4 ms·

It's economical to have low engine RPM at cruising speed, which necessitates high gearing. This has the side effect of a higher top speed. The engine might be a

It's economical to have low engine RPM at cruising speed, which necessitates high gearing. This has the side effect of a higher top speed. The engine might be at 1500 RPM at 60 MPH for maximum economy (and low noise), but it might have a maximum of say 6000 RPM... so gearing would limit the speed to 240 MPH. (side note: engine power is almost always the limiting factor in top speed instead of RPM, as power requirement scales with the cube of speed... though most cars nowadays have electronic limiters below the 'physical' top speed) Acceleration ability is needed and valued. Good acceleration is needed to safely execute a lot of manoeuvres: especially overtaking on a two-lane road with oncoming traffic (more acceleration means less time and distance facing oncoming traffic), and entering a highway (less risk of high speed differential when merging). Again, higher engine power is needed for good acceleration. Indeed at cruising speed, only a small fraction of "power capability" is utilized; short bursts of acceleration are the only time the engine is used to its full extent. Lots of engineering goes into making car engines capable of these short high-intensity sprints, but with low fuel consumption the other 99% of the time.

bayindirh 3y ago
Why not electronically or mechanically govern them (except speeds > 250KM/h), then? With today's electronics, you can use very smart feathering off acceleration curves, soft speed limit overrides for overtaking, etc. Gearing doesn't make sense. I can keep higher RPM but turn cylinders off to reduce power and cap the consumption and speed if I'd like to, but it's not implemented.
- heipei 3y ago
  Volvo has been doing this to all of its new vehicles for a while now, they're limited to 180km/h.
- 10u152 3y ago
  High rpm always equals more wear. More revolutions, less perfect oil film on lubricated surfaces, more parasitic losses, more oil burnt.
  - bayindirh 3y ago
    Well, again why not governing it at a modern 5th gear at 120 KM/h, with a soft bump at 90 KM/h?
    - 10u152 3y ago
      Oh I can’t answer that. Was just pointing out advantages of lower RPM.
    - itsoktocry 3y ago
      >Well, again why not governing it at a modern 5th gear Why not add a 6th, 7th, 8th, 9th or 10th gear? Wait, modern transmissions do.
      - bayindirh 3y ago
        Yeah, which depends on the torque band of the engine which drives the transmission. Generally 8 gear systems are paired with ~1.5L force induction engines to keep them in a very narrow RPM band, similar to Diesel engine dynamics. However, speeds nearing 180 KM/h is a natural result of these mathematics. What I was proposing to keep the same engine, reduce the gear count to 5-6 and keep engine RPM around 2-3K which maps to 90-120 KM/h band generally. You'll either keep 8 gears with closer ratios for a smoother acceleration, or shave two gears from top, because 8th gear on 120KM/h results in RPMs too low for the engine (on the cars I have driven though).
- itsoktocry 3y ago
  >Gearing doesn't make sense. I can keep higher RPM but turn cylinders off to reduce power and cap the consumption and speed if I'd like to, but it's not implemented. "Why doesn't trillion dollar industry with millions of engineering hours devoted to optimizing performance just do X, Y, Z?" is classic HN.
  - Eisenstein 3y ago
    There should be a 'rule' (in the style of Godwin's or similar) regarding people making overly reductive, grand statements in areas way outside of their expertise.
    - bayindirh 3y ago
      Sometimes knowledgeable people ask dumb questions on purpose. :D
      - Eisenstein 3y ago
        That's not what I mean though -- what I mean is like saying 'just add more lanes and that will fix traffic', and not knowing how wrong you are since the reason it won't work is not intuitive.
