Clean Air

Facts & figures

Bio-diesel and low-sulfur diesel

RTD remains on the leading edge of the transit industry with respect to our work with alternative fuels and pollution reduction technology. The RTD fleet operates on ultra-low sulfur diesel fuel, which contains 95 percent less sulfur, and on Compressed Natural Gas. Sulfur produces particulate emissions during the engine combustion process. The use of ultra-low sulfur diesel fuel results in noticeable Particulate Matter (PM) emissions reductions.

Compressed natural gas

Over 20 years ago, RTD began to experiment with various alternative fuels including methanol, propane and Compressed Natural Gas (CNG) in the dally operation of our standard passenger buses and support vehicles. In 2000, RTD introduced a fleet of 36 new mall buses that operate on CNG in electric-hybrid configuration. To support RTD’s CNG buses, RTD has a CNG fueling station at its District Shops facility.

Electric hybrids

RTD is one of the pioneers in the use of true electric-hybrid buses in transit service with our 16th Street Mall Shuttle hybrid buses fueled by Compressed Natural Gas (CNG). These mall shuttle buses take advantage of the latest advanced technology series electric-hybrid propulsion systems. They use a combination of conventional internal combustion engines powered by CNG and electric motors. A small Ford 2.5 liter engine drives a generator which in turn charges a set of batteries. These batteries provide electrical power to propel two electric motors that drive the rear wheels. Each bus can carry up to 115 passengers under the horsepower produced by an engine that is as small as the engine in the popular Toyota Prius hybrid passenger car. The mall buses are quiet and have very low exhaust emission. This fleet of electric-hybrid buses is one of the most successful fleets of hybrid buses in the country. For our hard work with electric-hybrid buses, we received the prestigious award from the Department of Energy’s National Renewable Energy laboratories for leadership in promoting renewable energy and energy efficiency.

Next-generation electric hybrids

RTD is currently testing a new generation of hybrid electric mall shuttle vehicles. RTD has put in service two new mall shuttle buses powered by an all-electric drive train using the state-of-the-art lithium ion batteries for energy storage. The batteries are used to power the electric motors, which drive the bus, and all accessories such as air conditioner, power steering, lighting, etc. The batteries recover kinetic energy from the bus brakes via regenerative braking to extend range. The batteries are recharged by a 30kW micro-turbine. The turbine is shut down when the battery charge is sufficient for operation. The bus will run on all drive operation part of the day. It can also be plugged in at night for recharging.

To further enhance the use of hybrid technology, RTD now operates in regular city transit service nine hybrid buses equipped with the highly advanced parallel hybrid system designed and produced by General Motors. These buses are driven by both a small diesel engine typically found in a pickup truck and electric motors integrated in a transmission-like component. The hybrid buses are showing about 15 to 30 percent improvement in fuel consumption as compared to the conventional diesel buses.

Intelligent transmission shifting

To reduce fuel consumption, RTD has worked with the bus transmission manufacturers to implement an intelligent transmission shifting program into its transit buses. Taking advantage of the increased computer power of the transmission electronic controllers, RTD has programmed the transmissions to select the shifting points based on the terrain (flat or steep roads), bus load, acceleration power available from the engine, and road surface resistance. The intelligent program automatically selects the most fuel efficient shift pattern according to the road conditions to achieve a fuel savings of between 5 to 10 percent as seen in RTD operating conditions. RTD has implemented this intelligent shift feature into its bus fleet which has resulted in reduction of fleet energy consumption, green house and other gaseous emissions.

Replacement of high-emission buses

In other efforts to minimize vehicle emissions, RTD continues to phase out the use of old high-emission buses. RTD is in the process of replacing over 500 old buses with new, near-zero-emission buses. In 2014, RTD put into service over 200 new buses. All of the new buses are powered by clean burning engines, which are equipped with highly advanced emission reduction devices and certified to meet the most stringent EPA emission regulations at the time of manufacture. These buses reduce exhaust emissions as much as 90 percent when compared to some of the old, high-emission buses which they replaced.

ReFUEL research laboratory

RTD District Shops has housed one of the premier engine and fuel research labs in the nation, the Renewable Fuels and Lubricants (ReFUEL) Research laboratory. The lab is being operated by the Department of Energy’s National Renewable Energy laboratories. Many of RTD’s alternative fueled test buses were thoroughly tested by the lab for exhaust emission reduction as compared to regular diesel buses. RTD maintains high exhaust emissions standards of two times more stringent than the requirements by the state of Colorado. If a bus fails an emissions test, it is taken off the streets immediately to remedy the problem. To make sure RTD buses are the cleanest in the state, RTD maintains one of the largest diesel fleet self-certification stations in the state.

Updated January 2015