[Closed] Electric cars, why don’t they?

That'd be perpetual motion 🙂

Any benefit in energy produced would be negated by drag and friction needed to rotate the extra 2 wheels.

Could be thinking absolute pish

The fancy drive motor is also the fancy regeneration motor. You would add loads of weight in extra wheels and 'alternator'.

Why the extra axle, just connect the motor to the alternator

To be (a little) more technical than answers so far. Any significant regeneration adds an equally significant load to the forward motion. Think engine braking or dragging the brakes. This drag will be a big drain on the system. As with conventional vehicles, the best way to get good economy is with as little drag as possible and coasting. My Passat GTE coasts very well. If you have a manual car drive down the road and dip the clutch. You'll get an idea of what it's like.

Adding more wheels and more generators doesnt add more power - the power available to be generated is 'the power required to stop a car'. Putting an generator on each wheel doesnt result in 4x 'the power to stop a car' being harvested, it results in each wheel generating 1/4 of 'the power needed to stop a car'. Putting 6 wheels on would result 1/6th of 'the power needed to stop a car' by each wheel and so on.

Why the extra axle, just connect the motor to the alternator

Why not connect it to six alternators, then connect each of those alternators to a motor, each driving six alternators 🙂

Let me try and explain it another way. If you had extra wheels generating energy, that would I crease drag so you'd have to put more energy into the motor. More, in fact, than you'd make with the regeneration.

Regeneration only works when you are slowing down. In any car you put energy into the car to make it go faster which gives it kinetic energy. In a normal car, when you brake this energy turns into heat. In an electric car, instead of heat most of it goes into the battery. If you convert the cars kinetic energy into battery charge, it slows down.

You can't get more energy out than you put in,in fact you always get less energy out as some of the energy gets converted to other forms of energy such as heat.

This is why hybrids are so rubbish, especially self charging hybrids, all they do is recover small amount of energy otherwise disappated as heat during breaking, this energy recover is further negated as the extra weight of the regen system takes energy to move.

Bottom line, yes you're talking utter pish.

Ha ha, bottom line is don’t drink on a Friday afternoon, then look at your neighbours Tesla thinking “ tell you what, I reckon I can do better than that!” 🤪

But if you attached a treadmill to the axle you could cancel your gym membership!

If you had 8 wheels with 4 regenerating you could create a surplus and power your house from the car when it's sat on the drive.

If you had 8 wheels with 4 regenerating you could create a surplus and power your house from the car when it’s sat on the drive.

Google “V2G”

(some of what you find will be technically sensible)

This is why hybrids are so rubbish, especially self charging hybrids, all they do is recover small amount of energy otherwise disappated as heat during breaking

Incorrect.

There are several sources of inefficiency in a petrol car. To control the speed of a traditional petrol engine you use a throttle which decreases the size of the hole through which it sucks air, to slow it down. This introduces a lot of turbulence in the airflow which ultimately wastes energy, called pumping losses. The engine is therefore more efficient when running at higher load but low revs, this is why your fuel economy goes down when you drive through town because that's low load and higher revs (compared to road speed) as you drive in lower gears.

In a mild or self-charging hybrid, instead of closing the throttle to control the car's speed it activates a generator (through clever energy distribution means, it's not like a clutch) that makes the engine do work to charge the battery, rather than create pumping losses. This energy is then used when you need to accelerate. The petrol engine can then be smaller, which means when you are cruising at a steady speed it is running at a more open throttle. And thanks to very clever designs they can create effectively very high gears which allows more open throttle again and more complete burn in the cylinder.

On top of that, they allow the use of electric-only power when in situations like crawling traffic, which is where traditional engines are at their worst.

The regenerative braking is only a small contribution to the efficiency.

Mild hybrids are much more efficient than petrol engines, and generally somewhat more efficient than diesels on the open road without any of the NOx or particulate pollution. And in town they win hands down over traditional engines.

Thanks molgrips it needed saying

They should make them entirely from solar panels.

And replace the friction brakes with a wind brake. Perhaps a windmill that deploys from the boot? Could mill some wheat at the same time.

