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Boxer engines (also called flat or horizontally opposed) are used by Subaru, Porsche, BMW motorcycles, and aircraft because they are compact, have a low center of gravity, and are naturally well-balanced.
One disadvantage to this design is that the crankcase has a vertical split down the middle, with
bolts holding everything together. In order to rebuild the engine and replace the crankshaft or bearings, the engine first has to be removed from the vehicle and completely disassembled. Also, the vertical split compromises the rigidity of the engine block and can lead to problems like head gasket failures (just ask any Subaru owner).
Instead of a vertical split, we can cast the entire engine block in one piece, with only a small rectangular cut out on the bottom to access the crankshaft and main bearings. This cutout could be filled by individual main bearing caps, or it could be a single unit like those sometimes found on high-performance V8 engines. Cross-bolts would tie the block together horizontally.
This might be more expensive to manufacture, since there would be three mating surfaces between the lower bearing block and the engine block, all of which would have to be precisely machined to fit properly. And holes for the cross-bolts would be tricky to drill, but this has been accomplished with V8 engines. They could also be drilled at an angle for ease of manufacture.
The advantages would be a more rigid engine block with less complicated passages for oil and coolant. It would be easier to assemble and service, and more reliable.
Boxer Engine
https://en.wikipedia.org/wiki/Flat_engine
[discontinuuity, Jan 12 2019]
Cross-bolted main bearings
http://www.mustanga...ssbolt-blockbuster/
note the bolts going horizontally into the engine to hold the bearing caps more securely [discontinuuity, Jan 12 2019]
MS Paint Diagram
https://i.imgur.com/eJtn9pz.jpg
Red lines represent bolts holding the main bearing caps into the block [discontinuuity, Jan 12 2019]
Offenhauser engine rebuild
https://www.hotrod....ndycar-engine-step/
with unusual main bearings in a one-piece crankcase [discontinuuity, Jan 13 2019]
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Annotation:
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I see one problem with your engine as depicted in the third
link. If I understand correctly, you have a bottom
removable section that has to form a tight seal on three
sides (top, left, right). The top seal will be fine, but the
left and right seals can't be tightened up without distorting
the main block. |
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You might be better off with a wedge-shaped removable
part; bolts would pull it upwards, tightening the seal on the
two mating diagonal faces. |
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So, effectively, the caps would be on the side of the
engine were it conventionally oriented? Are these
cracked caps or simply machined? |
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This sounds very like some of the Lycoming (now TRW) boxer aviation engines, where the crankcase is a one-piece machining but the cylinders and liners are separate (air-cooled) items. |
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They are very simple, reliable, proven designs. Not overly efficient, not a staggeringly good power to weight ration, but sturdy and long lived and most of all will often keep plodding along when quite serious bits break. In a single-engine aircraft that is widely thought of as a Good Thing. |
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"You might be better off with a wedge-shaped removable part; bolts would pull it upwards, tightening the seal on the two mating diagonal faces." |
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Hey Maxwell, that's a good idea. It would probably be easier to machine, too. |
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In my mind the bearing cap(s) wouldn't seal the engine; there'd be a separate oil pan and covers on the front and back, similar to a conventional inline or V engine. |
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Rayford: yes, if this was an inline engine it would have bearing caps that bolt on from the side. Which could be an advantage depending on what direction the bearing loads are going. |
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8th of 7: IIRC Lycoming engines (and aircooled VWs) still have a split crankcase, but also separate cylinders. |
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Those aircraft engines are usually way overbuilt, using something like 5L of displacement to make ~150hp. |
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Yes, that's to try and ensure a more positive experience for users, often referred to as "not dying". It's one of the most important aspects of being a pilot. |
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We'll check the specs, and that's probably true for current production, but some of the early 4-cyl units might have had a one-piece crankcase. They were a sod to service as it meant taking the pistons out still in their cylinders, accessing the bearing endcaps through a small removeable plate. |
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//The advantages would be a more rigid engine block |
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I did not know rigidity of engine blocks was something that
would benefit from improvements. Perhaps other ways to
do this would be to make an engine block with XXXs welded
to the outer surface, or even just making engine blocks 10%
bigger and machining out the areas between the XXXs that
caused greater rigidity. |
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8th of 7: I wonder if they're like an Offenhauser engine, where the bearing blocks are assembled around the crankshaft and then lowered into one end of the crankcase (see link). |
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beanangel, I'm not sure what you mean by "XXXs." Do you mean external cross-bracing? |
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[discontinuuity] Right, cross bracing. A little like the idea
of how box-beams bend and flax less only the flat version. |
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// lowered into one end of the crankcase // |
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That sounds familiar ... being a flat 4, it's a stubby, two-throw crankshaft. The rear bearing is in a blind hole in the back of the case, and the front bearing is in the cover plate. There's just the centre bearing to bolt up; the big end caps are accessed through the cylinder bores before the pistons are attached to the conrods, then the cylinders are slid over the pistons and bolted down. |
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A bit weird, but no doubt there are sound technical reasons for the design. |
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