13 ms·

In modern cars, disengaging the clutch (or using neutral) when coasting (toward a stop or down a hill) is less efficient than staying in gear. This is because i

• baq 10y ago
  can confirm with anecdata: in my car the computer reports 0.0 fuel consumption when coasting in gear but non-zero when in neutral/clutch depressed.
  • ldev 10y ago
    Simple understanding on how a car works - "wheels turn the engine" when in gear, otherwise fuel is consumed when in neutral..
    • justin66 10y ago
      Not so simple. Until recently, a standard ICE couldn't stop injecting fuel while coasting (unless you turned the car off, which brings other problems...). I remember when I was learning to drive being taught about engine braking (with an automatic) and the tradeoff between using up the brakes or burning fuel.
      • ygra 10y ago
        For every fuel-injected engine the ECU should cut injection when coasting. So "recently" in your case applies to carburettors which will behave as if the engine were idle and thus still feed fuel to the engine. I'm not sure 25 years ago counts as "recently".
        • justin66 10y ago
          > For every fuel-injected engine Well, I own a 911 that says otherwise. It's an older fuel injection system (Bosch K-Jetronic), but it's most certainly fuel injection. Porsche wasn't the only maker to use Bosch fuel injection. I just don't think DFCO (deceleration fuel cutoff) was a feature of most of the early electronic fuel injection systems. Perhaps it was standard by the time EFI made it to economy cars. I'm fighting a massive urge to go down this historical rabbit hole and read about it, but there's work to be done today.
  • ptaipale 10y ago
    Of course it works that way, but just for the note: what the car computer reports in dash is manipulated by software and it can be anything. It's not necessarily completely true.
  • PanMan 10y ago
    A counter point: In my previous car the screen showed 0.0 when idling, which obviously was false (as the engine was still running). It should probably show infinite instead. (km/l for 0 km).
    • _rav 10y ago
      Some cars show liters per hour when stationary, which always seemed logical to me.
    • petre 10y ago
      Or change to l/h when you're stationary or with the clutch disengaged.
  • kbart 10y ago
    another anecdata: I once drove a car that reported negative fuel consumption when driving down the hill. EDIT: no, it was neither electric nor hybrid car, but some old petrol car with manual transmission (Saab 900 or similar).
    • mannykannot 10y ago
      A hybrid?
    • TeMPOraL 10y ago
      Electric cars generally do that ;).
    • deleted 10y ago
      [deleted]
• nostrademons 10y ago
  How modern is modern? I could believe this of my 2009 Honda Fit, but I'm fairly certain that my dad's '92 Civic, '99 Civic, and '92 Accord got better mileage when coasting with the clutch disengaged.
  • dogma1138 10y ago
    92 was 25 years ago, this isn't a modern car, it should not be on the road if you care about economy and road safety.
    • nostrademons 10y ago
      Oh, it isn't on the road, my dad totaled it in 2008. It was, however, what I drove for my first 3 years in the workforce, so I got a pretty good feel for how to drive it economically.
  • jfitbsidbfb 10y ago
    My 1998 Saturn noticeably cut fuel on deceleration, it was easy to tell since the exhaust system was perpetually breaking
    • bborud 10y ago
      Come to think of it: the car I drove in 1988 cut fuel when coasting and it had a rather unrefined ECU mapping that would cause somewhat hard transitions from the fuel-cut state to feeding the engine go-juice.
  • semi-extrinsic 10y ago
    Basically any car that has electronic fuel injection does this. So at least the '99 Civic definitely does it.
  • Ntrails 10y ago
    Think about what is required to turn the engine over and prevent stalling. -When the clutch is engaged whilst going down hill the wheels turn the engine. No Fuel Required. -When the clutch is disengaged the engine has to turn itself over - which needs fuel. *Disclaimer, i'm sure carbourettas (sp?) could well be doing weird ass things so whilst conceptually the above holds they may well be different.
• yason 10y ago
  This used to be the case with fuel injected engines from 80's to 00's but because of new regulations current ECUs might, again, inject fuel while coasting just to keep the catalytic converter warm and operational. The amount is obviously negligible but it's quite crazy to spend more fuel in order to reduce emissions :)
  • Retric 10y ago
    Smog is a real health issue, and US city's are vastly better now. So, IMO it's well worth a tiny bit more global warming.
    • gregors 10y ago
      Check out what's going to go into Chinese cars ..... https://www.youtube.com/watch?v=S3cFfM3r510 https://www.youtube.com/watch?v=S3cFfM3r510
  • crottypeter 10y ago
    > The amount is obviously negligible but it's quite crazy to spend more fuel in order to reduce emissions :) That is what Volkswagen are being forced to do.
