View Full Version : advantage of bore=stroke
Just wondering about if there's any significant theoritical advantage of an engine having bore equal to stroke?
Any reference i could go to or book, that could help me understand the concept of bore/stroke ratio?
I was under the impression that it was just a function of torque versus revolutions (or power) - and that the "square" bore/stroke just came about mathematically from where the lines would meet, if you plotted bore against stroke (or the other way around, it doesn't matter)
One possible advantage is that B=S maximises the volume/surface area ratio. With lower surface area less heat is lost, increasing thermal efficiency.
Theoretically there's no relation to torque output (ie altering bore and stroke but keeping the same volume), unless you start working out the friction involved, but I can't remember how that relates...
ToyotaNut
10-05-02, 01:40 PM
The advantage of a larger bore (oversquare) is that the conrods don't move too far from a horizonal position. This helps the engine stay together under higher revs.
The advantage of a longer stroke (undersquare) is that the piston will stay at TDC for a longer period, so torque is increased.
So bore=stroke is a nice middle ground.
Billzilla
10-05-02, 05:25 PM
Up to a point, you're always better off having a shorter stroke and a bigger bore.
The reasons are -
- Slower average piston speed for the same revs, so the bottom end work a little easier. (Though the instantaneous G's are higher)
- Allows bigger valves
- The ratio of a longer stroke V's the piston area for a fixed capacity always favours a bigger bore making more torque.
That being said, turbo engines, for some reason, seem to like an even bore/stroke ratio, eg, 3SGTE, SR20DET, etc.
toyotanut, that is part of the answer for rod angularity, is also seriously affected by rod/stroke length...all alters frictional loads and thus mechanical effeciency.
Larger bore means greater pressure on the piston, usually lower piston speed for the same revs (compared to longer stroke) which again contributes to mech eff (ie less stroke less drag and also less acceleration).
Larger bore also allows larger valves reducing pumping losses and guess what...better mech eff.
Please note this is all simplified so I understand it...it is probably far more complicated thaan I understand!
Try N2performance.com and look at the lectures there. It all seems good info. First chapter in guy crofts book, tuning fiat/lancia twin cams also covers this well, but see if the book is local library and don't buy it. Bells four stroke performance tuning isn't bad either, but doesn't cover this from my memory.
Let me know if I can help more
Ah bill...theoretical argument....the memories!! Have you heard from Dave Andrews recently?
Most makers opt for an acceptable compomise between emissions, fuel economy,and manufacturing compatability with their current range of parts first and the theoretical luxuries come a poor second.
As far as my limited knowledge goes all the suggestions mentioned so far above are valid but car makers are in the business (key words) of making cars.
BTW Bill, Have you read the biography on Sir Harry Ricardo called "Engines and Enterprise" ?
I was going to email you O/T but a severe crash lost me all my email database.
Cheers, Pete.
Billzilla
10-05-02, 06:05 PM
Hi Neil, yes, I still get the odd email from him. Business as usual in ol' Blighty. ;)
Might be heading back there myself shortly ... not sure yet.
Good point, Pete, I forgot to mention that typically a long-stroke engine will tend to be better on fuel. I have no idea why this happens, but Honda exploit it quite a bit.
Haven't got any Ricardo books, but am seriously meaning to add some to my collection! Might do some shopping in the Uk if I get a chance.
You can catch me at billzilla@billzilla.org Easy, huh? :)
Bill would know this well,
The engine i'm refering is Toyota 4AGE, std bore 81mm, std stroke 77mm.
I came across a race kit for it, oversize piston, bore=83mm, stroked crank of 83mm
and another one is to use crank(non-forged) from a similar engine with stroke of 85.5mm
Therefore i presume the one with 83mm stroke crank would be a good compromise, allowing a broader torque range with the use of longer duration cam
under square engines tend to rev higher also than over square.
bore x stroke...
torque is defined as Force X Length of the lever.. the bigger stroke (lever in this case) = more torque... This means the piston has to travel further hence, you end up with very high piston velocities.. not good for high RPM work..
The other situation where bore is bigger than stroke (oversquare?) piston speeds are reduced, therefore the engine can rev a lot better (all formula one engines are oversquare for the 18,000 rpm they use)
then you get into the grity stuff like rod length / stroke relationships which .. when using longer rods reduces piston velocities again, reduces the lateral forces on the crank shaft, and allows cam shaft timing to be altered...
the only downside from what i'm aware of is that overall max timing advance is reduced... however this should be offset by the extra RPM one can pull out of the engine reliably....
in my engine we had a bore / stroke of 84.8 x 90 a bit under square.. heaps of torque standard but it wouldn't want to rev much past 7000rpm... the standard rod length was 5.7"
we ordered a set of billet rods 3mm longer, custom pistons to suit and this changed the characteristics of the engine completely.. we still had the low down torque, but the damn kept bouncing off 9500rpm... amazing difference...
cheers
Dave
If you go back to the fifties road tax (rego) in the UK was based on the bore of your engine......hence a lot of British motors were small bore/ long stroke.
