Naked Science Forum
Non Life Sciences => Technology => Topic started by: Gray33 on 24/04/2010 13:30:02
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Gray asked the Naked Scientists:
Hi Naked scientists, love the show (http://www.thenakedscientists.com/HTML/podcasts/).
I would like to know why the Top Gear (Jeremy Clarckson & Co) idea of charging an electric car with a small generator on board wouldn't work?
It seems, given the low operating cost of a small generator, it would be a very cheap alternative to charging the car from the plug everyday.
Thanks Naked Scientists
What do you think?
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I think that's what the Chevy Volt does. The engine only drives a generator. Essentially, it's a gasoline/electric with a battery.
It will almost certainly be less expensive to charge the battery using power from the electricity grid than running a generator to do it.
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Let me get this straight..
You remove a car’s internal combustion engine and replace it with an electric motor and batteries, that sounds reasonable.
But then you are suggesting that you put an internal combustion engine back into the previously all-electric vehicle to charge its batteries.
Because of conversion losses the internal combustion engine used to charge the batteries would either have to be more powerful than the one you took out of the car, or smaller and run for a longer period, say overnight when the vehicle was stationary. Either way the conversion losses would make this inefficient : a bigger fuel bill either way than just running the car exclusively from internal combustion.
The only advantage of this particular hybrid would be you could run the vehicle where you had access to gasoline but not electricity, and this advantage would be at the additional financial and weight costs of shoving a internal combustion engine in an electric vehicle.
It's regenerative breaking (http://en.wikipedia.org/wiki/Regenerative_breaking) that enables hybrid cars to be more fuel efficient than internal combustion only vehicles
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I understand the Volt is a "plug in". The batteries can be charged from the grid, so it can travel for some distance without the engine turning on.
Quite right about the regenerative braking. The electric motor will also run as a generator to recharge the battery, so in that respect it is like other "hybrids".
Porsche also has a method to recover braking energy. The front wheels are driven by motors. During braking, they operate as generators to drive a motor that spins up a flywheel. During acceleration, the process is reversed.
I'm not sure if that makes it a hybrid or not! The term "hybrid" may not be very meaningful.
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BTW, ships and locomotives have been using diesel/electric drivetrains without batteries for a long time.
The generators and motors have many advantages over mechanical transmissions. There are even some very large diesel/electric trucks.
"Honey - Come and see the new car I bought you."
http://en.wikipedia.org/wiki/File:Liebherr_T282.jpg
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I think such a hybrid - a petrol-powered generator within an electric car - could have advantages under certain conditions. For instance the batteries could be relied upon solely in city environments or heavy traffic where pollution or noise is a concern or likely to have an impact on efficiency, and then the generator could be used to top up the battery at convenient moments.
I don't think it's such a bad idea. The principle's not that different from the electric bikes you see people riding which rely on a combination of electrical and pedal power, with the battery receiving a top-up when the motor's not actively engaged.
Chris
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I think such a hybrid - a petrol-powered generator within an electric car - could have advantages under certain conditions. For instance the batteries could be relied upon solely in city environments or heavy traffic where pollution or noise is a concern or likely to have an impact on efficiency, and then the generator could be used to top up the battery at convenient moments.
I don't think it's such a bad idea. The principle's not that different from the electric bikes you see people riding which rely on a combination of electrical and pedal power, with the battery receiving a top-up when the motor's not actively engaged.
Chris
The other advantage of this arrangement (when there is a battery) is that it is possible to only ever operate the engine at maximum efficiency. The engine is either running at its most efficient torque/speed point, or it's not running at all.
The Toyota Prius does something a bit like that by using the electric motor in combination with a differential to produce a form of infinitely variable transmission. That allows the engine to run close to its "sweet spot" most of the time.
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I think such a hybrid - a petrol-powered generator within an electric car - could have advantages under certain conditions. For instance the batteries could be relied upon solely in city environments or heavy traffic where pollution or noise is a concern
The original question was about economic not ecological costs.
