Vehicle Emissions Reduction

Natural Gas

Natural gas (which is 85-99% methane) has many desirable qualities as a fuel for spark-ignition engines. Clean burning, cheap, and abundant in many parts of the world, it already plays a significant vehicular role in Russia, Argentina, Italy, Canada, New Zealand, and the US Recent advances in the technology for natural gas vehicles and engines, new technologies and international standardization for storage cylinders, and the production of new, factory-manufactured natural gas vehicles in a number of countries have all combined to boost the visibility and market potential of natural gas as a vehicle fuel.

Most of the natural gas vehicles (NGVs) now in operation are retrofits, converted from gasoline vehicles. The physical properties of natural gas make such a conversion relatively easy. Typical conversion costs are in the range of US$ 1,500 to 4,000 per vehicle, and are due mostly to the cost of the onboard fuel storage system. At present fuel prices, many high-use vehicles can recover this cost in a few years, due to savings on fuel.

Natural gas engines can be grouped into three main types on the basis of the combustion system used.

These types are

• stoichiometric
• lean-burn
• dual-fuel diesel

Most of the natural gas vehicles now in operation have stoichiometric engines, which have been converted from engines originally designed for gasoline. Such engines may be either bi-fuel (able to operate on either natural gas or gasoline) or dedicated to natural gas. In the latter case, the engine can be optimized for natural gas by increasing the compression ratio and making other changes, but this is not usually done in retrofit situations because of the cost. Nearly all present light-duty natural gas vehicles use stoichiometric engines, with or without three-way catalysts, as do a minority of heavy-duty natural gas vehicles.

Lean-burn engines use an air-fuel mixture with much more air than is required to burn all of the fuel. The extra air dilutes the mixture and reduces the flame temperature, thus reducing engine-out NOX emissions, as well as exhaust temperatures. Because of reduced heat losses and various thermodynamic advantages, lean-burn engines are generally 10-20% more efficient than stoichiometric engines.

Without turbocharging, however, the power output of a lean-burn engine is less than that of a stoichiometric engine.

With turbocharging, the situation is reversed. Because lean mixtures knock less readily, lean-burn engines can be designed for higher levels of turbocharger boost than stoichiometric engines, and can thus achieve higher power output. The lower temperatures experienced in these engines also contribute to engine life and reliability. For these reasons, the great majority of heavy-duty natural gas engines are of the lean-burn design. These include a rapidly growing number of heavy-duty, lean-burn engines developed and marketed specifically for vehicular use.

Dual-fuel diesel engines are a special type of lean-burn engine in which the air-gas mixture in the cylinder is ignited not by a spark plug but by injection of a small amount of diesel fuel, which self-ignites. Most diesel engines can readily be converted to dual-fuel operation, retaining the option to run on 100% diesel fuel if gas is not available. Because of the flexibility this allows, the dual-fuel approach has been popular for heavy-duty retrofit applications. Current dual-fuel engine systems tend to have very high HC and CO emissions, due to the production of mixtures too lean to burn at light loads. However, new developments such as timed gaseous fuel injection systems promise to overcome these problems.

Because natural gas is mostly methane, natural gas vehicles (NGVs) have lower exhaust NMHC emissions than gasoline vehicles, but higher emissions of methane. Since the fuel system is sealed, there are no evaporative or running-loss emissions, and refueling emissions are negligible.

Cold-start emissions from NGVs are also low, since cold-start enrichment is not required, and this reduces both NMHC and CO emissions. NGVs are normally calibrated with somewhat leaner fuel-air ratios than gasoline vehicles, which also reduces CO emissions.

Given equal energy efficiency, CO₂ emissions from NGVs will be lower than for gasoline vehicles, since natural gas has lower carbon content per unit of energy. In addition, the high-octane value for natural gas (RON of 120 or more) makes it possible to attain increased efficiency by increasing the compression ratio. Optimized heavy-duty NGV engines may approach diesel efficiency levels.

NOX emissions from uncontrolled NGVs may be higher or lower than comparable gasoline vehicles, depending on the engine technology, but are typically somewhat lower. Light-duty NGVs equipped with modern electronic fuel control systems and three-way catalytic converters have achieved NOX emissions more than 75% below the stringent California ULEV standards.

In the last few years, a number of heavy-duty engine manufacturers have developed diesel-derived lean-burn natural gas engines for use in emissions-critical applications such as urban transit buses and delivery trucks. These engines incorporate low NOx technology used in stationary natural gas engines, and typically an oxidation catalyst as well. They are capable of achieving very low levels of NOX, particulate, and other emissions (less than 2.0 g/BHP-hr NOX and 0.02 g/BHP-hr particulate with high efficiency, high power output, and (it is anticipated) long life.

Owing to the difficulty of transportation, the costs of natural gas vary greatly from country to country, and even within countries. Where gas is available by pipeline from the field, its price is normally set by competition with residual fuel oil or coal as a burner fuel. Compression costs for CNG use can add additional costs, however, depending on the size of the facility and the natural gas supply pressure.

The cost of LNG varies considerably, depending on specific contract terms (there is no effective "spot" market for LNG). The cost of small-scale liquefaction of natural gas is uneconomic in comparison to CNG in most cases. Where low-cost remote gas is available, however, LNG production can be quite economic.