I was reading a chart (which I can't find now) showing the energy "capacities" of various RC model batteries. The chart also included fuel for the gas powered engines. The amount of energy in liquid fuel per unit of mass was much higher (as you would expect) than any of the battery technologies and that got me to thinking: What is the cross-over point where it would be advantageous to create small hybrid engines for powering model planes or helicopters (obviously cars and boats could carry more weight in batteries)?
I know that generators are very efficient and I could probably get out my physics books and some charts of weights of gas-engines, fuel, generators, and electric motors for RC models and figure it out, but I also figured there must be something off in my thinking because a quick bit of Googling didn't find any good sources where someone had tried it.
Simplistically (and I'm not a RC modeler), I was just thinking of a small gas engine for a model plane running at a constant speed turning a DC generator (perhaps through gearing) and the output of the generator powering brushless RC model-type motors (you would probably also need capacitors and regulators to provide steady power).
Even if the efficiency was slightly lower than batteries alone or gas engine alone, could you still get longer flight times, quicker relaunch, and possibly easier control over multiple motors with a hybrid electric system? Does anyone sell such a thing?
It's an interesting idea, but I'm wondering why one would introduce an efficiency loss into the model. If a certain amount of power is needed to turn a propeller quickly enough to move the plane, wouldn't the gas engine need to be able to generate that plus enough to overcome the losses in the energy conversion? If that's the case, one would need a larger gas motor than they would if simply driving the propeller directly.
Perhaps I'm misunderstanding the concept, so I apologize if that's the case.
This reminds me of a project I've always wanted to try building. I'd like to get a model car/boat gas motor and power a small generator with it. I'd put a very good muffler on the motor and enclose the whole thing in a rugged, sound-dampened box so that it's a portable 12 VDC power supply for powering camping lamps, a radio, cell phone charger, etc. I figure it'd be lighter and smaller than a lead-acid battery for a given amp-hour capacity, which would only be limited by fuel tank size.
The reason I went down this path of thinking was that I was looking at ducted fan motors for RC planes and the highest efficiency ones of those use brushless DC motors. To drive two or three of them for more than a few minutes at a time, you need a lot of batteries, which is a lot of weight. So, if the energy density of the fuel was, say, three times the energy density of the batteries and the efficiency was fairly high, you could run two electric motors for 33% longer (roughly) off of one engine/generator. And, as in hybrid cars, gas engines can be optimally tuned to run at a constant RPM rather than revving and idling continually (IIRC). Finally, except for cooling off time, the vehicle could be refueled and relaunched quicker than it could be recharged (of course, you can usually switch out battery packs as well).
In typing this, I realize that I'm not sure I'm saving that much weight since the engine and generator plus fuel probably isn't lighter than batteries, but if the numbers are right, you might still be able to get longer flight times.
Even if it was heavier and somewhat less efficient, it would be a cool model. It would also make for a neat garden-scale locomotive that simulates the modern diesel-electric locomotives of today. It could be self-powered instead of receiving power from the rails like traditional model railroads.
I see what you're saying about energy density of fuel vs. battery. Like in automobiles, there is a crossover point where gas-electric is more practical than pure battery electric. The same must be true for the smaller scales like RC models. A 15-minute flight might require three pounds of batteries or a 2.5-pound combination of fuel, motor, and generator.
Just remember your laws of thermodynamics. Energy is always conserved. Anything you drive from an exsisting power source will have some waste in the form of heat. So.. running a generator off the motor will only shorten your flight time. The best "free energy" would be solar cells on the wing surfaces trickle charging the batteries for an electric motor.
Here's a thought... Install and electric motor and batteries, add a charge controller and solar panels AND add some generators under the wings driven by a props. Every time you take it into a dive and cut power to the motor, the generators driven by the prop and airspeed will charge your batteries. Since you will only be adding the weight of the above mentioned item to the engine load, you may just come out ahead on sunny day with some aerobatics added to your flight plan!!!
Brushless DC motors are very efficient, yes. Combustible liquid fuels are more energy dense than batteries, yes.
So why not mix the two, theoretically with rough (fake) numbers?
Our gasoline engine is 30% efficient at converting gasoline to kinetic energy. The generator it is turning to convert kinetic energy to EMF is 80% efficient. Our system is now outputting 24% of the gasoline's total energy. The motor that the electricity is turning is also 80% efficient. Our system is now outputting 19.2% of the gasoline's total energy directly to the prop.
Suppose the gas engine just turns the prop directly now, without all the efficient stuff. It now weighs less, and outputs 30% of the gasoline's energy directly to the prop.
It sounds like a hybrid would be worthwhile if combined with some additional system for generating/receiving electricity, like photovoltaics or beamed RF of some sort. If you built the plane with flexible solar cells instead of mylar for the wing's surfaces, it might be doable... the gas engine could be used for climbing, then shift to electric for cruising.
