On Mon, 28 Nov 2005 12:48:59 +0000 (GMT), Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
So you're suggesting there are batteries around that not only store
more energy than petrol but also take up less space? Is this on the
planet Zog?
Not quite. But given the appalling efficiency and weight of IC engines,
overall it comes close.
But IC engines are getting better all the time and will continue to
improve. It's not so long ago all diesels were cast iron, rough, noisy and
slow - not so today.
No. There is a theoretical limit to how efficient a heat engine CAN get.
Its related to the ratio of combustion temperatuyre to exhaust temperature.
Generating plant goes to extreme lengths to condense the output working
fluid (steam) down to extremly low levels of temperature to extract the
most.
Short of puuting cooling towers on your car, or runnig them with sewater
colling te exhaust pipes, the limit with average combyustion temperatures
is around 30% thermal efficiency. Power stations creep over 50%.
Thats why thie 'hydrogen' cars are taliking fuel cells and alectric motors:
They can be far more efficient in extracting energy from the fuel.
State of the art electric powertrains cannot yet match the power to
weight of e.g. a racing 5cc model aircraft engine and fuel tank, but
they are within 50% of it easily. They exceed 'cooking' IC engine
installations easily.
Well yes. If you take state of the art anything and compare it with
cooking then what do you expect? You're also assuming the electric power
train will improve, but not the IC one.
No, I am saying that state of the art stuff - which can be bought for under
a hundred quid for a horsepower motor BTW - is already there up in the
sorts of conversion efficiencies that are completely unobtainable from IC
engines. Cooking grade motors are up around 80-85%. Even the worst mass
produced **** with crappo ferrite magnets will do over 60%,.they are te
equivalent of a lawn mower engine in cruduity..
Diesel is about 13 KWH/kg.
My Lithium batteries achieve about 150W/kg. Bigger ones achieve 350Wh/kg.
Err, rather a large gap to overcome? We're not talking about a few percent
but an order of some 37 times.
wattch an wait.
http://xtronics.com/reference/energy_density.htm
Now if you factor in the overall powertrain efficiencies, and the weight
of the IC option, things get far far closer.
Since a diesel engine is, at best, 30% efficient in road going mode
whereas a top quality electric motor is 90%, you end up with - in terms
of output power - 3.9Kw/kg of diesel and 315Wh/kg for the lithium
battery.
Better at only 12 times...
But your 30% figure is rather out of date.
Its not.
Now if you look at power train weights...A 200bhp diesel car probably
has about 60% iof a 1.5 tonne weight in terms of all the gubbins
associated with making it go...to whit.
Engine
Exhaust stystem
Gearbox
Cooling system
Transmission
and so on. ALL of which can be replaced with 4 custom motors, one on each
wheel and say a fairly lightweight bank of electronics to drive them.
Again, you're not comparing like for like. All those heavy components need
not be so heavy. They're built down to a cost. Use the extra cost of the
hybrid power train on lightweight materials and the results will be rather
different.
BUT te whole point is that te electric motors are so simple and cheap and
light - that that ot comes out by istelf. You do not have to engineer an
electric motor down in weight, its there aleready.
Thats 900kg of gubbins and about 60kg of fuel, being replaced by 600kg of
battery, and four electric motors.
I am seeing about 2Kw/lb or 4Kw per kg (roughly) in terms of motor weight
in the stuff I am aquainted with.
So to do 150KW of motors for our 200bhp car, we need about another 38kg
of motor(s).
so 900kg for the diesel, 638 kg for the electric, for similar range and
power output.
The difference is the diesel power train currently costs about 10,000
quid and refills in 5 minutes, whereas the lithium.electric currently
would cost about half a million quid and takes an hour to refill.
Various lihium manufacturers are working to get cost and recharge times
down. Irs te reverse of what we do in the model aircraft world - there
we accept an hour to recharge and 5 minutes of flight - cars need 5
minutes of recharge and 5-10 hours of travel...
So although its not there yet, the overall engineering of lithium
electric cars is possible and competive performance- and range- wise
with an average car engine.
Predicting the future is risky. And we've been promised cheap electric
cars since I was a kid. But for every development of these we get similar
improvements with IC engines.
No you do not. Electric motirs have wlayws been vastly more efficient than
IC engines for sound theoretical reasons.
IC engines have got beter by tuning te fuel air mixtures, and increasing
peak pressures, but you cannot escape the laws of phsyics. Only if you go
to extremely high temperatures with e.g. a gas turbine can you get
efficenices even approaching 40-50%. Or fit condenser to the exhaust..
You can do all that in a generating set, but its TOO HEAVY in a car.
The issues are safety, cost, and recharge times.
And, of course, how you generate the electricity to charge them. That is
the big fly in the ointment since charging any battery is an inefficient
process.
Its not. Abou 90% efficient. If you do it right
As I said before Drivel is usually wrong, but in this case randomly, he
is not.
Excuse me? He reckoned there were existing batteries that weighed the same
and had the same energy capacity as a tank of petrol. This is just plain
nonsense.
well I have him plonked, so I can't say what he said originally.
Ther are batteries in existence that will enable electroic cars to compete
in performance and range with a comparable wieght of petrol
powertrains...lets leave it at that..
However you need to do an overall analsyis of the whole vehicle power
train- looking at one or another compnonet leaves a false impression. You
are as guilty of that as Drivel.
How you convert that stored energy into useful power is neither hear nor
there.
If you want to predict that there will be batteries in the future that
will do, I could equally as well predict there will be some form of IC
engine developed that gets to 80% efficiency. But I'd be guessing, same as
you.
No, I wouldn't be. NO IC engine will ever do 80% efficiency unless its
combustion temperature is up around plasma temperatures or it has
condensors on it to bring the exhaust down to absloute zero :-)
The practical limits of a very good diesel operated at its best is about
40%.
Do a google on 'enthalpy heat engine efficiency' and see what I mean
The fact that a power station can do more, is because of thse massive
condesors.
I am not saying that an electric car would result in a huge change of
overall burning of fossil fuels..BUT if we take the fact that nuclear
electric generation is likley to be the most prctical way foward in the
next few decades, what is not relevant is to use that electricity to
generate chemical fuel, then burn that in IC engines. Its extremely
watseful. That may not matter in terms of actual overall carbon balance -
nuclear constriucted hydrogen will niot affacte the carbon one way or
another, and its probably possible to make even diesel from carbon dioxide
water and enough electriity - but the cost of so doing is likley to far
exceed teh cost of simply having vbattery cars.