Octane/Spark advance vs power 1

[davidfarmer]

A friend posted this elsewhere, and thought we might get better results here.

In general, is there a reasonable increase in power by simply increasing octane and spark advance. I realize higher octane fuels (lets say 100 unleaded vs 93 unleaded) often have less energy content and slower burn speeds.

Lets say we can go from around 29 degrees advance with 93 vs 31 degrees advance with 100.

Thanks!

 

[patprimmer]

Thats a hard question, and the answer depends on a lot of variables.

Certainly, there is an optimum combination of compression ratio vs octane rateing vs spark advance.

My feeling is, and I don't have data to back it up, is that for a small chamber with efficent flame travel, the maximum compression that runs best with the fuel you use with about 20 to 22 deg advance will give the best results.

For a large chamber and inefficent flame travel, I would expect that maximium power wopuld come with the max comp that you can run with about 38 deg all in timeing.

There seems to be a sudden cut off when dyno testing, for example

An engine likes 34 deg advance on 100 octane at 7:1 comp.
Raise it to 9:1, it likes 32 deg
Raise it to 10;1 it still likes 32 deg
Raise it to 11:1 it detonates at anything over 24 deg, and makes a lot less power than the 10:1 engine on 32deg.

Those tests were on an air cooled VW

A SB Chev gives a similar type of response at about 38 deg, but I never did any back to backs on the Chev


Regards
pat

 

[patprimmer]

Also, it's hard to do true back to backs, as it's hard to change compression without effecting squish or dome shape, which both effect flame travel in their own right, which skews the results.

I expect that a very tight piston to valve clearance with valve notches in the piston might also have some effect on airflow if you have any manifold pulsing effecting the engine at around TDC.

Regards
pat

 

[Chumley]

Without knowing your specific application, the general
answer is no, advancing the timing and using higher octane
fuel usually won't help overall. Engineers spend literally
1,000's of hours testing specific applications attempting
to establish and balance the best overall timing settings
for any given application. The chances of just cranking
in a few degrees of timing advance and increasing the fuel
octane just for the sake of doing it, in general, and
finding new or lost power are next to nil without a
corresponding hit in some other aspect of performance
or reliability.

That being said, there are many occasions where newer ECU
controled spark advance does respond favorably to higher
octane fuel. As an engine ages, carbon deposits tend to
increase the compression and to some extent insulate the
combustion chamber causing hot spots and/or increasing the
combustion temperatures. Knock sensors tend to retard
timing when that is the case costing you a few hp in order
to prevent egnine knock. Under those conditions high
compression computer controled engines can respond
favorably to a boost in fuel octane which allows the
timing to return to its original programmed values.

Hope this helps,

Chumley

 

[Marquis]

This is quite litterally a “How long is a piece of string” question.
There isn’t a general “yes” or “no” and it depends solely on your application and specific engine.

When looking at knock limit, in very simplistic terms there is the compromise of the following factors;
Engine speed
Static CR
Dynamic CR
Exhaust manifold design
Port design ( and how much charge motion is induced)
Octane etc etc
Volumetric efficiency
And I haven’t even begun to hazard into the complex areas that Pat touched upon. (For instance I know someone with an air cooled VW engine who was very shocked at how his engine was detonating at a very modest CR and our high octanes over here, we later summised that the apparent squish 1.3 mm clearance band he had specified AGAINST my advice was making an “end gas” trap that was not only acting as a flame front quench zone but a point for det to initiate)
For a given combustion chamber type and cam timing etc etc, there is definately an optimum CR.
Manufacturers undertake a lot of single cylinder testing the to determine this.
The kind of studies that are undertaken are ignition loops at every engine speed for a given octane.
An ignition loop is bascially a plot of parameters measured on the test bed:
With ignition along the X-axis and Torque or BMEP along the X-axis. This will give a curve that a polynomial will fit easily. If the CR is then too high, the engine will hit knock before it reaches the peak of the curve. The peak of the curve is known as MBT, or Maximum brake torque.But then again this depends on the engine speed that is being measured and the amount of Volumetric Efficiency. There is more REAL time at low engine speeds for knock to occur , but conversely high speed knock is far more destructive. Having higher Volumetric Efficiency increases cylinder pressure and is somewhat analogous to increaseing engine load, again bringing the onset of knock sooner.

An engine with a very high CR and high VE will have a steep ignition loop that runs into knock well before MBT on low octane gas. With this kind of engine at this kind of condition a few degrees ignition will makes HUGE different in torque. Contrary to popular belief, there COULD well be engines out there like this, because this phenomenon might only occur at a certain engine speed, or the engine might have specified a high CR for good part load operating fuel economy. Supercharged or turbo charged engines typically have very steep ignition loops.
On the other end of the scale, tame engines such as the low specific output stock old style American V8s have very mild ignition loops, or VW beetles. These engines typialy also need quite a lot of ignition compared to say, a modern pent roof chambered Variable cam phased high volumetric efficiency quad cam V8. For comparison a V8 of the later variety might need a maximum advance of going from just 7 degrees at 1500 rpm to 25-26 degrees at 6500 rpm on 95 RON octane fuel.

A car with steep ignition loops WILL benefit from running a higher octane fuel. An engine again that is still knock limited at peak power ( that is –knock is reached before MBT is on the ign loop) will gain peak power also. Older engines were seldom specified this way, and American engines still tend not to be . With the advent of precision calibration and active knock control, more and more engines often run steep ignition loops.
Modern knock control systems even actively venture into knock to get the best possible performance.

 

[Michael996]

Higher Octane gas has the same heat energy as low octane gas. The octane only alters the pre-ignition properties. Timing is a function of engine speed and mixture settings.

A typical O-360 (lycoming) has a timing setting of 25° BTDC. (2500 rpm). Faster engines will have earlier settings.

http://www.sacskyranch.com/bvalve.htm

http://www.sacskyranch.com/glean.htm

http://www.sacskyranch.com/combustionClean.htm

http://www.sacskyranch.com/photopis.htm

Detonation is not a spark related function.

 

[Chumley]

Michael996 writes:

"Higher Octane gas has the same heat energy as low octane gas."

Not true. It depends on how the octane rating was "built."
There are hundreds of different molecules used to build
fuel octane. It is the fuel blenders choice how they plan
to obtain the desired octane specification for any
particular type of fuel. The aromatics while having
generally higher octane ratings also have slightly lower
heat content than straight chain or branched chain
alkanes. Oxygenates such as diethyl ether, ethanol, MTBE
all have high octane ratings but less heat engergy than alkanes.

see:

http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html

http://www.me.mtu.edu/~slpost/CLASS/hcprop.html

Micheal996 also writes:

"Detonation is not a spark related function."

This is simply not true. Most every ECU (Engine Control
Unit) which can control spark timing through knock sensor
input retards spark timing at or soon after the onset of
engine knock. As Marquis mentioned some engines run right
on the fence and allow some minor detonation but clearly
limit major detonation by promptly retarding the ignition
timing. Emission control issues arise at detonation also.
There are many other factors which affect detonation, spark
timing being only one of them.

(nice photos on the links though)

Chumley