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.

 

[SwinnyGG]

Dude friction exists. Heavier parts mean more of it, period. The physics are not complicated at all.

 

[BrianPetersen]

Friction exists - but the contribution from piston inertia increasing the hydrodynamic losses in the con-rod bearings will be minuscule compared to the combustion loading. At high revs, the piston inertia going through TDC between compression and power strokes offsets the combustion loading.

Don't get me wrong, as a general principle, the concept of having pistons being lighter and thus having less reciprocating mass is A Good Thing but it's from the vibration point of view, and the mechanical stress at high revs point of view, and perhaps the slight knock-on positive effect of allowing the whole engine to be a smidge lighter (lighter pistons will allow lighter crank counterweights, and MAYBE lighter con-rods if the inertial loading is dominant over the combustion loading).

Don't expect miracles in terms of BSFC, and don't forget that there are a whole lot of other design aspects that need to be gotten right.

 

[CWB1]

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.

The question is, will this lighter piston result in a small power increase as less work done by reciprocating the piston?

I politely answered your original question after reading the entire thread and said nothing about goals. FWIW, I'm one of maybe 2-3 on this forum with experience in both piston design and combustion development.

Bottom line - A material change alone affects both the dynamics and chemical-kinetics, and you have made both material and geometry changes so making an A-->B bc of C comparison is impossible. Given the high efficiency of modern pistons however its likely you will see a performance loss.

 

[novateague]

I politely answered your original question after reading the entire thread and said nothing about goals.

Telling someone they made an intern's mistake isn't exactly polite...Here is the goal as stated earlier in the thread:

"@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."

FWIW, I'm one of maybe 2-3 on this forum with experience in both piston design and combustion development.

Exactly who I was hoping could explain mathematically the effect of piston mass reduction.

 

[novateague]

Can we agree that lower piston mass will reduce the torque due to inertia?

Can we also agree that the torque resulting from gas expansion is equal for each engine (one with a lighter piston, the other stock)?

And that the total torque generated (not including resistive torques for simplicity) is the sum of these torques?

It seems that the effect of lower reciprocating mass is an increase in the rate of torque. Can anyone confirm or refute this mathematically? Not with conjecture

 

[novateague]

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.

Maybe not on tugboats...In this thread alone there is a video where Mahle used 3D printed pistons (topology optimized) to gain 30 horsepower. Not to get another tangent started - most of those gains came from innovative piston cooling and rpm increase.

https://media.porsche.com/mediakit/porsche-innovationen/en/porsche-innovationen/3d-printed-pistons

 

[BrianPetersen]

Now, integrate those graphs over a complete cycle. In the idealised cycle (neglecting friction), the instantaneous inertial torque integrates to zero over a complete cycle. The gas pressure torque integrates to the net output crankshaft torque when integrated over a complete cycle.

Reducing the mass of the piston reduces the magnitude by which the inertial torque fluctuates above and below zero, but the result of the integration over a complete cycle will be the same: zero.

The results of reducing piston mass are in the vibration, stress, and secondary knock-on effects.

 

[3DDave]

“This has allowed us to increase engine speed"

“An extra 300 rpm equates to around 30 PS more,”

From the Porsche link.

 

[novateague]

“This has allowed us to increase engine speed"

“An extra 300 rpm equates to around 30 PS more,”

From the Porsche link.

Unreal lol.

Not to get another tangent started - most of those gains came from innovative piston cooling and rpm increase.

 

[TugboatEng]

Also, 30 HP is only a 4% increase on that 700 HP engine. When you have to hyper-refine every component to get single digit gains, you're in the mature stage.

 

[3DDave]

Just as long as you know that only RPM increases are responsible for increased HP output.

 

[SwinnyGG]

the result of the integration over a complete cycle will be the same: zero.

This is only true if you neglect friction... which if you want to actually identify the affect, you cannot do.

 

[TugboatEng]

On big engines that's how we calculate power. You can take the measurement by hand. It's called Indicated Horsepower. It's accurate enough to compare engine performance.

 

[GregLocock]

One month ago " The only (I think) piston mass related effect that absorbs power over one cycle is friction". Have we made any progress?

Cheers

Greg Locock


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

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

 

[SwinnyGG]

It seems we've time traveled since this thread is only 9 days old

 

[3DDave]

It certainly seems like a month ago at this point.

 

[gruntguru]

Seems like last year.

je suis charlie

 

[novateague]

Let's ask ChatGPT:

 

[3DDave]

Show us the sources ChatGTP used. There must be a study where the only variable is piston mass. Better yet, there must be a study where the change in operation due solely to piston mass was predicted and then a double-blind test done with pistons of various masses to generate data to compare to the predictions.

Not a bad effort for a Regurgitron.

Check this out:

https://www.physicsforums.com/threads/3d-printed-piston-lower-mass-power.1048704/

 

[TugboatEng]

It doesn't take power to accelerate or decelerate a rotating assembly... It takes work.

The semantics are extremely important here.

I think we did ChatGPT wrong.

Remember, if there is no deflection or displacement there is no work done. Even if the engine is out of balance, a counterbalance shaft can prevent displacement of the engine so that no work is done. You'll notice that many new engines such as cross plane inline 4 and flat plane v8 are now available due to the use of counterbalance shafts. Of course, this has been a mature technology as GM's Detroit Diesel figured it out in 1937 or so.