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Old 3 Jun 2014, 23:31 (Ref:3415005)   #751
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Originally Posted by cokata View Post
Why would you have to make a V6 4l engine with ~5000rpm redline oversquare, or am i missing something
Most likely the high chamber pressures of the diesels require a more robust connecting rod. The longer the stroke the longer the connecting rod needs to be and that makes beefing it up to take the force from the piston difficult without adding unwanted mass. As long as good combustion quality and aspiration can be ensured it benefits to run oversquare since you have higher rpm potential.
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Old 3 Jun 2014, 23:53 (Ref:3415012)   #752
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Also, more and more road diesel engines are going oversquare. Peugeot's 908 V12 was claimed to have a 84mm bore and a 82mm stroke.

It has to do with revs and possibly compression ratio.
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Old 4 Jun 2014, 00:21 (Ref:3415020)   #753
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Originally Posted by chernaudi View Post
...more road diesel engines are going oversquare.

It has to do with revs and possibly compression ratio.
How is over square related to flat plane?
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Old 4 Jun 2014, 00:43 (Ref:3415024)   #754
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Again, probably revs (namely acceleration of a rotating mass).

Audi's 4.2 liter V8 used in the RS 4, RS 5 and R8 has pistons that at 8000 RPM or so travel at near F1 speeds, but that's over a huge stroke length and half the RPM of a V10 or V8 F1 engine of the past 20 years of so (in specific 2004-06 when the Audi RS V8 was developed).

And even it had a crossplane crank. The issue with the flatplane crank isn't RPM itself, but speeding up/slowing down. Flatplane cranks don't have the counterweights like crossplane cranks have. Lighter crank equals faster spin up. Flatplane cranks, though, do have a disadvantage in terms of high RPM vibrations, usually above like 8000 RPM and it gets worse the higher the RPM without good crank balancing.

Over-square cylinder dimensions allow for more RPM due to a shorter stroke distance (less distance for the piston to travel). A flatplane crank in a V8 will allow (in theory) faster spin up/slow down times.

Of course, this does matter for Toyota, who have a flat crank V8. For the Audi and Peugeot V12s, and Audi V10 and V6 diesels, they have their own crank designs that have no major variations from that would be expected from such engines.
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Old 4 Jun 2014, 00:46 (Ref:3415027)   #755
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Originally Posted by chernaudi View Post
Again, probably revs (namely acceleration of a rotating mass).

Audi's 4.2 liter V8 used in the RS 4, RS 5 and R8 has pistons that at 8000 RPM or so travel at near F1 speeds, but that's over a huge stroke length and half the RPM of a V10 or V8 F1 engine of the past 20 years of so (in specific 2004-06 when the Audi RS V8 was developed).

And even it had a crossplane crank. The issue with the flatplane crank isn't RPM itself, but speeding up/slowing down. Flatplane cranks don't have the counterweights like crossplane cranks have. Lighter crank equals faster spin up. Flatplane cranks, though, do have a disadvantage in terms of high RPM vibrations, usually above like 8000 RPM and it gets worse the higher the RPM without good crank balancing.

Over-square cylinder dimensions allow for more RPM due to a shorter stroke distance (less distance for the piston to travel). A flatplane crank in a V8 will allow (in theory) faster spin up/slow down times.

Of course, this does matter for Toyota, who have a flat crank V8. For the Audi and Peugeot V12s, and Audi V10 and V6 diesels, they have their own crank designs that have no major variations from that would be expected from such engines.
And to make it easier to understand why an oversquare engine tends to yield higher power than an undersquare engine of the same displacement, you can think of the oversquare engine rotating faster than its undersquare counterpart.

This means more fuel and air is ingested in a given amount of time for the undersquare engine, leading to more power. Simplistic explanation but its true for the most part, all else being equal.
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Old 4 Jun 2014, 01:35 (Ref:3415041)   #756
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Originally Posted by chernaudi View Post
Over-square cylinder dimensions allow for more RPM due to a shorter stroke distance (less distance for the piston to travel). A flatplane crank in a V8 will allow (in theory) faster spin up/slow down times.
I think I am understanding you. I'll provide an example which I think exemplifies what you are saying...

I grew up in the late 60s during the times of the American 'Muscle Cars'. There was an explosion of cylinder dimensions. I mainly followed Chevy back then and we had 283, 327, 396, 409 and then 454 cubic inches. Bigger seemed to be better and the way of the future. Then one night, this little plain looking Camaro Z-28 with a 302 cubic inch engine came loping through our hangout. We laughed but for those that thought bigger was better, this little short stroke engine proved them wrong.

