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In a petrol vehicle you rev the engine and the harder you rev, the more juice you use.

You can rev up to 6,000 rpm and still only do 20mph. You are wasting that juice to get to that speed.

What about an EV? Is the amount of output (speed) directly connected to input (revs and petrol used)? No gearbox to soak up excess rpm.

Do I need to keep a light right foot or can I floor it?
 

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Do I need to keep a light right foot or can I floor it?
You dont "need" to keep a light right foot, it depends how much acceleration you want to get to the speed or place where you want to be before you back off to go at the speed you wish to travel at. If you do floor it it will take off like teh best acceleration you'd get from an ICE in the optimum gear, but immediately and without any loud noise which can give a funny surprise to your passengers. So a mate of mine told me :whistle: EDIT; I see you have an i3 so you should know this?

The faster the engine spins, the more energy it's using. It will all other things being equal*, go faster in direct and constant proportion to revs, since there is only one gear ratio, eg if engine spins twice as fast car will go a fixed % faster since the wheels will turn at a fixed % faster. It's as if your ICE was always in say 4th gear but had the torque to pull away at any rev however low.

If you put your foot down and held it there you would hit one of two events (1) reach max speed due to reaching max engine revs vs air resistance, (2) a limit imposed by the ECU, lets say 99mph or whatever, or I suppose (3) something in front of you :eek:

Just like an ICE if you roar up to the lights then ram on the brakes at the next set, you'll use more energy than if you cruise up to them, except that you'll get some energy back when you brake which you wont with ICE. So the lighter right foot the longer range. Same as an ICE except range isnt generally an issue with them so you dont care.


* floor it too much and either the wheels win spin losing energy or traction control will kick in and back off the acceleration
 

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Yeah that's why eco mode is a bit of a con. It numbs the throttle but does that actually lead to reduced energy consumption? Don't think so...
 

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I'm sure that the car will only accept so much regeneration and there is also system loss (so no perpetual energy : ( ). On the instrument cluster there's the half semi circle that has the blue bars which indicate whether you are charging (regenerating) or using power from the eDrive. What's interesting is that with extreme regeneration the white bar goes past the blue bars on regeneration side meaning that there's too much energy to be regenerated at once. This encourages sensible acceleration And deceleration.

...So floor it but not quite and make sure you have enough room to slow down gradually lol.
 

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There may be more heat generated n the battery from higher rates of discharge which is of course wasted energy
 

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In an ICE revving the engine higher makes you go faster. Its not "wasted". The engine is mechanically connected to the wheels, via a gearbox.

RPM alone isnt the be all and end all of fuel consumption. Crusing along at 5500rpm will quite likely use less fuel than accellerating hard at 2000rpm for instance. engine load is important.

Higher engine speeds have more losses, so for a given road speed, a higher gear is usually (but not always) more economical.

In both an ICE and an EV, the throttle doesnt have ANY relation to speed. the throttle is essentially a torque demand. Your foot is requesting an amount of power. What that amount of power does depends on many variables.

25% throttle on an unloaded ICE might well see it round against the rev limiter like a chav in halfords car park. whereas equally 25% throttle in 6th gear up a steep hill might have it only doing 1200rpm. Next time your driving up a slight hill in an ICE, without changing the throttle position, depress the clutch and see what happens.

I'm not really sure what your actually wanting to know?
 

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It depends what you want the energy to do. It is not necessarily wasteful to use up more energy so long as you go proportionately faster.

(edit - see the last post for more on that comment)

The question therefore hinges on the peak efficiency of the two systems. For both, the peak efficiency for converting stored energy to kinetic energy is not at zero speed but some finite speed. The question has to therefore be put in the context of the profile of speed, acceleration and distance you are needing to follow.
 

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In a petrol vehicle you rev the engine and the harder you rev, the more juice you use.

You can rev up to 6,000 rpm and still only do 20mph. You are wasting that juice to get to that speed.

What about an EV? Is the amount of output (speed) directly connected to input (revs and petrol used)? No gearbox to soak up excess rpm.

Do I need to keep a light right foot or can I floor it?
With an EV, you have instant torque from the electric motor. When you put your foot on the accelerator, only light pressure is needed to activate the electric motor which drives the wheels forward compared to a petrol car or IC engine where you have to put a lot pressure (relatively) on the pedal in order to get the fuel pumping through the engine carburettor or fuel pump. There is much more friction in an IC drive system so that's why it needs more pressure to get up to a high speed but it also means it wastes a lot of energy and fuel. An electric car, through the inverter and motor will deliver power to the wheels much more quickly than IC engine can. Some good vids by Learn Engineering on You Tube explain this. It's why the all electric Rimac can leave the even the top spec Lamborghinis and Ferrari in the rear view mirror as Jeremy Clarkson found out recently.
 

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With an EV, you have instant torque from the electric motor. When you put your foot on the accelerator, only light pressure is needed to activate the electric motor which drives the wheels forward compared to a petrol car or IC engine where you have to put a lot pressure (relatively) on the pedal in order to get the fuel pumping through the engine carburettor or fuel pump. There is much more friction in an IC drive system so that's why it needs more pressure to get up to a high speed
I think this is not a great description.