I can understand why no extra wheels, but as a major barrier to acceptance seems to be short range precluding occasional longer trips, why not come up with a common power connection and battery trailer to increase range and carrying capacity on a temporary basis.
These could be hired and mean the vehicles remain efficient over everyday short commutes and can step up when the PMs spokesperson wants to visit her mum or for holidays.

short range precluding occasional longer trips

Because there isn't really a problem in this regard.

To control the speed of a traditional petrol engine you use a throttle which decreases the size of the hole through which it sucks air, to slow it down. This introduces a lot of turbulence in the airflow which ultimately wastes energy, called pumping losses. The engine is therefore more efficient when running at higher load but low revs, this is why your fuel economy goes down when you drive through town because that’s low load and higher revs (compared to road speed) as you drive in lower gears.

This is basically nonsense. At any given throttle opening, there will be a most efficient engine speed, which is where you achieve peak torque. To turn that around, for any required power output, you will have an optimum throttle opening and engine speed. In general, for cruising, this is at fairly low revs with a small throttle opening. This is why modern cars have very tall overdrive gearing.

Modern direct injection engines are a bit different, but older carburetor and port fuel injection required turbulence to mix the fuel-air mixture. At low revs, more turbulence is better, so smaller ports and less cam lift helps with low down torque. For high power outputs at high revs, you need much more airflow so large ports and higher camshaft lift is needed. This will give poor fuel-air mixing at low revs though. This is why older performance cars were so awful to drive around town.

The biggest issue for fuel economy around town is stop-start driving. It takes a lot more fuel to accelerate to 30 mph than to cruise the same distance at 30 mph because you have to convert chemical energy of the fuel into kinetic energy of the vehicle. A cruising vehicle already has kinetic energy, it's just a matter of overcoming mechanical friction and aerodynamic drag. When you stop, your brakes convert that kinetic energy into heat, so that energy is wasted. Hybrids convert some of that back into electrical energy, so they are more efficient for stop-start driving.

At higher speeds, aero drag massively outweighs mechanical friction so cruising at a low speed is much more efficient than cruising at high speed. Pumping losses in the intake system have nothing to do with this.

The answer to the original question is simply that a 6 wheeled car would look weird and no one would buy one.

Why don't electric cars have a small wind turbine on the roof? As they drive along this would generate free electricity which could recharge the battery.

I dont think anyone was suggesting it would be able too replenish the battery completely, or even produce enough of a charge to replace what is used up. But I cant see why it couldn't be used to extend the range to a small degree.

But I cant see why it couldn’t be used to extend the range to a small degree.

Because: energy in = energy out.
You get additional losses through friction and inertia (and mirriad others) which reduce the amount of energy out. The regen you would get with the additional axle or whatever will be no different to the regen you get with the normal on board system.

One area where hybrids gain economy is their ability to shut off the engine and power them (to varying degrees) with the battery and motor. Basically transforming the energy loss of braking heat that you get in a normal car to electrical energy to then start the engine again for free, power the vehicle or both.

Why don't they make them with really big back wheels and tiny front wheels, then they'd be permanently going downhill?

This is why hybrids are so rubbish, especially self charging hybrids, all they do is recover small amount of energy otherwise disappated as heat during breaking, this energy recover is further negated as the extra weight of the regen system takes energy to move.

Petrol hybrids aren't a silver bullet (nothing is) but this is misinformed.

1) the gains aren't "negated" by the extra weight because hybrids are still more efficient than their nonhybrid equivalents

2) petrol hybrids have created an immediate and significant fuel saving opportunity with zero real change in user behaviour and zero investment in infrastructure

3) Toyota hybrids have been stunningly reliable and durable, the exact opposite of "rubbish"

for any required power output, you will have an optimum throttle opening and engine speed.

Right but what governs that optimum? Toyota created that hybrid drive system to harvest energy that would have otherwise been lost by running the engine inefficiently, not just from regenerative braking.

It works anyway, petrol hybrids are more efficient than non hybrid petrols at pretty much all speeds and in both stop start and cruising, in my experience.

something like this but electric 🙂

(yeah I know, not quite.  it's fun anyway though)

You get additional losses through friction and inertia (and mirriad others) which reduce the amount of energy out.