Then up untill the early 90's there were taxes based on engine capacity which accounts for things like 518i BWMs and E class Mercs with 2L donks. This rule was scrapped and now tax is based on fule consumption. Other parts of Europe have wierd capacity rules as well. .....this leads to 50cc replicas of Mick Dohans race bikes and Alfa GTV's with 1.3 L motors.
Originally posted by fiat
bore x stroke...
torque is defined as Force X Length of the lever.. the bigger stroke (lever in this case) = more torque...
not true: Force = pressure x area, and area =pi*bore^2/4... ie torque output is only related to engine volume, not bore/stroke relationships.
matty, your defining the force exerted on the piston..
i can assure you two cars with identical capacity, but totally different bore/stroke relationships will NOT have the same torque.. that's simply laughable.
i'll say it again
TORQUE = force x Lever length
TORQUE = (pressure x area) x lever length
explained in english, torque is a function of force on the piston acting on a lever, being the crank whose STROKE is the effective lever.
Hypertune
10-05-02, 09:49 PM
:D
This is great...
Nero, Fiat... You guys rock... hahahahahaa
you're welcome. Good luck with the info search and let us know if you finds anything good! Oh look at the ENDYN site or the site called theoldone.com, I think from memory. Some stuff used to be on there about rod/stroke info...some of it is a little dodgy though as the guy wants to sell his honda tuning ideas.
Fiat did you end up using Argo rods? Not sure if I agree on the acceleration and rod length issue. If same stroke then same acceleration but less lateral drag on the bore? Agree on crank whip/forces reducing flexure, but fiat 2lt reputedly has balance weight issues anyway?? Was this solved with rods/pistons?
Nero,
yup, used argo rods... 4340 nice..
As for acceleration, i never mentioned it. I was talking pure velocity.. V = D X T Longer rods reduce the D, hence reduce the V..
D = distance
the rod/piston combo helped us immensely..
Dave
Originally posted by fiat
matty, your defining the force exerted on the piston..
i can assure you two cars with identical capacity, but totally different bore/stroke relationships will NOT have the same torque.. that's simply laughable.
i'll say it again
TORQUE = force x Lever length
TORQUE = (pressure x area) x lever length
explained in english, torque is a function of force on the piston acting on a lever, being the crank whose STROKE is the effective lever.
Assuming both engines have the same BMEP and capacity, then the bore/stroke relationship is irrelevant. It's that assumption that you're ignoring.
as you said: torque = pressure x area x lever length
if pressure (BMEP is the same), and we know lever length = stroke/2, then
torque = BMEP x (piston area x stroke)/2
= BMEP/2 x displacement volume
I don't think it gets any simpler than that.
As Bill Sherwood said though, most long-stroke engines will be running cams that are more effective at increasing BMEP at low rpm, hence get higher torque. But that's altering other factors in the equation...
karl_2ltGC
10-05-02, 10:33 PM
Jees boys, im gona hafta get ye old physics books out.
but im pretty sure its
Force = mass*acceleration
Torque = force*radius (stop saying distance and all this other shite its RADIUS)
As for the calculations themselves....in real life they mean nothing unless your Mr CarMaker with every possible fact nessecary for the calculation and heaps of people working/testing it realy is useless for you.
Cheers karl
karl_2ltGC
10-05-02, 10:35 PM
Originally posted by spamgirl
If you go back to the fifties road tax (rego) in the UK was based on the bore of your engine......hence a lot of British motors were small bore/ long stroke.
Then up untill the early 90's there were taxes based on engine capacity which accounts for things like 518i BWMs and E class Mercs with 2L donks. This rule was scrapped and now tax is based on fule consumption. Other parts of Europe have wierd capacity rules as well. .....this leads to 50cc replicas of Mick Dohans race bikes and Alfa GTV's with 1.3 L motors.
LOL @ Poms
poor bastards
Billzilla
10-05-02, 10:37 PM
SQMAX - For all the cost of trying to turn a 1587cc 4AGE into a nearly 2000cc engine, wouldn't be a heck of a lot cheaper & easier to use a real two litre engine, like the 3SGE in the first place?
sssgtr - I believe that it's not so much the bore-stroke ratio, but just the stroke itself as the 'traditional' limit is around the 4,000 ft/min average mark. (Though the Honda S2000 completely blows that away with it's 5,300 ft/min!!!)
I haven't mentioned rod ratios here, as that wasn't part of the question as such. Again, the 'traditional' best ratio is around the 1.7:1 mark (rod length about 1.7 times the stroke) and there are advantages and disadvantages to have long or short rods. I personally prefer longer rods, as I believe that they cause less frictional losses than short rods, even though they weigh a little more and so there's more rotational inertia to deal with.
