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Generally, the goal of a motor vehicle pollution control program is to reduce emissions from motor vehicles in-use to the degree reasonably necessary to achieve healthy air quality as rapidly as possible or, failing that for reasons of impracticality, to the practical limits of effective technological, economic, and social feasibility. A comprehensive strategy to achieve this goal includes four key components: increasingly stringent emissions standards for new vehicles, specifications for clean fuels, programs to assure proper maintenance of in-use vehicles,(I/M) and transportation planning and demand management. These emission reduction goals should be achieved in the most cost effective manner available.

Elements of a Comprehensive Vehicle Pollution Control Strategy

Over the course of the past 30 years, pollution control experts around the world have come to realize that cleaner fuels is a critical component of an effective clean air strategy. Fuel quality is now seen as not only necessary to reduce or eliminate certain pollutants directly (e.g., lead) but also a precondition for the introduction of many important pollution control technologies (e.g., lead and sulfur). Further, one critical advantage of cleaner fuels has emerged its instant impact on both new and existing vehicles. It is important to know that newer engine technology only will work as expected if matching fuel quality is used.

In addition to conventional fuels, gasoline and diesel fuel, many countries have identified significant benefits associated with a shift to alternative fuels, especially compressed natural gas (CNG), liquefied petroleum gas (LPG or propane) and ethanol.

Conventional fuel improvements should clearly distinguish between primary steps - removing lead from gasoline and dramatically reducing sulfur in gasoline and diesel, and the addition of detergent additives - and secondary steps - reducing Reid vapour pressure (RVP) and benzene content of gasoline.


The pollutants of greatest concern from gasoline-fuelled vehicles are carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), lead and certain toxic hydrocarbons such as benzene. Each of these can be influenced by the composition of the gasoline used by the vehicle. The most important characteristics of gasoline with regard to its impact on emissions are - lead content, sulfur concentration, volatility and benzene level.


Diesel vehicles emit significant quantities of both NOx and particulate (PM). Reducing PM emissions from diesel vehicles tends to be the highest priority because PM emissions in general are very hazardous and diesel PM, especially, is likely to cause cancer. To reduce PM and NOx emissions from a diesel engine, the most important fuel characteristic is sulfur because sulfur in fuel contributes directly to PM emissions and because high sulfur levels preclude the use of the most effective PM and NOx control technologies. Secondly, reduction of aromatic and polyaromatic hydrocarbons (PAH) in the diesel fuel directly reduces PAH emissions and thus the cancer risk.


Alternative fuels include compressed natural gas (mainly composed of methane), biogas (mainly methane), methanol, ethanol, dimethyl ether (DME), hydrogen, electricity, vegetable oils, liquefied petroleum gas (composed of propane or butane), synthetic liquid fuels derived from coal and various fuel blends, such as gasohol. Depending upon the feedstock and the process used to make these fuels, they can be very low or very high in greenhouse gas (GHG) emissions. For example, methanol made from coal would approximately double GHG emissions compared to conventional gasoline, whereas methanol made from natural gas would be slightly lower than gasoline and made from cellulose would be about 60 percent lower.

Natural Gas

Because natural gas is mostly methane, natural gas vehicles (NGVs) have much lower non-methane hydrocarbon emissions than gasoline vehicles, but higher emissions of methane. Since the fuel system is sealed, there are no evaporative emissions and refuelling emissions are negligible. Cold-start emissions from NGVs are also low, since cold-start enrichment is not required. In addition, this reduces both VOC and CO emissions. NOx emissions from uncontrolled NGVs may be higher or lower than comparable gasoline vehicles, depending on the engine technology, but are typically slightly lower. Light-duty NGVs equipped with modern electronic fuel control systems and three-way catalytic converters have achieved NOx emissions more than 75 percent below the stringent California Ultra Low-Emission Vehicle (ULEV) standards.

As a substitute for diesel, NGVs should have somewhat lower NOx and substantially lower PM emissions unless the diesel vehicle is burning ULSD and is equipped with a PM filter.

