Fuels

While proponents of a particular alternative fuel system will point out the merits of one particular system over another, there is broad agreement that alternative fuels as a group represent a tremendous improvement over older more traditional diesel fuel and standard engines.

Clean Diesel

The words "clean diesel" strike many people as an oxymoron. But a combination of sulphur reductions in diesel fuel, engine advancements and the addition of catalytic exhaust treatment has sparked a technological revolution that has dramatically reduced the environmental impact of diesel powered vehicles. Clean diesel refers to a diesel powered engine that meets current emission standards and regulations adopted by the U.S. Environmental Protection Agency. A study comparing clean diesel and compressed natural gas (CNG), by the Harvard Center for Risk Analysis (HCRA) at the Harvard School of Public Health, found that clean diesel is better for reducing greenhouse gasses. This view is shared by European policy makers and vehicle manufacturers who have considered the threat of global climate change for longer than North Americans have. Bus fleet operations require that safety, service, cost and reliability be factored into every decision. Diesel has long been the choice of transit operators to deliver best on all these demands. In safety terms, diesel scores high marks because, unlike gasoline which is highly flammable, it is extremely stable and difficult to ignite. Its long history of service also means that servicing diesel engines is relatively easy for most transit operators. The existing infrastructure and wide use of diesel means that diesel costs remain competitive with other fuels. Diesel also tends to win high marks in terms of reliability of supply and operation. And while even its most vocal supporters will admit that diesel can never become a completely clean energy alternative, a combination of clean diesel and solid public transit ridership adds up to a profoundly positive impact on the environment.

Diesel

Diesel fuel is a complex mixture of hydrocarbon molecules produced by blending byproducts of crude oil refining. After crude oil is distilled into different components, usually several refinery streams are recombined along with appropriate additives to produce commercial diesel fuel. Other than slight variations in the choice and amount of additives, for the most part the composition of diesel fuel remained unchanged until the advent of clean diesel. Given appropriate resources, public transit systems could convert their fleets to clean diesel and the puff of black smoke that many of us associate with municipal buses would largely be a thing of the past. Particulate matter (PM), a more technical label for this puff of black smoke, is tiny solid or liquid particles that come in many shapes and sizes. PM, like bacteria, is invisible to the naked eye and small enough to be breathed into our lungs.

Biodiesel

Biodiesel is the name for a variety of fuels made from soybean oil, other vegetable oils or animal fats. The concept of using vegetable oil as a fuel dates back to 1895 when Dr. Rudolf Diesel developed the first diesel engine to run on vegetable oil. Biodiesel is the only alternative fuel that can be used directly in any existing, unmodified diesel engine. Because it has similar properties to petroleum diesel fuel, biodiesel can be blended in any ratio with petroleum diesel fuel. The most common blend used today is B20, a blend of 20% biodiesel and 80% regular diesel fuel. Many U.S. federal and state fleet vehicles are already using biodiesel blends in their existing diesel engines. Biodiesel has many advantages as a transport fuel. For example, biodiesel can be produced from domestically grown oilseed plants such as soybeans. Producing biodiesel from soybeans and other domestic crops reduces dependence on foreign petroleum, increases agricultural revenue, and creates jobs. Biodiesel can be CO2 neutral, meaning that the CO2 emitted from burning the fuel is absorbed by the plants being grown to produce the fuel. Major advantages are: it is made from renewable resources; biodegradable; nontoxic; does not smell like a petroleum product; is esssentially free of sulfur. Anyone looking to switch to biodiesel should check with the bus/engine manufacturer before making the change.

