Author Topic: Peak output and flywheels  (Read 17766 times)

Procrustes

  • Sr. Member
  • ****
  • Posts: 332
    • View Profile
Peak output and flywheels
« on: August 12, 2006, 07:49:04 AM »
I'd like to understand better how flywheels help start induction motors.  I understand that they store inertia and thus are well suited to the peaks that electric motors require.  Why then does the SOM have a 2.5kW head?  A standard 6/1 ought to be able to output 2.5kW continuous, so I don't see what advantage the oversized flywheels on the SOM confer, other than steadier output.

Also, I've heard that a 6/1 can output 3.8kW at sea level.  Is this peak or continuous?  If not, what is peak power?  How would heavier flywheels affect peak output?

Rtqii

  • Full Member
  • ***
  • Posts: 152
    • View Profile
Re: Peak output and flywheels
« Reply #1 on: August 12, 2006, 02:16:30 PM »
Flywheels store energy. This energy can be unloaded very quickly as opposed to waiting for a power stroke from a piston. The faster the flywheel is spinning, the greater the rate of energy delivery to the load.

Peak power demands by a large load... Say a big induction motor on an air compressor can be extremely hard to start without a flywheel system. In order to start these kinds of loads, you either need to oversize the engine and the generator head, or you add flywheel mass. From an expense point of view, the cheapest place to add flywheel mass is on the generator head.

It is my view that the Lister orginal equipment, such as the SOM, were underated. These engines were really designed to serve industry. I do not have a 6/1, but it is my understanding that they can output 3.8kW continuous.  Peak power on something like that can exceed 5kW for a very short period of time... Initially the load would be served by the flywheel, then the rack would open up.

Heavier flywheels permit higher peak power to be delivered. Faster spinning flywheels permit the power to be delivered at a faster rate.

Continous power is the rated HP output of the piston. Peak power is the rated HP output of the piston, plus the stored energy in the rotating mass: crankshaft, flywheels, generator rotor, and any additional masses such as pulleys or shafts.

I am currently designing a jackshaft for PTO from an engine system. The jackshaft will rotate at 1800 RPM and permit direct coupling of the generator head. The rest of the high speed shaft is available for mounting flywheel mass and for additional PTOs. Because of the higher speed, I am planning on boring and milling out junk steel flywheels from trucks and tractors and stacking them on the keyed jackshaft.  This shaft will not increase the total horsepower, but the peak power would be increased dramatically, as well as the peak power delivery rate... Because of the stored energy in the shaft. Hard to start motors and other loads with high torque requirements can be started in many cases before the rack has a chance to respond.

One of the problems with having a generator without a flywheel in the system is with starting big motors and air conditioners, especially capacitor start electric motors. The generator may have a continous output rating sufficient to run this type equipment, and still be unable to start it. This really tears up the motor because they get very hot quickly if they are not brought up to speed rapidly. The commerical equipment that does not use flywheels get around this by running the engines and generators at higher speed, usually 3600 RPM.  The increased shaft speed increases the delivery rate and makes starting big loads a little easier... But even at 3600 RPM, most generator set are not going to have a great deal of peak power available because they lack flywheel mass.

Another way to look at this is compare a Lister type engine with a gas or diesel modern engine running at higher speed, but with the same HP rating.  The modern engine will have a faster delivery rate, but the Lister type will have greater peak power due to the flywheels. If you look at the SOM you will see the orginal equipment has added flywheel mass on the generator pulley. This provided an increase of peak power, and in delivery rate.

slowspeed1953

  • Full Member
  • ***
  • Posts: 153
    • View Profile
Re: Peak output and flywheels
« Reply #2 on: August 12, 2006, 07:17:50 PM »
I'd like to understand better how flywheels help start induction motors.  I understand that they store inertia and thus are well suited to the peaks that electric motors require.  Why then does the SOM have a 2.5kW head?  A standard 6/1 ought to be able to output 2.5kW continuous, so I don't see what advantage the oversized flywheels on the SOM confer, other than steadier output.

