Why YSK: People seem to, on average, think that a car takes a lot of fuel to start up. In reality, it takes on the order of a few millilitres of fuel to start an engine. That means if your car isn’t equipped with an automatic start/stop system to stop your engine instead of idling, it saves fuel to turn off your engine and start it back up when you need it.
Caveat: air conditioning and radio might not work with the engine turned off.
Scenarios where this might be useful include waiting for trains to pass at rail crossings, waiting for food at drive-throughs, dropping off or picking people up on the side of the road when they need to load stuff, etc. May not be a good idea to use this while waiting at a red light because starting the engine does take time which would annoy drivers behind you when the light turns green.
Some cars are equipped with systems that will automatically stop the engine when you are idling for a while (e.g. waiting for a red light). If yours is, then manually turning off your engine will probably result in reduced fuel savings compared to just relying on the car to do it for you.
Caveat: For cars not equipped with automatic start/stop, the starter and possibly the battery might not be specced for it so it could cause additional wear. Cars with start/stop systems often assist the process with precise camshaft position measurements and the ability to squirt fuel pretty much right away so the starter doesn’t need to do as much work.
Also don’t do it with a cold engine - it’s better to get the oil up to temp faster, it’ll also reduce fuel consumption as the engine heats up.
Thanks for reminding everyone of this. The gas savings over time will probably end up being the same, or less, than a starter on an older vehicle. Of course, if you’re not planning on keeping the vehicle until it dies, this is less of an issue for you.
It’s the battery prematurely dying that’s also an issue. Especially if the battery needs coding to the car. Could be a £300+ job. That’s a lot of fuel that’d need to be saved to be close to worth it.
It’s all about emission testing anyway. Keel start stop off.
It’s amazing how much batteries have increased in price since Covid.
I bought a car battery for my GF while I was out and she was upset at how much money I spent until she went online and saw that $200 for a car battery was totally normal.
If memory serves me right, it wasn’t long ago that many were well below $100.
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Stop start cars have more expensive agm batteries typically
I saw Matt needed a lithium battery for his M5. £750. Yes it’s a big and technology rich battery but wow.
Last I saw my M240 battery was about £200. Bet it’s £350 now.
Assuming it’s the battery shortage due to electric vehicles.
Wait, CODING? Are you serious? I can understand this with EVs, but for a standard 12 volt battery? This sounds more like a thing they do to keep you from doing your own work and allowing shops to charge more flag time for what should be a 10 minute job. Replacing a dead battery is one of the simplest jobs you can do. The hardest part of a battery swap should be finding your 10mm socket.
Yeh, you have to tell the car a new battery is installed to alter the V for start/stop mainly.
This is such absolute crap. The math didn’t check out in the slightest, and the backslapping on this topic alone could Flintstone a vehicle all by itself. 🤦🏼♂️
Cars with start/stop systems often assist the process with precise camshaft position measurements and the ability to squirt fuel pretty much right away so the starter doesn’t need to do as much work.
I always wondered why hybrids could start their engines instantaneously, when many conventional cars couldn’t. This is why, isn’t it?
I can’t speak for other cars, but my Prius uses the electric assist motor as the starter motor as well. Compared to a regular ICE car that’s a massively stronger electric motor than average starting a smaller than average engine.
My favorite thing about it though is I have the longer hatchback model and if you replace one of the back seats you can fully lie down for car camping. What about the heat you may ask? I can just leave the AC on overnight, the car will start up and use the engine like a generator to recharge the battery then turn back off autonomously. I always keep some spare gas in case but I’m always shocked how little it uses.
According to priuschat you can leave your prius “idling” with light duty power draws (a small lcd tv and a fan) for about 7 days on a full tank of gas.
Also I think on the prius the starter motor-generator is also the one that bleeds excess engine power to charge the battery, but I’m not 100% sure on that one.
This is why, isn’t it?
Regular old cars use weak starter motors from the 12V battery. They’ve gotten better, but it’s still just a small part used only to start the car, so it only turns over the engine fast enough to get it going.
A car like a Prius will put the car’s big, beefy generator in “reverse” to start the motor at whatever RPM the computer wants it at, and since it has abundant power from the synergy drive batteries, it can start the engine at whatever RPM the computer deems appropriate.
Also the fact that a hybrid has a huge electric motor to start the petrol engine instead of a small starter motor.
