Quantifying the Effects of Idle-Stop Systems on Fuel Economy in Light-Duty Passenger Vehicles 2012-01-0719

Vehicles equipped with idle-stop (IS) systems are capable of engine shut-down when the vehicle is stopped, and rapid engine re-start for the vehicle launch. This capability reduces fuel consumption and emissions during periods where the engine is not being utilized to provide propulsion or to power accessories. IS sytems are a low-cost and fast-growing technology in the industry-wide pursuit of increased vehicle efficiency, possibly becoming standard features in European vehicles in the near future. In contrast, there are currently only three non-hybrid vehicle models for sale in North America with IS systems, and these are distinctly low-volume models.

As part of the United States Department of Energy's Advanced Vehicle Testing Activity (AVTA), ECOtality North America has tested the real-world effect of IS systems on fuel consumption in three vehicle models imported from Europe. These vehicles were chosen to represent three types of systems: (1) spark ignition (SI) with 12 V Belt Alternator Starter (BAS); (2) compression ignition (CI) with 12 V BAS; and (3) direct-injection SI (DISI) with 12 V BAS/combustion restart The vehicles have undergone both dynamometer and on-road testing, and the test results show somewhat conflicting data. The laboratory data and the portion of the on-road data in which the driving is conducted on a prescribed route with trained drivers produced significant fuel economy improvement. However, the fleet data do not corroborate the improvement even though the data show significant engine-off time. It is possible that the effects of the verying driving styles and routes in the fleet testing overshadowed the fuel economy improvements. More testing with the same driver over routes that are similar for the IS system enabled and disabled modes is recommended.

There is anecdotal evidence that current Environmental Protection Agency (EPA) fuel economy test procedures do not capture the fuel economy gains that IS systems produce in real-world driving. The program test results provide information on the veracity of these claims and can help guide automotive manufacturers debating whether to include this system in future models.