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50Pesos

Active Member
I am no mechanical engineer. Why isn't a small diesel or centrifugal turbine able to generate enough power to continuously operate an electric drive motor?
 
Why?

What advantage would an electric motor provide?

Airplanes are a near max power for cruise, adding a electric motor is just an efficiency hit.
 
You could do what you want if all you ever needed to do was cruise. But you you need takeoff/climb power as well. So you need to upsize the electric motor for that, them either upsize the diesel or add batteries to get enough current to feed the electric. Now the system weighs more than just the gas or diesel engine.

Diesel Electric locomotives do it not because of the efficiency, but because of the torque. Running a constant speed diesel to feed electricity to the electric drive motors allows basically infinite gearing.
 
Perhaps the most brief answer is that you're adding mass (a generator and electric motor) with no increase in power. No energy exchange occurs without energy loss, either, so you now need a larger engine to achieve the same output power to the prop.

I drive a plug-in hybrid, and it's very good at my "mission". I can round trip to work without gas, yet if I need to go see the kids at college I can make that round trip with a combination of battery power and a bit of gas. The engine runs more often than not on longer highway trips, but the battery still takes the edge off going up hills or passing, while the engine cuts out and the battery recharges down hills or decelerating. Bottom line, a highly variable load (speeds, hills) suits the hybrid power train well.

If I just set out on the interstate doing 75 mph for hours across states like Illinois and Indiana (flat land), I get worse mileage than a car running purely on gasoline. That's because I'm hauling around ~500 pounds of electric motor and battery, but I *need* a nearly constant power output. This is analogous to the air plane example. With today's tech, you just can't match the energy density of hydrocarbons with batteries.
 
You could do what you want if all you ever needed to do was cruise. But you you need takeoff/climb power as well. So you need to upsize the electric motor for that, them either upsize the diesel or add batteries to get enough current to feed the electric. Now the system weighs more than just the gas or diesel engine.

Diesel Electric locomotives do it not because of the efficiency, but because of the torque. Running a constant speed diesel to feed electricity to the electric drive motors allows basically infinite gearing.

Yes/no. It could be/has been accomplished with transmissions and direct drives. There's a lot of reasons but friction is probably (one of) the biggest. Rail transport gets it's efficiency from the low friction but it's a two edged sword. You have to get that big mass moving. Low end torque is also a double edged sword. If the drive wheels spin, you aren't going anywhere.

Diesel electric simplifies getting power to multiple locations. More wheels = more friction. Simplifies that part of the control scheme as well.

I've noodled with a hybrid aircraft config as well. A smaller, higher rev'ing, powerplant with no transmission has a lot of benefits and flexibility. A lot of the backup systems could be eliminated and design margins decreased as batteries would be power source of the motor/propeller. Some waste heat available for system/cabin heating. Imagine full torque from an electric motor for takeoff or an aggressive climb.

Ultimately, like all other application attempts, the energy density of the batteries kills it. A negative charge rate during takeoff/climb could be engineered around. What would be the minimum kW-h -> time for flight if the IC motor failed?

A good thought exercise for sure.
 
What I was hopping to find possible a diesel engine or centrifugal turbine burning 3-8 ghp to turn a 700 KW motor. The advatanages I hope to find are constant power to climb, less fuel required increasing usable load, quieter, use jet A, possible reverse thrust.
 
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What I was hopping to find possible a diesel engine or centrifugal turbine burning 3-8 ghp to turn a 700 KW motor. The advatanages I hope to find are constant power to climb, less fuel required increasing usable load, quieter, use jet A, possible reverse thrust.

AN ICE, that can power a 900+hp electric motor, through a genset, that only burns 3-8 gph?

Probably not going to happen.

TANSTAAFL..."There ain't no such thing as a free lunch."
 
Using Siri

“Siri, convert 700 kilowatts to horsepower.”

“The answer is 951.73 horsepower.”
 
I am no mechanical engineer. Why isn't a small diesel or centrifugal turbine able to generate enough power to continuously operate an electric drive motor?

Simple answer is you can easily generate enough power to operate an electric motor. The complex part is in the trade space for weight, cube, cost, efficiency, performance, and endurance. If you optimize the turbine generator for cruise power, you need a supplemental power source for the max power operation, like a heavy battery pack. Electric motors can be run at +20% of rated power for short periods, which is great for takeoff and climb phases. Electric motors also allow for design flexibility, such as using a half-dozen small motor/props located somewhere other than the nose of the airplane. Don't forget the motor controllers, 3-phase high voltage wiring and motor cooling (most are liquid cooled). Hopefully by the end you still have some payload available for the pilot and a passenger or two. An H2 fueled Turbo-Rotary powered generator would make the climate change crowd happy and might be affordable for the rest of us non-turbine types.

The E-VTOL world has shown a lot of interesting concepts, maybe we will see a Van's RV-VTOL someday.:)

John Salak
RV-12 N896HS
 
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