Economics and ecologics of auto stop/start

In My Humble Opinion
1

I have a 2022 Subaru Forester with about 60k miles on it, and an auto stop/start feature. That’s the feature that, when you stop the car at a light or something, the engine turns off, then automatically turns on when you release the brake (or, in my manual Mini, when you put your foot on the clutch).

It keeps track of all the gas it has saved using this feature. The total, over 60k miles is 11 gallons. Yay, about $40 in gas.

The battery crapped out at around year 3 and I had to replace it with another special battery that can handle auto stopping and starting, which cost a couple of hundred more than a regular battery (the regular battery I tried first died within a few months).

I have to imagine there’s special hardware and software in the car, and there must be some excess wear and tear on the engine (and, certainly, the battery). All of that to save $40 over four years.

There’s no way this is economical, is there? Are there ecological reasons for it really? Maybe it would reduce emissions in cities, where most stop-and-go traffic is?

OK, maybe the tech is an afterthought bolt-on by Subaru – it certainly seems much less refined than my 2016 Mini stop-start. On the Subaru, you have to push the brake pretty hard to get it to work, and then if you adjust your foot at all, the engine turns back on (jolting the car a bit, too). On the Mini, when you put it in neutral, the engine turns off, and only turns back on when you put your foot on the clutch.

Maybe Subaru bolted it on to meet regulatory rules? Do engines emit extra emissions when they start, vs. when they’re running? It seems like saving 11 gallons over 4 years is a colossal waste of effort and money, for little benefit. What am I missing?

2

Here’s a 50-post thread from a year ago on exactly this topic.

Here in a different thread is another string of posts on-point:


Bottom line is 11 gallons is 2/3ds of a tank of gas is about 200 miles of driving. That at highway speed would take you 3-1/2 hours. And at idle fuel flow would take maybe 10 hours to burn.

Removing 10 hours of run time from your use of the car changes the city-driving pollution numbers a lot. And it was cheaper (and more marketing compatible) for automakers to meet those standards by stopping the engine than by, say, installing one with 10 or 15 percent less displacement & horsepower.

It’s all about meeting mpg & pollution regs the cheapest way possible.

3

I refuse to purchase a car with that feature. It’s just another thing that can go wrong and, thanks to your statistics, it is far from worth the risk.

4

My new truck (Ram 1500) has this feature and it is very sophisticated. AFAICT it stops the engine (Inline 6) when some of the cylinders are at TDC and then fires them when you release the brake. There is no sensation of the starter rotating at all, although it probably gives a boost. There is zero lag.

5

So long as it can be turned OFF by the operator I don’t see as much of a problem as it might be.

6

It can, but typically needs to be turned off each time you start the vehicle. There are OBDII dongles that turn it off permanently.

7

I did some research. They became very common after the Obama-era EPA put in regulations encouraging (but not requiring) them. The trump regime tore up those regulations.

It’s expected car makers will stop offering that feature soon. If they think they won’t be reinstated after trump is gone. If.

8

When I first heard about the concept, I assumed they would use an alternative type or beefed up engine starter suitable for this type of abuse. The reason a small electric DC motor actually works well in this application is because of the ordinary duty cycle is just a fraction of a second, and then it’s done for a while.

9

Thanks. Funny, in that other thread, someone else with a Subaru mentions having to replace the battery.

Maybe Subaru does this particularly poorly – the Mini, which is much older, starts up smoothly, as if the engine is running but muted, just continues on where it left off. The Subaru sounds and feels like it’s really starting from scratch again.

My impression of Subarus is that they are generally lower-tech than other Japanese vehicles or German ones (like the Mini by BMW). So, it could be that other cars save more.

In the other thread, it’s clear that it’s not about saving money, it’s about lowering emissions, but really about meeting CAFE standards. Saving one longish trip over four years isn’t lowering emissions much.

10

Not just the battery, the alternator isn’t really intended to re-charge a depleted battery. A very slight discharge is caused when starting, and then the battery is re-charged. This might take 5 minutes of run time. The alternator supports the electrical system. It isn’t going to cause too much trouble unless in stop and go type driving, short trips, or worse, a traffic jam. That’s why the ability to turn it OFF would be so important. Engine starter, solenoid or starter relay, alternator, and battery are all going to get roasted with excessive use.

Engineers make these things or “size” them just “big enough” to work in the intended application. Ordinarily that means starting the engine once, and then not trying that again for a while. The duty cycle on an engine starter might typically be a half hour cool down for 5 or 10 seconds of continuous cranking.

Back in the dark ages of automobiles, when you heard someone grinding on the engine starter for 30 seconds or a minute straight, you were hearing the death of that starter. It probably won’t fail that day but fail it will. The insulation is burnt off the motor windings.

11

But the starter motor isn’t used to restart the car. At least not on many cars.

Instead the computer stops the engine at TDC with a fuel charge in the cylinder. Fire the spark plug later and the nice warm engine engine is running. No starter motor involvement at all. Yes the computer will deploy the starter motor if the first method fails. And it all happens so fast you can’t tell what’s actually happening.

12

Right, but this sort of thing adds unnecessary complexity and expense to something that isn’t necessary in the first place. It’s one reason modern cars and trucks are prohibitively expensive to buy. And expensive to repair.

13

With some hybrids, like the Toyota design, there is no separate starter, the motor is started by the electric motor support architecture. The electric motors start the car moving from the stop and the gas motor kicks in when the car needs more power. Also, the gas motor often cuts out well before the car comes to a stop (steering and brake assist are electric rather than belt-powered).

14

The real motivation is (was now, thanks (ha) to trump) to reduce pollution emanating from stopped cars in big city traffic. To reduce ground level pollution for pedestrians and bicyclists and such.

When an entire phalanx of cars is sitting idling at a red light and instead is replaced by an entire phalanx of cars sitting at the same red light with engines off, the local difference is huge, even if the lifetime emissions of the cars aren’t greatly affected.

At least that was the idea.

15

I like to think I’m reasonably bright, but automotive mechanicals aren’t my strong suit at all. What is TDC in this context?

16

Sorry. My bad.

Top Dead Center. TDC is when the piston is at the very top of its motion with the piston rod and crankpin aligned directly below the piston.

Which on the compression stroke will have the combustion chamber at minimum volume = maximum pressure, all valves closed, and a full fuel charge at high temperature ready to ignite.

Actually you want to stop w one cylinder slightly past that point. So when you fire the sparkplug the burning expanding fuel propels the piston down & the crank in the forward direction.

17

With a Toyota style hybrid, the IC engine is switching on and off as conditions warrant, and does not necessarily need to be restarted after a stop light turns green, the electric motors may be good enough. In addition the AC and such are powered by electric motors, so the reasons to restart the ICE is either the battery is very low, or more power is needed to move.

Brian

18

Toyota has the most elegant approach to hybrid design, creating a CVT that is far lighter and simpler than anything else. As far as I can tell, it does not even have a clutch/torque convertor.

19

I have no expertise in how these things work, but in at least some implementations, judging from a rental car I had some time ago, there appears to be no involvement of the starter motor. Stop at a stoplight, and the engine stops. Take your foot off the brake, and it’s running again. If there’s any involvement of the starter motor at all, it’s not noticeable. There’s certainly no cranking, as I believe there was in earlier implementations of this “feature”.

I refuse to have this “feature” in a future car and, at a minimum, would insist that it be possible to disable it. But I’d strongly prefer not to have it at all since the buyer is paying more for extra complexity and more points of failure.

20

I mean, I guess so, but if it’s not mechanical, then so what?