How motorcycle Stop/Start works

Dailies -



Stop/Start is the kind of common sense, widely applicable, economy boosting technology we can all get behind. The concept is extremely simple — when you’re stopped at a traffic light or whatever, the engine turns off to save fuel; the trick is enabling rapid re-start so you can seamlessly pull away. All together, it’s good for an average seven percent improvement to fuel economy in urban driving. Here’s how Honda’s system works.

We haven’t heard much about Stop/Start in the US for one reason — unlike European Union and other standards, the way the official EPA fuel economy figures are measured doesn’t accurately model the benefits of the technology. If car and bike makers can’t advertise an improvement, then why pay to include the system on their vehicles? Measured using the EU’s ECE R40 urban cycle, Honda claims the PCX125 scooter’s fuel economy — already 125 mpg — improves seven percent. The American market PCX isn’t fitted with the system due to the EPA’s obstinacy.

When you’re idling, you’re burning fuel. Having the engine off during those times would save that fuel without impacting any sort of vehicle dynamics or performance or experience or anything. We’ve driven cars (in Europe) fitted with Stop/Start and they work seamlessly; it can feel a bit odd at first feeling the engine cut out, then cut back in when you open the throttle, but that’s it. We’ve never experienced any lag while pulling away from an engine-off, dead stop in a Stop/Start vehicle.

So the first part of the system’s function is easy. When you come to a dead stop for a set period of time (three seconds in the case of this Honda), the engine turns off. It’s what happens next that makes Stop/Start clever.

On the PCX, the starter motor is integrated with the generator. This eliminates the need for a separate starter motor to engage reduction gears and actually makes a Stop/Start engine more compact and lighter.

Now, overcoming compression to start an engine constantly during urban driving would normally be a huge drain on the battery. The PCX and other Stop/Start equipped vehicles reduce that energy requirement by opening the exhaust valve slightly on stop, decreasing compression while the piston compresses the fuel/air mixture for a restart.

The PCX’s piston also “swings back” to an ideal starting position on stop, meaning it only ever has to do one compression stroke before the engine fires, this further reduces the energy requirement.

Stop/Start requires nothing extra from the rider. You don’t need to think about it. Just brake to a stop like normal. Sit and wait for the green light, then roll on the throttle. The engine will instantaneously engage and you pull away like normal.

See why we’re excited? By integrating the generator and starter motor, the engine actually gets simpler with stop/start. Honda even gives you a switch to toggle the system off if, for some reason, you don’t want to use it. You save money on gas with no penalties. Neat.


  • Justin

    Could be unreliable. I think some brit bikes as well as Suzuki’s had this kind of starter system in the 60s before the compact starter motors came in with Honda’s, and they would fry stators. The mechanical engineer in me likes this starter set up a lot, but it’s had trouble in the past. It’d be more compact and you’d probably have a beefier charging system as well if this was integrated into bikes, not to mention the coolness of stop/start. I think the best thing would be keeping the bike from heating up, but you’d probably need an electric water pump or something with a hot running sportbike.

    • Case

      I don’t want to sound pedantic, but I’m guessing some engineering progress on this system may have been made in the past 50 years. :) Also not sure sportbikes are the best application – when are they intended to idle at a stoplight? Instead, it’s perfect for standards, scooters, tourers, etc. My sportbike sucks at stoplights. Instead of adding a stop/start feature, I think the solution is “don’t ride your sportbike in traffic if you can help it”.

      • Justin

        You use your sportbike on the street right? Stoplights are oftentimes 2 min+ and it can easily overheat in that time. There’s really no way to get around this, even when you lanesplit. The standards and scooters and whatnot cool MUCH better at idle, so this is less of an issue. The conventional starter setup is way beefier and more reliable. These guys had nothing but trouble with the stator/starter combo back then and Honda brought it in with few teething issues. Just because engineering has gotten better doesn’t mean that it’s not challenging and more failure prone. Tons of motorcycle stators die just from charging application, imagine if it’s also the field coil for a motor! Plus, if the starter dies, there goes your charging system!

        Not to say this isn’t a neat innovation, but I’m skeptical until they last 50,000 miles regularly. This starting mechanism has been failure prone in the past, and that makes me have to see some proof.

        • Andreas

          Check out the start motor set-up for Honda Zoomer (50cc). This is super-reliable, and is some the same as this setup.

          Its always makes me smile when I push the startbutton of a Zoomer, it’s actually a “magic”button. No starter sound at all, the engine just fires up.

    • David

      Did you just try to convince us that this type of system might be unreliable based on previous attempts made almost half a century ago?

      I suppose reliable CVT and DSG transmissions are pipe dreams too. Nevermind reliable turbocharging. That stuff was never meant to work on cars or bikes.

      • Justin

        I’m just saying I’m skeptical because it’s uncommon and has had problems in the past. Many bikes are commonly know to eat stators, and this is a much more technically challenging way of starting a bike. Also many of the new DSGs and CVT are reliability nightmares. Yeah they’re getting sorted out but there will be problems. Don’t discount the engineering difficulties of 50 years ago, we’re still facing many of them today. . Like aluminum fatigue on 737-800 fuselages. We’ve seen that before, and this is only a 15 or so year old airframe design (the 800 series, not the 737).

        Look, I’m rooting for it and think it’s cool, but I’m not going to buy a bike with one until they’re sorted and reliable.

