Single Cylinder Crankshaft Oil Drilling

[coseng]

Hi all,

I am new to the forum and have a crank related oiling question for anyone willing to chime in. I'm getting into my 3rd custom single cylinder roadrace motorcycle design and this one will have a custom crankcase/transmission assembly mated to a Ducati Panigale 1299 cylinder head, piston, and wet liner. I've done head/crankcase transplants before with good power and so-so reliability results so hope this build of a dedicated bottom end will increase the reliability aspect, and of course allow some more power to be squeezed out.

Most stuff is straightforward but one area I have less experience in is plain bearings/crank oiling. The crank will have plain rod and main bearings. I want to keep the overall engine narrow, the crank short, and the oiling system simple. To this end I want to use a mains-to-rod oiling system instead of feeding the rods through the crank nose as Ducati has done for a long time. This eliminates the need for an oil line through the side case and into the end of the crank and tidies up design, but I don't want to do it at the cost of oiling issues. Both mains would be fed directly from oil passages in the block. I would feed oil from the non-primary drive side using a grooved main bearing and a 'straight shot' oil hole to feed the single rod to an exit location forward of TDC/TDC. The oil hole can be skewed so that it's effective radius from the crank axis does not decrease that much to minimize its fight against centrifugal forces. I can have two feeds at 180 on the crank main bearing surface to ensure continuous flow. The engine turns 12krpm max.

Do you think this would provide sufficient oil to the rod? The section shown is through the oil hole axis and the front view shows the hole trajectories relative to the two bearing journal surfaces. I also use a large through hole at the big end for stress reduction instead of the Klose relief.



Thanks for any input.

[J.A.W.]

I'd suggest that there'd have been good tribological reasons for the Ducati
end-feed approach, & given the history of end-feed hi-po plain-bearing cranks,
(along with copious coolant oil-spray to the piston underside) you'd be game to 2nd guess 'em,
& do so, at your own risk.

Perhaps a perusal of current hi-po single-cylinder 4T's such as MX 450's may be of value?
Especially given that the last Ducati racing single was a 'defacto twin' crank dynamics-wise...

[coseng]

Thanks for commenting.

>>I'd suggest that there'd have been good tribological reasons for the Ducati end-feed approach, & given the history of end-feed hi-po plain-bearing cranks

This is Ducati's first engine with plain main bearings. All of their previous designs have used ball roller bearings on the crank mains so they never had a convenient source of high pressure oil to draw from for the rod bearing so it was easiest to go in from the crank nose. The (spur gear) oil pump from a 2006 999 will fit into an air cooled 2 valve carb 600cc engine from the late 80s, as does I think the crank end oil feed fitting, so I think they make a lot of design decisions on what parts they already have that work. Up to around 2010 or so most of the Japanese 4 cyl sport bikes all fed the rods from the mains. I'm only feeding one rod instead of two and can understand the issues of oil starvation at the end of a 4 cyl or more oil galley but think (hope) that one rod on a short oil galley on a somewhat low revving engine can avoid those issues for simplicity's sake.

>> Perhaps a perusal of current hi-po single-cylinder 4T's such as MX 450's may be of value?

All of the MX stuff (2T and 4T) has ball roller main bearings, as do the street singles, like the KTM690 and the Supermono. The SM is the only single with a plain rod big end, a result of its recycling the twin's crankcases and crank.

[63l8qrrfy6]

The flow rate through the big end bearing is dictated by the feed pressure and the restriction.
The restriction is in turn driven by the bearing clearance (the drilling area is normally very small compared to the clearance flow area). I think the 2 main drillings are overkill - one should be more than adequate. If anything maybe you would want to have 2 drillings in the crank pin.

In terms of the oil position on the crank pin - I would probably bring the hole even higher - that way the centrifugal effect pulling oil out is greater. The important consideration here is that the oil hole is not too close to the high pressure area during cylinder firing. Maybe another 30° up on pin axis should be safe ?

Also, don't be afraid to use multiple drillings rather than straight through main to pin. You can have a drilling from the main somewhere to the middle of the pin and then another breakout drilling into the pin. This way you avoid potential issues associated with acute breakout angles (cavitation and high stress at breakout fillets). You just need to make sure that drilling intersections are neat - for example use a smaller dia drilling to break through into a larger dia drilling, ball nose any sketchy intersections or even extrude hone if you can.

Any chance you have access to software than can do basic bearing hydrodynamics ?

[J.A.W.]

@ coseng, perhaps a rolling element main/plain big-end crank set-up is worth considering?
It would reduce the demands on your oil pump, commensurately.

[coseng]

Thanks for the critique, mudflap.