Given equal energy efficiency, GHG emissions from NGVs will be approximately 15 percent to 20 percent lower than from gasoline vehicles, since natural gas has lower carbon content per unit of energy than gasoline. NGVs in perfect condition have about the same GHGs as diesel fuel vehicles. However, for heavy duty NGVs, service intervals must be significantly shorter than for diesel vehicles. Spark plugs and ignition system must be 100% functional, otherwise GHG emissions in the form of unburned methane may increase and up to double total GHG emissions.

Obstacles to the widespread use of NGVs include the absence of transportation and storage infrastructure, cost, loss of cargo space, increased refuelling time, and lower driving range.


Biogas for vehicle use has approximately the same characteristics as natural gas and consists mainly of methane. It is produced by anaerobic digestion of organic waste, and thus has a considerably lower contribution of GHG than fossile methane. The amount of biogas that can be derived from the organic waste for an average city is dependent on the presence of food industries and agriculture.

Liquefied Petroleum Gas (LPG)

Engine technology for LPG vehicles is very similar to that for natural gas vehicles. As a fuel for spark-ignition engines, it has many of the same advantages as natural gas, with the additional advantage of being easier to carry aboard the vehicle.

LPG has many of the same emissions characteristics as natural gas. The fact that it is primarily propane (or a propane/butane mixture) rather than methane affects the composition of exhaust VOC emissions and their photochemical reactivity, and its global warming potential but otherwise the two fuels are similar.

The costs of converting vehicles from gasoline to propane are considerably less than those conversions to natural gas, due primarily to the lower cost of the fuel tanks. As with natural gas, the cost of conversion for high-use vehicles can typically be recovered through lower fuel costs (depending on taxes) within a few years.

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.

LPG, if a by-product from refining can contain substantial amounts of ethane and propene. As those substances are suspected mutagens the LPG standard should limit these compounds.

LPG's major disadvantage is the limited supply, which would rule out any large-scale conversion to LPG fuel.


Ethanol is produced primarily by the fermentation of starch from grains (mostly corn) or sugar from sugar cane. It is most commonly used as an oxygenate in reformulated gasoline and in a gasoline blend called "gasohol." These fuels can be burned in gasoline engines. Specialized engines are necessary in order to burn pure ethanol.

Vehicles burning gasohol will emit slightly more GHG emissions than conventional gasoline fuelled vehicles. Reductions associated with burning pure ethanol depend on the feedstock. Ethanol produced from corn has life cycle GHG emissions about 15 percent less than gasoline vehicles. Ethanol produced from woody biomass (E-100) has GHG emissions 60 to 75 percent below conventional gasoline.

In engines burning reformulated gasoline using ethanol, VOCs and CO are reduced but NOx tends to increase slightly.

A gasohol-fuelled automobile costs no more than a comparable gasoline vehicle. Since ethanol is derived from grains and sugars, the production of ethanol for fuel is in direct competition with food production in most countries. This keeps ethanol prices relatively high, which has effectively ruled out its use as a motor fuel except where, such as in Brazil and the U.S., it is heavily subsidized.

The high cost of producing ethanol (compared to hydrocarbon fuels) remains the primary barrier to widespread use. The limited NOx increase is also a concern. Emissions of toxic compounds but for some aldehydes are substantially lower for alcohol fuels compared to gasoline.

Synthetic fuels

It is possible to produce different synthetic fuels such as methanol, so-called Fischer-Tropsch fuels, di-methyl ether (DME) or hydrogen using a process called "Gas-To-Liquids" (GTL). Both renewable (biomass) and fossil (natural gas, coal and low-value refinery products) feedstocks may be synthesized to the different specifications. Energy efficiency varies for the different combinations and for different production plant sizes. Local emissions can be made low as the resulting fuels can be tailored to the end use.


Biodiesel 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 in a diesel engine .

Engines running on biodiesel instead of or blended with petroleum diesel tend to have lower black smoke and CO emissions, but can have higher NOx and possibly higher emissions of particulate matter. These differences are not very large, however.

The cost of biodiesel fuel is one of the principal barriers making it less attractive to substitute for diesel fuel.


It is now well established that cleaner fuels must be an integral part of a comprehensive and effective motor vehicle pollution control effort. The elimination of lead in gasoline as well as reduction of sulfur from both gasoline and diesel fuel are now well-established base elements of a clean fuels program.

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