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No wonder idle stop is so important. Idling at 900rpm or 15cycles per second is a tremendous waste. Somehow I always assumed it was slower. 0.9k RPM is way to

by Danieru 15d ago

No wonder idle stop is so important. Idling at 900rpm or 15cycles per second is a tremendous waste. Somehow I always assumed it was slower. 0.9k RPM is way too fast to be the low end of a machine.

Grombobulous 15d ago
True, idling is wasteful, but at the same time “spinning fast” does not necessarily equate to “high energy consumption.” The CPU fans in my computer spin at about 1000 RPM all day long but that doesn’t mean they’re consuming a lot of electricity.
huijzer 15d ago
Well.. as so many things in engineering: it’s a tradeoff. Mechanically, it’s also very important to let the engine warm up and cool down before and after heavy load. Primary reasons for warmup are that the oil is more fluid on higher temperatures and metal is also less fragile when 50 or so degrees above freezing. Cooldown is especially important on turbo engines as the turbo on hundreds of thousands revs per minute and high temperature needs oil to be pumped through the bearings. If you turn it off after heavy load on engines that do not keep the oil running after motor shutdown, there is a chance that oil burns and gets stuck in the bearings, degrading performance or worst case breaking the turbo.
- huijzer 15d ago
  Also for the warmup case, tolerances inside the engine are designed assuming a hot engine. At lower temperatures, the metal hasn’t expanded yet causing also more stress on various parts such as the piston (seals). And for the turbo I forget to mention that if you leave a turbo engine idle for a few minutes that this allows both the temperature and speed of the turbo to reduce. Many trucks have a sticker in the dashboard saying “do idle engine x minutes before shutdown to avoid turbo damage”. (But also do not idle for extended periods which I’m not 100% what’s the reason. I guess it’s due to diesel engines in some cases getting too cold while idling.)
  - fwlr 15d ago
    When a combustion engine is idling, it’s usually not burning anywhere close to 100% of the fuel that it draws, which means higher quantities of unburnt fuel are reaching downstream parts of the engine system (like the exhaust system) that are only designed to be moderately resistant to fuel.
mschuster91 15d ago
At idle, it doesn't take that much fuel to keep the engine alive. Idle stops are more important for noise abatement and local toxic exhaust gas emissions - a lot of exhaust gas treatment depends on the exhaust gas having a minimum amount of thermal energy. The downsides of idle stops are, however, that they tend to put a lot of load on the engine, especially from the lack of oil pressure at the very few first strokes [1], and that the low temperature deposits all kinds of gunk in the exhaust pipework instead of transporting it outwards. If you run an engine with idle stop, it really really is important to properly take care of the engine, including preventative maintenance and regular oil changes. [1] That is even more of an issue in large engine blocks, hence that high pitched noise you hear from ships and locomotives for a while before they start up - that is an ancilliary oil pump, priming the entire oil pipework until all air is gone and full oil pressure is present at all ports in the system.
- dragontamer 15d ago
  Unless you have a hybrid vehicle, like the Prius. Which basically switches to electric during idle stops and barely has any downside.
- SoftTalker 15d ago
  Yeah I disable idle stop on my cars, for exactly those reasons. If I'm going to be stopped for more than a minute or two, I'll let the engine idle so that temperatures can normalize, then shut it off manually. Back when turbochargers became common on passenger cars, there was even advice to be sure you let the engine idle for a minute or two before shutting it off, so that the turbocharger could cool. Otherwise the oil in the turbocharger would get baked into a goo. Modern synthetic oils, and turbochargers, handle this better today but I still like to give any hot spots a chance to cool off.
  - tristor 15d ago
    > Back when turbochargers became common on passenger cars, there was even advice to be sure you let the engine idle for a minute or two before shutting it off, so that the turbocharger could cool. Otherwise the oil in the turbocharger would get baked into a goo. Modern synthetic oils, and turbochargers, handle this better today but I still like to give any hot spots a chance to cool off. Yes, it was common to install a "turbo timer" that would leave the engine running at idle for a time after you shut off the ignition to let your turbo cool. I drive a 2024 factory turboed vehicle and it still recommends idling for 30 seconds to 2 minutes after parking before shutting the vehicle off and it continues running an electric cooling fan on a timer after you shut off the ignition. The same set of challenges still exist in 2026 as did in 1986 for a turbo, for the most part.
deleted 15d ago
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bob1029 15d ago
Idle stop == cam death rattle. The externalized consequences of drivetrains failing faster (or being abandoned because they give the perception they might) easily offsets whatever gains are achieved with this nanny tech. Auto start/stop is a great example of weirdly aggressive nerds failing to explore nuance, which is found everywhere.
- MisterTea 15d ago
  In the early days it also killed starter motors as they weren't rated for that duty cycle. I feel these kinds tech are more symbolic than functional and exist to simply check off boxes to obtain environmental ratings. I would even guess the environmental ratings are also short sighted and exist as symbolic gestures of environmentalism. Basically, environmentalism performance theater.
- margalabargala 15d ago
  > easily offsets whatever gains are achieved with this nanny tech That's a matter of opinion, which may not be shared by anyone living near places that have seen decreases in noise and air pollution due to idle stop.
  - bob1029 15d ago
    Starting a motor is much noisier than idling a motor. It's "idle start/stop". not "idle stop". At some point the light will turn green.
    - margalabargala 15d ago
      Having stood next to many cars starting after an idle stop, the start is barely louder than the idle. The area under the curve is far smaller for idle start/stop than it is for leaving the vehicle running.
PreownedPlaid 15d ago
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by Danieru 15d ago

No wonder idle stop is so important. Idling at 900rpm or 15cycles per second is a tremendous waste. Somehow I always assumed it was slower. 0.9k RPM is way too fast to be the low end of a machine.

