Alternative Fuel / Advanced Technology Vehicles
Emission Control Technologies for Alternative Fuel / Advanced Technology Vehicles
Alternative Fuel / Advanced Technology Vehicles
- Components for Fuel Cell Technology: A fuel cell is an electrochemical device that generates electricity by reacting a fuel (usually hydrogen gas) with a catalyst. In a hydrogen fuel cell reactor, hydrogen molecules are broken down to protons and electrons. Protons flow through a membrane and combine with oxygen from the air to form water. The electrons flow from the anode to the cathode to create electricity. As long as pure hydrogen and a source of oxygen are supplied to the fuel cell, it will produce electrical energy. Fuel cells can operate at efficiencies two to three times that of the internal combustion engine, require no moving parts and produce only heat and water as by-products.
- Hybrid Vehicles: Hybrid electric vehicles (HEVs) typically combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle. This combination provides lower exhaust emissions, with the power, range, and convenient fueling of the conventional gasoline and diesel vehicles. HEVs are powered by two energy sources: an energy conversion unit, such as a combustion engine or fuel cell; and an energy storage device, such as batteries or ultracapacitors. The energy conversion unit may be fueled by gasoline, methanol, compressed natural gas, hydrogen, or other alternative fuels. The batteries in HEVs are recharged using regenerative braking or by using an on-board generator.
A subset of hybrid vehicles that are getting more attention as fuel prices are increasing are plug-in hybrids or PHEVs. There are no mass produced vehicle models using this technology at this time, however, there are custom manufacturers who will convert a HEV to a PHEV. The conversion basically involves additional batteries and modification of the control system to allow higher speed operation under only electrical power before calling on the combustion power system. Typically, PHEVs can operate up to 100 miles under purely electric power and can be recharged by plugging into a domestic power source. PHEVs can achieve up to 100 miles per gallon which is about double the fuel economy of conventional HEV power vehicles.
- Flexible-Fuel Vehicles: A flexible-fuel vehicle (FFV) has a single fuel tank, fuel system, and engine. The vehicle is designed to run on unleaded gasoline and an alcohol fuel, usually ethanol, in any mixture of the two. The engine and fuel system in a FFV must be adapted slightly to run on alcohol due to its corrosivity and combustion characteristics. There must also be a special sensor in the fuel line to analyze the fuel mixture and control the fuel injection and timing to adjust for different fuel compositions. Flexible-fuel vehicles employ three-way catalyst systems to reduce exhaust emissions of CO, NOx, and hydrocarbons, including aldehydes.
- Ethanol Fuel: Ethanol is an alcohol-based fuel that is produced by fermenting and distilling crops that have been converted into simple sugars. Typically in the U.S., it is made from starch crops, primarily corn. It can also be made from sugar crops or from agricultural waste and "cellulosic biomass" such as trees and grasses. Because of its high oxygen content, ethanol is cleaner burning than gasoline. In the U.S., ethanol is sold in two gasoline blends: E85, which is composed of 85% ethanol and 15% gasoline (formulated for flexible-fuel vehicles), and E10, which is composed of 10% ethanol and 90% gasoline (formulated for conventional gasoline vehicles). The emission control technology suitable for engines operating on ethanol and gasoline blends would be similar to emission control technology used for gasoline-fueled vehicles.
- Biodiesel Fuel: Biodiesel is a renewable fuel that can be manufactured from new and used vegetable oils and animal fats. It is produced by reacting vegetable or animal fats with methanol or ethanol to produce a lower-viscosity fuel that is similar in physical characteristics to diesel, and which can be used neat or blended with petroleum diesel for use in a diesel engine. Biodiesel is commonly blended into petroleum-based fuel at low levels, i.e., 20 percent (B20) or less. Biodiesel can be used in its pure form (B100), but may require certain engine modifications to avoid maintenance and performance problems. Recently ASTM has developed specifications for B100 used as a fuel for mobile sources and specifications are under development for biodiesel blends like B20. Typical emissions benefits of B20 include a 10 percent decrease in CO, up to a 15 percent decrease in PM emissions, a 20 percent decrease in sulfate emissions, and a 10 percent decrease in HC emissions. In some tests, B20 has shown a slight increase in NOx emissions (up to three percent) on some types of existing heavy-duty engines. The emission control technology suitable for engines operating on biodiesel blends would be similar to emission control technology used for diesel-fueled vehicles.
- Hydrogen: Engine manufacturers have developed prototype internal combustion vehicles that can operate on hydrogen or dual fuel version that can use gasoline fuel as a second fuel source. These engines, called hydrogen internal combustion engines or HICE can operate on either compressed or liquid hydrogen using a slightly modified internal combustion engine. The resulting emissions from HICE are mostly water and NOx because they still involve a high temperature combustion process. The engine out emissions can be controlled to low levels requiring only a TWC emission control device to achieve California SULEV emission levels.
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