I walk, cycle, or longboard places whenever possible, and when these automatic cars started coming out, I thought they were manually starting and stopping their cars at each intersection. It really tripped me out.
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When it’s cold it can be beneficial to let the car idle for about 15 seconds before moving, to get the oils moving. They don’t usually need much more than that, unless it’s very cold of course.
Also cars will warm up faster driving around minimizing wear while engine is cold. Just don’t stomp on the gas go easy on it
Indeed! It’s a balancing act. It’s good to wait a while right at start, but then it’s better to go than wait on idle.
How cold are we talking? It routinely falls below 0F where I live in the winter. Are we talking anything below 32F (0C) requires more than 15 seconds?
I’ve heard mixed information from the car people in my life.
There are no hard rules, cars are different and so are oils, but I personally would give it a few minutes under 0F and maybe up to a minute if it’s just a bit under the freezing point, but not too cold.
Of course, something like a 0w30 oil will get pumped around the engine much faster in the cold than a 10w30 or 10w40 and therefore won’t need as much time.
Also, idling isn’t terribly good for your engine either, you should only do it enough to get the oil flowing and the blower to start putting out a tiny bit of warmth. Then you’re better off driving, because that warms up the engine faster.
Everything I’ve read says that newer engines (like last decade or two) only need a couple seconds. All the warming up for a minute plus advice was literally from carburator-era engines. Newer engines will be damaged from running cold far more than anything else, so it’s best to give them a second or two and then drive to get them warmed faster. That DOESN’T mean to push the engine or red line it, but easy driving.
But I’m not a mechanic, just a super type-A scientist that looks into things at a semi-ridiculous level for the sheer joy of knowledge.
The few seconds is bare minimum, but you can improve on it. Again, if you live in a warm climate, this doesn’t apply. I give my engine a maximum of 5 seconds of idle time in the summer too, because I know the oil isn’t too viscous at those temperatures and I don’t need to demist my windshield. But in the winter, you can tell from the sound the engine’s making that it’s not happy at -30C, even in a fairly new car with good oil. So you start off idling a bit and then start driving gently. I let it idle till I can see the mist starting to disappear from the windshield on its own, then drive real gentle - luckily I usually drive on low-traffic roads in the morning, so I don’t have to gun it to the speed limit in one second to keep everyone else happy.
There is truth to the damage from excessive cold idling, which is why I don’t recommend doing it too much on newer cars either - in direct injection cars in particular, I believe you can wash oil film off the cylinder walls with excess cold idling. But a minute or two is fine. We’re talking about temperatures that people from warmer climates might consider deadly cold. Not just mildly freezing temperatures
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Mine takes 5w20, so probably somewhere in between then. I usually let it idle for about a minute in extreme cold, but sometimes for several minutes if it’s super, super cold outside.
Sounds completely reasonable to me. 5w20 is pretty thin, which is good for winter driving. Again, don’t idle too much, but a little bit is good.
Yeah, I wouldn’t recommend long idling*, but I also wouldn’t recommend shutting down the engine for short stops when it’s cold. Just keep it running if you’re stopping for less than 5 minutes.
* Where I live, the winters can get to around -30C, though normally it won’t get colder than -25. You’ll want to let your car idle for a few minutes because otherwise it’s not going to be blowing any warm air at the windshield, and your visibility will be shit.
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Starting and stopping a car is the worst thing you can do to it, doing it repeatedly on purpose is just asking for expensive problems, like a burnt out starter, missing or worn teeth off the flywheel, broken stater mount on the block, dead batteries, coked up and worn out turbo bearings, bearing and knock issues due to lack of lubrication, soot buildup in diesel engines, failed emissions systems, etc.
The few pennies you save in fuel is not worth it, upping the time to 5 minutes would make more sense.
In some cities in my country (maybe all) it is regulated and fined to idle your car over 1 min, the point of it I think is to make people used to turning off their cars for train crossings and bridge openings.
How does that work with stop lights? I’d say most stop lights are longer than 1 minute, so does everybody turn their car off while waiting? Does that cause a noticeable delay when the light changes?
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Makes sense. The biggest takeaway from this post is that long idling is wasteful. If your vehicle isn’t designed with auto start / stop, then don’t turn it off and on in traffic. If you are waiting in a driveway for someone to come out, then turn it off.
It’s not necessarily an issue of fuel, but the overall wear on the components and engine when you start a car. A starter motor only has so many “starts” it can do before dying. The battery too.