  • justin66 10y ago
    This is interesting. How do hybrids handle this? Use electricity to warm the converter?
    • morganvachon 10y ago
      This may be a bit too much information, but it's a good read if you're interested in the technology behind it: https://hal.archives-ouvertes.fr/hal-01062316/document https://hal.archives-ouvertes.fr/hal-01062316/document
  • mikepurvis 10y ago
    Emissions per mile, miles per gallon, capital cost. You can only optimize two of them.
• 3poundkilograms 10y ago
  When you coast with engaged clutch, you will engine brake. Energy doesn't go to the engine for free. Adding the fuel to counter the engine braking is less efficient than letting the engine run idle while coasting due to losses in the power-train. Plus, more efficient cars still have to inject fuel. The electro-hydraulically controlled conventional gearbox of the Lupo 3L disengages the dry clutch while coasting for a reason. Of course, engine braking is obviously more efficient than friction braking, but that's not coasting, that's engine braking. Another funny note is that "conventional" automatic gearboxes have the highest gear (usually the only overdrive gear) have a coasting clutch on it, so it's not even technically possible to coast with engine engaged. You're always idling if you let go of the speeder, losing energy to the torque converter that doubles as oil pump for the transmission.
  • Yetanfou 10y ago
    When you coast with engaged clutch, you will engine brake True Energy doesn't go to the engine for free. Adding the fuel to counter the engine braking is less efficient than letting the engine run idle while coasting due to losses in the power-train It is a bit hard to understand where you're trying to go here but assuming that you state it takes fuel to 'counter the engine braking' you're wrong. Engine braking is just that, using the engine as a brake. The engine is used as a pump, air (and only air, no fuel is added) is compressed, the air heats up, this heat is dissipated by the cooling system. That is where the energy actually goes, as heat. Losses in the power train only add to the braking effect. This is also why it is a bad idea to use engine braking with a 2-stroke petrol engine: 2-strokes rely on oil mixed in with fuel for lubrication. When using the engine as a brake no - or hardly any - fuel is added to the incoming air while maintaining high engine speeds, starving the engine of lubrication.
    • Dylan16807 10y ago
      They're talking about the situation where you're coasting and don't want to brake.
    • 3poundkilograms 10y ago
      It's a bit hard to understand where you're trying to go here but assuming that you state it doesn't take fuel to "counter engine braking", you're wrong. Engine braking is just that, using the engine as a brake. If no fuel is added, pumping losses will make it act as a (relatively weak) brake. If you want to coast without braking, with the clutch engaged, you will need to inject fuel to counter the pumping losses of the engine, losses which are higher if the engine is not idle. Losses in the power train add to the braking effect, which must further be counted with additional fuel. If you're going downhill with clutch disengaged and maintaining your speed, engaging the clutch will very likely increase your fuel consumption for the same task (maintaining speed). Automatics have a coasting gear (usually the last or only overdrive gear) for this very reason. (Sorry, I had to do that.) Side notes: 1. Coasting is not braking. People seem to be mixing "coasting to a stop" and "coasting" in general. Engine braking, when your intention is to reduce your speed, is more efficient than friction brakes, but coasting does not mean that you intend to reduce your speed. 2. Who drives a car with a 2-stroke petrol engine? :)
• hanoz 10y ago
  The fact that you use extra fuel when coasting isn't enough to make it less efficient, you'd have to be using less extra fuel than the extra fuel you would otherwise have to use to replace the benefit you extract from your potential energy by coasting.
• J-dawg 10y ago
  Some ECUs also cut fuel on braking. I remember reading a review of a car where the reviewer (obviously an "enthusiastic" driver) was disappointed that he couldn't do left-foot braking because it kept cutting the fuel. (Left-foot braking is a technique where you keep the power down in a bend but dab the brake to adjust the fore-aft balance of the car. It is common in rallying - there is some awesome footage somewhere on YouTube of the driver's feet in an 80s rally car. His feet move so fast it looks like he is dancing on the pedals.)
  • pryelluw 10y ago
    This is one of my pet peeves. Sometimes I use left foot braking to transfer chassis weight from one side to tve other. Having a throttle cut off sucks. I won't buy a car with it anymore.
    • ohyoutravel 10y ago
      I can't think of any situation outside of a race course where this would be a necessary maneuver. If you're going so fast on public roads where you need to left foot brake to transfer weight to avoid skidding off the road, it sounds like something is wrong. Where are you doing this enough where having a car with throttle cut off is enough of an annoyance where you won't buy a car with it anymore? For reference, I got pretty good at this technique while driving autox, which for those unfamiliar is basically a race course around cones in a giant stadium parking lot, one car at a time. But in my 10 years of driving on public roads, I've never felt the need to use it even once.
