Classes are back in session, and school bus fleets are out in full force. School districts have upgraded existing vehicles and added new buses to their fleets. Cost and reliability continue to play key roles in buying decisions, but protecting the environment with cleaner, lower-emission buses has added a new component to the bus-purchase equation.

This has led some schools to experiment with buses that use compressed natural gas (CNG) to fuel their engines. At the same time, environmental improvements in traditional diesel-fueled engines have made diesel buses a viable choice for ecologically conscious school systems.

When deciding between CNG- and diesel-fueled engines, school officials must not only consider each fuel's advantages based on environmental effects, but also evaluate cost and reliability. Here are some of the key topics school officials should consider.

Environmental benefits Often, the assertions that CNG-fueled engines generate less emissions are based on comparisons with the diesel engines of yesteryear. Natural-gas engines are not cleaner than modern-day diesel engines in all the regulated emissions components. Today's diesel engines have lower levels of carbon monoxide and hydrocarbon emissions than most of the available CNG-fueled engines. In addition, both fuels meet the Environmental Protection Agency (EPA)'s emission standards.

Proponents of CNG, such as The California Air Resources Board, declare that diesel-run bus engines produce more exhaust emissions than the alternative-fueled engine. It states that diesels built prior to 1977 emit three times as much oxides of nitrogen and four times as much particulate matter as the CNG engines.

However, modern diesel engines manufactured since 1998 can provide emissions levels that rival their CNG counterparts. The introduction of low-sulfur diesel fuel and the application of electronic controls to today's diesel engines have resulted in particulate emissions 75 to 90 percent lower than earlier diesel engines. Current diesel engines are smokeless, and new developments are emerging.

Cost factors Most school bus manufacturers do not offer CNG-fueled engines because the benefits-such as lower emissions and diversification of fuel resources-do not justify their higher cost. A CNG-fueled school bus with enough capacity to support a normal day's operation can cost $30,000 to $40,000 more than a comparable diesel-fueled bus. In addition, the charging station required for refueling costs at least $250,000, depending on how quickly the bus operator wants to refuel.

CNG initially offers apparent fuel cost savings. However, often diesel-equipped buses have significantly higher fuel economies than similarly equipped CNG buses. When these fuel economies are factored in, the cost advantage of CNG is not so clear.

Schools often pursue conversion of buses to CNG based on the availability of subsidies. Some federal and state government agencies offer tax deductions and low-interest loan programs for clean fuel vehicles. The government also provides funds to support the extra cost of the CNG vehicle, and often the gas supplier provides the charging station at little or no cost. With subsidies, the school district can get a good deal, and the cost of owning and operating the buses may be economically feasible.

Reliability and protection Diesel-fueled school buses have little downtime. They can run continuously for extended periods of time. Diesel engines also have long oil-change intervals, electronic-controlled no-maintenance fuel-injection systems and no ignition system, resulting in simple maintenance and infrequent servicing for the most part.

Diesel engines can be more secure than CNG-fueled engines. Diesel is not explosive, has low flammability and is not stored under pressure. In contrast, natural gas can be a potentially dangerous fuel. CNG-fueled school buses need additional, costly equipment because of the volatile nature of the fuel. Methane sensors are used in both the engine and passenger compartments to alert drivers to leaks.

In California, the Energy Commission's Safe School Bus Clean Fuel Efficiency Demonstration Program requires that all engine compartments on the program's CNG buses be equipped with automatic fire-suppression systems. Activated by temperature sensors, these systems flood the engine with a dry-chemical fire retardant material if an engine fire occurs.

In addition, personnel who fuel, service and maintain the CNG-operated buses must be specially trained. Fueling a CNG bus is a delicate procedure and must be performed only by a qualified service technician.

The cylinders also are sensitive to surface damage, which can weaken the structure and can lead to fatigue failures after repeated discharging and recharging. Federal regulations require that storage cylinders be removed and subjected to extensive testing after no more than 15 years of service to determine their current strength and durability. Many school buses are in service for this time and longer. The cost of inspection could exceed the value of a bus at that age and lead to forced replacement.

The future Engine manufacturers are committed to lowering emissions. This is evident through the evolving technologies of competing fuels, such as CNG and diesel. Key elements of this evolution include continued refinements of electronic engine controls, fitting engines with cleaner diesel fuels and the use of effective after-treatment devices. Other technologies now in development include electric, hybrid-electric and fuel cell buses. These technological investigations will lead us into the future with cost-effective, viable solutions that will protect student passengers, as well as the environment.