(I think Dave might agree with me here, too. ;) )
Matty, again a gut feeling that a bored out engine will make more power because of the greater piston area, less valve shrouding, and also allowing slight bigger valves anyway.
(And the engine weighs less! :) )
BMEP is something TOTALLY different.
When you are comparing BMEPs you are really comparing the engine's EFFICIENCY in producing power. This IMHO is off topic to the main conversation.
You can't re-write equations.
Torque = force x lever
you say engine volume is the only factor in torque, and that is correct, what you don't realise is that STROKE is in the equation of volume. hence DIRECTLY effects torque.
in addition, diff bore/strokes combos with the same capacity will inherently ALWAYS have a different BMEP (frictional loses etc)
Billzilla
10-05-02, 10:47 PM
Quite right, Dave, If you leave me until tomorrow, I'll write up something that shows the torque at the crank with two different engines, big/small bore and short/long stroke.
I'll assume the same gas pressure on the top of the piston, to make it even.
Say, start with an 86mm x 86mm engine, eh? Then go up & down 5mm in the bores ....
Originally posted by Bill Sherwood
Matty, again a gut feeling that a bored out engine will make more power because of the greater piston area, less valve shrouding, and also allowing slight bigger valves anyway.
(And the engine weighs less! :) )
not denying that, but you're changing capacity which is unfair!
Bill - look forwards to what you can come up with - remember to keep capacity the same though... and you'll end up with the equations I've given.
fiat - sure T = F*R, but HOW do you get that FORCE? pressure times area, that's how. area is related to bore^2, and pressure (though changing during the stroke) can be averaged by substituting BMEP instead of integrating...
Billzilla
10-05-02, 11:05 PM
Okay, thought I'd get it over and done with.
An 86mm x 86mm engine will have a capacity of 1997cc. I gave a pressure of 1,000psi on the top of the piston, and came up with an instantaneous pressure of 9003 lbs from that. With a lever arm of 43mm (86mm/2) or 1.69" I get 1267 ft-lbs.
A 91mm x 71mm engine, with nothing else changed comes out to 1273 ft-lbs.
An 81mm x 97mm come out to 1270 ft-lbs.
I'm a bit tired, is that right?
Billzilla
10-05-02, 11:07 PM
Matty, yes, but I did mean for the same size engine. ;)
In fact, it's even better as the crank would be a little smaller and so weigh even less again! :)
BMEP: a measure of how effectively an engine utilizes its piston displacement to do work.
your argument revolves purely around BMEP and displacement volume considerations in determining Torque.
But you still are refusing the most basic of all equations. Torque is nothing unless it has a lever to act upon. you can have 10 trillions Newtons of force, but if you don't have 1mm of lever to apply it to. you ain't going nowhere.
more so, the longer that lever, in any application under the sun, the more torque you will produce.
integrating BMEP to REPLACE the lever is wrong.
Bill, Your calculations are on the money.
Dave
Billzilla
10-05-02, 11:22 PM
Thanks Dave, I'm a little too tired to do anything that requires precision right now. :)
I think that perhaps the slight variations I got in the ft-lbs were perhaps me not going into enough decimal places, and so they may all be exactly the same in fact.
TurboEscort
11-05-02, 04:16 AM
Originally posted by fiat
bore x stroke...
torque is defined as Force X Length of the lever.. the bigger stroke (lever in this case) = more torque... This means the piston has to travel further hence, you end up with very high piston velocities.. not good for high RPM work..
bigger stroke = more torque? not necessarily.....Sure, larger the stroke, longer the lever as you said above. But also consider this:
Torque = Force x Length.....Force in this case; F= average pressure inside the cylinder x Piston Area. So larger the bore, larger the piston area, hence larger the force.
The other situation where bore is bigger than stroke (oversquare?) piston speeds are reduced, therefore the engine can rev a lot better (all formula one engines are oversquare for the 18,000 rpm they use)
then you get into the grity stuff like rod length / stroke relationships which .. when using longer rods reduces piston velocities again, reduces the lateral forces on the crank shaft, and allows cam shaft timing to be altered...
How does longer rod affect piston velocities?
the only downside from what i'm aware of is that overall max timing advance is reduced... however this should be offset by the extra RPM one can pull out of the engine reliably....
Yes, you'll need less timing because piston spends more time near TDC (in relation to crank degrees), but thats not a bad thing.
in my engine we had a bore / stroke of 84.8 x 90 a bit under square.. heaps of torque standard but it wouldn't want to rev much past 7000rpm... the standard rod length was 5.7"
we ordered a set of billet rods 3mm longer, custom pistons to suit and this changed the characteristics of the engine completely.. we still had the low down torque, but the damn kept bouncing off 9500rpm... amazing difference...
