$1 billion BMS Ooops...

On Thu, 04 Mar 2021 11:55:17 -0800, Mark Mocho wrote:

Energy density is the "elephant in the room" that determines a lot about
electric vs. internal combustion. The Tesla S uses a 100 kWh battery
that weighs 1,375 lbs. 100 kWh is approximately the same energy
contained in 2.1 gallons of AvGas. So, you have a 2 gallon capacity
(about 13 lbs.) in a 1,375 lb. container. Makes perfect sense to me.
NOT!

And we aren't even bringing up the environmental impact of producing the
battery and then disposing of it when its life is used up.

Here are some free-wheeling thoughts inspired by what I've read about the
Aptera hybrid road vehicles: the prototype used an all-electric drive
chain connected to a battery with a 40 mile range when fully charged. It
also carried a small ICE generator pack - on ling trips the rig was said
to average 120 mpg.

So, how would a similar set-up work for us?

Say, use a pylon-mounted electric motor coupled to a battery capable of
take-off olus a 2000 ft climb (i.e. a somewhat higher than normal winch
launch) and carry a small ICE generator pack to be run during and after
launch to recharge the battery.

A modern 20cc 2-stroke can knock out 2.5 hp at 9000rpm (around 1.8 kWh,
so with a 40% efficient generator you can recharge the battery at a 0.7
kWh rate from a unit with a guestimated weight of 1.5 Kg (750 g motor
plus the same weight for the generator) plus fuel at around 9,7 kWh/litre
(thats 12 kWh/kg) so something like 1.2 kWh/liter of fuel can be put back
into the battery after launch (assuming motor efficiency 25% and
generator efficiency 40%). Now, scale the system up a bit and use a 200cc
generator set and you've got an equipment weight of 15 kg plus fuel and a
recharge rate of around 12 kWh. Time to recharge a 20 Kwh launch battery
is around 1.7 hours, so a fair time to be listening to the (muffled)
engine behind you, but a much lighter system than a pure electric system
(launch battery + 15kg generator set) would be.

What did I miss?


--
Martin | martin at
Gregorie | gregorie dot org

9,000 RPM makes quite a racket, no matter the muffler. Have a good
noise canceling headset...

Dan
5J

On 3/4/21 2:07 PM, Martin Gregorie wrote:
On Thu, 04 Mar 2021 11:55:17 -0800, Mark Mocho wrote:

Energy density is the "elephant in the room" that determines a lot about
electric vs. internal combustion. The Tesla S uses a 100 kWh battery
that weighs 1,375 lbs. 100 kWh is approximately the same energy
contained in 2.1 gallons of AvGas. So, you have a 2 gallon capacity
(about 13 lbs.) in a 1,375 lb. container. Makes perfect sense to me.
NOT!

And we aren't even bringing up the environmental impact of producing the
battery and then disposing of it when its life is used up.

Here are some free-wheeling thoughts inspired by what I've read about the
Aptera hybrid road vehicles: the prototype used an all-electric drive
chain connected to a battery with a 40 mile range when fully charged. It
also carried a small ICE generator pack - on ling trips the rig was said
to average 120 mpg.

So, how would a similar set-up work for us?

Say, use a pylon-mounted electric motor coupled to a battery capable of
take-off olus a 2000 ft climb (i.e. a somewhat higher than normal winch
launch) and carry a small ICE generator pack to be run during and after
launch to recharge the battery.

A modern 20cc 2-stroke can knock out 2.5 hp at 9000rpm (around 1.8 kWh,
so with a 40% efficient generator you can recharge the battery at a 0.7
kWh rate from a unit with a guestimated weight of 1.5 Kg (750 g motor
plus the same weight for the generator) plus fuel at around 9,7 kWh/litre
(thats 12 kWh/kg) so something like 1.2 kWh/liter of fuel can be put back
into the battery after launch (assuming motor efficiency 25% and
generator efficiency 40%). Now, scale the system up a bit and use a 200cc
generator set and you've got an equipment weight of 15 kg plus fuel and a
recharge rate of around 12 kWh. Time to recharge a 20 Kwh launch battery
is around 1.7 hours, so a fair time to be listening to the (muffled)
engine behind you, but a much lighter system than a pure electric system
(launch battery + 15kg generator set) would be.

