I was curious so I did a search to try and figure out where the 3x
rule came from that so many EV quote and I think it is here:
http://books.
Seem to be a well written book I might have to buy it. And the
chapter listed it talking about exactly the subject at hand!
On Jul 27, 2009, at 4:37 AM, James Sizemore wrote:
> I don't know a thing about the E-pod 3000. So I am not defending there
> analysis or figures, but you do have to understand that combustion
> motors and electrical motors HP is not rated the same way. Combustion
> motors HP is rated at maxim RPM with out load. Electric motors are
> rated at contuse power under load. So the two "HP" are measuring two
> different things. So no combustion HP does not equal electoral HP.
> The rule of thumb I see listed the most is electrical HP equals 3 X
> combustion HP. So using that rule 5.5 electrical HP = 16.5
> combustion HP. So you would be right that they are fudging a bit.
> But again comparing the two is not simple!
>
> Here is a small quote from the really long wikipedia page on HP (http://en.wikipedia
> ) please take special note of the last sentence about "objective
> performance" HP rating for combustion engines are really a marketing
> number and don't translate well to the real world work! Electrical
> HP rating are more useful to determine real world work!
>
> "Measurement
> The power of an engine may be measured or estimated at several points
> in the transmission of the power from its generation to its
> application. A number of names are used for the power developed at
> various stages in this process, but none is a clear indicator of
> either the measurement system or definition used.
> In the case of an engine dynamometer, power is measured at the
> engine's flywheel (i.e., at the crankshaft output). With a chassis
> dynamometer or "rolling road", power output is measured at the driving
> wheels. This accounts for the significant power loss through the drive
> train. As an example, an early-production BL Mini 850 cc (51.9 cu in)
> engine produced about 34 bhp (25.4 kW) at the flywheel, of which only
> 18.2 bhp (13.6 kW) reached the driving wheels.[citation needed]
> In general:
> Nominal is derived from the size of the engine and the piston speed
> and is only accurate at a pressure of 7 lbf/in².[11]
> Indicated or gross horsepower (theoretical capability of the engine)
> minus frictional losses within the engine (bearing drag, rod and
> crankshaft windage losses, oil film drag, etc.), equals
> Brake / net / crankshaft horsepower (power delivered directly to and
> measured at the engine's crankshaft)
> minus frictional losses in the transmission (bearings, gears, oil
> drag, windage, etc.), equals
> Shaft horsepower (power delivered to and measured at the output shaft
> of the transmission, when present in the system)
> minus frictional losses in the universal joint/s, differential, wheel
> bearings, tire and chain, (if present), equals
> Effective, True (thp) or commonly referred to as wheel horsepower
> (whp)
> All the above assumes that no power inflation factors have been
> applied to any of the readings.
> Engine designers use expressions other than horsepower to denote
> objective targets or performance, such as BMEP (Brake Mean Effective
> pressure). This is a coefficient of theoretical brake horsepower and
> cylinder pressures during combustion."
>
> On Jul 27, 2009, at 3:20 AM, bmhfercot wrote:
>
> > Present time I prefer to pass more time to build my Tiki46 than
> > going to the forums but I can't keep the silence concerning the
> > abusive adverts of Re-Epower.
> > From its website I copied the technical specifications of the E-pod
> > 3000 system :
> >
> > Technical Specifications of the E-POD 3000 System
> > Operates on 36-48 volts DC (48 volts recommended)
> > AMPERAGE: Peak 200
> > Max continuous 100
> > Recommended Cruise......
> > 40-60 amps @ 48 volts
> > (Performance = to 20 hp outboard / 25 hp inboard)
> >
> > In reading that I can't know what is the max continuous power at the
> > shaft. I have to estimate it : absorbed power 48Vx100A=4800W - a
> > minimum of 15% of losses in the engines and its controller = 4080W
> > or 5,5Hp. So by which miracle a 5,5Hp electric engine can do the job
> > of 25 hp inboard!!!!!
> >
> > I know the advantages of an electric engine are to deliver a good
> > torque at low rpm and to have a quick reply when you request the
> > full power and can even during a very short time deliver a peak
> > power as example to stop the boat (if the controller accept that).
> > So it's easier to handle the boat with precision when docking. To
> > have a good reply with a such little engine on heavy boats you need
> > a large propeller with a little pitch.
> > All that seems marvelous only if you have good conditions with no
> > wind and or no current.
> > As soon as you have to motor against the waves and the wind, the
> > torque at low rpm and the quick reply don't change anything and
> > you'll never get the push of a 25 hp inboard engine with a 5.5hp
> > electric engine.
> > If you try to reach a shelter in a gale against the wind and waves
> > in a long rocky entrance, you'll push the throttle at the maximum to
> > get the maximum continuous power of 5,5hp at the shaft (in fact the
> > propeller has an efficiency of 50% and the useful power to push the
> > boat is only of 2,25hp). As you draw a high amperage, the real
> > capacity of your battery bank will be lower than if you draw a
> > little amperage. Your speed is very low, so you have no hull speed
> > inertia and each gust or wave will stop the boat. As the speed is
> > very low, the angle of attack of the propeller blades increases, so
> > the propeller stalls reducing its efficiency. The engine and
> > controller warm more and more and so the useful power is going
> > down........
> > The boat don't progress and in less of a half hour (if you have a
> > good battery bank) you loose the control of the boat due to lack of
> > electric energy ( if the controller or engine don't shut down
> > before). The boat wrecks on the rocks in hoping you don't loose your
> > life!!!!!
> >
> > So I can say IT'S CRIMINAL to affirm to the customers that an
> > electric engine of 5.5hp can does the job of a 25hp inboard
> > engine!!!!!!
> >
> > An hp is an hp, there is no difference if the power is produced by
> > electricity, or by humans, or by animals or by petrol or by nuclear,
> > so and so. Behind the power delivers on the shaft the difference
> > depends of the efficiency of the propeller and if the propeller is
> > well adapted with the hull. This kind of comparison is only
> > available about facilities to handle a boat in an harbour but it's
> > not possible at full throttle. With the same propeller efficiency,
> > with the same boats, the bigger power engine will produce the bigger
> > propulsion power.
> >
> > Bertrand
> >
> >
> >
>
> [Non-text portions of this message have been removed]
>
>
>
[Non-text portions of this message have been removed]
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