Because mechanical components are not the only things that fail, even if they’re the most common failure causes. Sensors, wiring, computer components and even programming logic can be failure points. Complexity inherently creates many different points of potential failure.
But car engines have multiple cylinders. Won’t one of them always be at or near that position?
Yes, and the computer will know which one it is. That’s a main difference between a cold start and a hot restart–the engine is in a known state.
Sorta.
There are 720 degrees = two revolutions in a complete single-cylinder cycle of intake, compression, power, and exhaust. Most engines are designed with a uniform distribution of cylinders around that circle. Not all, but most. Assuming an even distribution (commonly termed an “even fire” engine), a 4-cylinder engine will have TDCs that are 180 degrees apart, while an 8-cylinder engine (typically in V8 shape) will have one every 90 degrees.
In an 8-cylinder engine w 90 degree spacing, there will always be one cylinder between TDC and halfway down the power stroke. And just as one is passing / past that 90 degree point, the next is passing TDC.
My bet is that igniting a cylinder at almost 90 degrees past TDC is going to be pretty weak. But as long as it ignites at all it’ll give some push. And then you immediately fire the next one in sequence that’s just past TDC and you get a full power deflagration and you’re running. You might need a bit of boost from the electric starter motor, but only in the worst case geometry or engine not yet warm.
In a 4 cylinder that stopped at completely random positions I’m thinking you’d need a boost from the starter motor maybe half the time. But that boost will be very, very brief.
Certainly there must be a time limit on the “pause,” though, since pressure & temperature are decreasing.
I do not drive like an American: my braking profile tends to be a lot shallower than most people, slowing down gradually instead of stopping hard (which sometimes gets me through a light that the average person would have had to stop for). An Escape that I was driving a lot seemed to be built for the typical American driving style, because my stopping habits often failed to shut the engine off.
And it kinda makes you wonder how good this auto-shutoff/restart is. Does the engine sometimes shut off when it should have kept running for a brief stop? It cannot realistically take the most appropriate action all the time. It seems like an iffy feature.
Ah, right, I was thinking 360º for a cycle. I forgot that internal combustion engines are fermions. For 4 or 6 cylinder engines, that would often put all cylinders far from TDC.
That seems very low - a quick search suggests savings of 2-15%, depending on the car and how often you’re in stop-and-go traffic or gridlock. Is the vast majority of your driving on a highway or on roads where stopping is very infrequent?
Not really. We live in a medium size suburb with plenty of stoplights around. I think the Subaru implementation is especially terrible, since the Mini seems to take advantage of it much more. My sister has a BMW and that seems much better than the Subaru, too.
At 30 mpg, which is optimistic for this car, I’ve saved 330 miles out of around 60k, so more like 1/2 percent.
Here’s a youtube explaining how the Toyota CVT works. It uses a planetary gear system instead of belts or cones of earlier systems.
The whole engine is quite hot. Some of the heat of compression will dissipate in the minute or so the engine is stopped. As to pressure, the cylinder is sealed, net of what leaks past the piston / cylinder sealing rings. Again over the minute-ish the engine is paused I expect that to be negligible.
IME if you’re on an extended stop, say parked and waiting outside at a store while somebody else runs in, the engine will auto start every couple of minutes to run for 30-45 seconds then stop again. Mostly to keep the HVAC blowing cold. So the cooldown & bleeddown don’t have too long to occur.
In any case, if for whatever reason the residual cylinder charge can’t fire the engine, a few milliseconds later the computer turns on the electric starter motor and the engine starts that way.
I’ve done that same face-palm more than once.
I have a 2026 Toyota Camry and this has been my experience. It’s a very smooth transition between the ICE engine and the electric motor. The engine will cut out if the batteries can move me at 45-60 MPH. I sold my 2014 Camry to my brother with 109k miles on it, and it’s got at least another 75k in it. I never had an issue with excessive wear and tear with the 2014 Camry beyond routine maintenance, new tires, and one 12v battery replacement.
Right, hybrid systems designed as such from the beginning will have a better start-stop solution than when you just add start-stop software to a preexisting conventional powertrain to try to squeeze out CAFE numbers.
And yeah, we could have lower emissions/higher mileage w/o start-stop (or cylinder deactivation or 3-cyl mini-turbos). Just that we’d have to accept puttering about in econoboxes with double-digit 0-60 times.
I have a Subaru of the same vintage as the OP. I hate the implementation of this feature. AC turns off when I’m stopped. Sometimes the car will stop when I’m stopped, and then re-start while I’m still in the same stop for no discernable reason. Re-ignition is inconsistent- sometimes quite quick, other times sluggish. I once had to turn off the car and re-start at a light that turned green when the engine wouldn’t re-start (took it to the shop, where they ran some tests, shrugged, and said everything looked fine).
Also, while I applaud any micro-advance in efficiency, color me skeptical that this tech is having a measurable effect on global emissions.
That is/was not the goal. The goal was to reduce immediate local pollution on crowded downtown streets with lots of cars idling at lots of lights. Righr next to throngs of peds, residents, shopkeepers etc.
For that narrow goal it works pretty well. If people don’t disable it.
I think that is debatable. Local pollution impact might be an outcome, but I don’t believe it is a driver of the tech. It’s certainly not in any of the easy-to-find marketing or user guides from auto manufacturers, nor in most articles I can find explaining the technology. They all talk about eliminating fuel waste, and general pollution/emission reduction.
Doing some reading, it looks like the tech became ubiquitous as a response to EPA incentives around greenhouse gas emission reduction- reduction in localized air quality in urban areas is maybe a nice side effect, but not an intent.
Also, automakers have really used up all the low hanging fruit as far as emissions and efficiency are concerned. They’re chasing single digit gains at this point…
My recollection is also that was not how it got “sold” to the public when it was first widelyimplemented, but rather more in the general emissions and fuel use sense. However of course it is something that applies more to stop-n-go urban traffic conditions. I suppose the car press and marketers preferred to not emphasize that this would most affect city commuters.
“Top Dead Center”. Way back in the day, when Model T’s practically ruled, a similar effect was observed. Hand cranking an engine was a bit of a dicey affair, or could be, if it wasn’t tuned up well.
But a reasonably in-tune engine that was warmed up, and a given piston in the cylinder lined up just so, (probably at, or just past TDC, could get a “free start” by merely exciting the ignition. A spark plus a little residual fuel in the cylinder, and no hand cranking required! Pretty fancy. This is basically the same principle used today.