However the petrol-electric hybrid described will be more polluting* than petrol only vehicle because it will use more fossil fuel.
An internal-combustion-engine charging a battery which powers an electric motor will use more fossil fuel than an internal-combustion-engine only vehicle because of the conversion losses, (you'd only get a fraction of the energy from the battery of the petrol energy it took to charge it).
[* Isn't CO2 considered to be a pollutant ? (http://news.bbc.co.uk/1/hi/8004975.stm)]
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An internal-combustion-engine charging a battery which powers an electric motor will use more fossil fuel than an internal-combustion-engine only vehicle because of the conversion losses, (you'd only get a fraction of the energy from the battery of the petrol energy it took to charge it).
I think it's a bit more complicated than that, so it's going to depend on a lot of factors. In practice, the engine is likely to be running only when the vehicle is moving, so a large amount of the power it produces is not charging the battery, it's going to the wheels. The battery is then capturing any excess energy from the engine, and any braking energy.
Any double conversion through the battery has to be offset against potential improved efficiency of conversion at the engine.
But I think you are right in that the best way to charge a battery will always be from the power grid. In this part of the US, the thermal energy available from a gallon of petrol costs just about the same as the same amount of thermal energy from the electric power grid, so, if you have a choice, it's much better to charge a battery from the grid. Charging it using a petrol generator will waste more than 60% of the energy.
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I understand that the highest thermal efficiency to be expected from an I.C. engine is about 40% (Diesel).
The efficiency of a modern electric motor can be 90%.
The efficiency of a modern generator can be 90%
The charge/discharge efficiency of battery systems is probably about 90% (I’m guessing there). Overall efficiency then might be about 30%.
But remember that:-
The I.C. engine efficiency stated will only be true over a small speed range, and will vary from maybe 20% to it’s maximum. It needs a box full of changeable gears to cope with this and the fact that usable torque also only occurs over a limited speed range (the gearbox loses a little more efficiency). It needs a clutch of some description because it is not self starting. Overall efficiency from an engine when moving a road vehicle is quite low, I have seen 15% reported, although its probably higher with modern cars.
A modern electric motor is self starting (needs no clutch), is able to generate its maximum torque over a very large speed range (needs no changeable gears), and maintains its high efficiency over a very large speed range. Add to this the fact that electric motors can be small enough that an axle can have a motor for each wheel so that the differential gear (another energy waster) is not necessary.
All in all, the concept of using an on board generator to charge a battery powered car is admirable, and is now called a Series Hybrid, as opposed to Prius etcetera, which is a Parallel Hybrid, in which the electric motor and the I.C. engine both share the load via a complicated system of gears, and is thus less efficient than a Series Hybrid.
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so that the differential gear (another energy waster) is not necessary.
A differential hardly wastes any energy at all. Independent motors seem like a good idea, but I think it's probably more efficient to use a single motor and a differential.
I believe the Volt and the Tesla use a single motor. In part this is because, with two motors, a lot of the expensive electronic power controls have to be duplicated. There is a certain economy of scale associated with a single larger motor too.
Also, if something goes wrong, and one motor suddenly stops producing power, some very bad things can happen. Mechanical diffs have been around for ever, so by using them the engineers working on electric cars have one less thing to worry about, and they already have their hands full.
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Porsche also has a method to recover braking energy. The front wheels are driven by motors. During braking, they operate as generators to drive a motor that spins up a flywheel. During acceleration, the process is reversed.
Er, sorry to jump back a bit in this thread, but ... this Porsche thing - why don't they spin up the flywheel directly? Would the gearing losses be higher than gen-to-motor conversion?
The Toyota Prius does something a bit like that by using the electric motor in combination with a differential to produce a form of infinitely variable transmission. That allows the engine to run close to its "sweet spot" most of the time.
Agreed. This "sweet spot" thing is the only reason hybrids make any sense.
