Slow Speed Changfa Project

[mobile_bob]

i forgot to add this
my drive pulley is a cast iron 4 groove B section, and is mounted with a 9"od spacer made of steel
so i would expect that the weight of this combination is about 20lbs over that of the oem pulley?
i would have to weigh everything but i bet i am close i the estimate.

i also have the st7.5 as part of the reciprocating mass, along with another 3groove pulley, the drive hub and element
so if i had to guess the added weight is probably about 75lbs over the oem pulley, but the genhead has its own brgs to help
support the load.

bob g

[veggie]

Roger that !

So, an additional modification will be injector timing.
When the time is right, I will bring up that subject and make the necessary adjustments.

Regarding belt arrangement and tension, the automotive alternator will be taking the place of the Voltmaster AC head.
The Alternator will be charging two 225 amp Trojan T-105's and then feeding a 5KW inverter.
Therefore the belt tension will help support any additional flywheel weight.

The bottom line question is weather the additional of slow speed inertia will assist in the compression/combustion cycle?
My assumption was that the additional mass would reduce the the deceleration pulse during compression and also dampen the acceleration pulse after ignition. Less hammering overall.
(also the main reason for your suggestion to adjusting the timing.)
Do my assumptions hold water ?

veggie

[mobile_bob]

your assumptions are pretty close, however

we must remember that just because there is a delay angle after injection followed by ignition which is evidenced by
a sharp pressure rise (the diesel knock)
the added weight of the larger flywheel can dramatically effect this pressure rise, if the timing is retained at the oem spec
i would expect some severe knocking and damaged brgs or worse in a short amount of time.

while the added mass is a "plus" while coming up on compression, it is a liability on "ignition" if one does not offset the injection timing
to keep the pressures in line with the design of the engine.

but that is easy enough to do when the time comes, just have to add a few thousands of shims under the injection pump.

iirc it amounts to .002" per degree or thereabouts

bob g

[veggie]

but that is easy enough to do when the time comes, just have to add a few thousands of shims under the injection pump.

iirc it amounts to .002" per degree or thereabouts

bob g

Just to be clear.....are we advancing the timing by adding shims? or retarding it ?
I Assume the object is to retard the timing.

veggie

[mobile_bob]

adding to retard the timing

bob g

[veggie]

Ok,

So here are the things which can be modified:

1] Adding flywheel mass to smooth out the pulses at the lower speed.
2] Modify the timing to accommodate the additional flywheel mass and low speed operation.
3] Monitor oil pressure at the lower rpm's to ensure adequate lubrication.

A good start......

veggie

[rcavictim]

Veggie,

I would worry about adding extra flywheel mass with the three bolts used to mount the V-belt shieve.  I suspect you could fracture the bolts.  The torque pulse on the stock flywheel is intense.  I did some experiments a few years ago trying to drive a heavy (~175 lbs.), 20" flywheel end on with a rubber damped coupler from the flywheel of a JD175A engine.  I kept breaking the steel pins inside the rubber pieces and also beating the rubber to pieces in very short run times measured in minutes.

[JLMTECH]

Hi, Veggie,

Your low speed project is interesting. I plan to slow down a 170B Changfa with a design speed of 3000rpm. I hope the following comments help with sizing the flywheel(s).
      
At design speeds the flywheel has a specific rotational energy. This energy slightly increases and decreases in an engine cycle (two revolutions). This rotational energy is determined by the square of rotational velocity (rpm's) and linearly with the rotational mass (called: moment of inertia)

Reducing the rotational speed from 1800rpm to 900rpm is a two to one ratio. A 195 engine runs smoothly at 1800rpm. To keep the rotational energy constant (hopefully retaining  smooth running at 900rpm) the rotational mass must be quadrupled (One-half squared).

To increase the rotational mass. (to have the same rotational energy at 900 rpm) requires an increase of flywheel radius and/or increase in mass.

For example: doubling the radius of the flywheel increases the rotating energy by a factor of four (without changing the mass of the flywheel). Alternately increasing the mass of the flywheel by a factor four (without changing the radius) will increase the flywheel energy by a factor of four.

A combination of increase in both flywheel mass and radius is usually best.

A practical first step to flywheel change to increase the rotational mass is to determine the maximum increase in flywheel radius possible and the maximum increase in flywheel mass acceptable.

The ways to increase the increase the rotational mass are endless. A few ways are below. Increasing the diameter of the flywheel may require raising up the engine mounts for clearance.

          Fit a heavy pulley and bolt to existing flywheel. Contact friction from the force the
          connecting bolts will prevent the pulley from slipping.   
   
          Fit a ring around the existing flywheel. Use a press fit and/or hold in place
          with screws.

          Rap flywheel with heavy copper or brass(much denser than steel) wire. Fix with
          screws.
   
          Fit a large radius steel plate to the existing flywheel.

