Natural gas powers about 175,000 vehicles in the United States and roughly 23 million vehicles worldwide. Natural gas vehicles (NGVs), which can run on compressed natural gas (CNG), are good choices for high-mileage, centrally fueled fleets that operate within a limited area. For vehicles needing to travel long distances, liquefied natural gas (LNG) is a good choice. The advantages of natural gas as a transportation fuel include its domestic availability, widespread distribution infrastructure, low cost, and clean-burning qualities.
The horsepower, acceleration, and cruise speed of NGVs are comparable with those of equivalent conventional vehicles. And, compared with conventional diesel and gasoline vehicles, NGVs provide emissions and environmental benefits.
Depending on the miles per gallon and the average miles driven per year, the return for converting a vehicle to CNG operations can be realized quickly. Even though the initial investment can be expensive, a large portion of the investment (the pressurized portion) can be reused on multiple vehicles since it has a twenty-year lifespan. This transfer of equipment can decrease the cost of the next conversion to as low as 25% to 30% of the original conversion cost.
Engine Conversions
A converted engine is one modified to use a different fuel or power source than the one for which it was originally designed. In an alternative fuel vehicle (AFV) conversion, a conventional vehicle engine from an original equipment manufacturer (OEM) is altered to run on an alternative fuel like natural gas.
Dedicated and Bi-Fuel
OEM engines can be converted to “dedicated” configurations, meaning they operate exclusively on an alternative fuel. They can also be converted to “bi-fuel” configurations that have two separate tanks—one for conventional fuel and another for an alternative fuel. The desired fuel is usually accessed by flipping a switch to change tanks.
Conversion System Installation
A conversion system is installed by the system manufacturer or by a qualified system retrofitter (QSR), also referred to as an upfitter or installer.
Vehicle owners and fleet managers interested in pursuing a conversion must work with the manufacturer or an authorized representative. The actual conversion work must be performed by a licensed technician associated with the manufacturer that holds the relevant emissions-related certifications and tampering exemptions.
Manufacturers should provide a comprehensive training program and detailed documentation to their own technicians as well as to QSR technicians to ensure that equipment and components are installed properly. Technicians installing equipment and components should be experts in the fuel or technology they are working with.
The QSR is accountable for the integrity of the conversion system components. It is the manufacturer’s responsibility to ensure the equipment meets the appropriate emissions standards.
Emissions
Natural gas burns cleaner than conventional gasoline or diesel due to its lower carbon content. When used as a vehicle fuel, it can offer life cycle greenhouse gas (GHG) emissions benefits over conventional fuels, depending on vehicle type, drive cycle, and engine calibration. In addition, using natural gas may reduce some types of tailpipe emissions.
Tailpipe emissions result from fuel combustion in a vehicle's engine and are emitted from its exhaust system. The emissions of primary concern include the regulated emissions of hydrocarbons, oxides of nitrogen (NOx), carbon monoxide (CO), as well as carbon dioxide (CO2). Due to increasingly stringent emissions regulations, the gap between tailpipe emissions benefits from natural gas vehicles (NGVs) and conventional vehicles with modern emissions controls has narrowed. That is because the U.S. Environmental Protection Agency (EPA) is holding all fuels and vehicle types accountable to the same levels of air pollutants emitted from vehicle combustion. Still, NGVs continue to provide emissions benefits—especially when replacing older conventional vehicles or when considering life cycle emissions.
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