Chevy Volt - Electric Car

[Mintmaster]

Mass does have an effect on constant velocity and ancilliary to that efficiency of any drive system. Unless, of course, you have steel tires that are perfectly non-deformable and relatively low friction (a train for instance), and even then mass will still influence it, although to a much lesser extent.

I thought this was a tiny part of highway resistance, but googling some data it seems like it can be over 20%. I didn't know hysteresis was so bad on tires.

So, _xxx_, while I still stand by what I said about physics, acceleration, and hills, I will concede that mass can make a significant difference in highway fuel economy. Each 100kg could cost 2%. In any case, we were always in agreement about regular hybrids being pointless for highway mpg.

 

[Arwin]

Glad I checked if there was a new post - I was writing this big post and reading up on my physics to make sure I explained it right, which is tough for me because I lack a physics background and all my knowledge comes from hanging out with too many car bufs, but you saved me the effort.

Of course now I still don't quite agree about regular hybrids being pointless for highway mpg. Regular cars will brake on the engine when you're not pressing the accellerator. Hybrids like the Prius don't, though they do simulate it a little in order not to make the difference between regular cars and hybrids too jarring. This simulation however works similar to regenerative braking and charges up the battery. Sure it's not an enormous amount, but it's still there.

Now because of this, going downhill even slightly will have a stronger (positive) effect on fuel economy in such a hybrid. Also driving in traffic, even if that traffic is mostly around the maximum allowed highway speed, still involves adjusting your speed. In order to avoid unnecessary braking you do this by pushing and lifting the accellerator. Whenever you lift ... you get my point.

Of course, hybrids can also choose engine types that are stronger in some parts and weaker in the parts where the electric engine helps out. Citing Wiki on the engine type in the Prius, "The Atkinson cycle is designed to provide efficiency at the expense of power and is beginning to see use in modern hybrid electric applications."

 

[Sxotty]

SB is spot on, and that is why I put the formula up ages ago to show it was proportional to normal force...And the document that lists a huge number of tires so you can calculate what it is yourself. The proportion does depend on speed since rolling resistance is constant (if the downforce is not increasing with speed).

 

[Mintmaster]

Of course now I still don't quite agree about regular hybrids being pointless for highway mpg. Regular cars will brake on the engine when you're not pressing the accellerator.

You shouldn't have any need to be engine braking on the highway, as there is plenty of air resistance to slow you down. If you do need some light engine braking, though, you're still saving the gas needed to maintain engine speed without load.

For a typical car at 65 mph, air and rolling resistance should handle a 4% grade, and engine speed maintenance maybe another percent. That covers most highway slopes.

 

[_xxx_]

Meh, sometimes it feels like my foot spends more time on the brake than on the throttle pedal. It's rather 80-100 mph average over here and many steep hills, so I indeed have to break often when rolling downhill because there is always some senior or housewife blocking the left lane with their 50-60 mph.

Not really related to the topic, just venting.

 

[Arwin]

You shouldn't have any need to be engine braking on the highway, as there is plenty of air resistance to slow you down. If you do need some light engine braking, though, you're still saving the gas needed to maintain engine speed without load.

No, but the point of the matter is, you will lift that pedal sometimes so that you don't accellerate at all, and at that point you will be engine braking. And that's more than you think too - just press the clutch instead and see how much less you slow down. Once you do, you're regenerating energy rather than just saving gas.

You won't hear me say it's a lot, mind you. But it's enough not to be completely pointless at least.

For a typical car at 65 mph, air and rolling resistance should handle a 4% grade, and engine speed maintenance maybe another percent. That covers most highway slopes.

But, again, a non-typical car like a hybrid (or at least the Prius) can benefit more from such a slope (the fuel engine will often shut down completely)

And yes, apart from that _xxx_ and I both live in areas where there are quite a few significant slopes.

 

[Mintmaster]

No, but the point of the matter is, you will lift that pedal sometimes so that you don't accellerate at all, and at that point you will be engine braking. And that's more than you think too - just press the clutch instead and see how much less you slow down. Once you do, you're regenerating energy rather than just saving gas.

