Pound for pound.. electric vs gas

How far off from hp/running time are electric powered craft vs gasoline powered.

 

I believe someone took numbers for an RV3. 30 gallons of gas about 180 pounds of fuel. Batteries, 3000 pounds. That's on top of the 1200 lb airplane. One for one it's not close to doable with range of gas. Just getting in the air for 15 min is doable.

 

Depends on what you mean by "fix" and what you mean by "ever".

Yes, gasoline is always going to have more energy density than a battery as we know them. However,batteries are improving. Also, electric motors are much more efficient and lighter. Let's take a look at the kind of ultralight I think about, meant for motorgliding. Say it needs to have about 20 hp for takeoff, 6 hp for just flying around. Say we're going to cruise around for an hour. 5 minutes of 20 hp followed by 55 minutes of 6 hp. 1 hp hour is about 2.7 MJ. Total of about 19.4 MJ.

Let's say I'm using a 4 stroke with redrive, that weighs 60 lbs and uses 0.4lbs/hp-hr or about .15 lbs/MJ. That's only about 2.9 lbs of fuel for our 19.4 MJ! Maybe it's a bit optimistic, but we'll stay with it. We probably need a 5 lb gas tank, so now we've got 68 lbs.


A couple of brushless motors that give about the same power, from t-motor, weigh only about 7 lbs. (And, don't forget, the motor mounts will be lighter for the electrics. But maybe that's diminished by having to support heavy batteries. ) ESC weight is less than a pound! According to Wikipedia, lithium batteries are around 1.8 MJ/kg, and the motor efficiency might be 85 percent. Works out to 28 lbs of batteries and a total of only 36 lbs!


Of course, with a two stroke maybe we have 25 lbs of engine and redrive, a 5 lb tank, and 4 lbs or so of fuel and oil, for a total of 34 lbs.

So for a really efficient ultralight the electric power is almost as light as a two stroke for 1 hour flights. Add a bit of drag or weight, and it's likely the break even point is as low as 10 or 15 minutes. But a big improvement in batteries could stretch that out to a much longer time.

To tell the truth, I expected the batteries to do a bit worse than this.

If I've screwed up the calculations or assumptions, please point it out. Maybe I could pull some numbers for real batteries and see if the figures agree.

 

So, 5 gallons is 35 pounds, ( I'm using 7 pound per gallon ) and 1/6 the battery is 500 pounds. Take away 100 pound for a realistic V-twin 4 stroke & add 35 pounds for a realistic motor & controller.

535 pounds electric, 135 pounds gasoline, seems reasonable for a two hour range plus reserve to compare.

Assume a Rotax 503 as the same mass 40+ hp as a Big twin, with less fuel economy, so those numbers pass the smell test for 2+ hours.

How realistic is 500 pounds of batteries for 2 hours?

I'm assuming a minimal pt103 like the Airdrome Dream. Simply change the wingspan for the 3 place equivalent electric with no weight penalty. Not realistic, but I'm giving an edge to the battery version.

Or, how many pounds of battery for 2+ hours? Plug that number in instead of 500.

Design for the mission. The mission is equal range to 5 gallons of gas in a draggy, slow, pt. 103. How much too heavy do we need, this year?

 

High capacity batteries can't be discharged at high rate. Have a look at available batteries. I'd suggest sticking to 18650s from the big producers for reliability and reduced meteor risk.

The 3Ah LG HG2 is 0.864MJ/kg, 19.4MJ is 22kg. That's 500 cells. The max power available from that pack is 36kW. You may be able to find some higher cap cells, but all the ones I know of over 3Ah are only rated at half the current of the HG2. Which means you are going to struggle hitting the 20hp with a lighter pack and that energy storage.
There are other weights to add. interconnects, casing etc and you are likely looking at a 30kg pack. Much the same as the engine and gas tanks, but the motor is on top of that. if you only want 20hp for 5 minutes, there are higher current cells with reduced capacity. I'm sorta tempted by a hybrid system with an IC engine producing abit over cruise power, and a motorgen and batt pack for climb etc. The cost is a dampener. Those HG2s are $5.65 ea in bulk.

1hp-hr of bare HG2s is 3.15kg, 7lb. Add losses, pack casing, interconnects and hardware and you could be at 10lb per hp-hr. With that number, you can see why succesful electric plane builders put so much effort into efficient flight. A draggy conventional ultralight is an awful choice for an electric plane. 1hp-hr of HG2s is agoing to cost you $420, it gets very slightly cheaper if you buy more.

