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]

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?

The piston-related friction losses are due to.. friction. So by adding .0000005 gram (or whatever) of coating, you impart a nano-level power loss. But by reducing friction you make up that loss with a larger positive delta.

I'm sort of shocked this has to be explained. It's a very simple concept.

There is ZERO work done reciprocating the piston - ZERO.

Not true. Pistons do not move themselves; 100% of the piston's kinetic energy is not turned into crankshaft kinetic energy.

There will be a small power increase due to reduced friction.

....yup.

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.

This has zero impact on the discussion. The point I've been making this entire thread is that piston mass is an extremely small but non-zero factor in brake power for a given BMEP. There are about 5,000 other factors that would also be terms in the BMEP-to-shaft-power transfer function; 4,995 or so of those are more influential than piston mass; but that does not mean that the piston mass effect is equal to zero.

 

[gruntguru]

There is ZERO work done reciprocating the piston - ZERO.

Not true. Pistons do not move themselves; 100% of the piston's kinetic energy is not turned into crankshaft kinetic energy.

That is not what I said.
The net work done reciprocating the piston per cycle is ZERO. Every Joule used to accelerate the piston is recovered when the piston decelerates. Absolutely incontrovertible.

je suis charlie

 

[TugboatEng]

gruntguru, SwinnyGG is right that there is work done reciprocating the piston. There is also work done by gravity on the same piston. As engineers, when defining a problem we do have to make it simple enough to solve. We do that with assumptions. We assume that the increase in friction due to the mass of the piston or the weight of the piston by gravity are so insignificant they don't need to be included in our calculations. Arguing about trivial contributions is very freshman...

 

[novateague]

Free research papers on piston mass ONLY are hard to come by...

Just came across a very well-done study that showed the dynamic effects of a lightweight titanium con-rod and wrist pin (MXRR) vs Stock. The wrist pin is 100% reciprocating, and the connecting rod is some ratio of reciprocating vs rotating; so at least some of the conclusion applies to pistons as well.

"The OEM piston rod mass was 173 g while the piston and pin masses were 158 (including rings) and 34 g, respectively.

MXRR model presents an I-shaped rod design made of a Grade 5 titanium, which enabled a lower rod mass (125 g). The stock piston was used, but the titanium rod had lower mass (27 g). The total weight saving on the titanium versus the OEM rod was therefore 55 g."

Here are some of the findings that apply to this thread:

"In phase 2, maximum MXRR crankshaft acceleration is in average 5% higher compared to the OEM engine. The difference in acceleration increases up to 9000 r/min due to more available engine torque. In phase 4, the difference is 8%. This is not as obvious since available engine torque drops 20% between 9000 and 14,500 r/min as seen in the HONDA CRF250R torque curve in Figure 7. The MXRR accelerates faster causing a faster drop in available torque which should decrease the difference. However, the OEM reciprocating mass requires more effect to accelerate/decelerate as shown in the KPI-8 section. The net effect is higher engine acceleration for the hole power band when lowering the connecting rod mass.

Through FTB, it was possible to clearly demonstrate the importance of minimizing the reciprocating mass of connecting rods.13 The kinetic energy variations were calculated for the OEM steel and MXRR titanium rods as shown in Figure 21. 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).

The peak power required to accelerate the OEM rod at constant speed (9000 r/min) was 1.1 kW while the MXRR titanium rod only required 0.85 kW (Figure 23).

Motor power (KPI-9)
Just before 9000 r/min, the dynamometer brake was switched on and the output power was measured for both engines as shown in Figure 24. The left curve shows that the MXRR crank is accelerating faster to 9000 r/min due to less reciprocating mass and hence has a quicker throttle response. The MXRR crank therefore initially produces more output power (1.5 kW). The difference in output power reduces to 0.4 kW when constant speed is reached (the different inertias have no effect at that point).

I attached a PDF version and here is a link to the study:

https://journals.sagepub.com/doi/10.1177/1687814017726921?icid=int.sj-full-text.similar-articles.1

Dynamic test bench for motocross engines
Terje Rølvåg and Matteo Bella
Advances in Mechanical Engineering 2017 9:10

 

[TugboatEng]

Research papers on piston mass only are hard to come by because pistons mass has a trivial contribution not worth researching.

