S65 engine weight - and some surprises!

[swamp2]

Your additional parenthesis indicate the correct formula I derived. It was only a typo on my behalf.

Originally Posted by CanAutM3

I disagree; I still believe the previous conclusion is accurate where an increased torque arm is offset by a smaller piston area. I thought I could demonstrate this through instantaneous torque, but I was mistaken. The math is more complex and my previous demonstration was oversimplified and inaccurate. I now realize that to complete the demonstration, the full cycle of the engine needs to be considered.

That's what I did...

Using the exact formula, and approximating the engine pressure vs. angle (so called indicator diagram). I performed a numerical integration of torque vs. angle for a full power stroke, since the pressure is negligible outside of this region, the integral represents a full 720° combustion cycle. Again altering nothing but the stroke (by a very significant change) changes the integrated (averaged) torque by a couple of percent. Did you miss this? Do you think the results would be in anyway affected by the exact form of the indicator diagram (of which I clearly do not have an analytical expression).

Originally Posted by CanAutM3

So for an engine operating at steady state, the inertial terms should cancel out through an entire 4 stroke cycle, wouldn’t they ?

Very good point, which I have overlooked. Since those terms generally have sin(nx) terms in them, they should indeed average out to zero over one or two crank revolutions.

Originally Posted by CanAutM3

I’ll try to present it from a different perspective using the conservation of energy principles instead. This gets rid of all the geometric and inertial factors.

Assume two isentropic engines operating at the same constant RPM for which all parameters are equal (cylinder count, displacement, compression ratio, volumetric efficiency, thermal efficiency, con-rod length, etc...) except for the bore and stroke.

Both engines are sucking-in the same amount of intake charge (same displacement, RPM and VE). So, for a given amount of rotations, the exact same amount of energy (chemical here) is input to the system. The output energy is measured by the torque produced over the given amount of rotations (work). If the engine with the longer stroke generates more torque, where is the other engine loosing its energy to? I know, this is still very simplified, but provides food for thought.

Equal torque does not seem to (trivially) follow from equal energy. Perhaps equal power but at a fixed rpm that would be equal torque. I really can't make any conclusions myself based on this line of reasoning.

Originally Posted by CanAutM3

From a design perspective, varying the bore and stroke ratio allows other parameters to be altered such as volumetric efficiency, valve size, blow-by losses, friction losses and many more that will vary the torque output depending on operating conditions (low vs high RPM for example). But IMHO, the conclusion remains, for a given displacement, the mechanical benefit of a longer torque arm is offset by a smaller piston area.

Agree with the first part, but again, I've satisfied myself with the numerical integration that "offset" is not exact due to the cylinder pressure variation during the power stroke.

Either way, I'm now pretty satisfied that for an engineering approximation, the original statement is largely true. Now if someone qualified ran a simulation with the Ricardo engine software and found otherwise I would trust the simulation over our somewhat basic analytical approach.

[CanAutM3]

Originally Posted by swamp2

Using the exact formula, and approximating the engine pressure vs. angle (so called indicator diagram). I performed a numerical integration of torque vs. angle for a full power stroke, since the pressure is negligible outside of this region, the integral represents a full 720° combustion cycle. Again altering nothing but the stroke (by a very significant change) changes the integrated (averaged) torque by a couple of percent. Did you miss this? Do you think the results would be in anyway affected by the exact form of the indicator diagram (of which I clearly do not have an analytical expression)?

No, I haven't missed this. A 1%-2% is relatively small error for an approximation.

I have not seen your full integration, so it is difficult to comment. The volume of the cylinder as a function of crank angle will not be the same with a different stroke/bore ratio; which in turn affects the cylinder pressure vs crank angle function. Did your integration take this into consideration? Or maybe it is as you say, that a more accurate representation of the gas expansion is required.

The compression stroke does consume a fair bit of torque. But IMO, the same principles apply and the bore/stroke ratio should not impact the torque consumed in the other strokes.

Originally Posted by swamp2

Since those terms generally have sin(nx) terms in them, they should indeed average out to zero over one or two crank revolutions.