    - bryanlarsen 3y ago
      The OP didn't break that rule. They asked a question, they didn't make a statement.
      - szundi 3y ago
        Actually he wrote “gearing does not make sense” - something based on every car having gearing makes me wonder this might make sense Also he asked a question
        bryanlarsen 3y ago
        "does not make sense" means "I do not understand", implying to me he wants to be enlightened. If you're trying to be generous, which you should be.
- michaelt 3y ago
  > Why not electronically or mechanically govern them Some vehicles do come with built in speed limiters. Trucks, vehicles used by delivery companies etc will often have speed limiters; presumably it lowers their insurance? And some production cars offer them as well. As I understand it, this allows them to scam the emissions/range tests, because no matter how easy to disable or how rarely used the speed limiter is, so long as it leaves the factory with the 60mph speed limiter enabled, then it goes through the emissions tests with it enabled.
  - bayindirh 3y ago
    I know. I'm asking why the hard limit is at 250KM/h for most cars, but not at 120. I use cruise control and speed limiters regularly, otherwise.
    - Thiez 3y ago
      Because many countries have speed limits higher than that? In the Netherlands 130 km/h is allowed (usually only at night), in Poland they have 140 km/h in some places, and in Germany there are many roads where a maximum speed of 130 km/h is recommended, but there is no limit (provided you drive responsibly).
    - nikau 3y ago
      Speed restrictors are often set to the maximum tyre speed minus a bit of margin. eg. a "W" rated tyre is 270kmh max rated, so the manufacture might set the max vehicle speed to 250. Tyres get exponentially more expensive as speed ratings go up.
- Arnt 3y ago
  One of the ways to make an intractable problem seem easy is to underspecify it. Easy to do by accident. In this case, try enumerating who would govern, and what each possible actor might do to govern, or suggest to govern. There's a tree of options. For example, it might be a single car manufacturer, and might introduce the limit either globally or in some region. Or it might be a country, except not, because if it's a country it has to start with a department of the state or with a political party. If it's a political party, it can be in government or opposition. The tree is deep. I'll do one of the leaf nodes: A political party that's currently in government, including the relevant minister if there's a coalition. Such a party has to put forward a concrete suggestion, perhaps a hard limit 120km/h with mandatory 100% responsibility for any accidents if the limited is modified, with supervision by forcing the navigation and mobile phone companies to report user locations. Or it chooses some other concrete proposal. It doesn't matter, because whatever it is it'll get a collection of shitstorms about the concrete details. The people who think 120km/h is too high would flame it for having an obviously wrong speed limit, the ones who think it's too low would do the same, the data protection/privacy crowd would flame it for the big-brother-ish data collection (but without that, another crowd flames it for lack of enforcement), and so on and so forth. That a particular change is generally desirable doesn't imply that there exists anyone that can put forward an actionable proposal. Making the change actionable means providing concrete details, and those attract shitstorms. (On a semirelated note: a politician, a sometime finance minister of a large country, struggled for 10+ years to push through a worldwide reform that most people would consider good. The reform was put into force this year. I haven't seen that politician's name in any reporting in the past year.)

