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]

I think we all get that in theory, reciprocating the piston, wrist pin, and connecting rod is net-zero work done - this is due to equal amounts of kinetic energy being given to and received back from the crank assembly over 360 degrees of rotation.

This does not seem to mean that there is no effect on engine performance according to the study referenced in the thread. Otherwise, why would the study mention this effect?

The kinetic energy was filtered with a 100 Hz low pass filter to remove energy fluctuations and visualize the energy (work) that is required to bring the MXRR (95 J) and the OEM (125 J) connecting rods up to maximum speed (14,500 r/min). Compared to the OEM rod, the MXRR titanium rod requires 24% less energy which has a major impact on the throttle response and gas consumption (Figure 22).

 

[novateague]

The power shown in that chart is not BRAKE power. You will get a similar result by lightening the flywheel but zero increase in BRAKE power. See my post above.

Yes, a steady state dyno won't show increases in BRAKE power because the reduced reciprocating inertia isn't being accelerated. But I don't see why this matters. That extra initial 1.5 kW (2hp) does have an effect during acceleration. The engine will be used dynamically on a dirt bike, not sitting at a constant speed.

 

[novateague]

This thread is definitely a strange read. I think most of us agree on concept but disagree on terms so we're arguing for nothing. Regardless, it is educational. I appreciate a good argument.

It's like the new "torque vs HP" argument - although I think that one's been resolved

 

[SwinnyGG]

How does friction and force relate under fully hydrodynamic lubrication conditions? This is the case at all times except the few degrees around TDC and BDC

Hydrodynamic lubrication doesn't mean drag drops to zero, or anywhere near it - it just means less wear.

The simplest function is:

COF = [N(rev/s) * absolute viscosity] / pressure

Once you have that, COF*normal force gets you drag, just like any other time.

If the fluid is undergoing shear (which oils in engines often are) then the drag goes up and the calculation is much, much more complicated.

 

[LionelHutz]

novateague - In case you haven't realized by now, TugBoatEng Is nothing more than a forum troll. A know it all asshole who will argue anything just to stir the pot.

 

[LionelHutz]

Without testing or calculation (or even research), how can you say that is true?

Why are you doing the horseshit practice of quoting part of one of my sentences so you can apply a completely different context to it?

 

[CWB1]

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

Assuming your only independent variable is mass, lighter pistons mean either lower density material or less material which almost certainly means higher heat transfer affecting combustion. I'd wager changing the combustion has a larger negative effect on power and transient response than frictional improvements for most any modern stock engine. The exercise most every OE did in recent decades was comparing cast aluminum to similar forged steel pistons, which showed steel being preferrable for the thermal properties as well as durability.

Mathematically every independent variable has quite a few dependent variables in engine design, and dependent trends rarely improve/worsen together. The classic if-->then assumption makes for good sales bc the public likes simple, even if its commonly wrong. If-->then is terrible engineering tho bc its not reality. Regardless, I'd caution against publicly available papers on combustion not published via SAE as they tend to be more advertising nonsense than fact.

 

[TugboatEng]

Our engine supplier recently switched to forged steel pistons. I believe Mahle Monotherm is the brand. The previous was an articulated piston with steel crown and aluminum skirt. I don't know how the weights compare.

 

[enginesrus]

A thread I finally agree with SwinnyGG on. And great posts.

 

[novateague]

For anybody interested, I finally got around to machining the piston on a harbor freight lathe.

I posted the pictures here:

https://www.practicalmachinist.com/forum/threads/line-boring-metal-3d-print.408998/page-4

Went to get advice from that forum and ended up getting the usual, BAHHHH it's impossible...put your bets in now for how long it'll survive in the engine.