 

[novateague]

Research papers on piston mass only are hard to come by because pistons mass has a trivial contribution not worth researching.

You haven't added anything to this conversation, so you might as well bow out. You sound like you do more tugging than engineering lol.

If you had even skimmed that post, you'd have seen that reciprocating mass (pistons are included in this) does have an effect on power. 55g less reciprocating mass increased the power by 1.5kW (2hp) on a 41hp engine.

This is just the first FREE paper I found dealing with reciprocating mass with engine performance.

 

[TugboatEng]

None of us are hear to pay for your education. We only offer free knowledge. Perhaps it's your time to bow out.

Don't add words to my quotes please.

 

[novateague]

None of us are hear to pay for your education. We only offer free knowledge.

I would think you're the one being educated "hear". Again, you nitpicked an unimportant sentence and made a useless comment.

Please don't "contribute" further unless there is actual substance.

 

[GregLocock]

On an acceleration test lighter pistons will increase the hp, because you have a smaller flywheel in effect. But on a normal fixed speed test, or a slow accel, the only effect they'll have is on sidewall friction, which may increase or decrease depending on the skill of the designer.

Cheers

Greg Locock


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

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

 

[3DDave]

I have enjoyed reading the blog from wiseco.com, a company specializing in aftermarket pistons. You might go over it to gain a better understanding of how complex the manufacturing considerations and the dynamics are in the operation of a piston.

The skirt area is a red herring for piston mass. One can eliminate the skirts completely and make a piston of similar mass.

Q1: The question is, will this lighter piston result in a small power increase as less work done by reciprocating the piston?
A1: No. There is no work done "reciprocating the piston."

Q2: Theoretically, the rate at which work is done will be increased?
A2: Transient acceleration would be affected, but not steady state. Note that some of that change alters the ability of the engine to run as one can see if the flywheel, the largest component in transient response, is removed.

Q3: With the reduced mass, the redline could be increased slightly due to less inertia load, but besides that?
A3: Only if all other components are able to withstand the increase in redline.

Ignoring power increase due to higher RPM, just purely the reduced work on the piston; Here's my line of thinking and correct me if I'm wrong:

F = ma
The force from combustion pressure is not changing. The reduced piston mass will increase the acceleration of the piston.

a = dv/dt
The increased acceleration means it will hit higher velocities along the stroke.

P = fv
If the combustion force remains the same and the piston velocity increases, Power should also increase?


If I dyno tested between the 2 pistons, the power/torque made at each RPM should be very similar (if the 3D printed piston is machined well) but maybe the RATE of horsepower will change?

The pressure in the combustion chamber is not only accelerating the piston - one needs to include the reflected mass of the entire power train.

Check this out - " Before the test run was executed, the crankshafts were balanced in a separate FTB model. " The improvement in that study is also due to changing the masses of other components besides the crank. Seems like this was mentioned in this thread - "At the lower stress the crank could lose some weight, certainly from any counterweight."

I don't understand "The 22% peak stress reduction for the MXRR titanium rod is achieved due to a better stiffness-to-mass ratio" as the stiffness to mass ratio of most metals is relatively constant - specific modulus is identical between steel and titanium and slightly higher for aluminum.

You quoted the part you liked, leaving off the part that is important:

The peak power required to accelerate the OEM rod at constant speed (9000 r/min) was 1.1 kW while the MXRR titanium rod only required 0.85 kW (Figure 23). This energy is taken and returned to the crankshaft at TDC and bottom dead center (BDC) and hence not representing a loss in itself. However, due to structural damping and friction effects, the MXRR rod will transmit more output power and less vibrations.

One is looking at the far smaller fraction lost to structural damping and friction. The delta above is about 1/3rd HP, and damping and friction are likely 1% of that, so 1/300th of a HP difference in losses; probably on the same level as eating a burger before the race adding weight to the rider.

So, if all you do is change the mass of the piston, vibrations will increase and there will be greater damping and friction losses. Contact the authors of that paper if you want mathematical support. It's far more complex than f=ma.

