3D Printed Piston Lower Mass = More Power? 8

[novateague]

This is a piston I had DMLS printed from AlSi10mg - the geometry is derived from generative design using some basic inputs and a starting shape. It's really just an experiment whether a consumer grade design and print can be bolted in and work for a time.

After machining, it is predicted to weigh ~50 grams vs the OEM 79 grams (1970's Honda XR75). The question is, will this lighter piston result in a small power increase as less work done by reciprocating the piston? Theoretically, the rate at which work is done will be increased?

With the reduced mass, the redline could be increased slightly due to less inertia load, but besides that? Of course, the engine should be more responsive (quicker to spin up) and with a much smaller skirt area, less friction too.

By my calculations, at maximum piston acceleration (~TDC) @ 11,000 RPM, the inertia forces on the piston are about 33% less.

 

[novateague]

@Lou Scannon The real goal of this experiment is to take a consumer grade generative design software (Fusion 360), and 3D print an output design without proprietary alloys (unlike Porsche/Mahle). Then run it in this test rig and see how long it will function.

If the piston holds up well with the acrylic engine cylinder, I'll test in the real engine under load.

 

[novateague]

My question I am trying to ask is, is this line of thinking incorrect? Mathematically is seems that with less interia force resisting the force due to gas combustion, there must be an increase in total force and therefore engine torque.

The equivalent mass of the reciprocating components (piston and some proportion of the connecting rod) will be reduced by a small amount, due to the piston mass being 33% less.

The Inertial Force, FI will DECREASE with a lighter piston.

Because the Inertial force FI OPPOSES the force due to combustion FG, there is a larger positive total force in the direction of FG. The total downwards force propelling the piston and crankshaft has less force resisting it.

More of the combustion pressure is available to drive the crank instead of accelerating a heavier piston. This is not much different than increase power output due to thinner, low tension, coated piston rings in my thinking.

With effectively more force reaching the crank, this in turn will then create increased total torque by some small amount.

 

[TugboatEng]

How can we explain it more clearly: YOU WILL NOT SEE ANY MORE POWER BY SIMPLY REDUCING THE MASS OF THE PISTON.

You're problem is that you're only looking at one side of the equation. f=ma applies to accelerating the piston in the first half of the stroke but -f=ma during the second half. All of the power accelerating the piston gets returned during the deceleration phase.

Always remember, internal combustion engines are a "mature" technology. Any time you think you have a revolutionary idea you are most likely ignoring or ignorant of some very important principles.

 

[novateague]

"f=ma applies to accelerating the piston in the first half of the stroke but -f=ma during the second half. All of the power accelerating the piston gets returned during the deceleration phase."

This makes sense - couldn't see it until then.

This helps visualize it as well:

 

[TugboatEng]

Yes. The next step is to take the area (integral) under the curve, that is that is the work done or the useful work

 

[CWB1]

FWIW, you're also making the intern's mistake of assuming only one variable is changing when in reality you're changing quite a few. Changing material properties and geometry (both internal and external unless you have an OE print to copy) means that you're changing the mass, balance, friction, combustion heat release, heat transfer to the ring pack and pin, not to mention strength/durability. Unless the designer has years of piston experience, IME projects like this normally result in a loss of power and efficiency despite being "better."

 

[gruntguru]

Nice project. The disadvantages of a printed piston can of course be overcome by printing a pattern or a mould then cast the piston.

je suis charlie

 

[enginesrus]

This reminds me of a friend in high school building a vacuum cleaner.

 

[SwinnyGG]

This is a wild thread.

Given the exact same combustion chamber conditions (this is a huge, glaring condition which is highly unlikely in the real world) making the rotating assembly lighter yields a direct increase in shaft power. This gain, in the real world, is going to be very small - small enough that depending on what specific dyno technology is in use, it's very likely to be impossible to measure consistently- but it's there.

For a particular combustion chamber configuration operating at 100% load, the amount of energy available due to each individual combustion event is fixed. If you start at idle RPM and accelerate under any load, the piston velocity on the compression and expansion strokes are going to be different; the energy to create that difference in velocity comes directly from the fuel being consumed. If the piston is lighter, creating that velocity difference takes less fuel.