Hydrogen

Canadian public transit authorities have a long history of being early adopters of new technology. Today, fuel cells rank as one of the most innovative developments taking place in the public transit industry. They have either very low levels of emissions or none at all, and are extremely quiet and energy efficient, thereby making an important contribution to sustainable mobility. Although hydrogen fuel cells are at the forefront of environmental innovation in public transit, the concept of a fuel cell is over 100 years old. It saw its greatest practical application when it helped put a man on the moon in 1969. Hydrogen fuel cells combine hydrogen and air to make electric power. Their only emission is water clean enough to drink. Hydrogen gas for the fuel cell can be produced by either electrolysis (the chemical breakdown of water) or by the reformation of natural gas or methanol. In either method, the emissions impact is still considered low. Hydrogen has drawbacks such as lower top speeds and shorter range than diesel. However, the technology still holds promise for municipal bus fleets because vehicles are not usually required to travel above 80 kms/hour and they can return regularly to a central filling station. The low-noise quality of fuel cell buses also adds to their suitability for city traffic. Canada's Ballard Power Systems is the world leader in developing, manufacturing and marketing zeroemission proton exchange membrane (PEM) fuel cells for use in transportation. Ballard's PEM fuel cell consists of two electrodes, the anode and the cathode, separated by a polymer electrolyte. At the anode, in the presence of a platinum catalyst, hydrogen fuel dissociates into free electrons and protons (positive hydrogen ions). These free electrons are conducted in the form of usable electric current through the external circuit. The protons migrate through the membrane electrolyte to the cathode. At the cathode, oxygen from air, electrons from the external circuit and protons combine to form pure water and heat. Individual fuel cells produce about 0.6 Volt and are combined into a fuel cell stack to provide the amount of electrical power required.

Propane

Propane is a premium fuel that is portable, clean, versatile and priced competitively in many markets. It is extracted from natural gas and oil refining as a colourless, non-toxic, heavier than air vapour. Propane is not a readily available option for heavy-duty buses. LADOT chose 200 Autogas (propane buses) for its downtown DASH service line. Some grant funding defrayed the marginal additional cost. According to LADOT, the benefits over CNG include: lower infrastructure costs; easy-to-install above ground tanks; and longer range between refueling. Service and maintenance costs are comparable to CNG. Fuel efficiency was slightly below what LADOT had anticipated. Overall, more cost-effective to operate than CNG, electric and hybrid-electric fuel technologies. Vehicle reliability is comparable to CNG, and more reliable than electric and hybrid-electric. Options are limited by the fact that the only current engine available is the Cummins 5.9L, which is not powerful enough to propel a 40-foot bus.

Natural Gas

The first natural gas engines for buses began appearing in 1985. The biggest advantage of natural gas over diesel and gasoline is that it burns more cleanly. Natural gas is primarily methane; a relatively simple molecule, which exists in a gaseous form at room temperature. Like oil, natural gas is a naturally occurring fossil fuel often found in underground reservoirs in conjunction with oil, but sometimes found in separate deposits. Like oil, it is produced from wells drilled at the surface. Unlike oil, however, natural gas does not require extensive refining.

Natural gas: a clean alternative

Natural gas mixes more uniformly than diesel fuel in an engine's ignition chamber, leading to more complete combustion and fewer emissions of several types of pollution. Natural gas is the cleanest burning alternative fuel. Exhaust emissions from natural gas powered vehicles (NGVs) are much lower than those from gasoline-powered vehicles. In addition to these reductions in pollutants, NGVs also emit significantly lower amounts of greenhouse gases and toxins than do gasoline vehicles.. While NGVs do emit methane, another principal greenhouse gas, any slight increase in methane emissions is offset by a substantial reduction in CO2 emissions compared to other fuels. Dedicated NGVs can reduce CO2 emissions by 20 to 30 percent.

Compressed natural gas (CNG)

Canadian buses using natural gas rely on compressed natural gas (CNG). In the U.S., 80 per cent carry CNG, while 20 per cent use liquid natural gas (LNG). CNG uses cylinder tanks that are generally filled with the gas to pressures of 3000 to 3600 pounds per square inch. Even at these high pressures, a CNG storage system takes up between three and four times the space of a conventional gasoline fuel tank and weighs from two to three times as much. This means CNG transit buses weigh several thousand pounds more than diesel buses, although new tanks made of composite carbon fiber are lighter than the older metal tanks. Natural gas in the compressed form continues to be the prevalent choice for transit agencies that operate natural gas buses. . 9% of new buses being made for transit in the US are using CNG. "Operating costs per mile, such as fuel costs and overhaul costs are significantly higher," says Harold Zuschlag, MCI. CNG storage systems add about 3,500 pounds to the weight of a motorcoach, and lowers the range about 50%.

Liquefied natural gas (LNG)

Temperatures of about -232º C are needed to liquefy natural gas. To keep it from warming and returning to its gaseous form, LNG must be stored in specially designed insulated containers that look and operate like large thermos bottles. The advantage of LNG is that it contains much more energy per pound or per unit of space than compressed natural gas. However, because the handling of super-cold LNG involves special procedures and training, its use is currently limited to a small number of heavy-duty truck and bus fleets, which have their own refueling equipment and trained staff. LNG is also not readily available in all cities.