Rod length to stroke ratio of v6 turbo engines.

[countersteer]

Here's some images and graphs I found while researching this thread.

[strad]

well if I was better with a drawing program I'd draw you a picture.
Although I would not have thought it necessary .

[PlatinumZealot]

Fede he is saying that the peak combustion pressure has the largest mechanical advantage when the moment arm is at its maximum length and in theory that would be when the angle between the connecting rod and the crank is 90 degrees on the downstroke. In theory! In real life the Combustion pressure is acting over a range of crank angles and the crank is also moving by very fast.... So the exact timing of where you want to 'centre' the pressure peak is a bit of experimentation with the spark timing. What does rod length have to do with this you ask? If tge rod is long you have a longer dwell time near tdc when the rod is shorter the dwell time at tdc is shorter. Depending in the combustion and engine speed this dwelling period affects when the peak pressure acts and also the side wall load of the piston and of course the stress that acts on the connecting rod.

For example... The honda type R engines have a greater than 1.6 rod to stroke rario versus the regular engines. They are renowned for really long connecting rods. This gives less sidewall loads at high revs because the rod is more vertical on the power stroke and it allows most of the foce to go downards to the crank. This downwards force must be applied at thw right crank angle for maximum effect of course. On the other hand a shorter rod can be lighter and it can allow slightly more aggressive timing because it has a slower velocity on the downstroke. This slower velocity gives more time for the pressure to push down.

As you can see there can be a bit off a trade off for the two philosophies. I didnt know what sort of rod ratios are typically used in f1 why i started this thread... But u learned know that even a "short" rod ratio in formula 1 is still freakishly long compared to a street car because the f1 crank stroke is so small. The counterweights are still in the way so the pistons cannot be made to be too close to the crank. So the rods are relatively long. So a short and long rod ratio is f1 is relative. I was really wondering how these new regulations affect the design of the rod length.

[gruntguru]

Truth is, all this mechanical advantage stuff is B.S.

The work done by the expanding gas in the cylinder is equal to the integral of P.dV This is "indicated work per cycle" - a close relative of indicated power and indicated mean effective pressure (IMEP)

The piston/rod/crank mechanism does not modify the total work done by the gas, except for friction losses which obviously must be subtracted. What remains is "brake work per cycle" - a close relative of brake power and brake mean effective pressure (BMEP)

So any increase in "instantaneous torque" obtained by changing the mechanical advantage at some point in the power stroke is offset by the increase in "rate of volume change" divided by "rate of crank angle change" (dV/dTheta). It is obvious that dV/dTheta (ie piston velocity/crankpin velocity) is just the inverse of the mechanical advantage.

[PlatinumZealot]

I am sorry but I cannot agree with your thinking that the "timing" of the explosion does not matter. That is essentially what you are saying.

There velocity of the piston and the position of the crank and rod will affect how the power is directed. Imagine pushing a cart that is pushing back on you, Vs a cart on level surface from standstill, Vs a cart that is already in motion... They all require different amount of effort to accelerate to the same final speed.

[Fede44]

Thanks for the plots and explanations guys

Just to be clear, lets fix the crankshaft big end journal at....10-13º ATDC This angle is no coincidence, but usually where peak cylinder pressure happens to be for maximum power output per cycle. And it also happens that, at that point around 50% of the fuel mass has been burned (which, also, happens to be good in terms of minimizing heat release!).

So, knowing where we have our maximum compressive forces we can only change the con rod length and analyse the different scenarios. In a few words, a short con rod will have a slightly bigger angle in relation to the cylinder axis. The normal component (sine of beta angle) will give the thrust load and define the friction in between piston assembly and cylinder wall.

A longer rod will reduce this thrust loads, reducing friction. Same thing can be done by offsetting crankshaft axis against cylinder axis, but only during half of the strokes (either going up or down, depending on the offset direction: in which direction would you move your crankshaft?)

This is friction, now we should look at the combustion dwell angles and so on. We will probably find out that the best thing we can do is make the engine as short as possible, but for aero and chassis gains, not really mechanical/combustion gains as this two will probably cancel out each gains - losses

[gruntguru]

I am sorry but I cannot agree with your thinking that the "timing" of the explosion does not matter. That is essentially what you are saying.

There velocity of the piston and the position of the crank and rod will affect how the power is directed. Imagine pushing a cart that is pushing back on you, Vs a cart on level surface from standstill, Vs a cart that is already in motion... They all require different amount of effort to accelerate to the same final speed.

Varying the Volume-Crankangle relationship by varying l/r ratio or by using different mechanisms like scotch yoke or pullrod cannot improve the conversion of combustion energy to mechanical energy unless you reduce friction (and friction losses in a modern engine are already quite low).

It really is simple. The p-v diagram defines how much work (energy) is done by the expanding gases. A torque-angle diagram tells you how much work the con-rod performs on the crankshaft. The difference between these two amounts of energy is the losses due to friction, windage etc. Energy must be conserved.

Changing the v-theta relationship will certainly change the Torque-Theta relationship but any increase in instantaneous torque at particular crank angles will be offset by a reduction in torque at other angles.

[Tommy Cookers]

......
A longer rod will reduce this thrust loads, reducing friction. Same thing can be done by offsetting crankshaft axis against cylinder axis, but only during half of the strokes (either going up or down, depending on the offset direction: in which direction would you move your crankshaft?)
This is friction, now we should look at the combustion dwell angles and so on. We will probably find out that the best thing we can do is make the engine as short as possible, but for aero and chassis gains, not really mechanical/combustion gains as this two will probably cancel out each gains - losses

is offset (as above, or rod offset by articulation as in radial, V, and W aero engines) even allowed ???

ie offset as above will be asymmetrical in eg the current F1 V6s - is asymmetry allowed ??
(symmetrical ie 'handed' offset helps package eg VW VR5, VR6, and 80s V twin Morini motorcycle, but has no sidethrust benefit)
the engine dimension rules are intended to give easy switching between makers

and do/don't the current rules on engine min height etc dimensions also strongly drive rod ratio ???

[Fede44]

......
A longer rod will reduce this thrust loads, reducing friction. Same thing can be done by offsetting crankshaft axis against cylinder axis, but only during half of the strokes (either going up or down, depending on the offset direction: in which direction would you move your crankshaft?)
This is friction, now we should look at the combustion dwell angles and so on. We will probably find out that the best thing we can do is make the engine as short as possible, but for aero and chassis gains, not really mechanical/combustion gains as this two will probably cancel out each gains - losses

is offset (as above, or rod offset by articulation as in radial, V, and W aero engines) even allowed ???

ie offset as above will be asymmetrical in eg the current F1 V6s - is asymmetry allowed ??
(symmetrical ie 'handed' offset helps package eg VW VR5, VR6, and 80s V twin Morini motorcycle, but has no sidethrust benefit)
the engine dimension rules are intended to give easy switching between makers

and do/don't the current rules on engine min height etc dimensions also strongly drive rod ratio ???

Hi, yes, I was just generally speaking. I went off the current F1 regulations, sorry for that.

I agree with you on the fact that within current engine block and cylinder head restrictions (length, CG, etc), the 4 engine manufacturers must probably use the same rod length. The stroke is given. So it's up to the piston design, then. Distance from compression ring to top, distance in between compression ring and oil ring, and then the piston pin as close as possible to the oil ring.

I will give a close look to the rule book and come back with a number, just a guess of course.