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]

 

[TugboatEng]

Please provide commentary with your picture so we don't have to guess at your interpretation or purpose.

I feel used right now.

 

[TugboatEng]

Everybody talking about losses due to friction needs to Google a "Sankey Diagram". If you find that friction is about 6% of the heat loss, a reduction of 6% is very much trivial. I guess you could compare it to the loss of energy from day time running lights.

https://www.dailymail.co.uk/news/ar...LL-day-cost-drivers-160-year-wasted-fuel.html

 

[3DDave]

A study by - which study? ChatGPT isn't supposed to be correct, it is supposed to be convincing.

I'll be more convinced if an engine is sold as producing more power with no change in rev limit because the pistons are lighter.

But here's the new question - why so much worry when you have the test stand right there? Just run the experiment.

 

[novateague]

Those aren't ChatGPT's studies, they're just ones it queried that came up with papers done on piston mass. I can't judge whether they're accurate or not but it would be highly coincidental the first results mention power/torque increases.

The study results make sense if you don't overthink it. Work done at faster rate equals more power. Work can be done faster with lighter reciprocating components, bottom line.

This was also shown in the plots I posted comparing identical engines with different piston masses.

 

[TugboatEng]

Work goes both ways. It can both add and subtract from the power of the engine. The point is, that regardless of the weight of the pistons the work required to accelerate it BDC to mid stroke gets returned to the crankshaft by TDC. If you look at this from a single variable standpoint you'll find that piston mass contributes nothing to the power output of the engine. As SwinnyGG said, that is entirely dependent on the amount of fuel burned.

Now, there are optimizations that can be made around a lighter piston. Most notably, the engine can spin faster which allows for more fuel to be burned.

But why bring AI here when it has obviously come to the wrong conclusion?

 

[novateague]

The key is that the work is being done faster.

But why bring AI here when it has obviously come to the wrong conclusion

Lol, the comprehension and confirmation bias on this thread is abysmal. My question to the AI was, "Studies done on piston mass".

It just so happened that studies done with "piston mass" keywords found power/efficiency increases due to piston mass.

 

[novateague]

Here is probably the real mechanism behind it: lower piston mass can increase the mean effective pressure, according to ChatGPT

Reciprocating Inertia: Reducing the mass of the pistons can decrease the reciprocating inertia of the pistons, which means the pistons can accelerate and decelerate more quickly. This allows for a more rapid combustion process and an improvement in the MEP.

Combustion Duration: A reduction in piston mass can also decrease the combustion duration. As combustion duration is reduced, the pressure inside the cylinder increases, resulting in an improvement in the MEP.

 

[3DDave]

Love the screen grabs from your phone. Can't copy paste so I'll wait to see a video of your piston in that test setup.

Until then, bye.

 

[SwinnyGG]

If you find that friction is about 6% of the heat loss,

You wouldn't find that.

One source of many:

https://www.mdpi.com/2076-3417/12/4/2155

Claims (with significant data if you read the whole thing) that piston/cylinder wall friction and crankshaft bearing (both rod and main) friction are responsible for consuming up to twelve percent of indicated power. That's a LOT.

So the question then is, what is the contribution that piston mass has on piston/cylinder wall friction, and rod/main bearing friction.

That contribution is not zero. Do the math. It's very, very small - but not zero. Which is what I've been saying this entire time.

 

[LionelHutz]

I recognized you have posted all along that the effect does exist but is so small it'd be hard to even measure.

I didn't believe it was mentioned, but less piston mass means less counterweight mass and that does mean more output shaft power from an accelerating engine.

 

[TugboatEng]

But what use is more power output from an accelerating engine? Once the engine is connected to a transmission and driving wheels, the acceleration of the crankshaft rotating assembly also becomes a very trivial consumption as angular acceleration of the crankshaft is also low.

 

[novateague]

the effect does exist but is so small it'd be hard to even measure.

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

Once the engine is connected to a transmission and driving wheels, the acceleration of the crankshaft rotating assembly also becomes a very trivial consumption is angular acceleration of the crankshaft is also low.

We mentioned before that in this case, the output is the flywheel.

 

[3DDave]

When is your test starting? Is the finish machining done yet?

 

[TugboatEng]

Why is output during acceleration of the flywheel so important if it's not connected to anything?

You're creating a specific scenario to give a result that doesn't have any relation to the actual use of the engine.

 

[novateague]

This test should be done in the next month or so. The machining has its own challenges...

Why is output during acceleration of the flywheel so important if it's not connected to anything?

You're creating a specific scenario to give a result that doesn't have any relation to the actual use of the engine.

Remember, this test is only to experiment with a 3D printed generative designed piston. It's a secondary curiosity as to what effect a much lighter piston would have on engine parameters, which is the question I asked here. I don't have a high-resolution small engine dyno, nor a data logging tachometer to really test for that anyway.

You're saying that lighter reciprocating mass has no effect once the driveline is attached. Have you changed pistons on a 140lb dirtbike and tested it to be able to make that statement? If not, what calculations have you done?

Anyone can say their opinion on a forum, but without proving it, it's just conjecture.

 

[SwinnyGG]

You're creating a specific scenario to give a result that doesn't have any relation to the actual use of the engine.

You're ignoring the actual question. No one ever asked if lighter pistons were worth 500% gain in brake power and 4 seconds of ET.

The question was:

In theory, does making the piston lighter yield more output power?

'More' just means 'more'. It doesn't mean 'enough to satisfy your arbitrary conditions' or 'enough to make it worth the dollars spent'.


The answer to the original question is yes. Period. It's binary.

 

[TugboatEng]

I'll reply with a question as well, what happens to the power output of an engine when I increase the mass of a piston by applying a low friction coating to the skirt?

 

[gruntguru]

You're ignoring the actual question. No one ever asked if lighter pistons were worth 500% gain in brake power and 4 seconds of ET.

Actually the OP question was -

. . 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? . . .

The answer is NO.
There is ZERO work done reciprocating the piston - ZERO.
There will be a small power increase due to reduced friction.


je suis charlie

 

[TugboatEng]

Emissions kind of mucks things up but the last two engines I had to make BSFC comparisons for were John Deere 6081 and 6068 engines, Tier 2 and Tier 3. At constant speed, the 6081 engine has substantial better BSFC numbers for all power levels above 25% (the charts for both engines did not go below 25%). The 6081 has a substantially heavier piston and connection rod.