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]

Porsche/Mahle have done this exact thing for the GT2 RS:

https://youtu.be/ztWsivHGL54

 

[TugboatEng]

Remember that work becomes heat so if there was work being lost due to "reciprocating the piston" then the connection rods would get hot. I don't think you'll find this to be the case.

 

[3DDave]

It will lower some loads on the crank - that will either lower the stress or allow a higher RPM. At the lower stress the crank could lose some weight, certainly from any counterweight.

AFAIK the power increase would be from allowing the motor to run at a higher RPM and therefore burn fuel faster, but then te valve train has to keep up as well as the ability to get more air in and more exhaust gasses out. I don't know if it will make a big difference in revving responsiveness unless many other components also lose weight.

 

[EdStainless]

And given the totally unknown fatigue properties (will it match a casting?) the stresses at the pin could become an issue.
Typically, the biggest benefit from lighter pistons is faster acceleration.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[novateague]

@TugboatEng
That work done by the connecting rods sometimes becomes light...when they punch through the block

 

[novateague]

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?

 

[TugboatEng]

The piston speed is fixed in relation to the crankshaft. Decreasing piston mass does not increase acceleration or speed of the piston.

In order for the piston to go faster the entire vehicle must go faster. I think you'll find the mass of the piston to be quite insignificant compared to the total mass of the vehicle.

 

[EdStainless]

What is the mass of your rods, crank, flywheel?
If those have all be minimized such as race parts, then you might see throttle response change.
If those are all standard parts, then there is no chance that you will see a difference.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[novateague]

A piston accelerating to and from a stop 22,000 times a minute is actually a huge amount of work. You're saying a light piston will not allow the crank to rotate faster, as miniscule as the change might be?

Yes, of course, there are many components you can cut weight from. Wrist pins, crank pins, dampers, flywheels, valves...let's focus on the piston.

I can't imagine reducing the piston mass by 33% wouldn't produce a theoretical difference. I'm not claiming the dirtbike will run 10's in the quarter mile now!

 

[novateague]

If less work is being done to move the piston back and forth, what happens to that work?

 

[TugboatEng]

When the piston is accelerating it is taking energy from the crankshaft. When it is deceleration it is returning energy to the crankshaft. These two balance each other out.

Something to keep in mind, when the engine is loaded, the speed of the crankshaft is relatively constant so inertia doesn't have a big effect on anything.

 

[TugboatEng]

A side story, in the Early 1990's International sought to design a camshaft-less diesel engine. They used hydraulic actuation for fuel injectors and valves. This lead to the very successful hydraulic electronic unit injector or HEUI that was used on many engines, most notably in Ford pickups, for the next two decades.

But what about the valves? International found that with a camshaft, the work required to compress the valve spring during lift gets returned to the cam as the valve closes. With the hydraulic actuation the energy to compress the spring is lost. The power requires to run the valve train was too high and hydraulic actuated camless valve train failed as a result. Koenigsegg has supposedly overcome this issue with their pneumatic system but I don't know how it works.

 

[novateague]

Interesting about that International project. I don't know much about the Koenigsegg Freevalve system, but I know F1 and MotoGP use pneumatics as well.

If less energy is used by the crankshaft/flywheel to accelerate the piston, what effect does that have down the line? The crank and flywheel must be able to release that energy slightly faster, correct?

 

[BrianPetersen]

Pneumatic valves in F1 and MotoGP still use conventional camshafts, they just use what amounts to an air cylinder as a substitute for a valve spring (less inertia in the valvetrain allows higher revs without valve float).

Re the lighter piston, yes, it will require less energy from the crankshaft to accelerate the piston but also return less energy to it at the other end of the stroke. That bit cancels out in terms of energy consumption or "power loss" (which is theoretically net zero) but allows the rotation of the crankshaft to be more uniform through the engine's cycle. If the crankshaft has counterweights that offset part of the weight of the piston (for balancing/vibration purposes), you could cut down the counterweights a little, so the engine as a whole could have less flywheel effect and less vibration and be a smidge lighter. It will rev faster, and come down off revs faster.

In the particular engine referred to by the original poster, that engine is not a high performance engine to begin with, and its rev capability and power output are limited by other factors.

 

[novateague]

So, it's really just the crankshaft RPM that will see a measurable result of lower piston mass - A more "responsive" engine and willingness to rev.

In the spirit of beating a dead horse, having a higher rate of angular acceleration of the crank should allow the engine to reach the powerband faster, even if it is an infinitesimally small amount?

 

[TugboatEng]

The crankshaft RPM is directly linked to the wheel RPM or vehicle speed through the transmission. Crankshaft speeds don't change quickly during normal acceleration. The willingness to rev is unimportant in terms of propelling the vehicle as the vehicle must also be willing to accelerate.

A lightened crankshaft may benefit acceleration in that it allows faster matching of revs for gear shifting, but that is all.

Otherwise, high rates of angular acceleration can occur within the crankshaft which tends to break things. That's why engines with significantly long crankshafts have torsional vibration dampeners.

 

[enginesrus]

It won't help produce power when it is destroyed, BMEP will help produce more power, small engines use RPM's to produce more power and that piston will not hold up if it is made from the wrong materials.

 

[Lou Scannon]

It is important to consider what is the mission of the engine in question. Is it to win 1/4 mile or similar type of acceleration race, some kind of circuit race, or simply to produce efficient and reliable power at a (fairly?) steady rpm, or...? The answer has a big bearing on what the important parameters are for optimizing the piston mass (and all rotating/reciprocating masses and inertias within the vehicle).

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[novateague]

With the assumed cylinder pressures and inertial loads, the FEA simulations showed high factors of safety with the additive alloy. But with prints you have possible porosity, poorly melted/adhered layers that may cause fatigue issues.

Generative design starting shape is in green, the red solids are the "obstacles" where material cannot be placed.

Material properties for AlSi10Mg: