IC Engine Connecting Rod Mass Moment of Inertia Measurements

[Dr.D]

Does anyone know of a source, on the Internet preferably, for component inertial data for a real IC engine? In particular, what I'd like to obtain (for some real engine, any engine) are these items:

1. Piston weight (including wrist pin)
2. Cylinder bore dimension
3. Connecting rod length (center-to-center)
4. Connecting rod weight
5. Connecting rod center of mass location
6. Connecting rod mass moment of inertia
7. Crank pin diameter
8. Crank throw radius
9. Main bearing diameter
10. Engine type (2/4 stroke), make, and model

The major sticking point is item #6. I'm familiar with the approximation of the con rod as two point masses, but I'm trying to avoid that and use an actual measured value if possible.

If anyone has a source for this sort of data, it would be much appreciated. Thanks.

 

[Ranger Mike]

When racers are restricted to sanction rules that say you have to use “stock” engine parts, we “blueprint” the engine.

This means the factory specifications are used to the maximum advantage…i.e we go for the maximum cylinder bore permitted, minimum rotating and reciprocating weight.

One thing to note on the con rod, we don’t care about the particular weight on the big end or small end, we look at the total and pick the lightest con rod of the 8 ( I am using the 350 cubic inch small block Chevy V8 as it is the most popular. Once we find the lightest weight of the 8 con rods, we have to measure the small end weight (wrist pin) of the lightest rod. We usually measure all 8 rods to see if the heaviest has enough material that can be removed to lighten it. We do the same process on the big end ( crank throw end). Production rods usually have a lot of material that permits this matching.
i inserted a photo showing the weight scale on the right and the big end holding fixture on the left. We have a small end holding fixture as well. Both have a bearing to remove rotating friction when we scale the rod.
I assume your reference to crank throw is the piston stroke.

1. Piston weight (including wrist pin) Piston weight 20.41 oz. wrist pin weight – 6.1 oz 0.92” diameter
   2. Cylinder bore dimension - 4 inch
   3. Connecting rod length (center-to-center) - 5.7”
   4. Connecting rod weight 20.8 oz
   5. Connecting rod center of mass location total weight is 20.8 oz (5.8 oz on small end)
2. 6. Connecting rod mass moment of inertia - see utube video below
3. 7. Crank pin diameter - 2.099 to 2.100”
   8. Crank stroke – 3.48”
   9. Main bearing diameter – in race engines we run 0.0025” to 0.0035” clearance so add this clearance to the actual crankshaft pin diameter – 2.100 + 0.003” = 2.103” dia. Bearing bore.
   10. Engine type (2/4 stroke), make, and model – GMC or Chevrolet 350 CID V8 4 stroke
4. utube video on balancing

What about the valves and their weight? Camshaft ? rod bearings?

 

[Dr.D]

Thank you for your response, RangerMike. I hope that your engine books provide some information when you get to them.

Let me clarify one point. My interest is not in improving engine performance per se, but rather in the mathematically correct, accurate description of engine dynamics.

I am certainly aware of the long established practice in engineering to model the connecting rod as a pair of point masses, one at the big end and one at the small end. The problem is that this is an approximation, not consistent with a rigid body description. I showed this clearly in an ASME IC Engine Division Paper (ICEF2014-5436) several years ago. This practice properly accounts for the mass of the connecting rod and the location of the center of mass, but it does not get the mass moment of inertia correct. In many cases, it over estimates the MMOI, but not always. This is widely practices; there are countless papers using this method, but it is still not mathematically correct.

This is why I am seeking actually experimentally determined (or FEA computed) mass moment of inertia values for actual connecting rods.

The video showing the weighing and grinding of a set of connecting rods simply shows efforts to lighten the rods and to put the center of mass of each in approximately the same position. The use of the term "balancing" seems strange unless it is intended to convey greater inertial similarity. It really does not achieve uniform rotational dynamic response characteristics.