23 July 2009

Design keeps buses cool, saves fuel

Moving forward, it will all be about how you can save energy, so a simple redesign of public buses used in hot and dry climates could make passengers more comfortable without the need to use extra fuel running air conditioning.

The majority of passenger trips occur in open window buses, said Sunil Kale of the Department of Mechanical Engineering, at the Indian Institute of Technology Delhi, India.

Worldwide, buses are the principal means of commuting within and between cities, Kale said. For cost reasons, particularly in the developing world, only a small fraction (less than 5%) of inter-city vehicles are air-conditioned. Air-conditioning is not a commercially or environmentally viable alternative to simply opening the side windows and even the doors.

Opened windows should improve airflow and keep passengers cool while the bus moves. However, the cooling effect of open side windows is inadequate for comfort in the hottest parts of the world on a crowded bus.

Kale’s team conducted an aerodynamic study of fluid flow in a 1:25 model. Their findings found a few simple modifications to conventional design could significantly boost cooling airflow with none of the cost or energy requirements of an air- conditioning system.

The turbulent flow into a standard bus does not allow air into all areas. Passengers sitting near or standing in the aisle do not receive any ventilation, while those in the front seats receive airflow from the rear. The team found a wide vent at the front and rear of a bus will draw air into the bus at a much better rate than side windows. A similar boost can happen with adjustable roof vents.

These modifications would allow cooling air to increase the comfort zone of the bus from a mere 11% of the interior volume to more than 50%. This means all passengers will experience some cooling airflow. In addition to improved comfort, there is an overall reduction in drag. However, by sacrificing some drag reduction, you could provide grills and filters to prevent the influx of insects and dust. In addition, a passive evaporative cooling system could fit into the vents to cool the incoming air and further boost the comfort inside the bus.

“In a long-term policy perspective of sustainable transport, buses form an important mode of transport that needs to be strengthened,” Kale said. “Besides improving fuel economy, passenger comfort is a major issue with such buses especially in tropical climates.”

For related information, go to www.isa.org/manufacturing_automation.