I doubt the above change was solely due to 3mm longer rods. To make the engine eager to rev at higher rpm, its volumetric efficiency must be increased at higher rpms, ie longer duration cams, larger turbo, etc....
cheers
Dave
Cheers,
YG.
bigger stroke = more torque? not necessarily.....Sure, larger the stroke, longer the lever as you said above. But also consider this:
Torque = Force x Length.....Force in this case; F= average pressure inside the cylinder x Piston Area. So larger the bore, larger the piston area, hence larger the force.
yup sure!! changing anything in the force x lever equation will increase torque.. you can change lever lengths, or you can increase the force as you said
How does longer rod affect piston velocities?
Ok, this is basically a Velocity = Distance x Time equation.
if you increase the length of the rod ( you must offset this length increase, by milling off the top of the piston) the distance the piston travels is reduced. the distance it is reduced by is only the increase in the length of the rod. (even thou stroke remains the same, mean piston velocity is reduced)
http://www.aros.net/~rbuck/rick/rodstudy.htm
these guys compared piston velocities with different rod lengths, they also measured dwell times at TDC etc etc.. you'll get the picture...
I doubt the above change was solely due to 3mm longer rods. To make the engine eager to rev at higher rpm, its volumetric efficiency must be increased at higher rpms, ie longer duration cams, larger turbo, etc....
yes definately true, we were a lot more comfortable revving the car due to better rods etc etc, but on the dyno the car was totally different (keeping everything else identical)
cheers
dave
I think that we are getting tied up on the details and not being holistic enough, an engine is a series of systems that make up a whole. What we currently have on the market and generally in our cars are engines that really (in some ways)only vary a little.
We can discuss changing one or two of the perameters but we also have to realise that there are other factors involved eg if we change the stroke, then we have to change the valve timing to suit the new perameter, so the net change in torque comes from the stroke or the timing...probably both.
Piston acceleration is a limiting factor only so far as materials are concerned, change the materials and the higher forces can be accepted as reasonable. A greater limitation is the rings, too much acceleration and velocity and they suffer hugely. But as with anything, you increase the forces and increase the wear and most importantly for the manufacturers you increase the emissions and decrease the profit!
The POMs had the tax arrangement limiting their engine development for about 25-30 years. The engine development was also halted by poor oils and very poor fuel @70-80 RON, the latter meant that long stroke engines running 7:1 compression made sense (also there was a lot of poor engineering!)
I have the forula's for BMEP and Mech Eff'y etc, but they may not help that much but I will post them if anyone wants them, except to say the mech eff'y one is perhaps the most telling, it takes a lot of work to spin an engine and approx 65% of the energy of the fuel is absorbed by this (in a crude way of reckoning I admit).
There's a few people who are getting there formulas mixed up here, and are needlessly arguing wiht people who aren't.
f=pa Force in Newtons=pressure in pascals*area in metres^2
f=ma Force in Newtons=mass in Kg's=accelleration (9,8m/s^2)
T=FL Torque in Nm=Force applied in N*length of lever in metres
With a 95 mm bore, 90 mm stroke and a 90cc conbustion chamber at 1000 psi of combustion pressure you have 48kN of force on the piston which equates to 4396 N of torque at the crank.
To get the same total displacement with a 90mm bore, the stroke must be increased to 100.3 mm, this gives a 43 kN force applied to the piston with 4397 N of torque at the crank.
This has all been calculated on a spreadsheet so if you have other numbers I can feed them in and give you an answer with zero effort.
Originally posted by Bill Sherwood
Okay, thought I'd get it over and done with.
An 86mm x 86mm engine will have a capacity of 1997cc. I gave a pressure of 1,000psi on the top of the piston, and came up with an instantaneous pressure of 9003 lbs from that. With a lever arm of 43mm (86mm/2) or 1.69" I get 1267 ft-lbs.
A 91mm x 71mm engine, with nothing else changed comes out to 1273 ft-lbs.
An 81mm x 97mm come out to 1270 ft-lbs.
I'm a bit tired, is that right?
accounting for the minor variations in capacity, you've got exactly the same torque output in all three cases. as did smeck. so fiat, I still argue that bore/stroke relationships are irrelevant from a theoretical standpoint. you then have to start arguing about the EFFICIENCIES of each design. Unfortunately, that is something beyond my knowledge level, but I'll defer to Bill S's expereince there.
Billzilla
11-05-02, 06:47 PM
Thanks Matty, my engines always seem to go quite well, so I guess I must have a bit on an idea! :p
I still reckon that if you prepped two engines of the same capacity, one with big bore and the other small bore, that the big bore one will always be better.
Again, for the reasons I've mentioned before, less valve shrouding, allows bigger valves, lower piston average speed, lower internal mass, etc.
i give up with this thread.....
Sorry Bill but thats a load of tosh. about the bore that is.
Fiat, it's unfortunate for your postion but unless you can bring new facts to light, the maths speaks for itself.
