4 replies to this topic

[Top Fuel F1]

Regarding purpose built engines it is well known that where high torque is desired it is important to have good leverage at the crank. That is the crank's throw (stroke/2) needs to be relatively long. The throw is measured from the centerline of the crank to the center of the crank's rod journal. In this case, because the throw will be relatively long, the rod will be relatively short (measured from the center to center of the circular opennings on the rod). This combination has the throw and the rod operating at a relatively steep angles to each other at times. For instance on the power stroke the rod is pushing the crank at a sideway angle placing greater loads on the rod, the crank's rod journal bearing and forcing the piston against the cylinder wall (piston/cylinder side loading). If such engine configurations are run at higher and higher RPMs there is more and more piston/cylinder friction (heat, power loss, wear), crank rod journal bearing axial loading (bearing wear) and stress on the rods. Further if such a design was run at high RPM for substained periods of time (just for example, like NASCAR stock cars) it would be difficult to keep the bottom end together. Also the engine would not be operating very efficiently.

So to run at substained high RPMs, whether the medium high RPM stock car or the very high RPM F1 cars, the crank to rod angularity needs to be decreased. To do this the crank throw must become relatively shorter and the rod relatively longer. A byproduct of doing this is an increase in the dwell time at both TDC and BDC. The extra dwell at TDC helps in affording the time necessary at high RPM for efficient combustion and pressure building. Another byproduct is that as the piston leaves either TDC or BDC it accelerates more quickly to a higher velocity. This can only help in maintaining volumetric efficiency and aid in the dynamics at play in the exhaust pipe collectors that interact with what is going on with other cylinders sharing that particular collector. Obviously the highest *R/S ratios used are in F1, where the very high RPM begs for reliability of the bottom end, the least amount of friction (heat, power loss and wear) and sufficient time for combustion and pressure building.

*Note: Numerically the rod ratio (some value X to 1) is calculated by dividing the rod length by the stroke (2 x throw) length: R/S = X.

The right R/S for an engine such as F1 is first evaluated with very comprehensive simulation comprised of many models derived from both mathematics and empirical data. In any event if one pursues a greater ratio too far the yield of the benefits good for combustion is not as great and the torque diminishes too much. Another important consideration in F1 engine design concerns the vibration of the crank. I imagine that the lower the R/S ratio, the greater the exposure to this.

Rgds;

[Halfwitt]

From the numbers being discussed for rod ratioon previous threads, wouldn't rod length divided by crank RADIUS (i.e. half of the stroke) be the correct formula?

[desmo]

No it's diameter (stroke). In fact in the old days rod ratio was often referred to as simply l/d. I haven't seen that used in a while though.

[Top Fuel F1]

Originally posted by desmo
No it's diameter (stroke). In fact in the old days rod ratio was often referred to as simply l/d. I haven't seen that used in a while though.

desmo:

Yes, you are correct. The Throw is (Stroke/2)

[marion5drsn]

The Crankshaft radius to rod C/L to C/L ratio (L/R) discussion has been going on for years. If you have a copy of C.F. Taylor’s book on engines Volume 2 look on pages 461 to 468 and you will find that no one agrees on any one ratio. They run all the way from 3.1 to 4.8. It would not surprise me that GP engines have even longer rod ratios.

Somewhere I found a table and analysis of con rod geometry and he claimed that a con rod only needed to be 3.78 to 1 ratio. He also claimed that was as long or short (Optimum) as it ever needed to be. Obviously a lot of people disagree. He took about five pages to do this and had all sorts of formula to “prove” it.
It seems that a lot of the designers feel a need to keep it as short as possible to lower the frontal area. Other engineers feel that rods need to be longer just to keep the engine together. M. Anderson
Edit Oct.Sun.09 2001 The above Optimum Ratio was by Kimball J. Franklin Inst.246(1948