Is this similar to what you are saying with the mass and piston distance?
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Old 4 Jun 2014, 03:46 (Ref:3415062)   #757
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I think I am understanding you. I'll provide an example which I think exemplifies what you are saying...

I grew up in the late 60s during the times of the American 'Muscle Cars'. There was an explosion of cylinder dimensions. I mainly followed Chevy back then and we had 283, 327, 396, 409 and then 454 cubic inches. Bigger seemed to be better and the way of the future. Then one night, this little plain looking Camaro Z-28 with a 302 cubic inch engine came loping through our hangout. We laughed but for those that thought bigger was better, this little short stroke engine proved them wrong.

Is this similar to what you are saying with the mass and piston distance?
Likely. The 1999 Mustang GT made 260hp from 4.6L. The same year the Honda S2000 was making 250hp from 2.0L. The Ford's square engine redlines at something like 5800. The S2000's over square engine redlines at 9000.

Extreme examples but I think the point is there.

When you rev high the limiting component is typically your connecting rod because the piston wants to rip it apart as it changes direction near the top of the cylinder. The shorter the stroke, the less intense the acceleration of the piston is near the top (and bottom) of the cylinder.

With that you can do two things if you were to use the same connecting rod. Rev it higher, or beef up the piston for a forced induction application
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Old 4 Jun 2014, 11:40 (Ref:3415147)   #758
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Why would you have to make a V6 4l engine with ~5000rpm redline oversquare, or am i missing something
ooohh..... juicy engine talk.....I do engines........the main reason the likes of F1 in the normally aspirated V10 & V8 era went massively over square was to reduce friction in the piston to cylinder interface, with a big bore which I think was around 98mm, and a small stroke, I think around 30mm or so, the side load emitted from the con-rod was minimal, friction is reduced with load (not area) and the rpm shot up as a result......the same applies to modern high performance and race diesels, hence they are now slightly over square, but only by a few mm.

the low compression ratio diesels (circa 14:1) are now preferred also because of a significant efficiency increase whereby the engine is not fighting the gas loads on the compression phase, all road car disels are now going this route for this very reason.

Regarding flat plane cranks, for the past 10 years or so the NASCAR teams have been running very light-weight cross-plane cranks with heavy-metal tungsten inserts, that are comparable in weight to that of a flat plane crank, so recently the weight issue is not such a big deal, but certainly a light weight crankshaft makes a race engine very very snappy and responsive.......the main and primary reason that people use flat-plane crankshafts is to get the exhaust pulsing correctly and evenly for power.......the cross plane cranks have a burble, due to the un-even firing, and the flat plane cranks basically sound like 4 cylinder engines, because thats exactly what they are!

But again even the NASCAR boys have figured the exhaust pulsing problem out, whereby they now run tri-Y manifolds that are coupled in such a fashion that the exhaust pulses are equal.......honsetly if I was designing a V8 LMP1 engine I would seriously consider using a cross-plane crankshaft, as the reliability issues and virbarions from a flat plane crank shaft are nothing but a nightmare for the chassis and general reliability.......I suspect Toyota know this and have probably very very carefully designed their engine in order that this is not a problem with a crankshaft that has critical resonant frequencies well away from the operating region, or a viscous damper on the crank-nose, or twin counter-rotating balance shafts........time will tell

somewhat off topic now!......regarding the R18 engine, well what can I say, the Audi design team have designed an engine that is 50kg too heavy and its hugely compromised their KERS choice......aww bless........ and thats just hard cheese as far as i'm concerned, I hope they get humiliated at LeMans.....amen.
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Old 4 Jun 2014, 13:53 (Ref:3415188)   #759
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ooohh..... juicy engine talk.....I do engines........the main reason the likes of F1 in the normally aspirated V10 & V8 era went massively over square was to reduce friction in the piston to cylinder interface, with a big bore which I think was around 98mm, and a small stroke, I think around 30mm or so, the side load emitted from the con-rod was minimal, friction is reduced with load (not area) and the rpm shot up as a result......the same applies to modern high performance and race diesels, hence they are now slightly over square, but only by a few mm.

the low compression ratio diesels (circa 14:1) are now preferred also because of a significant efficiency increase whereby the engine is not fighting the gas loads on the compression phase, all road car disels are now going this route for this very reason.

Regarding flat plane cranks, for the past 10 years or so the NASCAR teams have been running very light-weight cross-plane cranks with heavy-metal tungsten inserts, that are comparable in weight to that of a flat plane crank, so recently the weight issue is not such a big deal, but certainly a light weight crankshaft makes a race engine very very snappy and responsive.......the main and primary reason that people use flat-plane crankshafts is to get the exhaust pulsing correctly and evenly for power.......the cross plane cranks have a burble, due to the un-even firing, and the flat plane cranks basically sound like 4 cylinder engines, because thats exactly what they are!

