Pistons (Aluminum vs. Cast) @ 800 rpm (+ & -)

[Horsepoor]

I have a Metro 6/1 setup to run at 650 rpm or 800 rpm by using a two set pulley attached to an ST 7.5 generator head. I know the ST 7.5 is over kill but that is what I had. At least 90% of its projected life will be at 800 rpm because I need the extra power to start various pumps and other equipment. The engine is well balanced and I run diesel or waste jet fuel as the primary fuels. So I was wondering the next time I tear it down:

What if I replaced the cast iron piston with an aluminum piston?

I wonder what effect changing the piston would have upon balance at 800 rpm?

Would an aluminum piston screw up performance if I decided to run it at 650 rpm?

What are the projected positive and negative outcomes at 800 rpm and is this a worth the effort during the next tear down?

[mobile_bob]

if you have a well behaved engine now, in regards to balance, you will certainly aggravate the situation going to an aluminum
piston, unless of course you want to rebalance the rotating assembly.

if it ain't broke don't fix it

bob g

ps. i assume you have the stover wheels?   800rpm is probably too fast for spoked flywheels in my opinion.

[Crofter]

The lighter the piston the less reciprocating force created, so easier to smooth out, But if it is optimally balanced with the iron piston you may have to remove some counterweight or add some opposite to get the smoothest operation with aluminum.

The piston top to head clearance will have to be checked. Piston pin center to top measurements could be a bit different and thus affect compression ratio and piston to valve distance. Valve and piston interference would have to be ruled out. If you had to reset this it would involve messing with base gasket changes and redoing or renewing the O rings bottom of the sleeve.

A lighter piston will have no difference in performance other than in vibration since it neither adds to or subtracts from flywheel effect. The potential work involved though would seem to make a poor idea out of changing out a perfectly functioning cast iron piston.

Edit; Did not see Bob's post before I sent mine off. He has a real good "short answer"; if it ain't broke, don't fix it!

[playdiesel]

If it 'twas me and the only reason I needed to run 800 RPM was to start a couple motor loads I would slow the engine to 650 and add rotatiing mass to start the motors thus saving fuel and wear and tear.  For some reason when it comes to rotating mass people tend to only think about the engine flywheels where also it affects other things often to the minus. The English often added the wieght to the generator shafts where the extra mass is doing it's thing in a direct manner and does not affect the engine balance or stresses. If you have a bit of extra generator shaft and acess to subtable material and a lathe its a pretty straight forward process. 

Just something to ponder.

[LowGear]

I'm really surprised that it doesn't take a horsepower or two to throw that cast iron piston back and forth. 

How much weight are we talking about in pounds?  What's the throw in feet?  X pounds lifted X feet * 650 RPM / 33,000 = HP (right?)

Casey

[Crofter]

I'm really surprised that it doesn't take a horsepower or two to throw that cast iron piston back and forth.  

How much weight are we talking about in pounds?  What's the throw in feet?  X pounds lifted X feet * 650 RPM / 33,000 = HP (right?)

Casey

Most of the inertia that goes into accelerating the piston goes back into speeding up the crank when it decelerates. Kind of like teeter totter or pendulum. give and take. There will however be more losses though due to the angularity of the connecting rod. That creates side thrust, so frictiion losses onto the cylinder wall and that is affected by piston mass.  Probably small difference though compared to the necessary thrust of combustion etc. You would have to look closely too, to see what the relative co-efficient of friction is for cast iron VS high silicon cast aluminum. Another wild card!

[LowGear]

Crofter, 

I want you to think about that again.  It kind of smells like buttermilk and ginger ale mixed together.  I think you're over simplifying the conversion of uppy-downy to roundy-roundy and back to uppy-downy energy or power consumption.  Of course I stand behind my credentials.

Casey

[Crofter]

Lowgear, That drink may have a name; blintz, I think.