I changed the design to a 3 stage drill that gives 'circular' oil entry/exits and keeps away from the rotation center as much as possible. Brought the exit up a bit too as suggested. Extrude honing is a good idea, it should not be too dear on a small part like this. A second crankpin drilling is easy in this arrangement but would that be necessary? Would 2 smaller BE drillings be better? Most stuff I have seen was drilled as you indicated, one not too close to the max loading area.



I don't have access to any simulation software. For this project my ideas are centered in the chassis and the engine is just a packaging exercise to integrate tightly into my chassis/front end. To keep the project moving (or start moving again...) for engine stuff I try to reuse what works with minimal simulation.

To also address JAW's second post, I am reusing most of the Ducati Panigale oil system. For this new engine design they redesigned everything, including switching to a gerotor-style pump with scavenge and pressure stages. It will output slightly too much oil since it was designed for a twin but that is a small price to pay for reuse of the entire system. Having that pump system makes plain bearings an easy choice. They are the most durable choice, anyway. Rolling element main bearings are easy on the oil pump but hard on everything else. They don't tolerate debris or shock loads well and the large cross section makes design integration more of a compromise with overall crankcase size. Crankpins end up being smaller, which leads to a more flexible crank.

The Norton oiling diagram shown is about the same as previous generation Ducatis except for their spur gear pump in place of the Norton's piston plunger system. The Norton crank is a piece of work though, and not in a good sense! The center counterweight is huge (no center bearing) and the crankpins look like stork legs compared to it.

I'd really like to use an electric on-demand oil pump but will save that for the next engine/bike project, which is all about maximizing efficiency.

[J.A.W.]

Ta for the reply Chris, & yeah, actually its a Triumph (forged one-piece crank), poor old Norton
had to 'soldier on' with a 3-piece crank inc' bolted in cast flywheel, (which was prone to explode,
when its crank evolutions got too 'ropy' at high rpm), but at least they did have a proper gear-rotor pump
(albeit one which would allow the contents of the oil tank to seep past & flood its 'dry-sump').

Having noted those British engineering foibles, both designs allowed pretty exteme performances
for their day, from 24hr endurance runs to 200+mph records at Bonneville, & Daytona 200 mile wins,
to TC Christensen's 'Hog-Slayer Norton' dragbike of World Record time/distance achievement.

Interestingly, certain 2-stroke snowmobiles of very high specific output run sealed-grease main bearings!

[coseng]

Triumph, Norton, any 360 twin looks the same to me. BMEP has not increased much over the decades but the crankshaft speed at which it now can be achieved sure has.

>>Interestingly, certain 2-stroke snowmobiles of very high specific output run sealed-grease main bearings!

2 strokes need the crankcase to pull a vacuum on the piston upstroke so need a seal on all the main bearings. It sucks to have to press apart a crankshaft to replace a seal!

[63l8qrrfy6]

I would be concerned about having a hole (or worse, an interfered plug) that close to the top of the main fillet - keep in mind that the top of the main fillet and the bottom of the pin fillet are the some of the most stressed features on a crank.

The reason I recommend 2 breakouts in the pin is because in high speed aspirated engines where inertia loads dominate over cylinder pressure, the big end bearing is usually thermally rather than mechanically limited and so the additional oil helps remove the heat.

As a rule of thumb I would design for unit pressures below 100 MPa and L/D ratios between 0.25 and 0.35.

Best of luck!

[J.A.W.]

Triumph, Norton, any 360 twin looks the same to me. BMEP has not increased much over the decades but the crankshaft speed at which it now can be achieved sure has.

>>Interestingly, certain 2-stroke snowmobiles of very high specific output run sealed-grease main bearings!

2 strokes need the crankcase to pull a vacuum on the piston upstroke so need a seal on all the main bearings. It sucks to have to press apart a crankshaft to replace a seal!

I think perhaps you labour under certain misapprehensions Chris, see here below a Yamaha XS 650
crankshaft, its a 4 main fully rolling-element 360 degree ( or zero degree V-twin 4-Stroke):




& yeah, a single cylinder 2-stroke may only require a seal outside its bearings & crank.
(Seldom do crankseals fail either, even on multicylinder 2-strokes - esp' labyrinth seals).

BMEP advances allowed 125cc single cylinder G.P. racebikes to make 440hp/litre, @ 13,500rpm.

Sophisticated 2-strokes also feature pumped lube to main & big-end bearings, as it happens.

[coseng]

If that was a picture of a Norton or Triumph engine you would be correct but it is a Japanese engine, they know how to design one! A neighbor is involved in racing vintage British motorcycles so I see too many 360 twins to keep track of.