But again even the NASCAR boys have figured the exhaust pulsing problem out, whereby they now run tri-Y manifolds that are coupled in such a fashion that the exhaust pulses are equal.......honsetly if I was designing a V8 LMP1 engine I would seriously consider using a cross-plane crankshaft, as the reliability issues and virbarions from a flat plane crank shaft are nothing but a nightmare for the chassis and general reliability.......I suspect Toyota know this and have probably very very carefully designed their engine in order that this is not a problem with a crankshaft that has critical resonant frequencies well away from the operating region, or a viscous damper on the crank-nose, or twin counter-rotating balance shafts........time will tell

somewhat off topic now!......regarding the R18 engine, well what can I say, the Audi design team have designed an engine that is 50kg too heavy and its hugely compromised their KERS choice......aww bless........ and thats just hard cheese as far as i'm concerned, I hope they get humiliated at LeMans.....amen.
The sidewall friction is dependent on the connecting rod/stroke ratio too. So conceivably you could have low friction even with a very long stroke if you have a super long connecting rod =D

I really hope Toyota wins this year. I've been rooting for a second Japanese win and they have come close before!
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Old 4 Jun 2014, 15:40 (Ref:3415251)   #760
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but then the engine would be so tall the engines high centre of gravity would make any car handle like a boat!.......Toyota said their new LMP1 engine has a crank centre height something like 16mm lower than the customer version that they provide to Rebellion, so the short stroke engine theory also has a significant impact upon chassis dynamics and general handling
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Old 4 Jun 2014, 17:26 (Ref:3415301)   #761
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ooohh..... juicy engine talk.....I do engines........the main reason the likes of F1 in the normally aspirated V10 & V8 era went massively over square was to reduce friction in the piston to cylinder interface, with a big bore which I think was around 98mm, and a small stroke, I think around 30mm or so, the side load emitted from the con-rod was minimal, friction is reduced with load (not area) and the rpm shot up as a result......the same applies to modern high performance and race diesels, hence they are now slightly over square, but only by a few mm.
I am pretty sure that all high revving engines are oversquare because the bottleneck is piston speed, and because you can fit bigger valves, and at high rpm that makes a big diffrence
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Old 4 Jun 2014, 18:36 (Ref:3415341)   #762
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I found it interesting Kinoshita San said if they were to do a brand new engine, it would be 4.0L V8. I hope this is realized and they decide to go this route in the near future. Also interesting is that the electric power is limited by the supercap and its size/weight. They have definitely made gains from it to push out over 480hp while keeping the same size. Toyota also have a few new tricks that aren't currently applied to their car.

One is this- http://newsroom.toyota.co.jp/en/detail/2656842/ a silicon carbide power semiconductor. Reducing power loss from regenerative braking and electric power sent to the electric motor. This could add 5% to fuel economy on road cars and it reduces the PCU size by 80%. Theres definitely plenty to come from Toyota if they choose to do so.
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Old 4 Jun 2014, 19:51 (Ref:3415391)   #763
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I doubt that supercap is limiting factor, he said input/output to supercap, he could have meant the inverter (PCU).

Current PCU adds weight, silicon carbide could shave off some of that weight and also improve charge/discharge efficiency. The question is if this technology is mature enough to be used, it very well could be because the article states testing on a public roads within a year. Maybe they already have it?
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Old 4 Jun 2014, 21:02 (Ref:3415428)   #764
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I am pretty sure that all high revving engines are oversquare because the bottleneck is piston speed, and because you can fit bigger valves, and at high rpm that makes a big diffrence
The bottle neck is piston acceleration not piston speed. High piston speed is not necessarily bad, but is often so because it typically entails high piston acceleration.

What he said is correct, shorter strokes tend to lead to lower piston friction. First off when running at the same engine speed the piston speed is lower if the stroke is short. Secondly, if the connecting rod is a similar length to that of a longer stroke engine, the max connecting rod angle is smaller. This means the component of the force transmitted through the connecting rod that is pressing the piston into the cylinder wall is smaller.

Power used to overcome friction = friction coeff * force pushing piston into cylinder wall * piston speed

These two things lead to lower friction in a shorter stroke engine.
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Old 4 Jun 2014, 21:10 (Ref:3415433)   #765
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I am pretty sure that all high revving engines are oversquare because the bottleneck is piston speed, and because you can fit bigger valves, and at high rpm that makes a big diffrence

I would also agree with that, but as above its acelleration and therefore low mass, not speed.....really there are many reasons......all of them totally un-related to road car technology for the mass market!

Last edited by knighty; 4 Jun 2014 at 21:15.
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