Also, isn't the Prius a parallel hybrid? - where the IC engine can drive the wheels directly as well as the leccy motor. This brings the efficiencies up for motorway driving, but the Prius would definitely be less efficient than equivalent vehicle with a conventional drivetrain, if it only operated on the (clear) motorway.
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Re. the Porsche thing (I think it's only for racing cars BTW) you need some sort of variable ratio transmission between the wheels and the flywheel, otherwise you'd have no control over the braking force, or the amount of acceleration. (Actually, the wheels would stop dead if the flywheel was stopped when you connected the wheels to the flywheel.)
The motor/generators provide the "gearing" and necessary control.
Re. the (current) hybrids like the Prius, I think it's the other way around. They score during stop/go driving where you do a lot of braking and accelerating, but cruising on the open road, they don't have any advantage at all.
Of course the situation changes when you increase the size of the battery and you charge it from the grid. The idea with the Volt is that you can go for 50 miles on battery alone, which probably means that a lot of people will hardly ever use the engine. But if they need to drive a greater distance, it's there when they need it.
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The Porsche system AFAIK uses a old technology to store energy, the flywheel integrated into a motor/generator. This has been around in one form or the other since at least the 1920's, and was used for buses. This stores a lot of energy and allows it to be released fast when needed. Running the flywheel in a vacuum allows it to run with low losses and store energy for long periods. Good for a quick boost and able to store large amounts in braking without overheating or venting like a battery. Also used in F1 technology.
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This has been around in one form or the other since at least the 1920's, and was used for buses.
I know of trolley buses that spun up flywheels when they stopped at at bus stops, but I was not aware there were regenerative braking buses similar to the Porsche system. How did they control the energy transfer?
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Re. the Porsche thing (I think it's only for racing cars BTW) you need some sort of variable ratio transmission between the wheels and the flywheel, otherwise you'd have no control over the braking force, or the amount of acceleration. (Actually, the wheels would stop dead if the flywheel was stopped when you connected the wheels to the flywheel.)
The motor/generators provide the "gearing" and necessary control.
Good point! Should've thought of that!
Re. the (current) hybrids like the Prius, I think it's the other way around. They score during stop/go driving where you do a lot of braking and accelerating, but cruising on the open road, they don't have any advantage at all.
I thought that was what I was saying... mmmm, must Czech my English!
Of course the situation changes when you increase the size of the battery and you charge it from the grid.
Yes, should have said this disadvantage applies to non plug-in hybrids. ... Only there is the additional elephant-in-the-corner-of-the-room of massively upgrading the electric-grid infrastructure.
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Re. the (current) hybrids like the Prius, I think it's the other way around. They score during stop/go driving where you do a lot of braking and accelerating, but cruising on the open road, they don't have any advantage at all.
I thought that was what I was saying... mmmm, must Czech my English!
I looked again. Yes you did. The bit about motorway driving fooled me.
The charging load on the grid might not be that bad, particularly if you use smart meters that only supply energy during periods of low demand. Actually, the power suppliers might really like it because it allows then to get closer to a constant load.
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The charging load on the grid might not be that bad, particularly if you use smart meters that only supply energy during periods of low demand. Actually, the power suppliers might really like it because it allows then to get closer to a constant load.
We would still need a massive copper upgrade at the local level though. Domestic connections are woefully underdeveloped for a pure battery-electric in each garage.
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I find it interesting that a question 4 years old still has not received anything other than a bunch of knee jerk reactions.
The concept Grey33 is aiming for is a true hybrid not a partial hybrid like the Prius or a gasoline/electric like the Volt. A partial hybrid only uses the battery for things like energy recuperation from braking, but never takes full advantage of the reservoir the battery provides so results in a marginal fuel economy increase. Vehicles like the Volt are a chimera; it is both a gasoline and an electric crammed into the same vehicle with a price tag to match owning two cars and none of the fuel savings a hybrid can offer unless you stay close to home all the time. As a side note the Volts engine is mechanically linked to the drive-train while cruising at interstate speeds so does not run a generator only.
