Best bore-stroke ratio?

[Higheta]

I know there is a great well of accumulated wisdom here, and I would like your views on a somewhat theoretical as well as practical question--

What is the best ratio of bore to stroke, and why? Some stationary engines have B/S < .30. Some high-speed engines have B/S > 1.

 

[EngJW]

Here are some considerations:

At one time, I think in England, automobiles were taxed according to bore size, so the bore was kept small and the stroke increased to get the necessary displacement.

High rpm engines require a short stroke to keep the piston speed within accepted limits, so the bore has to be larger.

Larger bores have more room for valves.

An opposed engine as used in aircraft is limited in width, so the bore is made larger and the stroke decreased.

Larger bores increase engine length. More of a problem with inline 6's or the old straight eight.

Oversquare engines are not as favorable for emissions. The B/S ratio might be around 1.10 or less.

Larger bores have more area for heat losses to occur.

Increasing the bore may result in heavier reciprocating components than increasing the stroke only.

 

[SUPERKYLE]

The shorter stroke engine (displacement fixed) has less friction from reduced piston ring travel per cycle.

 

[EngJW]

Also in the stationary engines the bore/stroke ratio may have been selected to reduce heat losses in very large cylinders. Some are 2-stroke and that might require a longer stroke to fit the ports in.

 

[patprimmer]

Ummm

It really depends on what you want to do.

Why?

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Higheta]

Pat said,

>Ummm
>It really depends on what you want to do.
>Why?

Okay, let's take a particular matter, how about for cleanest emissions?

 

[patprimmer]

Smaller bore longer stroke tends to be better for emissions because of the better surface area to volume ratio, but there are many conflicting requirements. That is why there are such widely diverse designs when all applications are considered, but not much diversity among competitors with similar application and requirements.


Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[amorrison]

Slightly OT

Re: Stress loading on connecting rod etc...

At high rpm's the mass of the "system" causes more stress on the components than the combustion force loading.

 

[aorangi]

Higheta, have a look here, I think you'll find a comprehensive answer :

http://www.auto-innovations.com/site/document/power2.html

Cheers
Aorangi

 

[PowerDubs]

Can't look there... its a pay site.

 

[jkm333]

Super Kyle mentioned that the short stroke/big bore engine would have less friction due to less ring travel. I am curious though if the friction benefits gained from lack of stroke would somewhat be canceled out by the increase in piston ring contact area with a larger bore? Just a thought.

Josh

 

[aorangi]

No, it won't because actually there's less friction area on the cylinder walls with an increased bore for the same displacement. It's just a simple geometry question. A 5-liter V8 with 99 mm bore and 81 mm stroke has a what I call a "cylinder area" (cylinder circumference x stroke x number of cylinders) of 2020 cm2. With a bore of 103 mm and a stroke of 75 mm, the cylinder area is 1942 cm2.

Moreover, at equal rpm the mean piston speed is reduced with a shorter stroke, so that the reduction of friction is much greater.

On the other hand, the piston area and so the combustion chamber walls' area is increased with a larger bore and therefore the indicated efficiency drops. We have a piston area of 624 cm2 for the 99 mm bore engine vs 667 cm2 for the 103 mm bore 5 liter V 8.

Note that at identical mean piston speed and BMEP, the power is proportional to piston area, whatever the number of cylinders, stroke and displacement are.

Happy New Year to everybody !
Aorangi

 

[jkm333]

I follow what you are saying. I was asking more about the piston ring contact area with regards to an increase in diameter and ignoring stroke all together. Wouldnt a 1/16 tall ring with a 4.030 bore have more contact patch area that the same ring with a 4.00 bore?

Josh

 

[aorangi]

Of course if you keep the same stroke. But then you have a bigger displacement engine !

 

[HAD]

In regards to the B/S ratio of 1 is this considered the most efficient for a high rpm engine that is being built for power or is it better to aim at a lower B/S ratio?

