Hybrid Engine Systems and Sustainability

Yeah but electric mowers don't have as much power as the gas ones...
I've used both, they are not the same thing.

 

Of course they don't have as much power. They get the job done with LESS hp. that's my point! Just as the lawnmower, a propeller same size, same rpm, same pitch, can be driven by a LESSER hp electric motor, than an IC engine.

If the ic engine is devoted as a generator charging batteries, the lesser electric motor devoted to propulsion, you have improved efficiency and economy. How do you make a smiley with a lightbulb over it's head?

 

Electric drives are less efficient than a transmission at exactly the right gearing, and more efficient than a transmission at the wrong gearing.
The electric generator / motor combo is great because of the way it delivers power, but it is not better than a mechanical transmission in terms of being efficient.
They also have a huge benefit of not requiring a mechanical drive shaft system, which is why they ended up working out so well on locomotives.

As stated above in many posts by many people, the TQ output of an electric motor exceeds that of an internal combustion engine, but that does not mean an electric motor is capable of doing more work compared to an equal gas or diesel engine.

 

No, you could also install a tiny gas engine on a lawn mower, and it will cut the grass just as well as the electric mower (stalls excluded).

The major benefit of the electric motor, is the low end torque it is capable of delivering.
A 10 HP electric motor is not capable of doing as much work as a 15 HP gas motor, HP always = HP. Even though the 10 HP electric motor might be harder to stall, it is still not able to put down as much power.

 

We're going around and around.

does the fractional hp electric mower cut the grass as well as the multi hp gas mower?

If not, then why don't they market fractional hp gas mowers? like the electric mower?

 

I think we get hung up on "horsepower".
What we need is thrust to move a boat with props.
To get thrust, we need rpm and torque.
If a lower "rated" electric motor can provide the torque and rpm needed, then why should we use a higher "rated' IC engine to do the job less efficiently?

i'm not talking ski boat, but, sail auxiliary.

 

The gas mowers out perform the electric ones.

I assume the engines they use are about as big as they can use for lots of power, without getting super heavy.

 

Ok, let me invite you to choose a small inboard diesel, less than 25 hp. Match up a reduction gear and prop (diameter, pitch, number of blades, blade area and cup)

I'll find an electric motor capable of turning THAT prop and the required rpm.

Interested?

 

HP IS the product of RPM & TQ.

A gear box is 10-15% efficient, electric generator and motor is not as much efficient.

If you take 2 identical boats, completely identical, up to the transmission and engines, it is going to take exactly as much HP into the shafts to go the same speed.

The electric boat might have a slight off the line advantage, because it also acts like a transmission.

The electric motor drive would be ideal for boats or vehicles that spend a lot of time at less than cruising speed.

 

time at less than cruising speed.

we should then ask, 'What is cruising speed?"
For sailboats, primary propulsion is of course sails!
a motor driven prop is necessary in harbor, because at least SOME of those expensive yachts are owned by lawyers, who got rich suing people. You do NOT want to scratch HIS boat!

Cruising speed in harbor isn't hull speed, but "safe" speed.

The problem is dragging that prop around under sail, detracts from sailing speed.
folding and controllable pitch props reduce the prop drag, but don't eliminate it. An outboard you can tip up, out of the water is a solution many use.

I like spinning the prop JUST slightly faster than sailing speed. All drag is eliminated, and a small increase in speed is gained.
Racers spend lots of money on new sails to gain a tenth of a knot advantage.

so motorsailing at minimum amp draw, and slow harbor speeds for maneuvering control, are the 'cruising" speeds for a motorsailer or auxiliary sailboat, where the motor is concerned. :idea:

 

Don't get me wrong, I'm not saying an electric motor won't work, heck we know it will, they use them all the time, even in some ships.

However, TQ is not a replacement for horse power. Steam engines could pull more than they could track, and diesel electrics can track more than they can pull.
I suspect that would be a huge reason to use that system in a tub boat use, or other large boats where speeds are low.