Would friction really play such a large part these days ?. I mean, its not like those old fashioned dynamos on your front wheel.
There are systems that harness electricity and friction there plays a minor part if any.
On yachts we have fans to make use of the wind that crosses the deck normally to trickle charge boat batteries. So they must provide some power regeneration, or they wouldnt be in use.
Im not saying its a great deal, but these systems are pretty old technology wise, and maybe in todays tech the method for capturing energy and putting it back into a battery is far advanced.

Something is better than nothing is it not 😕

Because if you put a turbine on your roof you will generate additional drag which means you need to take more energy out of the battery to drive at the same speed than you would if it wasn't there.

Some of that energy will be recovered from the electricity generated by the turbine, but when you take out frictional, mechanical and electrical losses it will be less than the additional energy required.

You cannot use energy to generate energy without losses.

The reason it works on a yacht is because the wind is blowing anyway, you don't have to generate the power to get it to blow.

Why don’t they make them with really big back wheels and tiny front wheels, then they’d be permanently going downhill?

🙂

I was sceptical about self charging hybrid but it was the only option that currently fits our needs and comes in on budget. (It's not my car)

Our big self changing hybrid petrol monstrosity actually gets better mpg than our small diesel unless it's a motorway type trip.

We live up a hill and the first 10 mins of getting anywhere is down and then through town. So the self charge regens most of the time regulating speed and then using electric to move from junctions and in the town traffic.<span style="font-size: 0.8rem;">Where as the diesel just wastes it's energy pumping air and using heating the brakes, then starts to warm up once on the flat and out of town. </span>

On the way back up from a journey the petrol engine cuts in on the steeper parts of the hills and faster bits of road where the engine is warm and running at its most efficient.

The wind turbine on the roof could be setup so that it automatically lowers itself whenever driving at any kind of speed, and then raises up again, e.g. at traffic lights or in a big traffic jam.

It would also solve the problem of all those cables snaking across pavements tripping people up, at least for anyone living somewhere reasonably exposed.

Right but what governs that optimum? Toyota created that hybrid drive system to harvest energy that would have otherwise been lost by running the engine inefficiently, not just from regenerative braking.

The Prius and other hybrids use a drive-by-wire system with electronic throttle. The go-fast pedal isn't mechanically connected to the throttle. It just sends a signal to the ECU about how much power the driver wants to use. The ECU looks at the battery state and other variables from the sensors in the car and decides the optimum level of power to draw from the petrol engine (ICE) and battery. If the battery is fully depleted, it will use the ICE to partly charge the battery as long as the driver hasn't floored the throttle.

This is because the engine is optimized to run at a narrow rev band and uses a CVT transmission to keep it in the optimized rev range. That engine wouldn't work well as a normal non-hybrid engine with a regular manual gearbox, having the hybrid system lets them optimize the engine for efficiency and use the battery to boost it when the driver wants peak power.

Full-throttle ICE operation is really inefficient, so spending some part-throttle operation to keep the battery partly charged will generally improve efficiency because it provides a buffer to reduce full-throttle operation. Braking regeneration is free energy, so the system won't fully charge the battery just from the ICE. Programming the optimum mix of ICE and braking regeneration is the secret sauce. Ideally, it would remap itself based on user history and location/route data.

The wind turbine on the roof could be setup so that it automatically lowers itself whenever driving at any kind of speed, and then raises up again, e.g. at traffic lights or in a big traffic jam.

And where is the power coming from to keep raising and lowering a heavy wind turbine?

Solar panels could be a good solution as that truly is free but they don't really generate enough power to worry about.

kerley
Free Member

And where is the power coming from to keep raising and lowering a heavy wind turbine?

Put an extra pedal on the floor and have the driver pump the turbine up and down, easy.

The answer to the original question is simply that a 6 wheeled car would look weird and no one would buy one.

Except maybe Lady Penelope.

Going back to the Op. A 4-wheeled electric car is basically an electric motorbike with two extra wheels, but you have to remember that it's as wide as a car when filtering through traffic.

BTWm it's worth noting that after aerodynamic drag, the tyres on an EV are actually the biggest "consumer" of energy!

Not for nothing do the more "eco oriented" EVs have specific low drag tyres. This is taken to extreme by the BMW i3, which has 155 profile 19" tyres, super skinny, all in order to reduce rolling losses as much as possible

(It's also why the more sporting the EV, the greater its consumption. ie a Taycan S has huge, soft tyres, terrific for pulling your eyeballs out round the bends, but really, realy draggy to push around)

The wind turbine on the roof could be setup so that it automatically lowers itself whenever driving at any kind of speed, and then raises up again, e.g. at traffic lights or in a big traffic jam.

