different piston design for IC engine

[trumpy81]

Entropy455 Your tales of http://www.homemodelenginemachinist.com/members/entropy455the Aluminium rods etc... take me back to the '70's when most of the local hot rodder's thought big carbs were best. There was so many sick running V8's and inline 6's getting around at that time is was quite funny.

Your point about trade offs is also well taken.

 

[Niels Abildgaard]

niels

thats exactly the type of idea i had, nothing overly engineered, but reduces the weight and friction of the piston, allows for better response on revving the engine and possibly better performance too.

have you at all tried incorporating that design into an engine as of yet?

Hello Camnefdt

It was just an CAD experiment.
Somebody published drawing for a new Mercedes Smart Diesel engine, Diesels have much higher maximum pressure (push in rod) and not very great piston accelerations in top dead centre (pull in conrod) and the scheme could be wortwhile for singlecylinder genset.Not F1 where piston and rod will fly apart very soon.
The two conventional looking pictures are based on the Mercedes engine.
Happy Study and have You mastered to calculate piston accelerations yet?.
They are obscene in formula one.

 

[Goldflash]

Have a look at this file for working out Piston acceleration at different crank angles/ Rpm etc
You can play with the Inputs like Stroke , bore , and Rpm . It does not calculate piston side thrust at different crank angles but its a fun programme to play with
I just wish I could remember where I got it from.

View attachment Piston Acceleration.xls

 

[stevehuckss396]

I'm sure another thing considered by engine builders is the ease at which an old style barrel piston is made.

 

[camnefdt]

hi guys, again thanks for all the replies

just as a reminder, i am in no way trying to recreate the F1 piston in a scale engine with the same type of revving and speeds and everything, all i am trying to do is find a more efficient alternative to what is conventionally used.

such as these







these are average run of the mill pistons of which the one set was used in buiks almost 30 years ago as the standard production piston. but they are the kind of style and design that i am interested in trying for my engine design.

Please don't get too focused on the F1 aspect of the photo that i posted in my first post, it is just the first photo i got to for an example and i completely understand that it is an extreme in terms of the design of the piston.

some of the aspects i am looking at achieving for my engine is up to a max rev of 8000 RPM, a more 'exotic' sound to the V8 (using a flat plane crank) and i want to attempt using fuel injection with super charger.

I am in no way trying to achieve the performance of the F1 standards.

 

[petertha]

...thats exactly the type of idea i had, nothing overly engineered, but reduces the weight and friction of the piston

I wonder what the friction gain would amount to by simply lopping off a lower portion of the piston skirt? I would think the bulk of the piston OD is running moreso in clearance to cylinder ID anyway vs. the contact area of ring set responsible for sealing? (ie much closer fit, more rubbing contact therefore friction). Ive never really seen a proportional friction breakdown map quantified like: 60% rings + 40% piston = 100% friction, but it would be interesting.

I'm familiar with ringless pistons in (RC) high-perf applications, buts thats a different kettle of fish. Although, there may be some learnings there too... 'extra' piston material can't be shaved too far off certain miniumums before the piston shapes deform under load/temp & the magic clearances are lost very quickly. Interesting subject, good luck.

 

[Entropy455]

Drag force on the piston skirt is not a significant factor. There are much more appreciable design challenges in getting a V-8 to safely turn at 8000 rpm. For example, achieving a proper dynamic balance of the rotating assembly, and the ability to dampen crankshaft torsional harmonics. Also, keeping the lifters in contact with the cam-lobes, and preventing the valves from bouncing off their seats (aka valve float) requires high valve spring pressures. This means running case-hardened cam-lobes and lifters, with a positive-pressure oiling system, or running an all-out roller cam with case-hardened needle-bearing rollers.

Normally you’ll snap a connecting rod, long before you pull a wristpin out of a piston.

Piston failure is often the result of running lean, or running too much spark advance, or inadequate fuel octane, etc - - - not from running a piston with a heavy-duty skirt.

 

[Entropy455]

A bit of engineering discussion:

Piston friction force is equal to the normal force along the friction surface (cylinder wall), times the friction coefficient of the joint.