For a four stroke engine, it would have to be on 2 full crank revolutions, wouldn't it?

Originally Posted by swamp2

Equal torque does not seem to (trivially) follow from equal energy. Perhaps equal power but at a fixed rpm that would be equal torque.

Work, a form of energy, is equal to force over a distance. In the polar referential, work equals to torque over revolutions (just to clarify, we are talking revolutions here, not RPM which is an angular velocity). (Link to Wiki)

W = T * θ

Integrating the torque produced over two revolutions of the engine (from θ=0 to 720 deg or 4pi rads to cover the entire 4 stroke cycle) gives the energy output.

Originally Posted by swamp2

I really can't make any conclusions myself based on this line of reasoning.

I've satisfied myself with the numerical integration that "offset" is not exact due to the cylinder pressure variation during the power stroke.

Think about the energy model some more. If you are not inputing more energy into a system, you cannot get more out of it. The only way a larger stroke/bore ratio engine could produce more torque, is if it is thermodynamically more efficient; which I don't see how it can be by only changing this ratio (think of the P-v T-s charts of the Otto cycle).

Thoughts?

Originally Posted by swamp2

Now if someone qualified ran a simulation with the Ricardo engine software and found otherwise I would trust the simulation over our somewhat basic analytical approach.

Agreed, it would be interesting to see the output of a sophisticated simulator.

[Notnug]

Originally Posted by CanAutM3

No, I haven't missed this. A 1%-2% is relatively small error for an approximation.

I have not seen your full integration, so it is difficult to comment. The volume of the cylinder as a function of crank angle will not be the same with a different stroke/bore ratio; which in turn affects the cylinder pressure vs crank angle function. Did your integration take this into consideration? Or maybe it is as you say, that a more accurate representation of the gas expansion is required.

The compression stroke does consume a fair bit of torque. But IMO, the same principles apply and the bore/stroke ratio should not impact the torque consumed in the other strokes.



For a four stroke engine, it would have to be on 2 full crank revolutions, wouldn't it?



Work, a form of energy, is equal to force over a distance. In the polar referential, work equals to torque over revolutions (just to clarify, we are talking revolutions here, not RPM which is an angular velocity). (Link to Wiki)

W = T * θ

Integrating the torque produced over two revolutions of the engine (from θ=0 to 720 deg or 4pi rads to cover the entire 4 stroke cycle) gives the energy output.



Think about the energy model some more. If you are not inputing more energy into a system, you cannot get more out of it. The only way a larger stroke/bore ratio engine could produce more torque, is if it is thermodynamically more efficient; which I don't see how it can be by only changing this ratio (think of the P-v T-s charts of the Otto cycle).

Thoughts?



Agreed, it would be interesting to see the output of a sophisticated simulator.

Very interesting stuff guys. This is what forums should be about: Learning! Like others have said if you don't want "technical talk" go to the general section for all your desires. Great thread.

[k-lo]

I now officially miss the General M3 Section. j/k

Great thread and thanks for all the great technical posts.

[Nordstat]

I'll say, as an engineer myself, this has been the best thread I have seen in AGES! Real technical discussion without breaking into slander and insults.

Well done, sirs!

[swamp2]

Originally Posted by CanAutM3

The volume of the cylinder as a function of crank angle will not be the same with a different stroke/bore ratio; which in turn affects the cylinder pressure vs crank angle function. Did your integration take this into consideration?

This is another "hidden" variable a bit like our last gearing/losses debate. I did not account for it.

Originally Posted by CanAutM3

For a four stroke engine, it would have to be on 2 full crank revolutions, wouldn't it?

Those particular terms still average to zero over one revolution for a single cylinder engines. I am not sure about the math for multiple cylinders, likely not.

Originally Posted by CanAutM3

Work, a form of energy, is equal to force over a distance. In the polar referential, work equals to torque over revolutions (just to clarify, we are talking revolutions here, not RPM which is an angular velocity). (Link to Wiki)

W = T * θ

Integrating the torque produced over two revolutions of the engine (from θ=0 to 720 deg or 4pi rads to cover the entire 4 stroke cycle) gives the energy output.