Starter motors have gotten a lot better since the “bad old days” and engines start more smoothly thanks to fuel injection and computer control systems, so manufacturers have decided that it’s ok to start/stop engines as needed, but the reason for not doing it was never a matter of fuel savings.
Maybe not fuel savings in an economic sense, but definitely still in an environmental sense. Of course the emissions are going to be substantially lower when you are idling versus driving in a city for example, but when every car doesn’t idle for let’s say 5 or 10 minutes every day, all over the world, those little “non matters of fuel savings” tend to add up. Like all the grains of sand on a beach
I see this is NOT a place for open discussion. Noted.
The whole world has weighed in and explained why more starts and stops lead to excessive wear, but I’d like to take a moment and bring a little attention to the environmental damage that having to do a bunch of work to your car because you turn it off at every opportunity does.
Let’s say you need a new starter motor early. That’s copper, aluminum, iron, steel and a handful of rare earths for the solenoid. Melt em, smelt em, form and mold em, that’s more carbon from building the replacement part than you’d have kept out of the atmosphere by shutting off the car at stoplights.
The greenest car parts are the ones already in it.
Life Pro-tip: don’t turn your car off unless you’re safely parked. Not only is it insanely unsafe, but you’re actively blocking traffic even if it’s stopped around you; in the event of a wreck involving your car in said inert state, you’re in legal trouble in a number of directions. Don’t be a dumbass.
The infinitesimal amount of “saved” fuel is absolutely nothing compared to the mind-bogglingly enormous amount of commercial waste that pushes our civilization to the brink. You’re not “doing your part” in any way at all with this bullshit. Stop already and think, FFS. 🤦🏼♂️
Many new cars have start/stop features builtin. If the computer controller detects that the engine may have trouble starting (low temperature, low battery, starter motor failure, whatever), it won’t stop the engine for you.
Or that’s the theory.
Chiming in to agree that the scenarios listed are mostly ridiculous to think about turning your car off. Knowing that ~7 seconds of idle time is a reasonable threshold for just turning off your car is certainly useful, but how many times has someone turned the ignition key and the car hasn’t started due to battery drain or some other failure? Now imagine that happening at a stoplight, a drive-thru, or a rail crossing.
Seriously, this is some malinformed groupthink. How thafuq does anyone think that it’s safe at any point to be a stationary object in the middle of the damn road?! Assuming your car starts right back up again without any issue (non-zero chance of a wide array of complications there), why would you choose to add several seconds to your reaction time in an unforeseen emergency where a fraction of a single second could be the difference between life or death — and not just your own?! Fuck. This very notion is so disgustingly self-absorbed and short-sighted. Christ on a stick.
Man, this sounds so american.
Not very often one finds a low-effort burp of a comment in the wild here. Reddit is clearly leaking from its lower levels now. 🤗
I don’t think that stopping and starting the engine a lot is a good idea, even if it is not unhealthy to the engine. In the situations you had as examples, it is just inconvenient to turn off the engine and have to start it up again.
Right. And just think about being stranded at a stoplight because your car refuses to start again.
Yeah new cars usually have the ability to do that themselves and also usually do it when safe.
They auto start when power is needed or if the situation changes. Eg: touch the steering or gas pedal. Another example is my car will auto start it too many cars are around me.
Shutting your non-auto starting car off and then having an emergency happen could land your ass in trouble with insurance and the law. If you’re on the road, your car should be running (unless it was designed this way). Of course people mentioned wear and tear, so that too.
Cheers -Henry
Exceptions: if you live somewhere where you’ll die in 4 minutes in a car with no AC.
Sent from Texas, where it is currently 88 degrees in the middle of the night.
Starting the engine puts as much wear on it as driving for 50 miles. Automatic start/stop hurts engine longevity by doing that unnecessarily and should always be turned off.
My car uses 0.5l of fuel idling for an hour. There’s no way in hell that a start/stop system would even save 10$ a year, so there’s no benefit to using it.
That’s not true, cold starts cause wear not warm starts.
At most it’s starter motor and battery wear, start stop cars have agm batteries for that reason.
Cold starts cause more wear than warm starts, but a warm start still causes a lot of wear because the engine doesn’t have oil pressure when it’s off, so it doesn’t have good lubrication until it turned over a few times.
Besides that, my car and many others will happily stop the engine on their own even before reaching operating temperature. At least I can turn it off with a single button instead of going through a menu on a touch screen.
