In recent years, automakers have introduced engine stop systems in cars to tackle the problem of idling engines wasting fuel. But such systems aren't appropriate for buses and food delivery trucks that need the engine running to power onboard air conditioning and refrigeration systems. Now a team at the University of Waterloo has found a way to harness the wasted energy from a vehicle as it slows down, and redirect it to secondary systems to allow the engine to be shut down when a vehicle stops.

"An idling vehicle essentially operates at five per cent efficiency," says Amir Khajepour, professor of mechanical engineering at Waterloo, "meaning the vast majority of the fuel a bus or delivery truck uses when it is stopped is being wasted."

Concentrating on service vehicles that have significant auxiliary power needs (such as refrigeration), the study initially recorded the driving, braking and idling patterns of these vehicles. Computer models then simulated these routes in the lab with engines hooked up to secondary battery systems. Here the team developed a process to best collect and harness the wasted energy.

"By harnessing the energy a vehicle wastes as it is slowing down and redirecting it to a secondary battery system, these vehicles can be turned off without shutting off systems such as refrigeration and air conditioning units," explains Professor Khajepour.

The team estimates the cost of installing this secondary power system in a current vehicle would be offset in fuel savings within one to two years. On a mass scale a system such as this could save large organizations millions per year in fuel costs.

The researchers see this as a useful short-term step to increase fuel efficiency and reduce emissions before the inevitable larger scale shift to electric-powered vehicles.

"Given that most companies or governments cannot afford to transition their entire fleets over to cleaner vehicles all at once, this system could represent a cost-effective way to make current vehicles more fuel efficient in the short term," adds Professor Khajepour.

The team's study was recently published in the journal Energy.