What did I miss?

On Thu, 04 Mar 2021 14:27:53 -0700, Dan Marotta wrote:

9,000 RPM makes quite a racket, no matter the muffler. Have a good
noise canceling headset...

Yeah, I know - the engine I took weight and power numbers was a 20cc RC
aircraft engine, while I was iriginally thinking of thre 20cc petrol
engines you used to see on small chainsaws and big drills.

But, add a bit of weight and bulk for water cooling and put it in a sound-
absorbing box with the motor/generator combo sat on rubber mounts and I
think you cound reduce the sound level quite a lot.

But, the main poing of my piece was to show just how light and relatively
fuel efficient such a small generator set would be compared with an
battery of equivalent capacity.

A litre of gas or diesel fuel weighs 800g and has an energy capacity of
9.7 kWh.

Totally OTT: As an ex-free flight model flyer, I think the finest engine
sound I've ever heard was a 1cc Cyclon-06 glow motor with an open exhaust
spinning a 7" x 4" prop at 30,500 rpm on 25% nitro fuel mix.



--
Martin | martin at
Gregorie | gregorie dot org

On 3/4/2021 5:00 PM, Martin Gregorie wrote:
On Thu, 04 Mar 2021 14:27:53 -0700, Dan Marotta wrote:

9,000 RPM makes quite a racket, no matter the muffler. Have a good
noise canceling headset...

Yeah, I know - the engine I took weight and power numbers was a 20cc RC
aircraft engine, while I was iriginally thinking of thre 20cc petrol
engines you used to see on small chainsaws and big drills.

But, add a bit of weight and bulk for water cooling and put it in a sound-
absorbing box with the motor/generator combo sat on rubber mounts and I
think you cound reduce the sound level quite a lot.

But, the main poing of my piece was to show just how light and relatively
fuel efficient such a small generator set would be compared with an
battery of equivalent capacity.

A litre of gas or diesel fuel weighs 800g and has an energy capacity of
9.7 kWh.

Totally OTT: As an ex-free flight model flyer, I think the finest engine
sound I've ever heard was a 1cc Cyclon-06 glow motor with an open exhaust
spinning a 7" x 4" prop at 30,500 rpm on 25% nitro fuel mix.



I'd prefer a quiet engine, like the ones on small Honda generators.
Presumably 4-stroke, so a bit heavier, but very efficient (for a small ICE).

I'll leave it to the IGC folks to tear their hair out on how to deal
with the engine noise no longer being an indicator of (simultaneous)
propulsion.

Perhaps for a "sustainer" model you could run the engine only when you
decide you need propulsion, and have an engine large enough to supply as
much power to the batteries as the electric motor is using, or a bit more.

"There seems to be an anomaly with your numbers: The 100 kWh battery delivers about 400 miles of
range. How is it possible to go that far on the equivalent of 2.1 gallons of Av Gas?"

Eric- the main reason that the numbers seem skewed is the relative efficiency difference between modern brushless electric motors (often over 90%) and typical Internal combustion engines, which barely reach 30% efficiency.

For a reasonable overview of the gas vs. electric debate, I highly recommend an article in the January 2021 issue of "AOPA PIlot" magazine entitled "hp versus kW" by Peter Rez, "an Arizona State University physics professor from Scottsdale who flies a Mooney."

On Friday, March 5, 2021 at 9:30:31 AM UTC+10, Mark Mocho wrote:
"There seems to be an anomaly with your numbers: The 100 kWh battery delivers about 400 miles of
range. How is it possible to go that far on the equivalent of 2.1 gallons of Av Gas?"
Eric- the main reason that the numbers seem skewed is the relative efficiency difference between modern brushless electric motors (often over 90%) and typical Internal combustion engines, which barely reach 30% efficiency.

For a reasonable overview of the gas vs. electric debate, I highly recommend an article in the January 2021 issue of "AOPA PIlot" magazine entitled "hp versus kW" by Peter Rez, "an Arizona State University physics professor from Scottsdale who flies a Mooney."

I think what he was trying to get at, is that comparing on the basis of energy density isn't very meaningful. We should be comparing distance-retrievable/kg and height-climbable/kg across different ICE and electric solutions.