A true hybrid vehicle whether it be a hybrid electric or a hydraulic hybrid uses the hybrid system to establish an energy reservoir for energy recuperation and to even out spikes in demand so that the gasoline or diesel ICE only has to meet average demand. The earliest instance I can find of a true hybrid was a conversion done on an OpelGT in 1979 and it got 70+mpg on a vehicle that weighed nearly twice what a Prius does. In the 80’s there was a hydraulics class that hit the same idea. They produced a class project in an interdisciplinary assignment that could carry 4 people, accelerate from 0-60 in less than 6 seconds, and got 75mpg. It used a 5hp engine running at three quarters throttle all the time to charge a hydraulic accumulator, reversible motors to provide braking and energy recuperation to the accumulator, and wheel motors to drive all four wheels.
So to provide a direct response:
I know it is possible to power the drive-train directly from the generator and use the battery as a boost to cover peak loads. This is a true hybrid and can beat everything accept a range-extended-electric in gas mileage and range. Electric only could offer a better dollar per mile, but battery, and infrastructure limitations still make them infeasible if you drive more than 10-15 miles a day. As a point of disclosure I drive 135 miles commute every day so have absolutely no use for a “city car” that has less than 200 miles range and takes longer than 20 minutes to recharge between trips and my electrical engineering is not so good that I have built my own yet.
Here is what I have discovered about accomplishing what Gey33 is looking to do:
Take your donor vehicle and determine the average load on the engine. This is usually the effort it takes to cruise one the interstate plus a small margin of 10-15%. In my case that works out to about 15hp or roughly 11.5KW. Once that is determined a generator setup sized for that average load plus 10-15% would be acquired or constructed by finding an appropriate engine and mating it to a generator. Then the generator is connected in parallel to the battery at the motor controller. The battery controller would allow the generator to trickle power back to the battery to top it up while cruising and the battery to feed the motor(s) during peak load. Then the battery would only need to be sized for 10-15 miles range to cover climbs and acceleration while the generator provides cruising power.
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I find it interesting that a question 4 years old still has not received anything other than a bunch of knee jerk reactions.
Can you explain why you think these reactions are 'knee jerk'. In general the comments have sort to test the assumptions on which the question is based; which seems perfectly reasonable for a science forum IMO.
You are correct to point out that the Chevy Volt is not solely a series-hybrid vehicle. As mentioned, it has both series- and power-split- modes of operation. But to understand why such a complex drivetrain was chosen, it useful to think a bit about the typical driving cycle which these vehicles will face in their lifetime (and more specifically about the driving cycles on which the MPG will be calculated by the State).
For instance, if every journey made by road involved driving pretty much straight on to the motorway and sitting at 70 mph for an hour, before reaching the destination, then no one would have any use for hybrids - but how many journeys work out like that?
In fact any highway intensive journey, currently, almost certainly suits a well-geared diesel with good aero, better than any other solution out there. As BEVs continue to improve (and get cheaper) this is almost certain to change, but choosing any hybrid would be a waste of fuel and be pointlessly over complex.
[...a hydraulics hybrid] that could carry 4 people, accelerate from 0-60 in less than 6 seconds, and got 75mpg. It used a 5hp engine running at three quarters throttle all the time to charge a hydraulic accumulator...
Do you have a link to this Hydraulic Hybrid? I immediately wonder about the vehicle's top speed ... with a 5hp engine. That is, it takes about 7-8hp to hold around 60 mph continuously on the flat for all but the most slippery saloons, all else being equal. Also, the way that MPG is measured today is based on a more complete driving cycle, whereas previously it was simply a car's consumption at a near-ideal fixed speed, say 55.
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With regards to the efficiency of Diesel engines while car sized ones probably peak at about 40% large ones used in ferries and British aircraft carriers can reach 53%
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If one's driving was always the same, then you might be ok with your "average load" solution. But there are many cars that might putt around town for a month, then go out and hit the highway for a few hundred, or perhaps a few thousand miles.
