Series Hybrid into Plane, need help

Siwastaja said:

The idea is valid, but what really makes series hybrid worth it, is not a small amount of batteries but a large amount. Those days with expensive and heavy NiMH cells are over. Now it makes economically and technically sense to have 30-50 km of full electric range, and all hybrids should be made plug-in.

This also has the point most people forget to mention: if you buy a larger battery pack, you can get more power out of cheaper low-power cells. For example, if you built a pack that can run 2 km electric, you would need to buy special power cells that are more expensive. If you build a 10x larger pack with the same cells, you get not only 20 km electric range (actually a bit more!) but also 10x higher output power (for higher speeds & acceleration). In practice, if you don't need all that power, you can then switch to a cheaper cell -- from so called "power cells" to so called "energy cell", latter of which is optimized for low weight and low cost (EV use). Larger pack = better battery efficiency, more power (= better acceleration at higher speeds), less need for cooling, and what goes without saying, more range.

Also, the energy recoverable from regenerative braking and the benefit running the ICE at optimum RPM (traditional points of having a hybrid drive with a small battery pack) is not so much; you may lose it in the series hybrid losses. The large battery pack changes the game by making your car a plug-in car. Then you mostly drive with electricity from the wall and only need gasoline for longer trips, and even then only for a part of the trip.

Expect to spend $2000-$3000 in lithium battery cells and you'll get decent electric range and power. You'll get this investment back in about 5 years in saved gas. A proper lithium pack lasts for at least 10 years.


Well, to be really usable, your range extender would need at least 10 kW (13 hp) of electrical power. With your truck, that would probably have you go at maybe 60-70 km/h on average with gasoline. With a largish battery, this would truly be average speed; for example, it would charge the batteries while you stop for a break. (You could also charge from a public charging point at the same time.)

So I guess a 125-250cc motorcycle engine ain't enough, unless it's only for emergencies and you are fine with a "limp mode" or extending the range only a bit. I think you need to go up to around 500cc or more. The engine and the generator will weigh more than 100 kg.


Most seem to be, but I'm also thinking about converting a pickup to a plug-in series hybrid with a 10..15 kW generator and full electric range of about 50 km.

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The electric driving motor may run entirely fed by electricity from a large battery bank or via the generator turned by the internal combustion engine, or both. The battery bank may be charged by mains electricity reducing running costs as the range running under the electric motors only is extended. The vehicle conceptually resembles a Diesel-electric locomotive with the addition of large battery bank that may power the vehicle without the internal combustion engine running. The generator may simultaneously charge the battery bank and power the driving electric motor that moves the vehicle. The battery bank acts as an energy buffer. An advantage is that when the vehicle is stopped the combustion engine is switched off. When the vehicle moves it does so using the energy in the batteries. This reduces kerbside emissions greatly in cities and towns. Vehicles at traffic lights, or in slow moving stop start traffic need not be polluting when stationary.
In some arrangements when high levels of power are required, such as in vehicle acceleration, the electric driving motor draws electricity from both the batteries and the generator. With the Chevrolet Volt if the battery bank is depleted the vehicle may run entirely with electricity provided only from the generator. Some prototype vehicle designs such as the Volvo ReCharge and Ford F-Series pickup have electric motors in wheel hubs reducing the need for a differential saving weight, space and power being sapped by the differential. Series-hybrids can be also fitted with a supercapacitor or a flywheel to store regenerative braking energy, which can improve efficiency by clawing back energy that otherwise would be lost being dissipated via heat through the braking system.
Because a series-hybrid omits a mechanical link between the combustion engine and the wheels, the engine can be run at a constant and efficient rate even as the vehicle changes speed. The vehicle speed and engine speed are not necessarily in synchronization. The engine can thus maintain an efficiency closer to the theoretical limit of 37%, rather than the current average of 20%.[3] At low or mixed speeds this could result in ~50% increase in overall efficiency (19% vs 29%). The Lotus company has introduced an engine/generator set design that runs at two speeds, giving 15 kW of electrical power at 1,500 rpm and 35 kW at 3,500 rpm via the integrated electrical generator.[4]
As the requirements for the engine are not directly linked to vehicle speed, this gives greater scope for more efficient or alternative engine designs, such as a microturbine,[5] rotary Atkinson cycle engine or a linear combustion engine.[6]
General Motors in 1999 made the experimental EV1 series hybrid using a turbine generator set. The turbine weighed 220 lb (99.8 kg), measured 20 inches (50.8 cm) in diameter by 22 inches (55.9 cm) long and ran between 100,000 and 140,000 rpm. Fuel consumption was 60 mpg-US (3.9 L/100 km; 72 mpg-imp) to 100 mpg-US (2.4 L/100 km; 120 mpg-imp) in hybrid mode. Depending on the driving conditions, a highway range of more than 390 miles (627.6 km) was achieved. The results were highly successful, and would have promised to be more successful if a smaller microturbine was used, yet the EV1 project was dropped.