 

[TugboatEng]

My local motorcycle shop used to build the race bikes for Baja.

Wiseco? I wouldn't run those in my lawn mower

He had worse things to say about Kibbelwhite.

They found the best reliability using off the shelf Honda parts. I wouldn't count in Wiseco being experts in piston design. They're mostly piston copiers.

 

[Brian Malone]

Very interesting thread! I have picked up some great insights on internal combustion engine operation that I never really thought about, especially, the piston/con rod/crankshaft/flywheel system, and the limiting return on piston mass as related to total engine power and acceleration. One acronym that is being being used that needs definition: FTB. What is this? To me, FTB is Franchise Tax Board.

Novateague good luck with your piston project. Obviously, you have some development money available, and some expertise with combining standard machining with newer 3D printing tech and generative design. I hope you are able to continue your project to completion of the test runs. Your data will be the true source of your research paper that you can decide to keep as private technical knowledge for your TRO product line or put out as public info.

Love the choice of using an XR75 engine as a test bed. Back in'74/'76 my brothers and I raced MX on Honda CR125s and the early XR75 - they were all groundbreaking machines.

 

[3DDave]

FTB => "FEDEM test bench," from the paper.

 

[Brian Malone]

3DDave thanks!

 

[SwinnyGG]

The net work done reciprocating the piston per cycle is ZERO. Every Joule used to accelerate the piston is recovered when the piston decelerates. Absolutely incontrovertible.

For the one millionth time..

This entire thread is about an effect so small that in order to even attempt to observe it, you cannot disregard friction.

What you're saying is only true if you disregard friction. Period.

 

[novateague]

I have enjoyed reading the blog from wiseco.com, a company specializing in aftermarket pistons. You might go over it to gain a better understanding of how complex the manufacturing considerations and the dynamics are in the operation of a piston.

You may also want to check out Cosworth, Arias, CP, Mahle after you're done at Wiseco; I have.

You can make things as complicated or as simple as you want. I believe there was a Kiwi fellow who cast his pistons in tin cans. Burt Monroe ring a bell?

Personally, I'm using this multi-million dollar ellipse cam grinding machine

One is looking at the far smaller fraction lost to structural damping and friction. The delta above is about 1/3rd HP, and damping and friction are likely 1% of that, so 1/300th of a HP difference in losses; probably on the same level as eating a burger before the race adding weight to the rider.

Mostly scientific wild ass guesses on your part.

So, if all you do is change the mass of the piston, vibrations will increase and there will be greater damping and friction losses.

True, but will they entirely counterbalance any gains made? None of us know for certain.

Contact the authors of that paper if you want mathematical support. It's far more complex than f=ma.

I'll consult my algebra professor first, then the authors

 

[novateague]

Try to ignore this one 3DDave. Pay no attention to it, nothing to see here...

 

[novateague]

Novateague good luck with your piston project. Obviously, you have some development money available, and some expertise with combining standard machining with newer 3D printing tech and generative design. I hope you are able to continue your project to completion of the test runs.

Thanks! If I had the dinero, I'd actually test for performance increases. Just going to run it and see how it goes...

Love the choice of using an XR75 engine as a test bed. Back in'74/'76 my brothers and I raced MX on Honda CR125s and the early XR75 - they were all groundbreaking machines.

Air-cooled, small piston, low power - it gets no better for testing. Awesome, those were the beginnings of MX!

 

[3DDave]

I did see it and the description:

The difference in output power reduces to 0.4 kW when constant speed is reached (the different inertias have no effect).

No kidding, lower inertias store and return less kinetic energy as seen during transient conditions. That's why engines with really low component inertias tend to have really poor idle characteristics compared to those with higher ones. Most engines are designed to operate across a useful range, and it sucks to have the engine die whenever the throttle closes.

Have you calculated the amount you need to grind off the crankshaft counterbalance to keep the motor from shaking itself off the table?

That's a lot of stringy chips for a grinder.

 

[novateague]

No kidding, lower inertias store and return less kinetic energy as seen during transient conditions.

Look closely 3DDave, you might've missed another observation...

The engine with less reciprocating mass has higher peak power AND lower dips in power (during acceleration obviously).