The friction coefficient is pretty much a function of the lubricating oil. Note: piston manufacturers are now applying coatings onto piston skirts. These coatings are more of a wear-preventative measure, than a friction reducing measure. The friction coefficient has to do mainly with the shear forces within the oil film that is preventing metal-to-metal contact.

The normal force is primarily a function of rod-stroke ratio. Or more specifically, the smaller the rod-stroke ratio, the greater the piston will press into the cylinder wall. Thus longer rod-stroke ratios will directly decrease cylinder wall friction, and shorter rod-stroke ratios will directly increase cylinder wall friction.

Decreasing the piston skirt area will directly decrease the overall surface area of the oil that's preventing metal-to-metal contact. This means less friction, which means less power losses. It also however places higher shear-stress pressures on the remaining oil film, resulting in higher fluid-bearing stress on the piston skirt, and the cylinder wall. This directly increases the rate of cylinder wall and piston skirt wear. It also results in higher oil-film temperatures, causing accelerated thermal breakdown of the oil hydrocarbon chains.

Increasing combustion pressures will also increase the normal force along the cylinder wall. More cylinder pressure means more torque. It can be shown that crankshaft torque is ultimately a direct result of cylinder side-loading during the power stroke.

Increasing engine RPM will also increase cylinder wall drag forces, as the cylinder wall must “force” the connecting rod to change directions - as is stated within Newton’s Laws of motion.

Point being – there are literally dozens and dozens of engineering examples where engine longevity is directly sacrificed in the pursuit of increased “per-cubic-inch” power production.

On a related note: I’ve seen guys purchase 350 cubic inch small-block chevy engines, and then proceed to dump thousands of dollars into aftermarket performance parts. In the end, they have an engine that will make 1.25 horsepower per cubic inch - but it is octane sensitive, it runs with a narrow & elevated-rpm power-band, and it has elevated stress on pratically every moving part.

Whereas a wise man would purchase a 1970s Caddy 500 engine, do a low-budget stock-type rebuild, and make more overall horsepower, with a bone-crushing torque curve. . . . Saving thousands of dollars, and running for tens of thousands of additional miles between rebuilds. . .. . .

Point being - if you are not displacement limited, increase cubic inches first – then evaluate power-adding engine design modifications. . . . If you want to turn 8000 rpm "just becasue" then go for it. Otherwise slow things down, and stick with heavy-duty parts.

 

[wildun]

Just a couple of points, correct me if i'm wrong.
Could the low profile pistons be a help by enabling the use of longer conrods which will in turn reduce the sidethrust on the piston at mid stroke and also help to keep engine height to a minium when using these longer rods?
Also 8000rpm doesn't sound enough to obtain reasonable power from a model V8, after all the usual racing V8 ( big engines) today must surely be approaching that figure!
As an example, the little Honda four stroke 125cc 5 cylinder and the 50cc two cylinder GP motorcycles of the mid nineteen sixties revved to 22000 rpm, (quite extreme of course) but true nonetheless.

 

[Lakc]

The internal combustion engine in design and practice. Charles Fayette Taylor

http://books.google.com/books?id=mX1-OJBQ6ngC&printsec=frontcover#v=onepage&q&f=false

Read it, believe it, but dont outbid me on ebay when I finally find a copy with money in my pocket. I have had the pleasure of a copy at my disposal from a good engineering library in the past. Its indispensable when you want to get serious about designing your own engines. Since it was published it has been the bible of the industry.

It mentions piston speed being the basic design anchor for any engine power requirement. This makes sense, as its mentioned piston assembly friction is almost half of the whole engine, and speeds are generally 5-15 meters second.

This is the piston for my last engine. Its a compromise of the new and the old. Dia .980" and height about .6, 15degree dome with three .040 reliefs. I had plenty of room so no need to cut the skirt, but in a nod to more modern design I did move the rings and pin as high as possible. There are so many things to handle with a new design you dont tend to be so revolutionary with everything at once, but I do try to be evolutionary.

 

[wildun]

Great book, but really way over my head in many of the chapters.
However I do remember someone suggesting that in GP (motorcycle again) it would be better to base the classes entirely on mean piston speed! - thus having many different machines and engine capacities competing fairly against each other.
Of course this didn't happen, but I guess there was some wisdom there - even if it does blow my theory of 8000 revs not being high enough for a model V8 engine! - depends on the stroke/bore ratio used I suppose.

 

[camnefdt]

The internal combustion engine in design and practice. Charles Fayette Taylor

http://books.google.com/books?id=mX1-OJBQ6ngC&printsec=frontcover#v=onepage&q&f=false

Read it, believe it, but dont outbid me on ebay when I finally find a copy with money in my pocket. I have had the pleasure of a copy at my disposal from a good engineering library in the past. Its indispensable when you want to get serious about designing your own engines. Since it was published it has been the bible of the industry.

It mentions piston speed being the basic design anchor for any engine power requirement. This makes sense, as its mentioned piston assembly friction is almost half of the whole engine, and speeds are generally 5-15 meters second.

This is the piston for my last engine. Its a compromise of the new and the old. Dia .980" and height about .6, 15degree dome with three .040 reliefs. I had plenty of room so no need to cut the skirt, but in a nod to more modern design I did move the rings and pin as high as possible. There are so many things to handle with a new design you dont tend to be so revolutionary with everything at once, but I do try to be evolutionary.

thats an awesome looking book definitely worth tracking down

Have you tried your piston out in an engine? how does it perform?

 

[canadianhorsepower]

[Read it, believe it, but dont outbid me on ebay when I finally find a copy with money in my pocket./QUOTE]

Hi Jeff, thanks for that book reference, I just ordered one
here is a link where many used one are availlable, enjoy

http://www.amazon.com/gp/product/0262700271/?tag=skimlinks_replacement-20

 

[Lakc]

thats an awesome looking book definitely worth tracking down

Have you tried your piston out in an engine? how does it perform?

They havent broken yet

 

[canadianhorsepower]

[quoteThey havent broken yet

__________________
Jeff
][/quote]
do you have pictures or specs of the motor your running these in
thanks

 

[Lakc]

Here is the build thread from a few years back.

http://www.homemodelenginemachinist.com/f31/homebrew-boxer-twin-prototype-10465/

After switching to spark ignition I am still trying to get it running. Lack of spare time has been the biggest factor.

 

[cfellows]

Can't see any reason it wouldn't work. I could see that a short piston might be useful if you are building an engine with a really short stroke. However, unless you have a compelling design criteria that requires it, there is generally no good reason for a short piston (and cylinder).

Chuck

 

[Alec Ryals]

You have to remember that F1 engines are at operating temperature before they are even able to be started. They would disintegrate if started from cold, such are the tolerances of the components.

Such a piston in a model engine could easily 'twist' within the bore, and this is often the reason behind the extended skirts. Although in a lot of model engine designs I have looked at, I'm sure their pistons could easily be reduced in height without too many ill effects. Any offsetting of the crank to the cylinder center line will normally use an extended piston skirt to combat the increased piston to cylinder pressure. The premise being that the load is spread over a larger area and with greater moments of inertia.

How does the crank bore get off center from the cylinder bore?? and how does this increase the compression.
Alec Ryals

 

[windy]

On another ME forum I was asking a similar question about using a short skirt piston.
My engine is a 14cc two stroke flash steamer powering a tethered hydroplane (122.91mph over 500metres) and I am always trying to find that extra mph.
Having seen a JRM (Jawa) 499cc speedway piston with a very short skirt I wondered if I could do something similar to my piston to reduce friction?
My flash steam engine is a two stroke with a poppet valve to let superheated steam in and uniflow ports to let the exhaust out.
It has a narrow Dykes ring that if I used a short piston would it rock too much and destroy its seal?
11,000rpm under load, 15,000rpm with no load.
Piston and ring are cast iron running in a steel cylinder liner.
I have not seen any model competition engine with a short piston and wondered why.
[ame]http://www.youtube.com/watch?v=xdeUe9JcBVU[/ame]
This is my first meeting of 2012.
All suggestions on the merits or otherwise of using a short piston welcome.

Windy

 

[Till]

How does the crank bore get off center from the cylinder bore??

and how does this increase the compression.
Alec Ryals

It doesn't increase compression. It increases contact pressure per surface area because of bigger side forces.
The piston skirt is pressed against the wall of the cylinder liner. It's sidewards.