The thing about physics is opinions, loyalties, and commonly held beliefs don't count for squat if the numbers don't add up. Yes the lever is longer on an under square engine, but the force applied is significantly less and that's half of the torque equation.
Until doing the maths myself I was of the same opinion. I guess that's why just about all engines these days are oversquare.
Bill, if that were the case, why are F1 engines V10's and not V6'. Power is defined by the rate at which work is done, not the quality of work being done.
Billzilla
12-05-02, 10:25 AM
Good question, Smeck.
The reason is piston speed again - To keep the engine from flying apart they have to have some sensible limit to that speed, and they also know that they need as mnay revs as possible to make the HP they need, so they have to divide up the engine into as many cylinders as they need to keep the stroke sensible.
They typically run these days with about a 91mm - 95mm bore, so the stroke is from 46mm down to about 42mm.
Nero, I can't see why a lower piston speed, less shrouding, etc, can't be a good thing.
Bill there are SO many variables in a 4 stroke engine that saying 'more of X is better' is an argument of limited theoretical value eg, unshrouding the valves has its advantages to a point, that being at which the post valve closure homoginisating of fuel is affected by poor distribution/reduced 'squish'/less effecient combustion chamber and piston design, not to mention the mechanical effeciencies and emission/fuel economy, all of which has to be engineered to match the fuel used, with is differing volatile fractions that vary depending on what the companies have on hand on the day. All of this is the basics and I'm missing lots. Its like looking at VE and saying more is good, which is true only as long as so many other factors are considered.
You have to measure the merits of an engine (as far as performance and the changes to it through modification) via its mechanical effeciency, quoting the physics is fine, but you always have to have the proviso that it is a theoretical argument and not related to the real world. For an example, name one production engine that is seriously over square that has more that has a 3% advantage over its competitors (with the same capacity) in the same market segment, without a significant technical advantage in another way/area
Billzilla
12-05-02, 10:43 PM
I am trying to make in a real world situation, as I was trying to infer that if you were to bore an engine out, say, 5mm and destroke it the same amount it'll make more power than a 'stock' one.
I wasn't try to say that the ultimate engine is something that has about a 500mm bore and 2mm stroke ... ;)
Inertia is inertia and friction is friction, anything you can do to reduce those two will let the engine make more power. This is why engine that run synthetic oil make more power than regular mineral oil, etc.
Just in regards to the person who used the longer rods....
If you use longer rods aren't you effectively decreasing your stroke, which may help you rev the engine harder but will still give up some torque/power?
Cheers
Billzilla
13-05-02, 10:02 AM
No, the rod length doesn't affect the stroke, only the piston speed along the bore. Longer rods tend to make the piston accelerate and deccelerate a little slower than short rods, due to the geometry of the crank/rod/piston.
hmmm...ok.....It'll take me about a month or so to get my head around that one....
TRD-KINGSWOOD
13-05-02, 01:51 PM
Kalium, you change the compression height to make up for the rod length increase/decrease.
This is a good debate. Keep it happening boys.
The reason i started this tread is to find out if the following package is feasible for my 4AGE engine with stock bore/stroke of 81mm/77mm
Toyota 4age 1800cc Race engine package.
· 83mm Forged crankshaft c/w bearings
· 83mm HC Forged piston kit complete (use 4agze con-rod)
· 83mm bore Metal head gasket
Price US$1800.00
If i limit the max rpm at 7500RPM, it should still be durable for road use isn't it.
The way i see it, if i add some cams, quad throttle and after market ECU with the above package, i might end up with good power and a good spread of torque.
any comment?
Just a quick question, with a larger bore would that cause uneven burning / lagged burning? Being more surface area and for the flame front to move across or doen't it matter due to the mass of air is unchanged if you have shortened the stroke?
Race engines generally run as large a bore as possible to unshroud the valves. More air flow equals more power. Also boring over stroking maintains (or lifts) the RPM ceiling instead of lowering is as stroking usually does. Boring 1mm will add more displacement than stroking 1mm. Of course you are very limited as to how far you can bore a production block, so to add as much displacement as possible it is necessary to both bore and stroke. All things being equal, a bigger engine will always make more power than a smaller one. Also by adding stroke you add torque due to the extra leverage provided by the longer crank throw.
Because the engine's deck height is unaltered, when stroking you normally use a longer rod (to maintain the rod/stroke ratio and to allow the crank counterweight to clear the piston skirt) and a reduced compression height piston. It sould also be noted that boring and/or stroking adds compression which is an important consideration in a turbo engine. Dished pistons are used to overcome this.
On the subject of rod length lengths, even the world's best engine builders can't agree if a longer rod adds power (by increasing piston dwell and acceleration/cylinder filling at TDC and BTC). All documented gains I have seen have been miniscule and in the order of 4hp in a 800+hp motor. One thing they do agree on is to run as long a rod as possible to maintain a good rod/stroke ratio and to minimise side loading.
my 2c...