What about the fact that the oil drains to the pan in those few seconds that the engine is stopped?
This is my real concern. Sure you can upgrade starter motors and batteries to handle the extra cycles, but you can’t do anything about increased scoring and wear on cylinders in the milliseconds before the fluids start to circulate again.
Some start stop engines have auxiliary oil pumps. I don’t know much about them besides random research I have done in the past out of curiosity.
Napa also claims some vehicles have auxiliary water and transmission pumps as well.
That might be true, I’m not a mechanical engineer but despite that, my understanding is that within the engine block itself, cylinders are primarily lubricated via the system holding pressure. This pressure starts to drop the second the engine ceases.
You can notice the effect on cars that have realtime oil temp monitors. Mine does, and it’s digital. My stable oil temp is around 216 degrees Fahrenheit. After a start-stop cycle, even for only 5-10 seconds or so, the temp drops about 5-8 degrees. After a minute, the temp is down 25 degrees. That’s significant. Essentially the engine is no longer “at temp” for the first 30 seconds or so after it resumes. That’s 30 seconds of additional semi-cold, under pressure wear each cycle.
Then why do so many new car models have auto-stop features that kick in at red lights? They would not do that if it wasn’t more efficient.
Well I think I can answer that. It is more efficient for fuel consumption. They all have the systems because it allows them to hit better EPA fuel economy numbers. But better fuel consumption doesn’t mean there’s no effect on the engine.
I’m not saying I’m 100% correct btw, I’m waiting for a mechanical engineer to explain why I’m wrong. But my limited understanding hasn’t found an answer for my concerns yet.
My Ram start-stop automatically starts the engine back up once temps drop to a certain point for this exact reason.
Auto stop-start has had many years of advancements to work out these issues.
That’s makes sense but from an engineering standpoint, anything below operating temp and pressure fundamentally causes more wear.
It may be minimised with configurations such as you describe, but it’s still more wear than if the engine hadn’t stopped in the first place.
How much, I don’t know, but over the course of hundreds of thousands of miles and thousands of stop-start cycles, it adds up.
Believe it or not, they thought of that when they created start/stop systems.
In cars with these systems, the back pressure in the engine’s cylinders is greatly reduced via a variety of strategies including selective alteration of valve timing and purpose-built secondary valves. What this means in effect is that the torque required to re-start the engine is a fraction of a dead cold start, and even a fraction of a normal warm start. This should serve to minimize additional destructive wear on components.
In effect, well-designed start-stop systems do not create any additional wear on vital engine components versus the engine running for that same period of time.
The difference in wear might be too small to measure, but what we can measure is how much fuel it saves. And that’s usually less than 50l over the lifespan of a vehicle. Those systems don’t even offset their own cost unless you spend all day at railroad crossings.
They are there for two reasons: Getting better results in unrealistic tests and decreasing the amount of control that owners have over their vehicles.
I’ve always wondered where the line is that makes turning the car off more optimal than idling. Another poster talked about upping the time to 5 minutes because of the wear and tear on the engine from starting which I think sounds reasonable although 5 minutes at an idle is a pretty long time lol. The other thing I’ve always wondered is when it’s more efficient to use your AC rather than just roll the windows down cause at a certain speed having the windows down slows the car down more than the energy it saves in not using the AC I would assume.
I think mythbusters did this one. Ac is more efficient iirc. Dont quote me.
My car has the fuel mileage thing in the infotainment system (2019 Volkswagen)… so I’ve tried looking at it on days when I drive with my windows down vs with the AC on. Oddly enough, on a ~30 minute commute at mostly 50mph, I get better fuel mileage with the windows down and AC off. It’s even more odd because I feel like I drive more “spiritedly” when I have the windows down. It’s not a huge difference, but 2mpg is still better fuel economy.
That said, I still drive with the windows up and AC on most days because summer humidity sucks here.
This is awesome, thanks for the info! Have you noticed a difference if you go faster or slower? Like straight highway driving at like 70 or stop and go city driving?
Driving slower definitely gives better fuel mileage, as long as I’m going fast enough to get it into 6th gear. I don’t measure in stop and go city driving because I do so little of it, and I rarely roll my windows down at highway speeds because I don’t like the buffeting of the wind… so unfortunately I don’t have data on either of those. All my data comes from my drive home on backroads through mostly farmland.
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