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jk1981 said:

Don't get me wrong, I'm far from being anti electric or pro ICE for the sake of it. I just think that as a project at the moment it doesn't stack up. You're adding weight, cost and risk for no additional gain. You're effectively cramming 3 power plants (ICE plus 2x 3phase generator/motors), two energy sources and two high power inverters and a lot of bracketry to tie it all together in in place of one ICE... I know they can go pop unexpectedly but which of those two options sounds more likely to develop gremlins and realistically, which is going to be heavier

Personally speaking as as a glider pilot and an electronic engineer, given your stated goals I'd go for a nice standard 'off the shelf' self launching motorglider if it's the flying you're interested in. Second choice, with a much bigger budget and pretty well purely for the pleasure in the project and for quiet flight I'd go for an electric conversion on an existing self launching motorglider. Sell the ICE to pay for some bits. Seems like an expensive option to me but then I think the regulatory environment is a little different over there. A battery fire would be my main concern, pretty much every other powertrain failure in an SLMG you'll walk away from.

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Duncan said:

I think you will find most aircraft continue to use power to;

Flatten the glide slope
compensate for the extra drag of wheels/flaps

So there is nothing left to re-gen

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major said:

No problem. Just give it the proper command and it will generate all day long reliably. I've used ACIM drives as generators often and know PM types will also do the job.

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One principal reason for a possible efficiency increase in the case of a multibladed fan of smaller diameter, in the case you describe, is that we can load the blades in a manner more similar to the way our best wings are loaded, and let them behave more as an airscrew in terms of their AoA range over a flight speed range. Instead of a uniformly high blade loading, we can let high advance ratio and greater blade area lower the blade loading (and resulting induced drag) at high speeds, and yet overcome the higher resistance of large blade area at lower speeds by way of the available torque at the appropriate slow RPM called for by a lower airspeed.

This creates an "always flying" blade condition of higher blade CL at lower speeds, like our wings, rather than what often happens when we put 'high advance ratio blades' on an ICE: Often, root portions are fully stalled, creating negative L/D and demanding that we create and accept a smaller blade area in order to swing the prop. Props with that condition start off spinning at a higher RPM, then suddenly 'bite' as the forward airspeed unstalls the blades. Often this results in a brief lugging of the engine until the prop aerodynamic efficiency increase accelerates the aircraft, and the blade Angle of Attack moderates toward its higher L/D range thereafter.

The bandaid to provide the same result for an ICE is, of course, variable prop pitch. It adds complexity and weight, but it works well enough overall to overcome the fact that twisting a rigid blade causes totally non-optimum blade lift distributions and a loss of aerodynamic efficiency. A rigid blade can only be designed for one advance ratio, and getting each blade station to the correct AoA (or lift coefficient) for a new advance ratio would REQUIRE a local change in twist or camber, which is not a trivial engineering challenge.

Electric motors simply don't need any of this workaround complexity on their backs; they can turn faster to go faster and slower to go slower, like a servomotor driving a ballscrew. Trying it with a prop designed for an ICE doesn't work any better than using a rubber nut on a bolt: too much slip.

To get to where most people agree and follow this key strategy for electrically-driven props (efficient blades that slice and bite) is merely step one in a five or six step, whole-systems engineering process that leads to electric airplanes with nothing to be ashamed of. (On commercially available, off-the-shelf energy storage.) Another is motors of this caliber, which are only the beginning. Only when we adopt a zero-tolerance policy toward 'adding things' will we be on track to overcome today's battery limitations.

(BTW, I'm not sure you described the status quo torque/RPM relationships for an ICE correctly, and responders have replied accordingly with a degree of confusion/disagreement.)

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major said:

You said

So if the battery powered plane can only go 240 miles, and then you add the weight of an engine and generator, your range will be less on the batteries, right?

But I fail to see where any electric 'battery only' airplane has achieved 240 miles on a single charge. And it appears there has been some significant (read expensive) electric aircraft development lately. But I do not believe or see evidence that the parts and equipment exist today which you could buy and assemble to fly 240 miles on a single charge. And you used that as a premise in your logic.

It is like me saying: If my electric car can go 500 miles on a battery charge, ......

But it is just a pissing match. I think you use a lot of funny logic trying to convince us of something which doesn't really matter. Like I said; build your hybrid plane and enjoy it, no matter how far it travels.

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