The one problem I see with a hybrid model plane engine is that the gas-powered plane engines I've seen aren't self-starting; you get them going by spinning the propeller with a fairly heavy-duty motor driven by a relatively enormous battery. The gas engine would have to idle when the electric one was active.
A hybrid car is only beneficial because you can extend the range of an electric car when you need to go far or because you can re-capture energy that would normally be lost in braking. A hybrid aircraft shares neither of those advantages.
If you wanted to do photovoltaics, just make an aircraft out of something that weighs practically nothing (like foam) and fill it with a lightweight gas until it's near the weight of air. It won't fly on a windy day. The reason I think you'd have to take such drastic measures is the fact that photovoltaics don't capture much energy, something like 3% of the light on cheap models and 10-15% on expensive photovoltaics. The sun on a good day emits about 1 kW per square meter.
Multiply the efficiency as a decimal, in hundredths (.15 for 15%) times 1000W times square meters of the top of the plane. Example: a plane with a pretty large surface area, .5 square meters, absorbs 75 watts at 15% efficiency in high sun. That's a wing about 2 meters long and 250 centimeters wide. How much would that weigh, and would 75 watts really keep it up? A plane of that size with a gas engine would have at least a horsepower, which is 700 watts. That'd probably be way underpowered too unless the plane was made out of carbon fiber. Would that 75 watts mean anything in comparison after weight additions are taken into consideration? These are very very idealistic assumptions I'm making, and it still wouldn't be beneficial.
If you built it, it'd be cool to a lot of people, can't dispute that. To me it would be a pointless waste of money to try to make something look like it works better than it really does.
I think it's important to understand why hybrids are used.
An internal combustion engine, as already mentioned has a none too impressive thermal efficiency, but it is widely used in cars primarily because it can maintain that efficiency across a fairly broad range of load/power output. Your avarage car engine is actually at maximum efficiency with a wide-open throttle, but running it like that will quickly push you to speeds where aerodynamic drag predominates, as it increases with the square of airspeed.
A hybrid works because of two main points. 1) The engine/generator can be designed to operate the engine at its most efficient power level, without regard to the immediate requirements of the driver. 2) The engine can be sized to deliver the average power requirements, relying on the electrics to deliver the intermittently required maximum power, whereas a conventional car has to have the engine sized for maximum power.
Things are rather different in an aircraft however, once you've managed to take off, you don't have much need for varying thrust level [unless you're doing aerobatics], and of course weight is a far more serious issue.
One thing worth thinking about, is that gas turbine engines have a far higher efficiency, albeit a much narrower power band than internal combustion engines. Thus they would be well suited to hybrid applications. Unfortunately they are rather pricey.
yuffie sez: "why not make the airplane engine one too".
Indeed.
Going the hybrid route is only useful if:
1) Your average pwer requirement is X. 2) Your instantaneous power requirement is X + Y 3) You can generate X more efficiently than by throttling down a powerpalt designed for X + Y. 4) The weight or complexity penalty for outputting Y when required doesn't outweigh the efficiency gained in 3).
Always assuming you're not doing aerobatics, the highest power requirement for flight is at takeoff - after that the power requirements don't vary much. Probably the biggest efficiency boost you could get would be to size your powerplant for level flight, and have a jettisonable take-off booster. Any other system will likely lose as much or more through the extra weight penalty as you'll gain in efficiency.
practical or not, I love the idea... as previously said: for a mini generator or model train. Specifically, what generator would you use? Have you looked into that? Would you build one from scratch?
Hello All, I'm an RC airplane enthusiast and have been looking into this very thing, however I do not fly electrict models. Fuel only. What I'm interested in is running a small generator off the gas engine to produce the power for the servo's that operate the control surfaces. A typical size plane uses a 4.8 volt battery pack and the draw is about 250mh. My problem is that I know absolutly nothing about the electronics much less where to obtain a small generator that would withstand the 6,000 to 16,000 rpm of the engine. Mounting the generator and drive system would be a cakewalk for me but the electronics is my downfall. Anyone with info would be greatly appreciated.
Are you sure you'd want to do that? If you run out of fuel or something else stalls the motor, your electronics are dead, too. With the receiver's battery, you're still able to glide the plane back to the ground under control should the motor quit.
Also, I think the combination of generator, drive system, and filter circuitry would weigh as much as the rechargeable battery pack already in use.
I was thinking of making the mini-generator, but I haven't done it yet. I was thinking of using a generator from a bicycle headlight. There are those that are driven by a knurled wheel that contacts the bike's tire. I don't know what RPM they can handle, but I imagine pretty high, since the ratio of tire diameter to generator hub diameter is very large.
sick85 sez: "My problem is that I know absolutly nothing about the electronics much less where to obtain a small generator that would withstand the 6,000 to 16,000 rpm of the engine."
Instead of looking for a generator, look for a small PM motor. You'll also need some smoothing and a voltage regulator - that could be as simple as a capacitor and a suitably rated zener diode, or as complex as a switchmode regulator.