I think you are massively over estimating the amount of power a small turbine can generate.

I suspect you'd struggle to generate enough power to do much more than cover the additional energy needed just to drag the weight of it around with you.

Small turbines do very little, that's why we don't have them on our roofs at home. They have to spin very fast to do anything because of the low torque of short blades. This makes them very noisy. Longer blades will turn more slowly and work by high torque rather than high speed. This is much quieter.

However a car with a 10m high turbine on it is not going to be easy to sell. Or make.

@thols2 I think you just described the Honda system. The Toyota system doesn't technically have a CVT transmission. It has motor/generators that work alongside the ICE and energy can go to and from wherever it needs to. Too much to type out here, but it's all online.

The car doesn't simply drain battery until it's low then turn on the engine, unless you're crawling. At 30mph, if you cruise on flat roads it would alternate between battery and ICE driving the wheels and charging the battery. This is more efficient than simply driving the wheels at say 30mph, I'd assumed because of the amount of throttling required would reduce the efficiency of the engine.

I'm not convinced that that engine was optimised for a narrow rev band. It had VVT for a start.

OPs original idea is just kind of partial perpetual motion machine I.e. any power that the extra wheels generates would have had to be generated by the motor, so it’s just a roundabout way to recirculate power but with additional losses.

Is by no means a daft idea to explore this though, as this type of thinking can and has lead to other forms of energy recuperation and efficiencies (e.g use of waste heat and high co2 levels from natural gas engine exhausts in industrial greenhouses. As energy content in pipeline gas has tends to be quoted conservatively by the utility company you can end up with >100% efficiency albeit only economic rather then physical efficiency.

Plus don’t forget that many clever people has explored perpetual motion over the years.

Wilkipedia - perpetual motion</

The Toyota system doesn’t technically have a CVT transmission.

It uses a planetary gear system that is functionally equivalent to a CVT. The engine speed isn't directly linked to the road speed so it can be optimized to balance how much ICE power goes to the wheels and how much goes to the electrical system, and vice-versa. Technically different, functionally it serves the same purpose.

The function of a CVT gearbox is only to vary the torque and speed of the wheels relative to the engine.

The purpose of the Toyota hybrid drive is to send energy where it's best sent, and as a side effect this varies the ratio of road speed to ICE speed, so I reckon that it's quite different. It just feels the same under many circumstances (but not all).

I'm well acquainted with how the system works, where I'm filling in the blanks is the relationship between charging the battery whilst cruising and throttling back. If you have a SoX lower than the target (something like 70% or 6 bars out of 8 on the display) and cruise at say 60mph it will give about 50mpg for a few minutes and then jump to 75 or so. It's clearly stopped generating electricity at this point and list now be limiting engine speed the traditional way. The question is, is it doing this because the battery is effectively full and it has to, or just because it is now more efficient?

The fact that it generates whilst cruising at a cost of instantaneous economy suggests that it does this be wise the overall energy output (cruising energy +battery charring) is higher than simply waiting for regenerative braking to refill the battery.

I quite like the idea of towing an extra battery for longer journers.

Well sort of. I thought we could agree on 3 battery sizes and then swap them for a charged on at a service station. That solves the long journey problem and no charging point at home problem. It also solves the waisted battery problem. There is a risk i might buy a 200 mile range car and rarely use more than 15% of the battery. That battery would save more co2 split between more vehicles

Yeah I thought of that but it's not really necessary when you can do 250 miles between charges now.

I'd quite like a caravan with extra batteries in it to offset the increased consumption.

Emissions Elsewhere........

This is taken to extreme by the BMW i3, which has 155 profile 19″ tyres, super skinny, all in order to reduce rolling losses as much as possible

Front and rear tyres on an i3 are different - I was asked to check the size on one we have in storage, and I assumed they were the same size all round, but something made me check front and rear, and they’re different.
Still skinny, though.

Early i3's have 155 all round, later cars have an asymetric set up with wider rears (and the "s" has 20mm bigger section tyres)