Grombobulous 15d ago

True, idling is wasteful, but at the same time “spinning fast” does not necessarily equate to “high energy consumption.” The CPU fans in my computer spin at about 1000 RPM all day long but that doesn’t mean they’re consuming a lot of electricity.

huijzer 15d ago

Well.. as so many things in engineering: it’s a tradeoff. Mechanically, it’s also very important to let the engine warm up and cool down before and after heavy load. Primary reasons for warmup are that the oil is more fluid on higher temperatures and metal is also less fragile when 50 or so degrees above freezing. Cooldown is especially important on turbo engines as the turbo on hundreds of thousands revs per minute and high temperature needs oil to be pumped through the bearings. If you turn it off after heavy load on engines that do not keep the oil running after motor shutdown, there is a chance that oil burns and gets stuck in the bearings, degrading performance or worst case breaking the turbo.

huijzer 15d ago
Also for the warmup case, tolerances inside the engine are designed assuming a hot engine. At lower temperatures, the metal hasn’t expanded yet causing also more stress on various parts such as the piston (seals). And for the turbo I forget to mention that if you leave a turbo engine idle for a few minutes that this allows both the temperature and speed of the turbo to reduce. Many trucks have a sticker in the dashboard saying “do idle engine x minutes before shutdown to avoid turbo damage”. (But also do not idle for extended periods which I’m not 100% what’s the reason. I guess it’s due to diesel engines in some cases getting too cold while idling.)
- fwlr 15d ago
  When a combustion engine is idling, it’s usually not burning anywhere close to 100% of the fuel that it draws, which means higher quantities of unburnt fuel are reaching downstream parts of the engine system (like the exhaust system) that are only designed to be moderately resistant to fuel.

mschuster91 15d ago

At idle, it doesn't take that much fuel to keep the engine alive. Idle stops are more important for noise abatement and local toxic exhaust gas emissions - a lot of exhaust gas treatment depends on the exhaust gas having a minimum amount of thermal energy. The downsides of idle stops are, however, that they tend to put a lot of load on the engine, especially from the lack of oil pressure at the very few first strokes [1], and that the low temperature deposits all kinds of gunk in the exhaust pipework instead of transporting it outwards. If you run an engine with idle stop, it really really is important to properly take care of the engine, including preventative maintenance and regular oil changes. [1] That is even more of an issue in large engine blocks, hence that high pitched noise you hear from ships and locomotives for a while before they start up - that is an ancilliary oil pump, priming the entire oil pipework until all air is gone and full oil pressure is present at all ports in the system.

dragontamer 15d ago
Unless you have a hybrid vehicle, like the Prius. Which basically switches to electric during idle stops and barely has any downside.
SoftTalker 15d ago
Yeah I disable idle stop on my cars, for exactly those reasons. If I'm going to be stopped for more than a minute or two, I'll let the engine idle so that temperatures can normalize, then shut it off manually. Back when turbochargers became common on passenger cars, there was even advice to be sure you let the engine idle for a minute or two before shutting it off, so that the turbocharger could cool. Otherwise the oil in the turbocharger would get baked into a goo. Modern synthetic oils, and turbochargers, handle this better today but I still like to give any hot spots a chance to cool off.
- tristor 15d ago
  > Back when turbochargers became common on passenger cars, there was even advice to be sure you let the engine idle for a minute or two before shutting it off, so that the turbocharger could cool. Otherwise the oil in the turbocharger would get baked into a goo. Modern synthetic oils, and turbochargers, handle this better today but I still like to give any hot spots a chance to cool off. Yes, it was common to install a "turbo timer" that would leave the engine running at idle for a time after you shut off the ignition to let your turbo cool. I drive a 2024 factory turboed vehicle and it still recommends idling for 30 seconds to 2 minutes after parking before shutting the vehicle off and it continues running an electric cooling fan on a timer after you shut off the ignition. The same set of challenges still exist in 2026 as did in 1986 for a turbo, for the most part.

deleted 15d ago

[deleted]

bob1029 15d ago

Idle stop == cam death rattle. The externalized consequences of drivetrains failing faster (or being abandoned because they give the perception they might) easily offsets whatever gains are achieved with this nanny tech. Auto start/stop is a great example of weirdly aggressive nerds failing to explore nuance, which is found everywhere.

MisterTea 15d ago
In the early days it also killed starter motors as they weren't rated for that duty cycle. I feel these kinds tech are more symbolic than functional and exist to simply check off boxes to obtain environmental ratings. I would even guess the environmental ratings are also short sighted and exist as symbolic gestures of environmentalism. Basically, environmentalism performance theater.
margalabargala 15d ago
> easily offsets whatever gains are achieved with this nanny tech That's a matter of opinion, which may not be shared by anyone living near places that have seen decreases in noise and air pollution due to idle stop.
- bob1029 15d ago
  Starting a motor is much noisier than idling a motor. It's "idle start/stop". not "idle stop". At some point the light will turn green.
  - margalabargala 15d ago
    Having stood next to many cars starting after an idle stop, the start is barely louder than the idle. The area under the curve is far smaller for idle start/stop than it is for leaving the vehicle running.

PreownedPlaid 15d ago

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