      • pryelluw 10y ago
        Different driving styles. I focus on safe driving. Being able to transfer weight around bad bumpy roads helps me drive safe. Roads are not great in Puerto Rico and I drive a FWD car. Edit: Downvotes? For driving safely on shitty roads? I hope people don't assume Im being a jackass driver just because I use my left foot.
        • 10y ago
• TeMPOraL 10y ago
  Oh so that's what was happening... Two days ago my friend and I were driving a rented car and we've noticed that when coasting, the computer said the fuel consumption is 0, but when the friend disengaged the clutch, the fuel consumption actually went up (!) to some value. We've been wondering what's going on, and we actually suspected a software bug in the fuel consumption indicator. Thanks!
  • po 10y ago
    Not only that but staying in gear when going down hills or slowing down will reduce the wear on your breaks by using the inertia of the engine to slow the car instead of break pad friction/heat.
    • irishcoffee 10y ago
      I've always wondered about this. In my head, using the engine/transmission to "brake" when in gear going down hills or slowing down seems like a bad idea. Something is getting stressed in order to slow the car down. Having to replace my brakes makes sense, they get worn out, and its relatively inexpensive (especially if I do it myself). I have always been wary of staying in gear to brake. And, as it so happens, my manual has 130k miles on it, stock engine, transmission, and clutch. Never had any problems. I would love to understand the science behind why using the engine/transmission/clutch to brake is better for the car than just using the brakes.
      • JoeAltmaier 10y ago
        Just what the previous commentor said: saves on wear and tear on the brake pads. As for wear on the engine: the engine is actually designed to mesh with the transmission. Engine braking is not much different than accelerating.
        • at-fates-hands 10y ago
          >> Just what the previous commentor said: saves on wear and tear on the brake pads. When I was learning how to drive a manual, my Dad was a huge fan of them and all of our family cars were manual. I was taught that it makes a huge difference when you're driving in the mountains to use engine braking instead of your brakes. He also explained the difference in braking in traffic and using engine braking to do so and how it supposedly saves on gas as well - something about idling the engine versus continuing to have the engine running? I'd be interested to hear from people who regularly drive in the mountains and the advantages/disadvantages of engine braking versus using your brakes and if that was some myth or actually real.
• odonnellryan 10y ago
  > where staying in gear allows the computer to completely cut fuel flow I don't think you need a computer for this: it's called engine braking. https://en.wikipedia.org/wiki/Engine_braking https://en.wikipedia.org/wiki/Engine_braking
• rsync 10y ago
  "In modern cars, disengaging the clutch (or using neutral) when coasting (toward a stop or down a hill) is less efficient than staying in gear. This is because it takes some fuel to keep the engine idling, where staying in gear allows the computer to completely cut fuel flow, as long as the forward motion is keeping the engine turning." This is very interesting - thank you. I shift cars into neutral while going down hills because I strongly dislike the subtle lurching of the hill descent control ... I never thought I was saving any appreciable fuel. However, what does happen is that (provided you don't mind speeding) you exit the bottom of the hill with much more stored energy, allowing you to coast (in or out of gear) much further before it is necessary to apply the throttle again. So it's interesting that the direct effect of coasting in neutral does not save fuel - however I still think the net effect (again, provided you don't mind speeding when exiting a hill) is fuel-saving ...
  • rhinoceraptor 10y ago
    It's actually illegal in many states to coast downhill in neutral.
  • nawitus 10y ago
    Coasting down a hill in neutral definitely saves fuel compared to coasting in gear if you can utilize the kinetic energy. If you're coasting down a hill and then need to stop quickly it's better to coast in gear.
• gens 10y ago
  There is nothing preventing cars with manual clutches from doing that. People going down hills with manual transmission also keep the car in gear (they teach you that). And it doesn't matter as it is not much fuel compared to the fuel used when accelerating, where automatic transmissions are less efficient. If that is the only reason for automatic transmission to be more efficient, then i doubt it is.
  • user5994461 10y ago
    > People going down hills with manual transmission also keep the car in gear (they teach you that). It's taught for safety reasons. You have more controls of the car with the motor attached.
• mmagin 10y ago
  Yep. Where modern in some cases includes cars from 26 years ago. (My 1990 Chevy Corsica - inline 4, with a single throttle-body injector did this.)
• astrostl 10y ago
  Well, TIL on that, so thx! Still going to pop into neutral w/o engaging the clutch and coast a ton, as I've always done, though :D