The Latest Way to Save Gas: Auto Stop-Start

November 7, 2016 •

The Latest Way to Save Gas: Auto Stop-Start(image via Toyota)

Ever wonder how much gas you waste idling at red lights? Well auto industry engineers have, and their solution is “auto stop-start.”

A car or truck with auto stop-start shuts off the engine when the vehicle isn’t moving. Lift your foot off the brake, (or depress the clutch,) and it fires up the motor again. In the best systems this happens faster than you can move your foot to the gas pedal, and with not a sound or shake.

Real Savings

Already common on hybrids and cars in Europe, the technology is now appearing in the US. Interestingly though, the benefits don’t show up in EPA gas mileage numbers because those tests have very little idling. But for drivers spending too many hours at red lights, the savings could be significant.

Skeptical of the benefits, back in 2014 the auto experts at Edmunds.com tested three cars equipped with auto stop-start. On a route with lots of idling they saw savings of around 10 percent. In other words, a vehicle that got 20 mpg City without stop-start could achieve 22 mpg with it.

Hardware Upgrades

More frequent restarts demand a stronger starter, because it gets more use, and a heavier-duty battery. Not only must the battery more often deliver juice for starting, but it needs to keep a number of electrical systems running while the engine is stopped.

To avoid overheating, coolant must continue flowing through the engine, so an electric pump is needed. The transmission needs hydraulic pressure, so that’s another electric pump. And occupants want cool or warm air, so the fan must keep running. But what of the air conditioning?

Air conditioning is a challenge for auto stop-start systems because it draws so much power. That’s why most systems leave the engine running if the AC is on, or if they do shut it off, they only do so for a short time. It’s also why Edmunds did their testing with AC off. Turn it on and most of the gas savings disappear.

Get Used To It

Auto stop-start surprises drivers not expecting it, but a few minutes city driving is usually all it takes to reveal the benefits. For those who find the restarting intrusive most manufacturers provide a “disable” function: just look in the manual for instructions. But drivers who stick with it will almost certainly save a few bucks.