Octane rating vs CR

[PSA]

You could look at how an engine can start knocking when it's drawing hot air from under hood, so it's actually a lower pressure with less dense air, but higher temperature.

[hoodeng]

PSA, this is where the octane rating tests were changed some years back, from RON to the current in the USA pump indicated MON+RON÷2 designation.
Both MON and RON tests use an engine similar to a CFR to arrive at a given number for a test fuel sample. The MON test uses a higher induction air/fuel temp than the RON to better replicate under hood/engine bay conditions, hence a lower number for a same given fuel.

This brings up an argument over here [Australia], as people read American service manuals and see 95octane fuel recommended for their motor, then misinterpret that as our 95, this is not the case as we still use the RON only method, our mid fuel is called 95 which equates to US 91 or so, our 98 is US 95. No factory standard engine suffers if the manufactures recommendations are followed, it is only when the preamble of the recommended number is not read and understood that knock occurs.

You would be surprised at how many guys that you would think would spot this will argue black and blue that this is not the case. I can tell you, I see engines trashed after a run across the Nullabor plain at 40+°c on our 91 fuel, and still have the owner not put his hand up for any responsibility for incorrect fuel choice and instead blame crap fuel, they could probably get away with whatever they have been using when ride stints are interspersed with stops,,, but hold that sucker on load from one tank to the next, now we have a problem.

Cheers.

[BLSTIC]

Combustion chamber and inlet porting. You can get a lot of tumble or swirl out of modifying the ports if you're so inclined, although often at the expense of flow bench readings. Whether that affects power is a different story

[David Redszus]

Combustion chamber and inlet porting. You can get a lot of tumble or swirl out of modifying the ports if you're so inclined, although often at the expense of flow bench readings. Whether that affects power is a different story

While it is possible to produce charge motion (swirl, tumble) using inlet flow direction, neither lasts long enough to have a significant effect on combustion. Charge motion does have an effect on mixture preparation which is quite important.
But does inlet charge motion exceed the benefit of increased air mass flow?

If we pursue increased mass flow (at the expense of mixture preparation), is it negated by incompleteness of fuel burn?

Thermal efficiency results from high compression pressure which occurs at/near TDC. Now the shape of the combustion bowl becomes a dominant factor.

Another confounding factor is that rich mixtures often mask an incomplete burn.

[BLSTIC]

Combustion chamber and inlet porting. You can get a lot of tumble or swirl out of modifying the ports if you're so inclined, although often at the expense of flow bench readings. Whether that affects power is a different story

While it is possible to produce charge motion (swirl, tumble) using inlet flow direction, neither lasts long enough to have a significant effect on combustion. Charge motion does have an effect on mixture preparation which is quite important.
But does inlet charge motion exceed the benefit of increased air mass flow?

If we pursue increased mass flow (at the expense of mixture preparation), is it negated by incompleteness of fuel burn?

Thermal efficiency results from high compression pressure which occurs at/near TDC. Now the shape of the combustion bowl becomes a dominant factor.

Another confounding factor is that rich mixtures often mask an incomplete burn.

Bold is mine, it's what I'm answering.

I disagree. The Honda study I read made various tumble improvements to a partially squished hemi and it improved mass faction burned (can be read as burn speed) as tumble increased. However, yes, the modifications required to increase tumble without substantially altering the port layout did reduce power by 4% because of reduced airflow. They didn't detail any attempts to recover that power with regards to cam timing or similar. What they did do was increase squish (from 12% to 27% of combustion chamber area). That drastically improved the margin to knock for ignition timing, so they increased compression from 9 to 9.5 and that clawed back 1% (for a total loss of 3%)

Graphs from that same paper show that compared to just the squish changes, adding tumble improvements (to the squish changes) reduced MBT by some 15 degrees across the RPM band and improved the knock-margin by 0-6 degrees depending on RPM. Unfortunately they don't have graphs for MBT and margin to knock comparing stock to tumble-improvement-only, but the tumble absolutely improved upon the results from the squish-only.

But as for if it resulted in overall power? Not for a naturally aspirated engine where you can't drastically change the port layout. Power was reduced 3% with tumble and squish improvements plus raised compression, but fuel consumption dropped 6%, which is impressive.

I did some simulations on EA+ to estimate compression vs boost on knock, and figured that if you increased boost instead of compression ratio you could have gained 3-ish% overall (going by memory here) power via tumble, squish, and a boost increase instead of the 3% loss if you were stuck with raising compression and couldn't raise boost. So on a squished hemi with boost, sacrificing inlet flow for tumble and boost could work. But not an N/A engine where you don't have full control over port layout.

squishtumble.JPG

Base - stock
Sq-B + 0.5 - squish improvement + compression
Intake valve offset - Tumble improvement
Intake valve offset Sq-B+0.5 - Tumble + squish + compressio

[hysteric]

Thx BLSTIC.

Can you link to the original document or paper and the original author?

[HiPer Express]

While it is possible to produce charge motion (swirl, tumble) using inlet flow direction, neither lasts long enough to have a significant effect on combustion.

Diesels do magic then ?

[David Redszus]

While it is possible to produce charge motion (swirl, tumble) using inlet flow direction, neither lasts long enough to have a significant effect on combustion.

Diesels do magic then ?

Diesels squirt fuel at very high pressure and velocity. End gas ignition is of no concern. The combustion occurs at the outer edge of the fuel jet which is frequently directed into a bowl in the piston.

Yes, they do magic.