I used to be of the opinion that the energy used to create the movement in the piston was lost as well as the energy used to stop it being lost but apparently it merely bounces in and out of storage in the crankshaft with only frictional losses on cylinder and bearings plus miniscule loss into heat of flexing. Approaching the top of the stroke the piston wants to continue at the same velocity but it is tied to the crank pin. The piston momentum tugging on the crank pin which is out of line, actually then accelerates the angular velocity of the crank as it approaches TDC. As the crank passes TDC it has to put that energy back into accelerating the piston downwards but it is returned again at the BDC.  Remember that the energy you put into throwing something up, all comes back when it falls back down (minus air drag of course)

Here is one for you to think on. Would the piston acceleration be the same arriving/departing TDC as it is BDC?

[LowGear]

Would the piston acceleration be the same arriving/departing TDC as it is BDC?

If it did then we could pull the head and harvest some of that perpetual motion stuff.  I guess (that should have been Guesssss) that the forces of gravity would sum each other out.

Going down the piston is pushing half the time and being pulled down the other half (4 stroke) but is always being pulled up.  I'd still rather move an aluminum piston regardless if it were uppy-downy or roundy-roundy.  Your understanding of mechanical energy is superior to mine.

Casey

[Crofter]

Sorry Blintz is the pancake; the drink is a lassi

I pilfer some knowledge from my son. He is into hopping up chainsaws and pretty deep into 2 stroke engine dynamics and dyno work.

[Horsepoor]

Interesting replies, thank you all.  

• I agree on the spoked flywheel rim speed and only did so after careful inspection. But who knows what I may have missed hidden by paint and putty.
  

• I read someplace that the rim speed of a cast iron flywheel should never exceed 60 miles/hr. At 650 rpm I calculate a rim speed of 45 mph and at 800 rpm I get 56 mph. Clearly this an upper limit and I believe this is truely an upper limit. Last year I followed one of the links featuring a lot of pictures from the 1900s of flywheel explosions, very interesting, and worthy of caution.

• My little 6/1 at 650 rpm has no problem starting any single pump or similar device with the over kill ST 7.5 gen head mass, and two step cast iron pulleys (130 lbs) spinning. Cooling system and well tuned engine can sustain 3,100 watts output. The problem is I need to start another device when the first one is running. So at 800 rpm, I get 4,200 maximum output with a 4,000 watt sustainable load. Most of the time, the load runs between 2,500 to 3,500 watts, but there are periods of heavy load.
  
  
  
  • Solution was to push the 6/1 close to the upperlimit at 800 rpm. Now it can start and run a 2 1/2 ton air second story unit which services all three bedrooms and two bathrooms. It has enough spinning mass to also start a 1 1/2 hp above ground water pump with a few lights and some other constant incidental loads totaling about 500 watts in there also. This amazing little engine can just barely handle this intermittent high load when running at 800 rpm.
    

• The reason I push the 6/1 is efficency. I have a GTC 20/2 down rated to 850 rpm driving an ST 15. I have measured 11,200 watts as an absolute maximum output with a sustainable 10,000 load as more reasonable. This is my hurricane daytime, afternoon, and evening machine. My blond wife can run almost anything and I switch off the electic range, stove, and dryer, so she doesnt overload it. Amazingly, it will run and start 2/1/2 ton AC (up stairs), 3 ton AC (down stairs), 1 1/2 hp water pump, electric hot water heater, and reasonable fans and lights: But will not start everything all at once. But at night, I dont want to run this machine at 25% load. That is why I built and pushed my 6/1 up to 800 rpm.
  

I appreicate the comments and input on pistons. I was just wondering if there was a little more to gain next time I tear it down. I could spend $70 to $100 to buy the Al piston with rings, I know how to gap, test, and time the engine for maximum performance. Just wondering but I think I got my answer - not worth it - not broken dont fix - you're getting all you can out of a 6/1 which is really setup more like an 8/1.

[cujet]

There will be a slight, but actual decrease in thermal efficiency with the aluminum piston. It is far more capable of removing heat from the combustion chamber. In fact, cast iron engines are generally more efficient due to less thermal conduction.

Not sure you could ever measure this on a 6/1. However, every little bit helps.