bayindirh 3y ago

Why not electronically or mechanically govern them (except speeds > 250KM/h), then? With today's electronics, you can use very smart feathering off acceleration curves, soft speed limit overrides for overtaking, etc. Gearing doesn't make sense. I can keep higher RPM but turn cylinders off to reduce power and cap the consumption and speed if I'd like to, but it's not implemented.

heipei 3y ago
Volvo has been doing this to all of its new vehicles for a while now, they're limited to 180km/h.
10u152 3y ago
High rpm always equals more wear. More revolutions, less perfect oil film on lubricated surfaces, more parasitic losses, more oil burnt.
- bayindirh 3y ago
  Well, again why not governing it at a modern 5th gear at 120 KM/h, with a soft bump at 90 KM/h?
  - 10u152 3y ago
    Oh I can’t answer that. Was just pointing out advantages of lower RPM.
  - itsoktocry 3y ago
    >Well, again why not governing it at a modern 5th gear Why not add a 6th, 7th, 8th, 9th or 10th gear? Wait, modern transmissions do.
    - bayindirh 3y ago
      Yeah, which depends on the torque band of the engine which drives the transmission. Generally 8 gear systems are paired with ~1.5L force induction engines to keep them in a very narrow RPM band, similar to Diesel engine dynamics. However, speeds nearing 180 KM/h is a natural result of these mathematics. What I was proposing to keep the same engine, reduce the gear count to 5-6 and keep engine RPM around 2-3K which maps to 90-120 KM/h band generally. You'll either keep 8 gears with closer ratios for a smoother acceleration, or shave two gears from top, because 8th gear on 120KM/h results in RPMs too low for the engine (on the cars I have driven though).
itsoktocry 3y ago
>Gearing doesn't make sense. I can keep higher RPM but turn cylinders off to reduce power and cap the consumption and speed if I'd like to, but it's not implemented. "Why doesn't trillion dollar industry with millions of engineering hours devoted to optimizing performance just do X, Y, Z?" is classic HN.
- Eisenstein 3y ago
  There should be a 'rule' (in the style of Godwin's or similar) regarding people making overly reductive, grand statements in areas way outside of their expertise.
  - bayindirh 3y ago
    Sometimes knowledgeable people ask dumb questions on purpose. :D
    - Eisenstein 3y ago
      That's not what I mean though -- what I mean is like saying 'just add more lanes and that will fix traffic', and not knowing how wrong you are since the reason it won't work is not intuitive.
  - bryanlarsen 3y ago
    The OP didn't break that rule. They asked a question, they didn't make a statement.
    - szundi 3y ago
      Actually he wrote “gearing does not make sense” - something based on every car having gearing makes me wonder this might make sense Also he asked a question
      - bryanlarsen 3y ago
        "does not make sense" means "I do not understand", implying to me he wants to be enlightened. If you're trying to be generous, which you should be.
michaelt 3y ago
> Why not electronically or mechanically govern them Some vehicles do come with built in speed limiters. Trucks, vehicles used by delivery companies etc will often have speed limiters; presumably it lowers their insurance? And some production cars offer them as well. As I understand it, this allows them to scam the emissions/range tests, because no matter how easy to disable or how rarely used the speed limiter is, so long as it leaves the factory with the 60mph speed limiter enabled, then it goes through the emissions tests with it enabled.
- bayindirh 3y ago
  I know. I'm asking why the hard limit is at 250KM/h for most cars, but not at 120. I use cruise control and speed limiters regularly, otherwise.
  - Thiez 3y ago
    Because many countries have speed limits higher than that? In the Netherlands 130 km/h is allowed (usually only at night), in Poland they have 140 km/h in some places, and in Germany there are many roads where a maximum speed of 130 km/h is recommended, but there is no limit (provided you drive responsibly).
  - nikau 3y ago
    Speed restrictors are often set to the maximum tyre speed minus a bit of margin. eg. a "W" rated tyre is 270kmh max rated, so the manufacture might set the max vehicle speed to 250. Tyres get exponentially more expensive as speed ratings go up.
Arnt 3y ago
One of the ways to make an intractable problem seem easy is to underspecify it. Easy to do by accident. In this case, try enumerating who would govern, and what each possible actor might do to govern, or suggest to govern. There's a tree of options. For example, it might be a single car manufacturer, and might introduce the limit either globally or in some region. Or it might be a country, except not, because if it's a country it has to start with a department of the state or with a political party. If it's a political party, it can be in government or opposition. The tree is deep. I'll do one of the leaf nodes: A political party that's currently in government, including the relevant minister if there's a coalition. Such a party has to put forward a concrete suggestion, perhaps a hard limit 120km/h with mandatory 100% responsibility for any accidents if the limited is modified, with supervision by forcing the navigation and mobile phone companies to report user locations. Or it chooses some other concrete proposal. It doesn't matter, because whatever it is it'll get a collection of shitstorms about the concrete details. The people who think 120km/h is too high would flame it for having an obviously wrong speed limit, the ones who think it's too low would do the same, the data protection/privacy crowd would flame it for the big-brother-ish data collection (but without that, another crowd flames it for lack of enforcement), and so on and so forth. That a particular change is generally desirable doesn't imply that there exists anyone that can put forward an actionable proposal. Making the change actionable means providing concrete details, and those attract shitstorms. (On a semirelated note: a politician, a sometime finance minister of a large country, struggled for 10+ years to push through a worldwide reform that most people would consider good. The reform was put into force this year. I haven't seen that politician's name in any reporting in the past year.)