I thought about this more and I'm going to strengthen my claim: If you don't downshift then you're not losing anything in engine braking.

Regardless of the load, part of the fuel used in running an engine at, say, 2000 RPM is simply overcoming the friction, and you can see that with how much you have to hit the pedal to idle at that speed with the clutch disengaged. On a flat road, fuel is used to overcome that friction. Going downhill, you use less or zero. Just think about the energy flow. Some energy manifests itself as heat through the exhaust (carnot efficiency limits) and unused gas expansion. This will vary depending on the efficiency of the engine at different operating points (see BSFC charts), and will be affected by hills. Actually hills can benefit, because many engines (even the VW TDI) are optimal at power outputs greater than that needed for level highway driving.

Some energy is used overcoming friction in the drivetrain, friction in the engine, air resistance, and rolling resistance. All these are the same for a given car and engine speed. It doesn't matter if fuel is going to the engine and driving the wheels or vice versa, as the work done is identical. Thus engine braking during downhill portions does not lose energy that you wouldn't already lose otherwise during flat driving, but of course this is only true as long as you don't have a higher engine speed due to a very steep hill speeding you up or downshifting to increase engine braking.

However, I suppose you could improve flat fuel economy by accelerating to 70 MPH, hitting the clutch and coasting down to 60, and repeating rather than staying at a level 65 MPH. You couldn't do this as freely on hilly terrain. But this isn't constant speed so my argument still stands.

I actually did this once with a van that was stuck in second gear, thus causing overheating on the highway due to high RPM. Cycling between accelerating and coasting in neutral let me manage, though rev-matching with an automatic was a bit unpredictable.

 

[Mintmaster]

Meh, sometimes it feels like my foot spends more time on the brake than on the throttle pedal. It's rather 80-100 mph average over here and many steep hills, so I indeed have to break often when rolling downhill because there is always some senior or housewife blocking the left lane with their 50-60 mph.

In that case you could probably get even better economy than you already do.

BTW, stop making us feel bad with your 80-100 mph priviledge Even if I disobeyed the law I'd rarely see a stretch of road for that without cars in the way.

 

[Sxotty]

BTW, stop making us feel bad with your 80-100 mph priviledge Even if I disobeyed the law I'd rarely see a stretch of road for that without cars in the way.

Go west my son Or East if you are too far west already.

 

[_xxx_]

That privilege is as said very often theoretical, we have so much traffic that you'll rarely ever get to drive faster than 100-120 km/h on freeways. Often you'll even have the right lane (with all the trucks which are limited to 80 km/h) moving faster than the clogged up left lane.

 

[Mintmaster]

Often you'll even have the right lane (with all the trucks which are limited to 80 km/h) moving faster than the clogged up left lane.

That happens all the time, and has been one of my rules of lane choosing even before I got a car. The left lane is full of tailgaters, so it's really dense, and it doesn't help that they change lanes frequently too. When traffic slows down it gets nasty there (generally speaking, of course).

I guess I got the wrong image when you said 80-100mph average.

 

[Frank]

Buy an all-electric with brushless DC hub motors that also replace the brakes.

 

[Silent_Buddha]

Downforce also doesn't increase the mass or weight of the car, but is just a down pushing external force.

And mass acts as a force pushing down on a car.

The difference with the downforce generated by air resistance is that the downforce thus generated can be directional. IE - with the proper setup you can have the force pushing down only. But it still increases the apparent mass of the vehicle with regards to friction and all those other lovely physics calculations.

The mass of the car on the other hand not only produces downforce, but when cornering also attempts to keep the car going in the same linear direction.

If it were possible to weigh a F1 car at maximum speed, it would have an apparent weight many times what it actually weighs.

Regards,
SB

 

[Mintmaster]

The difference with the downforce generated by air resistance is that the downforce thus generated can be directional. IE - with the proper setup you can have the force pushing down only.

Downforce is always going to push backward as well, i.e. it will always give additional air resistance. I guess you can always pretend that the downforce is purely downwards by making a comparison with a not so efficient shape, but AFAIK for any shape producing downforce there will will be a similarly dimensioned one that doesn't but has lower drag.