 

Wish I had the answer to that. However, I had the pleasure of spending a couple of days @ the "Battery Expo" in Novi MI 2 weeks ago. 600 vendors of all stripes their to showcase their products in relationship to electric mobility. This is electrification of drive-trains is not going away folks, it is real, people attended from all over the world, and words do not do a trade show like this justice until you see it for yourself or attend some of the seminars. This year what a struck a chord with me was the vendors showcasing what you might consider mundane sub-components that would reduce vibration, temperature, or in one case let the current flow w/ less resistance via a winding type. All geared towards increasing driveline efficiencies in total around the margins. I don't want to get too in the weeds, I have an article I have to rap up on this show. However, does a 40+ lb 200 hp motor that would fit behind an RV spinner grab your attention that will be in a flight application in the near future, or a Solid State battery that will find it's way into the UAV space in 19'. Yes power densities are up in this space, and will be even more so w/ Solid State. Are we there yet? Not to the point of the RV-3 total replacement needs mentioned in this thread, I can't refute the math or the physics. I am curious as to how close we are to your typical $100 hamburg special aka fly out 1 hr away with your buddies in formation, and your bird charging while you are refueling .. Add your low-drag favorite 2 seater that might be amenable to this type of retrofit and imagine what the retrofit might look like or entail. The Gent here on HBA w/ the E-Belite might give us some data points soon, probably not far off from Mark Bierle's efforts. Anyway I look forward to hearing on all of these experimenters progress and wish them well.


My best,
Anthony

 

most EV electric motors are around 90+% efficient. At least the ones we use in cars.
When it is scaled up to a larger aircraft, the weight of the controller, charger, & electrical wires also increases.

not included in you calculations are the resultant chemicals spewed in the air by the I.C.E. It's been proven that the exhaust gases from the power company and E.V. may use is less than that of the I.C.E. Those of us with photovoltaic panels have even less.

I think the "break even" will rapidly expand up to 2-3 hours in the near future (10 years?) and we will hit another materials limit.

Ultimately, Dana is correct until such time as we get "flying saucer" technology. Then airplanes will only be a recreational hobby and I.C.E.s will play a small role in our lives. I realize now, that I am saying electric motors are a step toward the future, but this technology has its limitations and cannot completely replace the I.C.E., until a breakthru is discovered (and released to the public). I'll stop here, as my thoughts are starting to get political and off topic.

 

For 20 hp, you wouldn't need anywhere near 36kW. Maybe half that. For high specific power, some guy on eBay has been selling big packs of maybe 96 A124 cells, formerly used by the Navy. One cell is good for 60 amps when new.

 

No, you'd only need about 18ish kW. But higher capacity cells than 3Ah are only good for 1/2 the current, so you'd have less than 18kW with a your lighter pack with less cells. Inadequate. Running cells at their max ratings isn't great for life. Considering what you'd spend on a pack, you probably want to consider life.

If you have specs of better cells, list them?

A123 are 3.3V, 1.1Ah and 39g. Your 19.4MJ pack is now 57kg of bare cells, though the power will be awesome. The A123s are about the best specific power, though. I would shy away from used ones, myself.

 

I have been contemplating a similar calculation recently. Fuel tanks and fuel have a lower energy density than batteries - fact. But it is possible to get higher energy conversion density out of an electric motor. So where is the sweet spot? At what engine power and duration does there exist a point where the (starting) mass of the propulsion system is the same for IC as electric? (The answer might be there is no such point)

 

Dont forget that once gas is burned the weight is gone too

 

Well, there's more things to figure than just hp/running time. At it's Simplest form, say for a Part 103 Ultralight.
GAS = 6lbs per Gallon so Part 103 5 Gallons = 30lbs. Say $3 a Gallon = $15
For say a 1hr of Flight with a 40hp Engine.

A 40hp Electric motor is much Lighter than a 40hp Gas Engine, but the Batteries needed for that same 1hr of Flight might Weigh about 112+lbs. Then you have to Wait for them to Recharge probably 2+ Hours at each Airport!

Then you have to look at the Battery Cost, those Battery Packs for 1hr can Cost $7000, and will need Replaced in about 7 Years, some people say less.

$7000/$3 = 2,333hrs of Flight. Per Kitplane Mag the Avg Ultralight use is 50hrs a Year. 2,333hrs/50 = 46 years of Flying!

Try to Sell that Ultralight or Kitplane that needs New $7000 Batterys!

The Battery Pack Weight and Cost is what Kills Electric.

 

A123's were my suggestion for a hybrid system.

 

oh i know its bad i wasnt sure how bad.. not to mention just how inefficient it is.

 

I think the only batteries that might have a chance to rival ICE in a combination of weight, endurance and power within our lifetimes are batteries that will likely not be allowed... nuclear.

 

Get above the weather with a huge flying solar panel, and stay up for long time. Not practical but if you got the money and time?

 

Who doesn't want their own nuclear powered aircraft? No more gas and oil, just kick the tires and light the fires!

 

No, it doesn't. To run at half power for two hours, it will need a battery that weighs over a ton and costs $100,000.

That's reality today, not hyperbole. Next year isn't set to be 500lb and $10,000.