Billzilla
13-05-02, 09:32 PM
Sqmax - I don't think it's safe to bore a 4AG or 7A block out to 83mm (take a look at the thickness of the bores between the cylinders, there's naff-all there!) but I'd be happy to go to 82mm though.
Perhaps another suggestion; Look at a CA-18, as they're 83mm x 83mm from the factory.
7500rpm with an 83mm stroke is fine for piston speed, not a bother.
Nick - That's why squish areas are very important, they help control all that.
And wot Howy said! Cool, mate, ta.
Originally posted by smeck
There's a few people who are getting there formulas mixed up here, and are needlessly arguing wiht people who aren't.
f=pa Force in Newtons=pressure in pascals*area in metres^2
f=ma Force in Newtons=mass in Kg's=accelleration (9,8m/s^2)
T=FL Torque in Nm=Force applied in N*length of lever in metres
With a 95 mm bore, 90 mm stroke and a 90cc conbustion chamber at 1000 psi of combustion pressure you have 48kN of force on the piston which equates to 4396 N of torque at the crank.
To get the same total displacement with a 90mm bore, the stroke must be increased to 100.3 mm, this gives a 43 kN force applied to the piston with 4397 N of torque at the crank.
This has all been calculated on a spreadsheet so if you have other numbers I can feed them in and give you an answer with zero effort.
firstly looking at f=pa
I could be/probably am wrong though the way i see it is that if all factors such as friction etc are the same then the force is dependent on the capacity, which in this case is the same.
The area of the piston is different, therefore the pressure is different. Everyone else seems to have equal pressure in both examples, which is why i think the same torque figure is always calculated.
torque = force x radius.
as the force is the same in both examples as they have the same capacity, friction etc. Then the only variable is radius which would give the engine with the longer stroke the advantage in torque.
The downsides of a longer stroke have been covered by everyone else. So i wont mention them.
Trent
Originally posted by Trent
...
I could be/probably am wrong ...
you got that much right... read the above posted calculations by smeck and Bill
Originally posted by Matty
you got that much right... read the above posted calculations by smeck and Bill
check out the articles called "power and torque" on autospeed.
they backup what i said. written by a physicist as well.
Trent
gianttomato
15-05-02, 04:11 PM
Can someone please post (or point me to) the formula for calculating piston speed? Also what is an acceptable maximum piston speed? I have heard figure of 4000 ft/sec - is that correct or a load of bollocks?
Cheers Dave.
piston speed=(stroke mm x rpm)/ 153
or (stroke inches x rpm)/6.024
it's from Four -Stroke Performance Tuning by A. Graman Bell
it also mention for road car withstd cast crank and rod, max piston speed is 3500
road car with forged crank and rod, max speed is 3800
its has got other figure for rally, road race and drag
gianttomato
15-05-02, 04:40 PM
Thanks for that. Interesting, as I thought piston speed was partly determined by conrod length, and was inversely related to it.
I wonder if the denominator is related to a presumptive rod length:stroke ratio of around 1.7:1?
Originally posted by Trent
firstly looking at f=pa
I could be/probably am wrong though the way i see it is that if all factors such as friction etc are the same then the force is dependent on the capacity, which in this case is the same.
The area of the piston is different, therefore the pressure is different. Everyone else seems to have equal pressure in both examples, which is why i think the same torque figure is always calculated.
torque = force x radius.
as the force is the same in both examples as they have the same capacity, friction etc. Then the only variable is radius which would give the engine with the longer stroke the advantage in torque.
Trent
The force has nothing to do with capcity and everything to do with the bore size/piston area. The volume of the cylinder has not changed, so the pressure remains exactly the same in both examples. The force is then calculated with f=pa and if the pressure is the same but the area different, the force must be different, it's a fact, ask your physicist.
The reason every one gets the same torque figures is because a small force applied to a large lever equates to the same as a large force applied to a small lever. I am thouroughly confused as to how you have involved pressure in force/lever equation.
Don't get all wrapped up in friction because it's extremely variable and must be calc'ed on a case by case basis. That and it has no affect on the force applied by the combustion of the fuel. Gases are not affected by piston friction, especially when the piston hasn't moved.
Billzilla
15-05-02, 09:19 PM
gianttomato & guys, the 'usual' limit these days is around the 4,000 ft/min mark. If you look at a stock 4AGE or Suzuki Swift Gti, then at the red-line the piston speed is very close to that figure.
The Honda S2000 completely blows that away though with its 5300 ft/min!!
The formula above for piston speed also only gives you the average speed, and that will alter with different length rods. Long rods give more piston dwell at TDC, shorter rods tend to have higher acceleration around that areas, etc.
1.7:1 is a good ratio to have, hard to go wrong. ;)
gianttomato
15-05-02, 11:17 PM
Ahha. Thank you Bill.