As a side issue: I understand that decreasing flywheel rotational mass increases loads on the rod bearing for a given rpm. So increasing rotational mass is important for low rpm operation.

Another side issue: at half rpm the imbalances caused by the piston are quarter size. Thus the vibration causing forces are nearly quarter size and the counter rotating shaft might not needed. Also increasing the flywheel rotational mass will reduce the forces on the drive train for the counter rotating shaft.

Your results will be interesting,  Jlmtech                        
      
                           

[veggie]

Thanks rcavictim,

From what you described, it would seem that your fractures may have been a result of flywheel diameter.
20" is a very large flywheel when considering WR^2 and inertia forces.
The Changfa has a 16" flywheel and additional flywheel weight would be kept within this diameter.

Here's an example:
2 flywheels both the same weight but different diameters spinning at the same speed....

          Diameter    Weight    RPM      Centrifugal Energy  Inertia foot-lbs force
Fly#1       14"           80#     1500           16,242 kg         10,443 **                

Fly#2       20"           80#     1500           23,203 kg         21,314 **

** So an increase of 42% in flywheel diameter resulted in a doubling of forces.
    I think the trick to adding flywheel mass is staying within the diameter of the current flywheel.
    If your flywheel was 175lbs, then you had some serious forces at play on those 3 mount bolts.

veggie

[veggie]

Jlmtech,

Thanks for your comments. My calculations on the required additional flywheel mass are falling in line with your suggestions.
My thinking was along these lines....

1] Engine runs very smoother at 1500 rpm, so....
2]Calculate the rotating mass forces in the stock flywheel at 1500 rpm.
3]Then calculate the mass required to match the 1500 rpm forces when the engine is slowed down to 900 rpm.

When I did this I came up with some very large numbers for weight. Flywheel weights that were not realistic for my application.
I may have to fab. a lighter than optimal flywheel knowing that this is an improvement but not the best solution.

PS: I will post my flywheel design shortly and would appreciate any comments and suggestions you may have.

veggie

[veggie]

Here's the preliminary design of the secondary flywheel.
I will do some calculations to establish the desired thickness of the various parts and refine the drawing with more data added.
This design utilizes the sheave mounting holes in the existing flywheel for fastening.
At this point, feel free to critique and comment.

veggie

[mobile_bob]

in theory that looks good, however

the reality is going to be ugly in my opinion

the shear mass of the second flywheel being attached only by the three oem center mount bolts
is just not anywhere near enough, over time they will work loose, shear and you have a real problem on
your hands.

you might consider making the secondary flywheel larger in diameter, and machining a register ledge so that
it fits over the OD of the oem flywheel, then drill and insert dowel pins in several locations after the thing is bolted
up? or alternatively drill and tap to bolt the secondary to the oem flywheel about the periphery

with the ledge register and doweling to take the torsional stresses of the bolts, the resulting connection
between the two flywheel halves would be much stronger and safer in my opinion.

bob g

[veggie]

Based on the comments from Bob, rcavictim, and jlmtech, I made the following modifications...
Any other suggestions before I begin calculating the overall weight required and finalizing a machining drawing ?

(It appears the weight for the flywheel will be determined by the crank bearing carrying capacity.)

veggie

[veggie]

in theory that looks good, however

the reality is going to be ugly in my opinion

the shear mass of the second flywheel being attached only by the three oem center mount bolts
is just not anywhere near enough, over time they will work loose, shear and you have a real problem on
your hands.

you might consider making the secondary flywheel larger in diameter, and machining a register ledge so that
it fits over the OD of the oem flywheel, then drill and insert dowel pins in several locations after the thing is bolted
up? or alternatively drill and tap to bolt the secondary to the oem flywheel about the periphery

with the ledge register and doweling to take the torsional stresses of the bolts, the resulting connection
between the two flywheel halves would be much stronger and safer in my opinion.

bob g

Just curious bob,

Based on the rev.1 design, I wonder if the load on the three sheave bolts would be any greater than a 3 groove sheave driving an ST7 head pulling full 12HP where the 3 bolts must deal with pulse loads as well as the 12 BHP.

The rev.1 flywheel is just rotating mass and caries no additional torque. Agreed, the pulse loads are at a lower frequency and potentially more destructive. But I wonder if those loads exceed that of a 12 HP fully loaded engine?

veggie

[rcavictim]

Just curious bob,

Based on the rev.1 design, I wonder if the load on the three sheave bolts would be any greater than a 3 groove sheave driving an ST7 head pulling full 12HP where the 3 bolts must deal with pulse loads as well as the 12 BHP.

The rev.1 flywheel is just rotating mass and caries no additional torque. Agreed, the pulse loads are at a lower frequency and potentially more destructive. But I wonder if those loads exceed that of a 12 HP fully loaded engine?

veggie

The peak torque, or impulsive or shock torque would be far and away much greater and therefore much more potentially destructive IMO.  I've seen it happen first hand.