“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, March 21, 2009

The Moller Skycar

Since I was a little boy, there has been talk of flying cars. And since I was a little boy, Paul Moller has been a short few years away from going into production on such a vehicle. He still is, though now it's called the Skycar Volantor. It's reminiscent of the wag's remark about fusion energy: "fusion is the energy source of the future, and always will be."

But what about the M400 Skycar? It's "specifications" can be found here. What a fine way to get around! 275 m.p.h. cruise at better than 20 m.p.g. It's stated that the production model will employ eight rotary engines rated at 150 h.p. per engine burning any of a variety of fuels, but ethanol is suggested. This is the power required for the vertical take off capability. At this site, there are a variety of videos, including a hover test of the M400, it apparently will get off the ground. There are also some specifications wherein it's indicated that the "nominal continuous power" is 720 horsepower and the fuel is ethanol. This would be running at 60% power and makes sense. It's not stated whether the engines are turbocharged, but they must be since the operational ceiling is stated to be 36,000 feet (!).

So let's take a look at a vehicle utilizing internal combustion engines to get 20 m.p.g. (the specs. say ">20" so this should be conservative) at 275 m.p.h. This means it's burning 13.75 gallons of ethanol per hour. OK, we find here that ethanol has an energy density of 6,100 watt hours/liter, or about 83,100,000 joules/gallon. So the total available energy in 13.75 gallons of ethanol is 1,143,000,000 joules. Burning this in one hour or 3600 seconds at 100% efficiency will produce 317,395 watts or 426 horsepower. This means the Skycar's engines are using fuel with an efficiency of 169%. I rather doubt it.

Let's suppose, then, that the figures come from burning gasoline. Gasoline has a significantly higher energy density, and would mean the claim is only an efficiency of 106%. Still higher than the figures one typically sees for an internal combustion engine. All right, suppose that the 720 horsepower only applies to the "top speed," listed as 360 m.p.h. Now, with gasoline, we're looking at an efficiency of 81%. Well, at least it's no longer in the category of the "over unity" nut cases, but I've never seen a real engine with such a specification and, unless it's operating at a very high temperature and dumping into a very cold reservoir, thermodynamics won't allow it. If a heat engine is operating from 1500 K into a reservoir of 273 K (about 2240 degrees fahrenheit into 32 degrees fahrenheit) the absolute theoretical maximum efficiency is 81.8%. No real engine comes close.

Is 720 horsepower sufficient to produce a speed of 360 m.p.h.? It's likely that it is. The Piper Meridian, for example, utilizes a Pratt & Whitney PT6A-42A turboshaft engine running at 500 horsepower to cruise at 260 knots, or about 300 m.p.h. It burns on the order of 40 gallons of Jet A fuel each hour to do it though. Speed available goes up approximately with the cube root of power, so 720 horsepower should be able to give an increase of about 12.9% over the Meridian, all else being equal. This would be about 339 m.p.h. Maybe the Skycar is a little aerodynamically cleaner, maybe its ducted fans are slightly more efficient than the Meridian's propeller. I'll call it plausible.

So if the Skycar were to be a real product, what are its claimed advantages? Well, it's driveable at low speed from your garage to an approved takeoff location where it can take off vertically. It's being designed, ultimately, to be fully automated, no pilot intervention necessary, and thus able to be used by those with no flying skills of any kind. You were previously able to buy a place in line for a production Skycar at a cost for delivery of about $400K to $1M (according to some dated web sites) depending on where in line you were. You could put only part of that down to reserve your place. It seems that that's no longer the case though.

Dr. Moller is now stating that, in limited production, the M400 Skycar Volantor will sell for about $500,000 and be available in "about three years." In mass production (i.e., when everyone you know is buying one) they'll sell for $60,000 to $80,000. If you'd rather own the company than the Skycar, it's sold over the counter as MLER.OB on the OTCBB ("Over the Counter Bulletin Board"). You can peruse the financials here. Note the negative book value. No wonder they're not taking deposits. Its market capitalization values the company at $8.27M, with 54% owned by insiders and 5% holders. Pretty thin. You just can't beat the laws of thermodynamics.


Anonymous said...

Look up - Wankel rotary engine (Mazda RX8) - 1.3L = 200+ HP

King of the Road said...

I'm not sure what you're getting at. I can believe such an engine is possible and actually, I acknowledge that it's reasonably likely Moller's engines deliver the power he claims for them. But that won't achieve 20 m.p.g.

It's easy to figure what the efficiency would have to be in order to achieve a given fuel economy on a specified horsepower. After all, horsepower is only a matter of how much heat from fuel can be converted to mechanical energy in a particular amount of time.

I think I'll make a post on that subject.

Thanks for the comment.

Unknown said...

I drive an RX8 daily. As anonymous says: 1.3L low 200's HP. The kicker though is MPG. I think my new car sticker listed 18 or 19MPG, but I get about 14MPG with mostly highway driving at moderate RPM. Moller's claim of >20 MPG is rediculous on its face, especially since his rotary pods would be screaming at high RPM to get that car into the air.

Luke said...

*sigh*. The commenters don't seem to get it. A horsepower, is a horsepower... is a horsepower.

One horsepower is equal to around 750 watts (I say "around" because there are a couple of different standards, but they all come very close to 750). So an engine producing 180 horsepower will need to suck in at least 180 horsepower's worth of fuel (135,000 watts). In real life, engines are only around 33% efficient (and this is being optimistic). So you'll only get 33% of the petrol energy you put in, out of the crankshaft.

Working backwards, if I had an engine with 180bhp and it was 33% efficient, then I have to pump in 180*(100/33) = 545bhp worth of petrol. That means 408,750 Watts.

Now, a watt means "one joule per second", and petrol has 32 million Joules per liter. So for my engine to be happy, I have to feed in petrol at a rate of 408,750 joules per second.

1 liter = 32,000,000 Joules
? liters = 408,750 Joules?

408,750/32,000,000 = 0.01277 liters. Remember this is per second. So the engine needs 0.01277 liters per second of petrol to generate 180bhp (assuming an optimistic 33% efficiency - typically it's closer to 25% for petol engines, but I will give him the benefit of the doubt)

How much is that per hour? 0.01277*60(minutes)*60(hours) = 45.98 liters.

So each engine needs 46 liters per hour of petrol to give full power. And there are 8 of them, so every hour the car will need to burn 46*8 = 368 liters, or 97.21 US gallons.

Now, the top speed is 330 miles per hour, with all eight engines at full throttle (in other words, each outputting 180 horsepower).

So the fuel efficiency at full power comes to (drum roll please!):

330 miles per hour / 97.21 gallons per hour = 3.39 miles per gallon of petrol. Oh dear.

Everything I've said so far is based on the official specifications of the car (see the wikipedia page) and the standard data for petrol and the definition for watt and horsepower. The only thing I assumed is the 33% efficiency. And I gave its efficiency at full throttle, because that's the only data that is available.

At cruise it will obviously be more efficient, but since the thing has so far never been driven out of a hover, no one knows how much horsepower each engine will need to produce in cruise.

BUT, once that data is available, all you need is the horsepower at cruise and the speed at cruise, and you can easily use the steps I wrote up here, just change the horsepower and speed numbers and you can calculate the mpg yourself and see whether it will really reach 20mpg or not.

Thank you King of the Road for posting this article :)

kathy said...

This is good alternative to the very expensive flying cars, the likes of the moller skycar, however this is too small. I need it a bit larger good for two peoplr.