From what I gathered, Dr. Evil was just trying to say that if you're aiming for high efficiency then you're going to eliminate downforce because that's a natural part of reducing the coefficient of drag.

Interestingly, lift could actually increase efficiency at the expense of high speed grip. I wonder if any of these green cars look into that, especially since a car's shape looks a bit like an aerofoil anyway.

 

[DuckThor Evil]

If it were possible to weigh a F1 car at maximum speed, it would have an apparent weight many times what it actually weighs.

I agree basically with what you said here, I just mentioned that there is a difference between real weight and apparent weight created by downforce. At around 200 kmh you could drive An F1 car upside down due to the downforce pushing the car against the "roof" if it added real weight to the car then it would fall down.

From what I gathered, Dr. Evil was just trying to say that if you're aiming for high efficiency then you're going to eliminate downforce because that's a natural part of reducing the coefficient of drag.

Interestingly, lift could actually increase efficiency at the expense of high speed grip. I wonder if any of these green cars look into that, especially since a car's shape looks a bit like an aerofoil anyway.

Yes that's what I meant. I wonder if you can create lift without adding also extra coefficient of drag in the process?

 

[Dave Baumann]

Downforce is always going to push backward as well, i.e. it will always give additional air resistance. I guess you can always pretend that the downforce is purely downwards by making a comparison with a not so efficient shape, but AFAIK for any shape producing downforce there will will be a similarly dimensioned one that doesn't but has lower drag.

There are more efficient methods of producing downforce though. Diffusers are more efficient than wings, producing less drag while still providing downforce.

The most aerodynamic shape is a teardrop, which is why you see all the solar powered racers shaped that way (on a cross section from front to back) but thats not feasibly producable from a mass manufacture point of view (nor practicality). You'll find that a lot of the "green" cars are based on aerodynamics that follows a Kammback shape, that provides low drag coeficients while still providing usable and manufacturable shapes.

I think when we start talking about producing lift to incrase efficiency things start getting into dangerous areas.

 

[Mintmaster]

I agree basically with what you said here, I just mentioned that there is a difference between real weight and apparent weight created by downforce.

I think SB took objection because he was talking about rolling friction. That indeed increases with downforce, and there's no difference between real mass and downforce there.

There are more efficient methods of producing downforce though.

They're more efficient but still produce drag. A teardrop is efficient, but if you modified it by making it more of an upside down aerofoil shape, used ground effects, or changed the angle of attack to produce downforce, then drag would increase.

I think when we start talking about producing lift to incrase efficiency things start getting into dangerous areas.

Yup, although I think there is some lift in many car designs. Otherwise we wouldn't hear companies bragging about "zero-lift" design.

A quick google pointed me to this:
http://jalopnik.com/5148770/2010-mugen-zero+lift-honda-insight-photos-video-and-no-lift
I can only conclude that the Insight has lift without the modifications, and it probably decreases rolling resistance.

 

[Silent_Buddha]

Aye, if you're going to attempt to use a shape that promotes a "lifting" force, then you have to be careful to limit the top speed such that it never approaches the limit where lift is close to or greater than the weight of the vehicle pushing down.

Additionally cornering at any sort of speed now becomes a more precarious situation. So additional safeguards would have to be put into place such that speed of cornering doesn't overcome the reduced friction (tire to road) from any speed related lift.

In other words, I don't expect this to be persued in any consumer driven vehicle. At least not to any significant degree. Especially when you take into account silly consumers that love to tinker with their cars. Possibly then exceeding the limits of safety of such a vehicle. I see lawsuits ahead.

Regards,
SB

 

[Simon F]

Yup, although I think there is some lift in many car designs.

Just watch the roof on a soft-top. I don't expect it's terribly significant since my car has a very subtle spoiler.

 

[Dave Baumann]

Just watch the roof on a soft-top. I don't expect it's terribly significant since my car has a very subtle spoiler.

I don't know what you're driving, but that lift is likely caused by negative pressure as the airflow comes from the front windsreen; its probably negligable in comparison the the downforce from the windscreen.