 

[jkm333]

I was always taught that horse power is governed by nothing more than the revolutions of an engine and is restricted by volumetric efficiency. I am sure that it's a little more complicated than that. But I have seen many successful racers using a bigger bore to stroke ratio than 1 to 1. Mainly to reduce piston speed, decrease rod angle, and decrease piston side loading.

Josh

 

[crysta1c1ear]

I don't go for aorangi's cylinder area theory of piston friction much. I tend to favour jkm333's piston ring contact area idea. Friction is supposed to depend on coefficient of friction, perpendicular force, and contact area. The cylinder area idea is a bit like multiplying the width of a tire by the length of a journey, so I don't think the number it produces is that relevent although I'm not saying it is altogether irrelevent.

So bigger bore will give more of jkm333's piston ring contact area. That works against bigger bores.

Working against larger stroke engines - I've just noticed its the same name - we have jkm's piston speed. Like he says, answering his own question, if it was just volumetric efficiency, they would be chasing that exclusively, but in practice, they are seeking to reduce piston speeds, among other things. Kinetic energy of the pistons is ½mv². As RPM increases, piston speed increases and piston kinetic energy increases. You could calculate an RPM where the whole power of a cylinder combustion was used just to reverse the motion of the piston ,and there would be no power for anything else at all. So high piston speeds are bad.

Increasing the stroke increases the loses reversing piston motion. Increasing the bore increases piston ring contact area and so presumably piston friction.

The ideal bore to stroke ratio will be when the loses are a minimum: it will be a delicate balancing act between these (and other) conflicting loses. It cannot be as simple as saying the ideal ratio is X:1.

For example, if lighter piston materials could be found, then piston kinetic energy would reduce and the balance might tip in favour of higher piston speeds, ie longer stroke.

If engine cooling were improved, cylinder temperatures could drop: piston friction would be reduced and the balance might favour wider pistons, ie bigger bore.

To come up with an ideal bore to stroke ratio for a particular engine and particular operating conditions I think you'd need to have an idea of functions for all the bore to stroke ratio dependent loses, eg my two simple examples, piston friction and kinetic loses and choose the value that minimises these loses, ie maximizes efficiency: power if you prefer to use that word, but I think the word encourages waste!

 

[GregLocock]

" Kinetic energy of the pistons is ½mv². As RPM increases, piston speed increases and piston kinetic energy increases. You could calculate an RPM where the whole power of a cylinder combustion was used just to reverse the motion of the piston ,and there would be no power for anything else at all. So high piston speeds are bad."

Um, not really. The kinetic energy of the piston is not lost in each cycle. It is used to compress the gas at the top, and to accelerate the crankshaft at the top and bottom of the stroke.





Cheers

Greg Locock

 

[maverickgt]

hi could it be that the best bore to stroke ratio is not as important as the best rod length to stroke ratio? by having a higher ratio you increase the dwell the piston sees at the top of the cylinder and basically changes the sinusoidal graph of the piston movement. I also recall that this will make the engine more efficient by allowing more complete combustion. Piston wall forces should also be reduced by using the higher rod/stroke ratio. I also feel that piston ring friction is negligible. Personally I feel using the smallest piston possible and still achieving the desired airflow and displacement is your best bet. The ideal piston/stroke ratio may vary depending on all of the other variables in an engine. Just my half a cent.

 

[incandescent]

F1 engines have hugely oversquare cylinders with B/S ratios around 2.3. B/S ratio is a lot more important than R/S ratio within a very wide range of combinations - anything that we might run. A main reason F1 engines have such high R/S ratios (around 2.5) is because the as the block heights get shorter, there is not enough space for optimal intake manifold volume and/or runner approach (space and angle) to the port. Wide angle engines help this problem but create others.

The problem with running the smallest piston possible is that the desire for more airflow and power never dies. You can always make more power going to larger bore, shorter stroke.. till you run into combustion, compression, clearance, component strength problems just like F1 is.