Also, I've heard that a 6/1 can output 3.8kW at sea level.  Is this peak or continuous?  If not, what is peak power?  How would heavier flywheels affect peak output?

Procrustes,

On average a 6/1 will carry 3300 watts with a st5 gen head, that represents a loss of 1174 watts in the transmission (belt) and gen head windings. Not taking into consideration the slight loss of the belt drive the st head is approximately 73% efficienct in converting mechanical energy into electricty.

If maximum efficiency is the main priority the most efficient way of producing electircty with the 4474 watts (6hp) available would be IMHO to use a dc permant magnet generator as they can be as efficient as 90% then convert to ac using an inverter as they average around 95% efficient. That would be a total loss of 15% rather than the 27% loss incured by the st head.

Or the totals for the two systems look like this.

ST5 = 3300 watts          dc to inverter = 3803 (likely your 3800 #)

Peace&Love :D, Darren

GuyFawkes

  • Hero Member
  • *****
  • Posts: 1184
    • View Profile
    • stuff
Re: Peak output and flywheels
« Reply #3 on: August 12, 2006, 07:40:15 PM »
I'd like to understand better how flywheels help start induction motors.  I understand that they store inertia and thus are well suited to the peaks that electric motors require.  Why then does the SOM have a 2.5kW head?  A standard 6/1 ought to be able to output 2.5kW continuous, so I don't see what advantage the oversized flywheels on the SOM confer, other than steadier output.

Also, I've heard that a 6/1 can output 3.8kW at sea level.  Is this peak or continuous?  If not, what is peak power?  How would heavier flywheels affect peak output?

Procrustes,

On average a 6/1 will carry 3300 watts with a st5 gen head, that represents a loss of 1174 watts in the transmission (belt) and gen head windings. Not taking into consideration the slight loss of the belt drive the st head is approximately 73% efficienct in converting mechanical energy into electricty.

If maximum efficiency is the main priority the most efficient way of producing electircty with the 4474 watts (6hp) available would be IMHO to use a dc permant magnet generator as they can be as efficient as 90% then convert to ac using an inverter as they average around 95% efficient. That would be a total loss of 15% rather than the 27% loss incured by the st head.

Or the totals for the two systems look like this.

ST5 = 3300 watts          dc to inverter = 3803 (likely your 3800 #)

Peace&Love :D, Darren

Cut the shit darren, you don't own a 6/1 so you haven't got a fucking clue how efficient they are.

How efficient is my start-o-matic when producing 2 Kw of AC?
--
Original Lister CS 6/1 Start-o-matic 2.5 Kw (radiator conversion)
3Kw 130 VDC Dynamo to be added. (compressor + hyd pump)
Original Lister D, megasquirt multifuel project, compressor and truck alternator.
Current status - project / standby, Fuel, good old pump diesel.

bitsnpieces1

  • Full Member
  • ***
  • Posts: 247
    • View Profile
Re: Peak output and flywheels
« Reply #4 on: August 12, 2006, 09:23:38 PM »
  It all goes back to E = M V2.  For a flywheel, the amount of energy stored in it is applied to the gen head when you have a demand that is larger than the Listeroid can put out at that instant.  Meaning when you have a momentary load (starting a large motor) the gen head draws its energy from the flywheel rather than the engine and then the engine picks up the load.  Sorta like a capacitor in electrical loads.  The  M is the mass of the flywheel itself, the V is the velocity of the flywheel.  Velocity is worked out at each distance from the center for what ever mass is at that distance. 
  Velocity (might as well call it rpm) supplies energy as its square,  mass at a straight line.  Whatever energy you dump from the flywheel has to be put back into it from the engine. 
  This is why I'm planning on integrating extra flywheel into my setup.  On a jackshaft with its own bearings, setup wwith a clutch to take it out of the drivetrain when necessary. 
Lister Petter AC1, Listeroid 12/1, Briggs & Stratton ZZ, various US Mil. surplus engines. Crosley (American) 4cyl marine engine(26hp).