The oil system you showed is a drip feed. It was also used by Rotax in their 4 stroke singles (crankcase donor for my existing engine) to oil the rod big end roller bearing. The oil drips out of the main bearing, is caught in a hat-brim shaped oil collector on the crank cheek, and then the centrifugal force from the crank spits it out to the rod big end, so not exactly pressure fed.



Used in a 2T engine I would think the oil dripping from the big end bearing would throw off the premix ratio (and is also not premix oil).

[coseng]

>I would be concerned about having a hole (or worse, an interfered plug) that close to the top of the main fillet - keep in mind that the top of the main fillet and the bottom of the pin fillet are the some of the most stressed features on a crank.

I do have relatively simple stress analysis capabilities so will be doing some optimization for minimizing stress and will move that as far as possible away from the fillets.

>> The reason I recommend 2 breakouts in the pin is because in high speed aspirated engines where inertia loads dominate over cylinder pressure, the big end bearing is usually thermally rather than mechanically limited and so the additional oil helps remove the heat.

Interesting. Where would the second oil exit into the BE? Symmetric around the max loading area?

>>As a rule of thumb I would design for unit pressures below 100 MPa and D/L ratios between 0.25 and 0.35.

What is the D/L referring to? 100MPa seems a low stress level for a race engine crank not needed to last 100k miles, or is that just my inexperience showing? Initial stress simulations were showing max stresses in the 220MPa range in the fillet areas.

[63l8qrrfy6]

It is common practice to have the second drilling almost symmetric but slightly lower to bias the flow towards the lead oil hole (I also heard another explanation suggesting that at very high speeds the bottom of the crank pin gets hotter and having an oil hole closer helps).

L/D is the ratio between bearing length(width) and bearing diameter.

The bearing unit pressure is the maximum big end load (whichever is greater between the inertial and firing load) divided by the projected bearing area (around half the bearing area).

It is an indicative bearing performance parameter very helpful for preliminary bearing sizing - in reality the instantaneous hydrodynamic pressures can be as much as 10 times this value.

Proper racing PbIn bearings can go as high as 120 MPa but they have to be very well designed.

[J.A.W.]

If that was a picture of a Norton or Triumph engine you would be correct but it is a Japanese engine, they know how to design one! A neighbor is involved in racing vintage British motorcycles so I see too many 360 twins to keep track of.

The oil system you showed is a drip feed. It was also used by Rotax in their 4 stroke singles (crankcase donor for my existing engine) to oil the rod big end roller bearing. The oil drips out of the main bearing, is caught in a hat-brim shaped oil collector on the crank cheek, and then the centrifugal force from the crank spits it out to the rod big end, so not exactly pressure fed.

https://www.cosentinoengineering.com/wp ... -crank.jpg

Used in a 2T engine I would think the oil dripping from the big end bearing would throw off the premix ratio (and is also not premix oil).

Actually, no it isn't. The diagram I posted shows an 'injectolube' system in which the oil is provided
by a crankshaft driven pump, & it is quantitatively metered by a throttle-position quadrant (it may
also be pumped via discrete lines to 'pre-mix' with the fuel just prior to delivery), while at the crank,
the oil is slung by rotational force to flow through the crank-pin drillings & directly feed the big-end.



(In some multi-cylinder applications the pressure pulse from adjacent crankcases was also utilized
to redirect excess drip-pooled oil through reed-valves in each 'sump', via oil galleries & back to
the main bearings of the next cylinder, also).

As for the Yamaha, it was their 1st attempt at selling a big 4-stroke bike, so they naturally used the experience they had with 2-stroke rolling element crankshafts, but their XS 650 was way overweight, didn't offer any power/performance advantage over the British twins,
& when raced in hard-tuned form, it also proved a tad fragile, too.

Kawasaki & Suzuki both did better with their early rolling-element crankshaft 4-stroke fours,
but that's a bit more of a subject drift...

[coseng]

>>It is common practice to have the second drilling almost symmetric but slightly lower to bias the flow towards the lead oil hole (I also heard another explanation suggesting that at very high speeds the bottom of the crank pin gets hotter and having an oil hole closer helps).

I may stick with one hole in the rod initially. 12krpm is not that high a crank speed that I would have to solve problems that Ducati didn't.

>>D/L is the ratio.....PbIn bearings can go as high as 120 MPa but they have to be very well designed.

I thought you were talking crank material stress levels. I was planning on using the OEM Ducati bearings for both the mains and rod. The mains may be a bit oversize but that is better than too small. If it becomes an issue, the bearing width can be narrowed a bit and still be over D/L 0.25.