Hoegberg wrote:
Is "hypermiling" even applicable to an EV's motor? If so, do the same
practices lead to the same efficiencies,
Hmm, interesting idea! I suppose the logic says "yes" here?
Otmar, can you test? :-)
Hypermiling works because the car's power plant has an operating
condition that provides peak efficiency. Let's say your engine's peak
efficiency is 30% at 50 HP, and falls off to 10% at 10HP.
The vehicle as a whole also has an operating condition for peak
efficiency, too. Let's say it's at 30 mph on flat ground with no wind.
Going faster than this causes a sharp increase in wind resistance,
lowering efficiency. Going slower than this makes all the "fixed" loads
that are present whether the vehicle is moving or not (DC/DC converter,
lights, radio, heater, etc.) take a larger and large percentage of the
total power, and thus efficiency drops again.
Now you discover that the car only needs 10 HP to drive at 30 mph. You
can't operate the engine *and* the car simultaneously at their most
efficient point! Driving at 30 mph and 10 HP makes the ICE woefully
inefficient. Driving at 80 mph runs the ICE at peak efficiency, but you
are throwing away loads of power on wind resistance.
The solution is to hypermile:
- Run the engine at its peak efficiency (50hp). The car accelerates,
because this is more power than it needs for its peak efficiency.
- Shut off the engine when it reaches (say) 35 mph.
- Let it coast down to (say) 25 mph.
- Repeat...
Now the car averages 30 mph (most efficient), and the engine always
operates at its peak efficiency of 50 HP, or it is off.
This is impractical for a normal ICE. They do not like being constantly
started and stopped. There are substantial warm-up losses, higher
pollution, and wear and tear on the starting system. Car makers had to
go to great lengths to redesign their hybrid vehicle's ICEs to work
better with this sort of stop/start operation.
Now for EVs...
All the same principles apply. The car itself still has the same
efficiency curve, and will be most efficient at some low speed.
The electric motor and controller (happily) have a much higher
efficiency. They also maintain their efficiency over a much broader
range than an ICE, and are not affected by start/stop losses. For
example, if the peak efficiency is 90% at 50 HP, it may still be 80% at
10 HP or 100 HP.
So hypermiling still works; but you have a lot less to gain by it.
For example will the 10% "HF"-PWM>15kHz
+ a 50% VLF-PWM ( *very* low freq, maybe with up-down ramps) will it be
more range-efficient than a 5% HF-PWM @steady 100% =no "VLF-PWM" ?
I think so! :-)
You do gain a little efficiency by running a controller at a lower
frequency. This reduces switching losses.
But it also creates more acoustic noise, which some people don't like.
(As if ICEs don't make lots of noise) :-)
I see the no load-consumtion,
the eff.% at a constant low speed in inner city driving probably shows a very
oversized motor for this kind of light duty
The efficiency curve for an electric motor barely falls as you go down
in HP. It's still very efficient even at 10% of rated power. It also
falls fairly gradually as you go above rated power.
This means that standard practice is to size electric motors for the
*average* load they see. This way, they will be very close to their peak
efficiency during normal operation. The motor will still work
considerably above rated loads for a short time (to accelerate or climb
a hill), without the efficiency getting too bad. You don't operate under
these circumstances for enough time to matter.
--
Failure is only the opportunity to begin again more intelligently.
-- Henry Ford
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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