JT

There is a good read about different base stock oils affecting knock. From this preliminary study it looks like an Ester base is best at preventing knock.

Here is the link to BITOG
http://www.bobistheoilguy.com/forums...2409744&page=1

arghx

iTrader: (3)

Hard to comment on some guy on the internet's interpretation of what he thought he heard in a talk about a paper that has not been published.

We need to see the paper in its final published form. Knock and mixture formation are very complicated and application-specific. We don't know what kind of engines he was testing. He may be on to something or he may not. I read these kind of papers all the time--I have a couple thousand pages worth of this stuff in various books and collections.

For example--what kind of crankcase ventilation system was this using and how much oil was being recirculated back into the intake? What kind of load range did the engines operate in? What was the rpm and BMEP (brake mean effective pressure) for these tests? Are they port injected, gasoline direct injected, or carbureted? What kind of charge motion did they use--tumble, swirl, squish flows? Were these production engines or single cylinder optical engines? What fuel was he using in these tests?

JT

I agree with you arghx about not jumping to conclusions. Their initial findings are posted, I can't tell if you read through that or not. I think there is enough info for me to change premixes to a Ester base 2 stroke mix.

Link to preliminary findings:
http://www1.eere.energy.gov/vehicles...er11_alger.pdf

arghx

iTrader: (3)

I didn't see that link, thanks. It confirms what I suspected. They haven't done any testing on anything close to a production engine.



They're not anywhere near the final stage; they're just showing proof of concept. They are testing on a single cylinder variable compression ratio engine based on a diesel combustion chamber:



The thing you have to realize is that modern engines are all built around using charge motion and mixture distribution. Swirl and tumble effects are key to suppressing knock in the low rpm, high load range, and they work directly with the injection concept: open valve port injection, closed valve port injection (most common type), wall guided GDI, spray guided GDI, air guided GDI. There's also variable valve lift systems which affect fuel atomization and all sorts of other stuff like that.


tumble effect (random google image search)


swirl effect (random google image search)

The intake port plays a huge role on this... if you read about the Ford Ecoboost 3.5 engines there was a lot of attention paid to making intake ports with high charge motion to make a more homogenous mixture and suppress knock.

On a rotary the key is the squish effect in the trailing portion of the combustion chamber--basically pockets of concentrated mixture somewhat near the trailing spark. This is where knock occurs.



So if you are premixing for example, the way the fuel propagates in that area has a big effect because the premix is atomized with the fuel. The injection timing and spark timing play a big role. And then the amount of oil deposited in the engine from the crankcase ventilation system plays a big role, and that has a lot to do with operating conditions, sealing design, and oil separator design. It's pretty complicated...

JT

Thanks for that excellent explanation. I understand there are lots of variables and differences compared to the test cylinder, I still find that their concept is valid since they compared the different base stocks under the same conditions. While true there is no evidence to support that these same findings would be valid in a trichoid. I think(for myself anyway) I can make the inference.