Still interesting, as when I did the calculation using the above formula for my 2JZ at redline (6500), the average piston speed was above 4000 ft/sec.
I have a formula somewhere for critical rpm somewhere that is reliant upon conrod length to stroke ratio that I really must dig out.
Thanks again.
TRD-KINGSWOOD
16-05-02, 08:51 AM
Work it out for the 1J!! Almost perfect!!!
125mm rods. 71.5 mm stroke. Nice...
dont know if anyone else has corrected this, i havent read the whole thread, but what you said is back to front
undersquare is longer stroke than bore, oversquare is larger bore than stroke. longer stroke pushes you towards a torque bias and away from revvability
the poms used to make a bunch of long stroke engines iirc, they had a tax on bore years ago. shows what happens when dumbass pollies make rules on cars, a bit like the whole speed kills farce ....
j
edit: shit, nero beat me to the tax bit ..
2nd edit: holy crap, spamgirl beat me to the punch as well, i might as well just kick my sandcastle over and go home ..
i might as well add, since i gotta put some value back into this post somehow =) .. if you are interested in reading an exhaustive source of engine related calcs, look up the book series entitled the internal combustion engine in theory and practice by charles fayette taylor. he has written a couple of books under that title, splitting the topic between them... not exactly bedtime reading but they are each around 500 pages cover to cover with every calculation on every aspect you could think of. fantastic if you are writing a paper on it, or just a freak who likes to read that sort of stuff.
Originally posted by sssgtr
under square engines tend to rev higher also than over square.
honda isnt backwards in pushing the envelope, to be sure. and they have some nice engines to show for it. but edo's experience would seem to show that, on some engines, maybe theyre using just a few too many rpm ..
j
Originally posted by Bill Sherwood
gianttomato & guys, the 'usual' limit these days is around the 4,000 ft/min mark. If you look at a stock 4AGE or Suzuki Swift Gti, then at the red-line the piston speed is very close to that figure.
The Honda S2000 completely blows that away though with its 5300 ft/min!!
The formula above for piston speed also only gives you the average speed, and that will alter with different length rods. Long rods give more piston dwell at TDC, shorter rods tend to have higher acceleration around that areas, etc.
1.7:1 is a good ratio to have, hard to go wrong. ;)
i would just like to add, having read the whole thing now, that this is definitely a step in the right direction, and one of the best threads ive seen here for a while. good to see some real discussion
here's to more of the same ..
j
last post in a row, i swear ..
bill, im curious as to what prompted that last sentance... in what circumstances, particular application, etc .. afterall the F1 cars seemed to work fantastically well on small capacity, large bore/small stroke, massive rpm and massive turbos (relative to displacement anyway) .. not refuting, just wondering?
j
Originally posted by Bill Sherwood
Up to a point, you're always better off having a shorter stroke and a bigger bore.
The reasons are -
- Slower average piston speed for the same revs, so the bottom end work a little easier. (Though the instantaneous G's are higher)
- Allows bigger valves
- The ratio of a longer stroke V's the piston area for a fixed capacity always favours a bigger bore making more torque.
That being said, turbo engines, for some reason, seem to like an even bore/stroke ratio, eg, 3SGTE, SR20DET, etc.
Billzilla
16-05-02, 10:54 AM
The F1 turbo engines 'only' revved to about 12,00 rpm odd, so they could get away with a relatively long stroke compared to todays F1 engines.
There was a debate back then as to which was better, a four cylinder or a six cylinder. The 4's had less friction and less things to go wrong, but had more piston area and so suffered from the tops of the pistons melting faster. It was a bit of a trade-off, but ultimately the 4's won, as BMW proved with the fearsome 1470hp monster engine.
They still run a short stroke on those though, as they still need to rev the buggery out of them to make HP. Well, it's not so much the HP they're after, that's just big numbers to impress the kids, it's to make the whole thing so they can gear it down to make LOTS more torque at the wheels.
For example, the current F1 engines don't make a whole lot more torque than the old DFV of 1967, but instead of 'only' 11,000rpm they now run to 18,000rpm, so as they still 'only' have the same 200mph-odd top speed, they can gear the car to suit and so have about 63% (18/11) times more torque where it counts.
The turbo engines were a slightly different story, as they didn't have to rev them quite as hard, just run more boost! :) They still had the typical 4,000ft/min limit though back then, so I guess they worked the stroke of the 4's & 6's to suit that limit. Again, this infers that the 6's could rev more, but the 4's ended up being the winner in sheer HP.
And yes, this is a bloody superb thread!! Let's keep going!!
gianttomato
16-05-02, 12:27 PM
TRD Kingy
Dude I know I know. I lose, you win!
