Crank bearing carrier or caps ?

[Guest Martin F]

Not particularly Supra related, but one of my curious technical questions.

 

On some engines i notice they have a bearing\crank carrier which seems to bolt to the bottom of the block and hold the crank bearings in place compared to others which use individual caps for the crank bearings, i.e. like the 2JZ or 7M engines

 

Anybody know what the merits are of using one technique over the other.

[Chris Wilson]

Ladder frame caps tie the bottom of the block together fro more rigidity, givng more stable bearing housing alignment and better bore stability through strength. They can also dissipate bearing heat into the sump better. Most modern race and high performance engines have gone to this set up. On some race engines the sump itself is dowelled or keyed to the block and becomes an integral part, some even use the sump as the bearing caps casting.

[Guest Martin F]

Thanks Chris. It was actually the K series VVC engine that i saw this on (MGF), but i'm sure i've seen it on others.

[Chris Wilson]

Skyline engines use it, it's been around for years, peugeot/renault/volvo (PRV) V6 was one of the earliest mass production road car engines to use it. More common on ali blocks than cast iron.

[Digsy]

As Chris says it all about stiffness. As with so many things in life, the stiffer your bottom end, the better.

 

You can have a ladder frame which links the individual bearing caps together, and either a shallow-skirted or a deep-skirted block, depending on where you want the block / oil pan joint to be. Shallow skirted blocks stop at the crank centreline so the whole bottom face is machined flat in one operation. Deep skirted blocks carry the block structure down past the crank centreline and have cutouts for the caps or ladder. Deepskirted blocks are especially good if you want to cross-bolt the main bearing caps or ladder frame into the cylinder block as well as using vertical bolts.

 

Best of all is the "upper / lower crank case" design where you have a shallow skirted block and a second casting which incororates the main caps and deep skirt all in one. The whole cylinder block hoizontally splits in two along the crank centreline.

 

Again, as Chris points out, it is usual to close the whole block at the bottom with a structural oil pan which is made from cast aluminium. This gives the whole bottom end much more rigidity than an old-skool pressed steel oil pan and also allows you to tie the transmission case or bell housing into the block structure as well. You can also flare out the bottom edge of the lower crankcase into a wide, flat oil pan and simply close off the bottom with a flat pressed steel panel, which without going into a lot of detail is a hell of a lot easier to manufature than a 100% cast "winged" oil pan.

 

I think using the actual sump to carry the bearings is pretty rare on road cars at the moment. It may be more common on race engines with a dry sump where you don't need the depth in the oil pan.

 

If you really have your head screwed on at the design stage you can also stick the oil suction pipe in the ladder frame / lower crankcase. Its a good place to put the oil pump housing as well. Other components which you could also integrate if there is enough room are balance shafts and windage trays.

 

Aluminium ladder frames usually have cast iron inserts to take the direct loading from the crankshaft / bearing bolts.

[Digsy]

Edited to remove double post.

[Digsy]

The Rover I4 K-series is unique in that the head bolts go straight through the block and into the ladder frame. If you take the head bolts out (I think they are studs, actually) some non-structural bolts hold the ladder to the block and stop the bottom end falling off.

[Guest Martin F]

and thanks to you too.