NGVs themselvesoperate on the same f undamental concepts asgasoline-powered vehicles.

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Property Natural Gas Gasoline Diesel

Flammability

Limits (volume

% in air)

5-15 1.4-7.6 0.6-5.5

Auto-Ignition

Temperature

(°F)

842 572 446

Peak Flame

Temperature

(°F)

3423 3591 3729

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Fuel Storage

Most NGVs operate using compressed natural gas so the f uel takesup less space.

At a f ueling station, gas is compressed to 3,000-3,600 pounds persquare inch before being pumped into high-pressure, tube-shapedcylinders that are attached to the rear, top or undercarriage of the vehicle.

The storage tanks of early NGVs were bulky and took up much of the vehicle's cargo space, but newer, more lightweight cylindershave been developed.

Called Integrated Storage Systems (ISSs), these all-composite cylinders are contained within a fiberglass shell and impact-absorbing foam to protect them in the event of a crash.

The cylinders also have a smaller diameter so that three of themcan be housed together in a size and shape resembling a conventional gasoline tank

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Engine Modifications

When the engine in an NGV is started, natural gas f lowsfrom the storage cylinders into a f uel line.

Near the engine, the natural gas enters a regulator toreduce the pressure.

Then the gas f eeds through a multipoint gaseous f uel-injection system, which introduces the f uel into the cylinders.

Sensors and computers ad just the f uel-air mixture so that when a spark plug ignites the gas, it burns efficiently.

A natural-gas engine also includes forged aluminum, high-compression pistons, hardened nickel-tungstenexhaustvalve seats and a methane-specific catalytic converter.

An installed natural gas engine

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Chassis Modifications

Some modifications in the suspension of a NGV may be required to create space for the f uel-storage containers.

In the rear of the vehicle, a semi-trailing arm suspension sometimes replaces the lateral-link suspension that comes

standard in many gasoline-powered cars. This creates more open space in the rear undercarriage, yet

still provides a smooth, comfortable ride.

NGVs also remove the spare tire and jack, which allows fora f lat f loor plan.

"Run-f lat" tires, such as the Extended Mobility Tires fromGoodyear Tire, are installed to compensate for the f act that the spare tire and jack are missing.

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Diff erences in Ref ueling

Ref ueling a natural-gas vehicle may also be a little diff erent.

The f ueling point is typically at the front of the vehicle, although in some NGVs, such as the Honda Civic GX, the f ueling point is in the rear.

An NGV can be f ueled at a "f ast-fill" pump in about the same time it takes to f uel any gasoline- or diesel-poweredvehicle.

Alternatively, an NGV can be f ueled in five to eight hoursusing a "slow-fill" method.

The home ref ueling stations off ered by Honda are of the slow-fill variety, requiring car owners to ref uel their vehiclesovernight.

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Advantages and Disadvantages The biggest advantage of NGVs is that they reduce environmentally harmf ul

emissions. natural-gas vehicles can achieve up to a 93 percent reduction in carbon monoxide emissions, 33 percent reduction in emissions of various oxides of nitrogen and a 50 percent reduction in reactive hydrocarbons when compared togasoline vehicles. NGVs also rate higher in particulate matter 10 (PM10) emissions. PM10 particles transport and deposit toxic materials through the air. NGVs that operate in diesel applications can reduce PM10 emissions by a f actor of 10. ± Natural-gas vehicles also off er these benefits:

± NGVs are saf er. The f uel storage tanks on an NGV are thicker and stronger than gasoline or

diesel tanks. There has not been an NGV f uel-tank rupture in more than two years in the United States.

± Natural gas costs are lower than gasoline. On average, natural gas costs one-third less than gasoline at the pump.

± Natural gas is convenient and abundant. A well-established pipeline infrastructure exists in the United States to deliver natural gas to almost every urban area and most suburban areas. There are more than 1,300 NGV f ueling stations in the United States, and more are beingadded every day.

± Natural gas prices have exhibited significant stability compared to oil prices. Historically,

natural gas prices have exhibited significant price stability compared to the prices of petroleum-based f uels. This stability makes it easier to plan accurately for long-term costs.

± NGVs have lower maintenance costs. Because natural gas burns so cleanly, it results in lesswear and tear on the engine and extends the time between tune-ups and oil changes.

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Disadvantages

One of the biggest complaints about NGVs is that

they aren't as roomy as gasoline cars.

This is because NGVs have to give up precious

cargo and trunk space to accommodate the f uel

storage cylinders.

Not only that, these cylinders can be expensive

to design and build a contributing f actor to the higher overall costs of a natural-gas vehicle

compared to a gasoline-powered car.

lpg n ngv

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