  • NitroPye

    Due to owning many iffy vehicles whenever I can’t feel engine vibrations at a stop light my heart rate instantly shoots up.

    It may take me a bit to get used to a stop/start.

    • ursus

      Yes, same here. It would take alot of trust that a voluntary stop-start wouldn’t become an involuntary stop-walk.
      I do like the integration of starter / generator. It looks like a strong positive.

  • Kyle

    Its so stupid that our cars dont have this technology simply because it costs “a little bit extra” and since the EPA doesn’t account for it. Auto companies dont spend the money to bring it over here.

    Same for the bike world I guess.

    • jp182

      well they are in business to make money.

  • Scott Pargett

    Is anyone concerned about the need to quickly move from a dead stop in the case of an emergency? Is the start/stop fast enough for this?

    Aka: old woman running over everyone at a red light in a 7,000 pound mercury? I know two people this has happened to.

    • Wes Siler

      Well, I think I said it twice in the article, but it’s instantaneous. There’s probably a tenth of a second penalty, but you’d never feel that.

      • Devin

        I too was wondering how instant “instantaneous” was. One tenth is perfectly alright.

        • karinajean

          I’m sure it’d be a little different than the car system, but in my manual transmission honda insight when I pop it into gear the auto-stop shuts off so I can get the engine going if I want to really smoke everyone at a light. (which is hilarious when it happens, b/c it’s a 3 cylinder 60-odd hp engine.)

    • stephen

      Just allow lane splitting everywhere and problem solved!

      • Scott Pargett

        I always split (live in CA), but sometimes you’re the only one “in line”. Thanks for the reply Wes.

  • Mark D

    I’m wondering how the EPA tests city mileage. Does it not include long idling periods? Seems pretty unrealistic.

    • jpenney
      • Mark D

        Cool graph, but the breakdown says that it spends 18% of the time idling. Would the EPA just not factor any time the engine is off into its calculation?

        • jpenney

          I’m not sure why this wouldn’t factor in. I would think that the engine being off 18% of the time would have some measurable impact.

  • HammSammich

    This seems like a great system with one major drawback…The leather bondage pirates won’t be able to constantly blip their throttle at stoplights (you know, for safety)…

    • Gingerbeard

      (Un)fortunately throttle blipping is still possible with the start/stop engaged.
      Stop at traffic light–engine cuts out after three seconds–blip throttle–engine automatically starts, revs, idles for three seconds, cuts out again (if throttle has not yet been blipped again). It seems it will give pirates MORE reason to blip their throttle (you know, to make sure the engine’s still working)…on second thought, there’s one major drawback–the engine isn’t cutting out.

  • kidchampion

    My Honda Civic Hybrid does something similar. The engine restarts when your foot leaves the brake and it is running before your foot reaches the gas. It is very reliable.

  • noone1569

    How does this affect my being able to blip the throttle at every light? I mean, come on now, that’s part of riding right

    • Emmet

      did your local Harley dealer tell you that?

  • Brammofan

    My Brammo Enertia already has this. The gas savings are un-frikkin’-believable.

    • HammSammich


      There, I said it. (In reality, I’m just jealous, especially since it cost almost $14 to fill up my bike today). :)

      • Brammofan

        It costs about .25 to fill up my bike. But if it makes you feel any better, I can only go about 30-35 miles on that .25. And I probably can’t go quite as fast as you – top speed ~60mph. But I bet we both have a damn good time riding.

  • Chuck

    Am I the only one who thinks that a seven percent gain in fuel economy does not outweigh the added complexity and chances of things breaking?

    • kidchampion

      A 7% emissions reduction times a million vehicles is substantial and important in countries like China, India and Vietnam.

  • Penguin

    It is quite surprising how many new cars over in the UK actually have this system on them – I have driven a couple and there is a tiny bit of a delay at traffic lights if the driver isn’t paying attention, usually the car restarts as you depress the clutch so it is ready by the time the gear is selected. It is a neat system but I always find myself wondering just how much fuel a car/bike really uses when stopped. The biggest problem we have in the UK is – believe it or not – the government (in more ways than one). Green Wave traffic lights have a similar application by stopping the need for traffic to constantly be braking, stopping, idling and then accelerating a vehicle can save a lot of fuel, probably even more fuel than stop/start. However, a few years ago it came to light that the government has repeatedly ignored Green Wave for the specific reason that it would have a big hit on tax revenue from saving fuel (bear in mind we pay $9.80 a gallon). So even if this technology saves 7% of a vehicles fuel it won’t save anyone a damn penny because we’d just get bigger and bigger taxes. Man my government sucks.

    It’s the other uses I am, however, warming to. No more hot smelling bike at the traffic lights, wonderful stuff. My CBR hates warm days in traffic. Get rid of the starter motor? Great idea. Those things weigh a shit load. I suspect that in the near future there will be a Fireblade with something like this setup on it, tell a biker that it will save 7% fuel and they will roll their eyes, tell them it will save a kilo and watch them swoon over it!

  • Tom

    yeah try putting that on a bgi twin and see how long the battery lasts!!!

  • ryan

    Looks like it may still be a while until we get a version of this system in the US. While the PCX itself is coming here, according to the “idling stop system” is not:

    “One technological trick left out is an “idling stop system” [...] It’s claimed to net a 5% savings in fuel economy but was left off the US version because riding here tends to be a mix of urban, city and rural riding, which would minimize the benefits of the urban-intended system. ”

    Too bad, we may just have to wait.