GuyFawkes

  • Hero Member
  • *****
  • Posts: 1184
    • View Profile
    • stuff
Re: Peak output and flywheels
« Reply #5 on: August 12, 2006, 10:09:35 PM »
Consider a solid disc flywheel of radius 50 cm and mass 140 kg. How fast would it have to spin to have a store the equivalent amount of energy that is stored in just 10 kg of gasoline when burned in an internal combustion engine:

    * 10 kg of gasoline = 140 KWH
    * Engine has 15% efficiency --> 21 KWH of useable energy
    * Flywheel has a conversion efficiency of 80%
    * Flywheel must therefore store 21/.8 = 26.25 KWH
    * Kinetic Energy goes as 1/2*I*w2. For flywheels I =1/2MR2.
    * If we measure w in revolutions per second then the stored energy of a flywheel is approximately 6MR2 x w2 (RPS)

    * For M=140 kg and R=50cm this yields a required w of 500 RPS or 30,000 RPM


A CS 6/1 Start-o-matic flywheel *each* weighs approx 136 kg, and has a radius of approx 33 cm

Obviously 650 RPM = 10.83 Revs per second, hell of a lot less than 500, but still... do the math.
--
Original Lister CS 6/1 Start-o-matic 2.5 Kw (radiator conversion)
3Kw 130 VDC Dynamo to be added. (compressor + hyd pump)
Original Lister D, megasquirt multifuel project, compressor and truck alternator.
Current status - project / standby, Fuel, good old pump diesel.

Procrustes

  • Sr. Member
  • ****
  • Posts: 332
    • View Profile
Re: Peak output and flywheels
« Reply #6 on: August 13, 2006, 02:20:47 AM »
Thanks for the replies everyone.  However I still don't understand what good the huge flywheels are on a SOM if it has a 2.5kW head; it should be able to produce that much power continuously.  What am I missing?

GIII

  • Jr. Member
  • **
  • Posts: 67
    • View Profile
Re: Peak output and flywheels
« Reply #7 on: August 13, 2006, 03:34:02 AM »
  A single cylinder engine only produces 1 power pulse for every 2 revolutions so there needs to be heavy flywheels to carry the engine through the non-power strokes of the cycle without causing significant slowing of the generator that will cause flicker in connected lights.  This is a prodlem with ANY single cylinder engine and is more pronounced at slower rated speeds and particularly on 4 stroke cycle engines with the longer 'coast' times between power impulses.
By derating the output the manufacturers could also achieve two goals; the maximun output would appear higher on their 2kw unit and there would less likelyhood that the engine would slow enough to cause the enoying flicker.  This was especially important to Lister since they were marketing a high quality light plant.  Fairbanks-Morse also employed huge flywheels on their engine and derated them slightly when those engine were to be fitted to a generator for lighting for these same reasons.  Kohler was one of the first to offer relatively low hp engines with multiple cylinders for light plants as a way to overcome the need for huge flywheels to minimize flicker.

cujet

  • Hero Member
  • *****
  • Posts: 848
  • Lister power rules!
    • View Profile
    • www.cujet.com
Re: Peak output and flywheels
« Reply #8 on: August 13, 2006, 04:44:59 AM »
Listeroids can cause lights to flicker. The voltage climbs and drops in sync with the combustion stroke. A voltage regulator can help this, as do heavier flywheels.

While Darren has done his math and is correct, he makes one serious error. The 6-1 will make well more than 6HP contin. I have seen better than 4000 watts with black smoke and 3800 with minimal smoke. Any way you look at it the engine makes more HP.

Chris
People who count on their fingers should maintain a discreet silence

Doug

  • Guest
Re: Peak output and flywheels
« Reply #9 on: August 13, 2006, 03:08:25 PM »
  Kohler was one of the first to offer relatively low hp engines with multiple cylinders for light plants as a way to overcome the need for huge flywheels to minimize flicker.

This is a good point, considering twin Listers and triple Petters are available. If flicker is an issue....