All pedals these days [in new cars] are electronic, AFAIK, so the amount of 'pressure' or otherwise on the pedal is purely a software mapping which can be altered at will.

It very much depends on the algorithm used to map the pedal to the demand power. Getting something that feels intuitive and gives the driver good control is non-trivial.

For example, you could program a pedal to deliver an acceleration proportional to the pedal displacement. In this case it would be quite different to a conventional pedal but would actually be totally intuitive because if you push beyond a point the car will accelerate, and drop behind it and it will provide drive-train deceleration.

I believe, but don't really know, that in general EV pedals are set for 'mostly' speed control, this being the most similar to ICE. The thing with ICE pedal position is that it is a complex function of torque and speed control. You don't need to know anything about the mapping, all you need to know is the relative movement, i.e. go faster then put your foot more down. The thing that ICE does very nicely is that once the car is up to a given speed, the pedal effectively becomes a torque control device, so that the car will slow gently as the power requirement goes up. (Whether you actually like this effect or not is going to be to taste, because on the one hand it is why you will slow while going uphill which might be good for you to know when power is required, but there again you might just prefer to go up the hill at a steady speed for a steady pedal position.)

None of this has anything to do with the efficiency of EV versus ICE.

However, as is well known to hypermilers, the losses within the engine's moving parts are a function of the speeds of those parts, and how quickly they slide against each other. Optimum efficiency is therefore (generally*) with the lowest engine speeds at the highest loads for that given speed. i.e. change up into the highest gear possible that can deliver the required torque at the road wheels.

*(It's not quite like this, but you need to look up BMEP maximums and things for a fuller explanation.)

Hybrids exploit this 'behaviour' and will run the lowest engine speed that generally covers the 'average' power needed for the driving of the moment, with the spike demands being met by the battery and the load on the engine being kept high and constant by loading it with the generator, and through re-charging the battery.

An ICE with appropriate gearing and power management can keep its wheels at the maximum torque, short of losing traction, just as an EV can. It is easier for an untrained driver to use an EV to deliver its power instantly, but it is not impossible for an ICE to do that either, it just depends a little more on the driver.
 

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In an ICE its generally a torque demand, but its a 2D map with pedal position on one axis, and engine speed (rather than road speed) on the other.

Heres the pedal map from a VAG 2.0TFSI for instance:



Top line is physical pedal position. rows are engine RPM. Output is requested torque %, which is then fed into numerous other tables to work out how to achieve that torque request, IE throttle angle, boost pressure etc.

You will note that at low engine speeds, its actually very aggressive, and then backs off as the engine speed climbs. This allows the car to feel more willing at lower engine speeds where available torque is very low, but keeps it controllable at higher speeds. It also has the peculiarity that if you hold one specific throttle position somewhere in the midrange, your requested torque falls off as the revs climb. Probably good reason for that but i'm not sure why. Perhaps because the turbo comes on boost and available torque massively increases?
 

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An electric motor spinning fast, say at 7000 rpm has far lower losses than a petrol 4 stroke engine. A petrol 4 stroke has losses from its reciprocating pistons as well as pumping losses, eg from the oil and coolant, an electric motor doesn't really have any losses like that so requires very little energy to keep it spinning (think of a flywheel).
 

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So, I’ve been thinking about the variables involved to work out if gentle acceleration to 40MPH using 5kW or a quick blast of 80kW is more efficient in a BEV.

Is it better to get up to cruising speed quickly and then maintain that speed or accelerate slowly to the cruising speed.

In my real world tests it doesn’t seem to make much difference, especially if the anticipation when slowing down means little to no use of the regen/brakes.
 

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So, I’ve been thinking about the variables involved to work out if gentle acceleration to 40MPH using 5kW or a quick blast of 80kW is more efficient in a BEV.

Is it better to get up to cruising speed quickly and then maintain that speed or accelerate slowly to the cruising speed.

In my real world tests it doesn’t seem to make much difference, especially if the anticipation when slowing down means little to no use of the regen/brakes.
It depends on the car.

In my general experience*, and extensive hypermiling in nauseating levels of detail and attention, it is better to stick around 0.5~0.8C discharge rate, i.e. if you have a 30kWh pack, 15~20kW accels are around the most efficient.

Too little power and the motor is away from its optimum operating points, too much and you end up wasting energy due to I^2.R heating effects.

In general, if you are driving reasonably around urban areas, i.e. nice and gently, then you are more likely to be driving inefficiently due to a lack of power applied rather than too much. A steady solid accel to speed and then back off and allow the car to slow down gradually. Try not to operate the car in the 'dithering-around' levels of power, either accelerate solidly, or back off and let the car coast.

*My experience does not extend to cars above 30kW. Whether it is more efficient to run a 100kWh Tesla at 80kW accels, I have my doubts, but I have had no opportunity to test this out.
 

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I went from Morzine (alt. 1000m) to Avoriaz (alt. 1800m), starting from a full battery on the e-tron. Impressive how low the range can be. I don't think the weight of my foot had much impact on that ;)
(the weight of the car probably has though)
 
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