If you're operating at 100% load, you are still getting the exact same amount of power extracted from each combustion event - but less of that power is being consumed by accelerating rotating components, and more of it is making it to the output shaft and into the load being accelerated.

The difference might become large enough to become measurable if you were to spend big dollars to lighten every moving component; you'd create other problems (ie, NVH) along the way.

 

[GregLocock]

I hope you aren't arguing that fitting a bigger flywheel reduces the output power at constant rpm. The only (I think) piston mass related effect that absorbs power over one cycle is friction. Modern pistons have clever sidewall designs to reduce friction without creating slap.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[novateague]

FWIW, you're also making the intern's mistake of assuming only one variable is changing when in reality you're changing quite a few.

You're also making the veteran forum dweller's mistake of not reading the thread. The goal doesn't have to do with what you mentioned. I repeat, the goal is simply to experiment whether a generative designed piston can be 3D printed and function.

The original question here is what parameters will change in the engine with lighter pistons.

 

[novateague]

This is a wild thread.

I agree lol.

If you're operating at 100% load, you are still getting the exact same amount of power extracted from each combustion event - but less of that power is being consumed by accelerating rotating components, and more of it is making it to the output shaft and into the load being accelerated.

This is exactly what I was trying to explain. It would be nice to see mathematically whether my hunch is correct or not.

I went ahead and plotted these using the actual engine geometry and an assumed gas pressure of 300 psi.

Torque due to reciprocating inertia (piston and con-rod):

Keep in mind, a realistic plot of combustion would be phase shifted and look like the total torque chart I posted earlier in the thread. However, this does show the contribution of lower inertia on the RATE that the torque is produced.

Total Torque = Inertia Torque + Torque Due to Combustion (the gas pressure is equal for both pistons obviously):

 

[SwinnyGG]

I hope you aren't arguing that fitting a bigger flywheel reduces the output power at constant rpm. The only (I think) piston mass related effect that absorbs power over one cycle is friction. Modern pistons have clever sidewall designs to reduce friction without creating slap.

Not what I said at all.

There's a fundamental difference between flywheels (plus crankshafts) and pistons (plus wrist pins/connecting rods) - pistons stop 4 times per cycle; flywheels do not. At constant speed, more flywheel mass isn't going to contribute much to lost power (other than via increased bearing drag, speaking of tiny immeasureable effects).

At steady state though, pistons are still stopping and starting again 4x per cycle. That takes energy. 100% of the energy in the system comes from fuel. Lighter things that accelerate more easily use less fuel (or make more output power for the same amount of fuel consumed)

 

[GregLocock]

Hmmm

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Compositepro]

There is a fundamental principle of conservation of energy, not torque.

 

[gruntguru]

There's a fundamental difference between flywheels (plus crankshafts) and pistons (plus wrist pins/connecting rods) - pistons stop 4 times per cycle; flywheels do not.

Flywheels don't stop like pistons do, but their momentum and kinetic energy fluctuate just like the pistons. In fact the kinetic energy lost or gained by the pistons is gained or lost by other moving parts - mainly the flywheel. (neglecting friction of course)

je suis charlie

 

[GregLocock]

Indeed. The energy you 'waste' accelerating a heavy piston is recovered as it decelerates.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[TugboatEng]

Combustion engines are a mature technology. There isn't much that can be done to improve the components. Today's improvements are from modifying the cycle. Miller, Atkinson, HCCI, etc are modifications of the cycle that work on top of very conventional bottom end components.

 

[SwinnyGG]

(neglecting friction of course)

Yeah... but friction exists

Indeed. The energy you 'waste' accelerating a heavy piston is recovered as it decelerates.

Some of that energy winds up as kinetic energy in the crank and is available to help the piston/conrod go the other way - but not all of it.

 

[TugboatEng]

Where does that energy go? Connecting rod heating? If it were a significant loss, engines with heavy pistons would require connecting rod cooling circuits.