Kinetic to Electric Power Conversion

1. Mar 1, 2012

RoSmith

Hi all,

Having trawled the web for too long, I don't seem to be able to find any resources surrounding a question I've had regarding turning kinetic power into electric power.

Take the situation where an internal combustion engine's flywheel is connected to an alternator. The idea is that the alternator turns the kinetic energy of the engine into electrical power.

Assuming the load on the engine from the alternator is constant, what is the relationship between the horsepower, rpm and torque of the engine and the volts and ampage produced by the alternator?

This is obviously a hypothetical situation and I know that I'm probably missing some factors that need to be taken into consideration (such as flywheel diameter etc.) but, what are they? How can you calculate the relationship between the two?

Richard

2. Mar 1, 2012

Staff: Mentor

Welcome to PF!

The easiest way to figure this out is to just skip all the intermediate calculations and use a conservation of energy equation to compare power in to power out, figuring in an efficiency. If the output at the engine shaft is 1kW and the generator is 90% efficient, then you have 1*.9=.9 kW output. If you want to use horsepower from the engine, you'll of course need to apply the conversion factor for horsepower to kW of .746.

3. Mar 1, 2012

RoSmith

Hi, thanks :)

The engine will produce a kW output which will be reflected in the electricity output of the generator... but since Watts = Volts x Amps, how can you tell how many amps and volts you're getting in relation to engine torque, rpm and hp? I want to find out the connection between the two... so whether there's an equation;

Engine rpm, horsepower and torque ---------> Volts and Amps produced.

Is there any way of doing that?

4. Mar 1, 2012

Staff: Mentor

No, there is no direct connection. It depends entirely on the specifics of the motor of the generator and those specifics are generally just chosen. Ie, if you go out and buy a 1kW generator that outputs 120V it may have exactly the same gas engine as one that outputs 240V. ByP=VI, the second one puts out half the amperage, but they both put out the same kW.

It really looks to me like you are focusing on things that don't matter. What exactly is this for?

5. Mar 1, 2012

RoSmith

I want to investigate the conversion of an IC engine's power to electrical energy at source without battery storage. It's a personal project / investigation into passanger car hybrid systems.

I see what you mean about the generators. So, say for example, you had the engine running at 3000 rpm, producing 100 ft/lbs of torque (and concequently 57.1 hp). The generator is absorbing that kinetic power at 100% efficiency (let's dream!) and is pushing out 24 volts. How can you calculate how many amps are being produced by the generator?

6. Mar 1, 2012

sophiecentaur

1hp is the imperial unit for power. The Watt is the SI unit. The conversion constant is
746W = 1hp, as stated above

Electrical power is given by Voltage times Current P=VI but Power is Power, in whatever form - being the rate of transfer of energy (also Joules per second)
In Europe they tend to quote engine power in kW aamof.

The efficiency of the system needs to be considered and that will depend upon the specific system but the system you are discussing is far from optimal (it's designed to get Power to the road wheels as well as possible) and I could suggest you won't be getting better than around 60%. That means if you have 100W of lights turned on, you are probably taking around 150W from the engine (about 1/5hp). You can notice the effect of an electrical load - turning on the windscreen heaters in particular - on the tone of the engine at tick-over.

btw torque is not relevant here. it's all about Power. Power is torque times revs but this would be the whole Power and not just what's delivered to the alternator (and, in any case, it's the torque at the crankshaft that's specified).

[Edit: Our posts have crossed and now I see your particular application - if you want an idea of efficiency then look up "motor generator efficiency" or suchlike - I should imagine the specific info relating to automotive systems would be a bit commercial in confidence so you would have to look at more general applications.
How come you're doing a project like this if you haven't been taught about Power?]

7. Mar 1, 2012

RoSmith

Wait... have I just asked a really stupid question? Reading my post back I've just realised I've given enough information to answer my own question...

If it's producing 57.1 horsepower, that's 42.6 kW (42,600 Watts).

Watts = Amps x Volts, so 42,600 = Amps x 24

So the number of Amps produced is 1,775 Amps?

Can anyone tell me if that's right?

----------------------------------------------------------------------
Hi Sophiecentaur

Thanks. I only mentioned 100% efficiency just to simplify the equation. Is 60% typical? Thanks for the info on torque :)

[Edit: I'll take a look. I have learned about power in automotive systems, I'm just taking it back to basics and, in all honesty, I know very little about hybrid systems. I'm still doing my reading and I just wanted to ask some very broad questions.]

Last edited: Mar 1, 2012
8. Mar 1, 2012

sophiecentaur

60% is only a 'number'. How can you be ignoring the Battery issue if you are looking into Hybrid cars? The overall efficiency, involving batteries is worse, of course.

And, yea - watts is amps times volts and you can do arithmetic .