Think about the energy model some more. If you are not inputing more energy into a system, you cannot get more out of it. The only way a larger stroke/bore ratio engine could produce more torque, is if it is thermodynamically more efficient; which I don't see how it can be by only changing this ratio (think of the P-v T-s charts of the Otto cycle).

Thoughts?

Work isn't a form of energy... Doing work changes a systems energy.

Other than that your basic outline here seem plausible on the surface, less the massive amount of loss in an ICU...

One can take pressure to torque perhaps missing only frictional losses. Taking torque to work or energy not taking into account that large majority of the fuels energy goes to engine losses and the large majority of an engines losses are heat losses is invalid.

Originally Posted by CanAutM3

Agreed, it would be interesting to see the output of a sophisticated simulator.

For me this is probably the ultimate answer to this question. Of course properly operating such a simulator is highly non-trivial and will be prone to garbage in garbage out.

[swamp2]

Originally Posted by Nordstat

I'll say, as an engineer myself, this has been the best thread I have seen in AGES! Real technical discussion without breaking into slander and insults.

Well done, sirs!

This site has long been a place for some very good technical discussions. Glad some enjoy it, I certainly do. CanAut and I are buds (perhaps not obvious!) and we probably agree about 95% of things.

[acidandroid]

this is almost 3 years old thread, don't know if you guys are still here

i just spent an hour going through all 5 pages with formulas!
my head is smoking right now i think

conclusion:

With fixed displacement, longer stroke makes more torque, but not as much?
the major reason that LS makes great torque is actually more because of large displacement rather than longer stroke right?
longer stroke in LS engine contribute to some the better torque but not much right???

am i right?

[Leonardo629]

I guess it depends on your definition of "not as much".

[Malek@MRF]

Originally Posted by Billj747

The Porsche GT3 RS 4.0 has 339lb-ft (44lb-ft more than the S65).


The BOSS 302 has a 7,500rpm stock redline and has no problem at 8K (S65s seem to be fine at 8,750) -pretty close IMO.

Originally Posted by Billj747

991 GT3 is a 3.8L -So 29lb ft more torque and 200cc less.

The E46 has 269lb-ft out of a 3.2L

The 991 is direct injection and in a flat six configuration.

There is a substantial difference between engines that can spin to 7500 compared to one that spins to 8500+ rpms.

[BMRLVR]

Originally Posted by acidandroid

this is almost 3 years old thread, don't know if you guys are still here

i just spent an hour going through all 5 pages with formulas!
my head is smoking right now i think

conclusion:

With fixed displacement, longer stroke makes more torque, but not as much?
the major reason that LS makes great torque is actually more because of large displacement rather than longer stroke right?
longer stroke in LS engine contribute to some the better torque but not much right???

am i right?

The 7.0 LS7 makes less torque per litre (67.08 BHP/L) compared to the S65 (73.75 BHP/L) so I can't quite understand what you are saying above?

[TrevorM3]

my head hurts

[FiveOhFour]

Originally Posted by acidandroid

this is almost 3 years old thread, don't know if you guys are still here

i just spent an hour going through all 5 pages with formulas!
my head is smoking right now i think

conclusion:

With fixed displacement, longer stroke makes more torque, but not as much?
the major reason that LS makes great torque is actually more because of large displacement rather than longer stroke right?
longer stroke in LS engine contribute to some the better torque but not much right???

am i right?

After all that they couldn’t even answer you 😂

[Scharbag]

Good read, my head also hurts.

It seems as though engine building is anything but simple. Perhaps that is why, after 150 + years, that new and cool Otto cycle engines are being developed. And mad props to the builders of high performance NA engines. They do not get to cheat like FI designers. Every single ounce of torque must be cajoled from the laws of physics. Not disparaging FI engines, but there is just something special about a well designed NA engine. And one thing FI engines can rarely provide is the instantaneous throttle response of a high compression NA engine.

Kudos to all of the smart people in the world. And just imagine the look on old Otto's face if you showed him a NA V10 F1 engine from 2005.

Pretty cool stuff.