Doug

slowspeed1953

  • Full Member
  • ***
  • Posts: 153
    • View Profile
Re: Peak output and flywheels
« Reply #10 on: August 13, 2006, 03:12:14 PM »
Listeroids can cause lights to flicker. The voltage climbs and drops in sync with the combustion stroke. A voltage regulator can help this, as do heavier flywheels.

While Darren has done his math and is correct, he makes one serious error. The 6-1 will make well more than 6HP contin. I have seen better than 4000 watts with black smoke and 3800 with minimal smoke. Any way you look at it the engine makes more HP.

Chris

The peak horsepower of a 6/1 is not the point I was trying to get accross hense the word average.

The point was that if you are shooting for maximum efficiency of the transfer of mechanical energy to electrical power the st style heads are not the best choice.

If your set will carry 4000 watts with a st then it would carry approx 4480+ with a permant magnet dc genny and efficient inverter.

All the very efficient Honda portable gennys are producing power this way and yall probabally know as well as I do Honda doesnt fuck around.

Peace&Love :D, Darren

Doug

  • Guest
Re: Peak output and flywheels
« Reply #11 on: August 13, 2006, 03:35:59 PM »
Find me a PM generator that turns slow, a load sencing system that can varry the speed of a stationary engine to match, an inverter with a DC link ( AC-dc-AC ) and do this all under 500 bucks and you will have my attention. Untill then I think the ST head is dollar for dollar the most cost effective way to achieve the desired result.

Yes Darren it would be great to have an inverter generator with a low speed stationary engine. But I don't think anyone could fix it. The failure rate for even the best electronics still leaves you with the probability that the engine will long out live the Inverter and ther guy with an ST Lister combo will have lights on longer even if he burns more fuel.

Last but not least I don't know if the small increase in power is worth the trade off. I still think an ST will more reliably handle a surge than a bank of IGBTs....

Doug

Jim Mc

  • Sr. Member
  • ****
  • Posts: 330
    • View Profile
Re: Peak output and flywheels
« Reply #12 on: August 13, 2006, 04:19:51 PM »
  A single cylinder engine only produces 1 power pulse for every 2 revolutions so there needs to be heavy flywheels to carry the engine through the non-power strokes of the cycle without causing significant slowing of the generator that will cause flicker in connected lights.  This is a prodlem with ANY single cylinder engine and is more pronounced at slower rated speeds and particularly on 4 stroke cycle engines with the longer 'coast' times between power impulses.

Exactly. 

Now there are other options to keep lights from flickering.  The one I prefer is electronic regulation of the output.  Of course the frequency will still vary, but for household loads this is not always a problem.  There is one bugaboo with electronic regulation:  In order to remove the flicker, the electronic regulator has to have a fairly fast response time - faster than the flicker rate of 5 Hz, anyway.  This requires that it be able to rapidly change the current through the field winding.  I've looked at the inductance of the field winding, and have a design of regulator that should work. 

GIII

  • Jr. Member
  • **
  • Posts: 67
    • View Profile
Re: Peak output and flywheels
« Reply #13 on: August 13, 2006, 04:32:21 PM »
Do you think a fero-resonat transformer would work?  I have no idea of their frequency response.
George

Rtqii

  • Full Member
  • ***
  • Posts: 152
    • View Profile
Re: Peak output and flywheels
« Reply #14 on: August 13, 2006, 04:45:11 PM »
If you want stability in a stationary setup: the simplest and most reliable, longest lived solution is to add high speed flywheel mass to the shaft driving the generator.

http://listerengine.com/smf/index.php?topic=896.msg11668#msg11668

Note: this jackshaft is being designed for 2 PTOs. The shaft would be directly coupled to an ST15kw head on the left side.  The clutch engineering I am designing is for a hydraulic pump and would not be needed on a simple shaft designed for frequency & voltage stability and starting difficult loads.
« Last Edit: August 13, 2006, 04:50:27 PM by Rtqii »