9. Mar 1, 2012

RoSmith

I won't go into too much detail, but it's looking into using batteries in a supporting role, rather than a vital 'power storage' role... if that makes sense!

Heh I know 1,775 amps seemed a lot, just wanted to make sure it wasn't 1.775 amps! (And if there wasn't something embarrasingly stupid I was missing!)

Last edited: Mar 1, 2012
10. Mar 1, 2012

sophiecentaur

Perhaps you ought to look into diesel electric railway locomotive design then.

If you are talking of a 40kW load at low voltage then you don't be surprised at needing very high currents. This is why all domestic supplies use 110 or 240V. It means that you can use sensible thicknesses of cables with less loss. It must be a serious consideration in existing hybrid cars. They use around 200V. Look it up.

11. Mar 1, 2012

RoSmith

I will. I know that some trains have an enormous diesel engine at one end with a generator that powers the electric wheels... I didn't think of that. I'll have a trawl.

I know that EVs try to use as high voltage as possible and then utilise reduction gears in the 'in wheel motors' that power them along. I'm going to have to do some more reading. Thanks for your help, I really do appreciate it! :)

12. Mar 1, 2012

sophiecentaur

Re: "reduction gears" - this is not to do with the high voltage supply. You would obviously use motors with the correct voltage rating and operate them at an optimum speed. Reduction gears with a high ratio are inefficient and modern motors can operate at more sensible speeds, I believe. Motor speed control is also a big issue, if you want to do it efficiently. In the old days they used to use different battery connection arrangements plus massive hot variable resistors - not very green!

13. Mar 1, 2012

RoSmith

Oh... I was told that reduction gears were used in the motors to compensate for the fact that EVs made use of high voltage low amperage systems to avoid weight issues and 'bulking up' of insulation etc. Remembering my sources, I was probably informed wrongly!

14. Mar 1, 2012

sophiecentaur

I don't think there is any speed factors for low or high voltage motors. High voltage would certainly be lighter, though. But there are often 'special' factors in new fangled motor technology. Let us know if you find out about it.

15. Mar 1, 2012

RoSmith

I will... just to clarify... if in the above scenario, the generator was a 120 volt rated unit and it was connected to a 120 volt rated motor... the amount of amperage flowing would be directly proportionate to the speed of the motor, right?

16. Mar 1, 2012

sophiecentaur

Omg. Nothing as simple as that. The current will relate to the LOAD and not the speed. A motor will go very fast off-load. The highest current is when the motor is 'stalled'.
When they are spinning, they act as a generator and produce a 'back emf' which is against the sense of the supply, limiting the current. At high speed and high power (uphill or accelerating), the current will be high, of course, but this is often for a minority of time - as with most auto engines..

Look this up. In fact, you need to look a lot of things up. You want to search for "electric motors basics" and suchlike. If you don't then your project could risk being based on some serious misconceptions.
Immerse yourself in motors an you will find some it familiar.

17. Mar 1, 2012

RoSmith

I am really confused (I'm sorry, this is probably basic stuff). I know that if you put load on a motor, it draws more amps from the power source... but what if the kW are already predetermined by the output of the generator? If the generator produces so many kW then the motor will spin relatively regardless of it's load, right? The voltage will be fixed, so the amperage will have to be fixed.

18. Mar 1, 2012

Staff: Mentor

Like a car engine, a generator will put out only as much power as is demanded of it, regardless of RPM. Consider that you can make a car travel at constant speed at multiple different RPMs.

And consider what happens when you get to a hill: you push on the gas pedal to generate more horsepower and hold your speed. The speed regulator on a gas generator works much the same way.

19. Mar 1, 2012

RoSmith

When you say the generator will only put out as much power as is demanded of it, that's surely only when the generator as a unit is fitted with sensors to detect when it's current output is insufficient or excessive to the demand? If the IC engine is being given varying power demands by the driver, how does that translate into what the generator gives out in amperage?

So, if the load on the wheels and the motor stays the same, but the driver demands more power from the engine... what will change? I presume the amp output of the generator would increase... so would the speed of the wheels change?

20. Mar 2, 2012

pantaz

A common misconception is that a generator or alternator always outputs its maximum amperage (at a given RPM) all of the time. Current only flows if there is a load, and the amount of current varies with the load (for the same voltage).

This says it better than I can -- http://www.generatorguide.net/howgeneratorworks.html :
"If an external circuit is connected to the coil's terminals, this voltage will create current through this circuit, resulting in energy being delivered to the load. Note that the load current in turn creates a magnetic field that opposes the change in the flux of the coil, so the coil opposes the motion. The higher current, the larger force must be applied to the armature to keep it from slowing down."​

Many other examples can be found around the 'net. http://www.google.com/search?q=electric+generator+basics

As for formulas, there are many, depending on the specific question.