“Auto stop” is a car feature that turns the engine off when you stop the car (for example at a red light) and restarts it when you come off the brake or hit the gas, automatically, to save gasoline.
How long does a stop have to be, to make this worthwhile? In terms of total cost of operation, which would take into consideration gasoline and wear and tear on the engine, starter, battery, et cetera. I doubt a one second stop saves enough gas to cover the wear and tear, but imagine a one minute stop more than covers it. Where’s the break even point?
This should be useful to the driver, who usually has some idea how long a stop will last. On my car, lightly holding the brake enough to keep the car from creeping ahead does not trigger auto stop, but pushing the brake further does. How do I know when to trigger it optimally?
I don’t have an answer but cars are so efficient at idle I suspect it takes a while. In fact, it may be tougher on the engine if people unconciously hit the acceleraor harder because the engine is stopped rather than rolling into a light press on the gas.
I find I’ve already acquired a new habit. I relax the brake a bit, but not enough to let the car roll. This causes the engine to start without moving the car, avoiding that bump that’s like getting kicked in the rear. Then I come the rest of the way off the brake. So, it’s a very brief pause or step in the motion of releasing the brake. I bet it’s no more than half a second.
Why do you think there is any break-even? What is lost when the engine isn’t turning? 100% of the wear that occurs at idle isn’t occurring. 100% of the fuel burn that occurs at idle isn’t occurring.
Explain what thing(s) you think are being consumed by a stop/start cycle then perhaps we can discuss whether those concerns have merit.
There’s wear on the starter each time it’s used. Having to start the car dozens of times per trip will wear it out faster than if it only had to start it once. The gas savings will need to be balanced against the cost of replacing the starter more often. Replacing starter is going to be several hundred dollars.
Cars typically have a way to disable auto stop. It might be a button that you have to activate each trip or there may be a way in settings to permanently disable it.
Well, first of all, auto stop-start was never designed to save money. It might save money over the long term, but that’s not it’s purpose, which is to eke out a little bit more fuel efficiency for CAFE standards.
My guess is that it will just barely pay for itself. The starter is heavier-duty (so more expensive) and even so, it probably wears out faster. If you ever had to replace the starter due to AS/S, that would likely wipe out 10 years of gasoline savings.
AI says a car might burn 64 ounces of gasoline per hour at idle. Sounds about right. So, that’s, say, $2/hour of idling. If a starter costs $500 to replace, you’d have to have the engine shut off for 250 hours to pay for it. In my Bronco, the AS/S will only shut off for around 30 seconds before restarting, so that’s 500 full shutdowns. So, maybe that’s doable, especially if you do a LOT of stop-and-go driving (I don’t).
I drive around 12,000 miles / year - with a fair amount of highway miles. At an average speed of 40 mph, that’s 300 hours of driving/ year. I can’t imagine that the AS/S is operating more than a few hours/year, but as they say - your milage may vary.
One other aspect is that there may be cost savings from having other car components not running during auto stop, such as the alternator, water pump, valves, ignition coils, etc. It might be that auto start causes the starter to wear out more often, but other components might last longer because they are running less.
There would probably need to be a large scale, real world test to determine the overall cost impact of auto stop. For instance, a rental fleet that has half the cars with auto stop enabled and the other half with it disabled. They could compare the overall maintenance costs and fuel mileage of the two groups of cars to see what difference auto stop has.
Zactly. There are cars that stop the crank with one cylinder just past TDC. When it’s time to restart the car, they fire the one fuel injector and spark plug, that cylinder fires, and the engine is running. The starter motor isn’t even used.
@beowulff makes the valid point that auto stop is aimed at compliance with US CAFE regulations and also with EU “no idling pollution” regulations. They aren’t specifically about cost savings, although there is some.
Back when we were kids, Dad had to replace the starter motor in the 1963 Dodge about every 40K miles. The starter motor in a modern car is rated for umpteen hundred thousand miles’ worth of starting (and auto stopping). That extra life might add $100 to the purchase price of the car when bought new. It (most likely) isn’t adding anything to the lifetime maintenance cost of the car. The odds greatly favor that the car will be crashed, or crushed, with a fully functional starter motor with lots of good life left in it.
It’s a mistake to apply our 1960s-era instincts to 2020s era cars.
Amazing but (apparently) true. I had a VW Jetta as a rental that seemed to automagically start at stop lights without any sign of the starter motor cranking.
Next time any of us start our car from cold, pay attention to how long the starter motor runs before the engine takes over. It’s probably 1 or 2 cylinders = about 1/2 a crank rotation. Blink and you’ll miss it.
Think back to starting a 1960s or 1970s car: you’d crank for several seconds, perhaps pumping the accelerator pedal to get it to light. And maybe have to do that 3 or 4 times before it lit off. All that after setting the automatic choke (and giving a shot of prime) with one full pedal press before cranking.
IOW, one cold start on a modern car is about 5% as much cranking and hence starter motor wear as it was on an old car. If that much.
While I cannot answer the OP’s question, I will note that Trump’s BBB eliminated any and all penalties levied on automakers for not meeting CAFE standards. This makes me wonder what engineering changes will be upcoming in future model years:
I absolutely hate the “auto stop” feature and would not shed a single tear if manufacturers were to eliminate it completely.
Yeah I’d be interested to know too. My, possibly over skeptical, opinion on it….
The manufacturer will have very carefully studied the effect on MPG, and tuned it very carefully so it does give as big an MPG increase as possible. As they have to publish MPG numbers (so they will have tuned to maximize whatever circumstances is covered by the “City” MPG numbers)
They will have done very little to study or prevent wear or tear, as they don’t need to publish numbers for reliability. In fact an increase in replacement starter motors, etc is probably a good thing for them, as long it doesn’t get so bad it gives them a reputation for unreliability.
There is wear and tear on the battery. I have a 2024 Subaru Outback, and it was less than eight months old when the battery needed to be replaced under warranty. The mechanic said it was likely because of auto stop start being enabled and that it was not an uncommon problem.
For a vehicle with a starter designed for this use the extra starts are no big deal. Many golf carts have an engine that only runs when you press the accelerator (gas pedal). Every single time you move these carts the engine starts and then stops when you take your foot off. They are designed that way and last just about forever.
As others have noted some of the advanced cars don’t even use the starter for these hot starts. If you think that is an engineering marvel, you should see how a modern snowmobile can operate in reverse with out having a reverse transmission.
I find this auto stop feature on cars annoying, but not for any technical reason. It’s just something that is new and different that will take time to get used to.
I saw an experiment on this in a video and the experimenter came to the conclusion that starting a car is the equivalent of running the car for seven seconds. Sorry, can’t find the video anymore.