Originally posted by smeck
The force has nothing to do with capcity and everything to do with the bore size/piston area. The volume of the cylinder has not changed, so the pressure remains exactly the same in both examples. The force is then calculated with f=pa and if the pressure is the same but the area different, the force must be different, it's a fact, ask your physicist.
i don't think i explained myself very well. by force and capacity i am saying that if you have 500cc of a petrol/air mix, then when that is combusted it will give a force against the piston. the same volume will give the same force no matter what the bore/stroke relationship is.
this will result in different pressures being excerted on the pistons depending on their bore. everyone else has the same pressure being excerted on the top of the piston.
The reason every one gets the same torque figures is because a small force applied to a large lever equates to the same as a large force applied to a small lever. I am thouroughly confused as to how you have involved pressure in force/lever equation.
i understand that a small force applied to a large lever equates to the same as a large force applied to a small lever.
i have done an excel spreadsheet to show how bore/stroke affects torque.
the top 3 pistons are where pressure on the top of the piston is the same (which i believe is incorrect)
the bottom 3 pistons are where the force on the top of the piston is the same.
http://www.geocities.com/trentomatic/index.html
Originally posted by Trent
i don't think i explained myself very well. by force and capacity i am saying that if you have 500cc of a petrol/air mix, then when that is combusted it will give a force against the piston. the same volume will give the same force no matter what the bore/stroke relationship is.
And that's where you're still incorrect.
look, we take the same intake volume charge (say 500cc single cylinder) in our two imaginary engines of different bore/stroke.
we compress it to eg 50cc (combustion chamber shape doesn't matter) - and we get the same pressure by adiabatic compression laws.
we ignite the fuel. same calorific value, so same temperature (and therefore pressure) increase.
So, at the top of the stroke, we've got the same pressure and conditions, and the same combustion chamber volume.
Now, this pressure acts in all directions. The only USEFUL component acts on the top of the piston.
The FORCE on the top of the piston will be the pressure multiplied by the area.
Now, the piston travels down through a distance of the stroke. The pressure reduces during the stroke according (again) to adiabatic expansion laws. These, if you've studied thermodynamics, are related to VOLUME change, independent of shape. However we can assume in both cases (since the before and after volumes are the same) that the start and ending pressures are also the same.
Now, the useful work output of the piston is given by integrating the FORCE over the distance travelled (the stroke).
at the end, we see that the work output is a product of the piston area and the stroke, or to put it another way, the VOLUME!
OMFG, I can't believe I'm bothering to explain all of this. If you still don't believe it, go study any thermodynamics textbook, any engine building manual, any physics course, etc...
Your not understanding the formula Trent.
Force is measured in Newtons (N) and that is the force exerted by the piston.
Pressure is measured in Pascals (Pa) and is the pressure being applied to the piston.
Area is measured in Metres^2 (m^2) and is the area of the piston.
When you combust the same 500cc of gas , then you will get the same 1000 psi of pressure, that acts on the area of the piston and a force is applied
When the pressure is constant (and it has to be if the volume is unchanged), and the area is changed, then the force has to change.
I have looked at your spreadsheet and your formulas are very foriegn and seem unnecessarily complicated to me. Why are you dividing your stroke by 2 before converting it to metres and multiplying it by force.
Your top three equations are correct, you can see this with your own eyes. The bottom three are based on an incorrect theory and are subsequently wrong.
ok. i am wrong.
i always understood where you guys were coming from, i just thought that .... well you know what i meant.
the formulas are like that, because i worked them out, then converted them a unit that was easier to read.
maybe it is also the way pi and squares are used in excel.
yeah, i divided the stroke by 2 to get the lever length, converted it to metres and then multiplies it by the force to get the torque.
i now know its wrong, but it seems to be a commonly held belief.
i guess it is a case of people just repeating what they have heard.
that and a little knowledge can be dangerous.
Nick the issues of combustion and flame front travel are varied. An increase in the bore possible in a comercially available engine are not worth worrying about. The flame front can be aadversally affected by poor modification of the head (e.g. old wisdom of unshrouding the valves in a MGB head is now reversed and the squish is far more important to homoginise modern fuels), poor or uneven distribution of the fuel/air mix, poor distribution that does not suit the volatile fractions of the fuel used (leads to preignition) and the CR used. Distribution of the fuel in the CC can also be affected by poor porting, e.g. taking the small lump off the long radius of some VTEC engines stops the tumble into the bore and the distribution so carefully designed by honda is wrecked. Because the distribution is upset the power drops and the emissions go all to hell.
without looking at the spreadsheet, the dividing the stroke by two step in converting from force to torque is because the stroke distance is exactly double the distance between crank centreline and bigend centreline, ie the effective lever length is *half* of the stroke. hence divide by two.
hth, Nick
Remeber that the BMEP is the force at the crank so directly related to BHP, IMEP (indicated mean effective pressure) is the force generated by combustion. ie the force at the piston.
IMEP - mechanical losses = BMEP
Rob.
btw F1 engines have approx 40mm strokes and 80mm bores which, along with pneumatic/hydraulic valve closure is why they can pull 18000 odd revs...
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