Hybrid Engine Systems and Sustainability

Mr Cockey just showed why power is important.

You really need to understand the difference between torque and power. If your prop is turning it needs power. Torque on its own does not factor in RPM. You could argue that a certain prop needs X amount to torque to turn at X rpm. But all you would be doing then is talking about power anyway as you mentioned torque and RPM together, which IS power. Torque and RPM are the only variables needed to solve power.

 

Indeed and Jeremy's example of 100hp @ 2000rpm equals torque of 262.6 ft lbs
same prop needing 1.5hp @ 500 rpm equals 15.76 ft lbs.

This explains quite well why extreme low end power (torque) is not that relevant for spinning a propeller.

I believe that the confusion of better electric horses come from:

1. Comparisons are made to very badly matched combustion engine-prop combos. As fuel burning systems and fuel are relatively cheap (per hp) and simple to add there is less need for careful optimization. Whereas electric systems are energy storage starved and efficiency is a major issue.

2. The electric speed controller does act like a transmission and thus it can work more efficiently with partial loads.

But the whole full torque from zero rpm thing is somewhat a myth. There are a lot of electric motorbikes that use no mechanical transmission - but they are far from optimal at low rpm. The currents go through the roof when very high torque is demanded and a lot of energy goes to heat (relatively) - if the bike would have 2-3 gear transmission it could provide much better performance and much less heat at lower speed operation. Well I guess you could optimize 1 gear system for low speed but at the cost of max speed.
Jeremy knows these much better so he can correct my mistakes.

 

Yes. As has already been quoted for the example I gave, the torque needed to turn a prop at 500rpm is very small compared to the torque needed to drive it at 2000rpm.

I can't get my head around why the fixed relationship between torque, power and rpm seems to cause people so much angst; it's almost as if some seem to erroneously believe that you can have torque, as an entity in a rotating system, that's completely independent of power.

 

Sometimes it helps to point out that with gears, you can achieve any rpm or torque desired - all from the same engine.

 

This is a lot like electrical things.

Amps are like TQ
Voltage is like RPM
Watts are like HP

 

True. You can build machines that trade the first 2 values (transformers/gearboxes) but no machine can multiply power.

 

Ok, gentlemen.
then I have a conundrum for you.

IC engines cannot produce static torque, BUT, electric motors, steam engines and certain hydraulic motors DO produce static torque.

Many thousands of foot lbs of static torque at zero rpm, as when a diesel electric train is starting from a full stop, the X torque times 0 rpm equals zero power, yet that 100 car train starts moving!

 

> torque times 0 rpm equals zero power

Output power, yes. But it doesn't mean that lots of input power wasn't consumed in producing the torque. It all goes into heat until something moves.

 

My point is comparing ic engines to electric motors is apples and oranges.

the force in IC engines is internal, while electric motors have an external power source.
electric motors have high start torques that IC engines simply can't.
Which device is better suited to spinning a propeller to create thrust?
I believe the electric motor is remarkably more efficient at lower rpms.
While high speed boats benefit from highspeed IC engines.
sailboats, in particular motorsailers are not normally included in high speed classes.
The ic engine is WELL suited to turning a generator at the engines optimum economical rpm.
hence, diesel electric is functional.
I've run WWII era diesel electric tugs and ships. The Electric motors driving the shaft are a fraction of the diesels hp generating the electricity. Some companies kept the original generator diesels, and converted the tug to twin screws with clutches and reduction gears.
The same diesels driving smaller props produced less bollard pull and less max speed than the original diesel-electric massive single screw drive.


Modern electrics are much improved over WWII stuff!

 

http://www.shipscribe.com/usnaux/AT/AT077.html

One of several ATA's I've skippered.

She was named tug "Ranger' when I worked for Caribbean Towing, Inc.

Longest voyage I made with her was from New Orleans to New Orleans, via Gulf of Benin off Nigerian coast, Africa.
90 days at sea without touching land, to Africa and returned after rendezvous with a lay barge that picked up the oil platform I towed..

 

That must have been a pretty epic voyage. I would love to go on boat like that

Really though. It has been shown multiple times that the high torque at low rpm is not required when turning props! Therefore the electric motors torque curve is the opposite to what would be suited.

Look here. You can see even a highly tuned gasoline outboard has far more power than what is required to turn the props as the RPMs go down! What would the point be if you swapped it with another motor that had even more torque in the low RPM?

There would be no point normal IC engines already have more torque than what is required!



Here is a power torque curve of an electric motor. Now, overlay that on the prop power charts to see that the characteristics are not better than that of a IC engine for driving props. A prop needs max torque at max RPM and zero torque at zero RPM. The exact opposite of what an electric motor can deliver!






The fact that the modified tugs lost bollard pull only shows that the modifications were not done well.

 

It's a moot point; as the electric motors are making TQ at 0 RPM, they are also drawing power from a source.
Same thing as a regualr truck or car taking off, the axles are at first 0 RPM, but the energy is being spent on the torque converter or clutch. No different than an engine driving a generator and not moving.


TQ and HP are very much misunderstood; HP is a measure of total power, I.E power that doesn't depend on another variable to know what the output is.
If you take 2 engines, and they both have a peak power output of 500 HP, they can both do equal amounts of work, if run at peak output. Doesn't matter if it's a tiny 2 stroke gas racing engine VS a giant slow speed diesel. HP is HP.

When people talk about diesel torque, or how "torque gets the job done" they are not speaking of the peak output of the engine, but rather the powerband characteristics of the engine.
If you look at some engine output graphs, you can see that diesels usually have much more low end torque than gas engines, and run slower. This might give the illusion of being more powerful, but there are plenty of gas engines that can make the same HP, but not the same TQ, as a diesel.

I attached a graph, it is fords graph showing their new engines power outputs.
FYI, the 3.5 ecoboost is faster than a 5.0, and sometimes faster, sometimes slower than a 6.2, but you can see that the 3.5 doesn't have the most PEAK HP, but it holds its own, as the TQ is steady, and high.

The 2nd picture, is a power output from an electric motor.

 

Ohh more graphs

 

http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee11/bdeee11_7.aspx

According to this article, type B (most induction motors) and type c have nearly flat, actually slightly increasing torque through about 80% of max speed range.
Given any 2 different designed motors of equal horsepower, the motor type with higher torque can be replaced with a lesser horsepower motor of same type, and perform the work/torque of the less torquey motor, for any given application.

Since the electric motor developes MORE low end torque than an IC engine of equal horsepower, for a needed amount of torque, a LESSER hp electric motor will perform as well as a greater hp IC engine!

As long as speed requirements for the electric motor, are 80% or less than rated max rpms. right?

 

here's is an easy to understand everyday life example.

A 3 and half hp briggs n Stratton gas lawnmower, will stall in tall grass, requiring operator to back up, or tilt up the mower deck by lowering handles, to allow regaining rpms and torque.

The same yard can be cut with a 110 volt electric mower of half hp, or less, and rpms will be reduced in tall grass, but mower doesn't stall.


http://www.sears.com/black-decker-1...p-07137006000P?prdNo=1&blockNo=1&blockType=G1


http://www.lawn-mowers-review.com/murray-35-hp-classic-cut-20-inch-lawn-mower.html