The electric-electric transmission is considered one of the least efficient transmissions, and as Peppercorn mentioned, if speed (and elevation) was constant then a well designed direct drivetrain will beat them.
The Prius drivetrain is unique in which the engine is partially coupled to the drivetrain so that it gets both improvement in mileage through the direct drive, as well as improvment in mileage through the battery system and optimizing engine efficiency, perhaps very similar to what you're describing.
I have a BEV. It needs a little work. But, I was actually considering adding a "hybrid diesel generator" somewhat like what you're describing for range extension, perhaps as a towable trailer (not needed for in town, but good for on the highway, although a trailer does loose some efficiency). My thoughts was that the charging system charges at about 30A, but much of my driving is around 80A. I can run on batteries without the genset around town, but the range is limited. Charging at about 1/3 "duty cycle" would still get me out on the open road. A 40 mile two-way range would become about a 60 mile one-way range, where the vehicle could continue recharging during any stops or bathroom breaks.
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The internal combustion engine is not the ideal prime mover in this application. Better to use a turbine running at constant speed on jet fuel (kerosene) for maximum efficiency and use an electronic "gearbox" (essentially a multiphase stepper motor) for low-loss speed matching.
Matters of cost and environmental impact are often confused when people talk about electric cars. The low cost per mile of a pure-electric car is as much due to the low tax on mains electricity as it is on the inherent efficiency of a regen-braked electric system. There is some improvement in thermal efficiency between a small i.c. engine and a fossil-fuelled power station but the environmental impact is pretty much the same: the power station just dumps the CO2 and smoke in a different place.
The question in my mind is, if I charge a pure-electric car from the mains, I pay little or no tax on the fuel. I can also charge it at home from a small diesel generator running on tax-free agricultural fuel. If I carry the generator with me to my destination and recharge the car for the return journey, does it matter to the tax man whether I carried the agri-fuel with me, or bought it from my host? So why should I have to pay tax on "road fuel" which is exactly the same stuff carried in a tank in a hybrid diesel electric car?
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Because of conversion losses the internal combustion engine used to charge the batteries would either have to be more powerful than the one you took out of the car, or smaller and run for a longer period, say overnight when the vehicle was stationary
no thats not right
Consider the power required, not the power as given.
lets say our needs are modest, we expect to keep up with traffic with some reserve as a normal car, but we dont need to go very fast, say 70MPH
We are fighting the power required to move us through the air, the rolling resistance of the tyres, and whatever frictional losses there are in the system
For instance a small car like a Mini only needed about 6HP to propel it at 55MPH, but its engine was set to produce many times that.
So we fit our car with an electric drive, that has a generator capable of giving that electric motor power for 55MPH with some excess
The excess goes to a small battery bank, that is used for reserve power to accelerate, or to climb hills
The amount of excess will depend upon the drivers habits and the nature of the driving environment.
When we floor the throttle we have the combined energy of the generator and the battery bank
Out of the motor, we want the torque to drive off from zero to design speed, design speed is provided by the horsepower
This coincidentally is exactly how an electric motor is configured, max torque just above zero RPM, and max HP at max RPM.
As a consequence electric drives do very well with just one or two gear speeds and no clutch.
A typical VW conversion usually uses 2nd gear to 45-50MPH. 3rd on the freeway
The IC engine on the other hand isnt ideally configured and wastes a lot torque because its power curve doesnt provide enough torque at low speed without also providing an excess of torque at high speed. So an IC engine equipped vehicle requires more gears to be able to do the same things.
It is the motors inherent characteristics that make electric drives more efficient (smaller and more modest) even if they were meant to do identical things.
Smaller still if you were intent on doing somewhat less
the part that is at issue is the battery technology, which is why history has few battery driven trains, but a majority of diesel electric drives :)