What the electric motor is capable of doing, is delivering more HP lower in the RPM band (because of the TQ curve), which means it is better at putting out power at lower RPM's, but it is not capable of actually doing more work than an equally rated engine of any kind.

I'm not "anti" electric motor, they have a place, but they are not a direct replacement for every problem.

 

In the ways it sounds like would employ it, the electric motor system might work out really well for you.

For a trawler, houseboat, powerboat etc, there is little to be gained, I think.

 

Thankyou. It's gracious of you to see my pov. I tried to give you rep, but says I got to spread it around.first

 

Hi Yobarbacle,

I thought that we have already discussed that electric HP vs Diesel HP in the past, but it was probably not you.

Firstly, do not forget that it is the power (HP), not the torque which drives your boat (or your car) at some speed. A boat moving at a speed V creates a resistance R, regardless of the type of engine. The resistance is related to the hull shape, not to the engine type. In other words, for a given V, R is fixed. The power is given by the product R*V and is therefore fixed too, for a given speed.

Hence - you need power, NOT TORQUE, to drive a boat at a given speed.

Once you have accepted that basic concept, please read this post: http://www.boatdesign.net/forums/boat-design/tko-electric-solar-concept-43940-4.html#post568467 where a case study has been done between a 48 HP diesel and a 12 HP electric motor. If, after reading it and checking the attached graphs, something remains still unclear - we can discuss it.

Cheers

 

I took a quick look through the thread posted above, and decided to quote from one of the posters there :

"double the rpm and the power required increases by eight times."

With that in mind, imagine 2 identical boats, let's say they're 40 ft trawlers, and the goal is to power them with something so they can both cruise at 8 knots.
Boat #1 gets a 85 HP diesel, boat #2 gets an 85 HP electric motor, and diesel electric system to run it.

Lets pretend (cause I'm not witty enough to know REAL values...), that the design says we need to spin the propellor shaft at 1000 RPM to hit the correct speed, 8 knots. So, the diesel boat gets a 2:4 transmission put into it, so it cruises while running at 2400 RPM's.
OK, so now, we have the conventional diesel boat, running at 8 knots, 2400 RPM's, prop shaft speed at 1000 RPM. Thats a normal boat. Lets also assume, that the system was geared, so that the engine is putting out all 85 HP at 2400 RPM, because the propellor is sized to draw that much TQ at that speed. (186 ft-lbs is being provided from the engine, at full power, at 2400 RPM).

OK, now the electric boat....
We have an 85 HP electric motor, bolted onto the prop shaft, and it's rated at 85HP@ 1000 RPM.... putting out 446.4 ft lbs of TQ...

Both boats are spinning the propellor at the same speed, both go 8 knots...
But why does the electric motor have so much TQ? It must be better? Oh, wait... we forgot about the transmission on the conventional diesel....
186 ft lbs * 2.4 = 446.4 ft lbs.... the exact amount of TQ from the electric drive boat.

Well, looks like the electric motor doesn't really provide a free lunch after all. And considering the gear box is more energy efficient than an alternator / motor setup, you'd actually need to burn MORE fuel in the electric boat to keep up.

What about the "low end TQ" the electric motor can "put down"? Since it can provide massive stall TQ numbers compared to the conventional power train, it must accelerate faster?! Oh oh, theres a huge problem with that....
Look at our 2 identical boats again (-drive train)...

Both require 446.4 ft/lbs @ 1000 RPM on the prop shaft to move at 8 knots.
Using " double the rpm and the power required increases by eight times."
That means, if the boat is running slower, let's say half shaft RPM, we only need 55.8 ft lbs of TQ to turn the prop shaft. (446.4 ft lbs / 8 = 55.8 ft lbs.)

So wheres the advantage of an electric motor in this instance? The huge amount of TQ it could potentially provide are impossible to transfer into actual work from the propeller.
Despite the diesel conventional power train's weaker TQ output compared to the electric, it can still potentially deliver more TQ than could actually be applied to the prop shaft.

The RPM range where the electric motor excels at providing massive TQ, the prop shaft demands almost none.