Effect of fuel octane number on FE

[theunchosen]

In that case we totally agree

[aerohead]

Originally Posted by theunchosen

I would argue with that. Its not really uncontrolled any more than normal. It is suspected that it happens if/when the flame front strikes a pocket of stoich mix while the surrounding area is lean. the stoich burns faster and causes an uneven pressure spike on the bottom of the cylinder(against the piston) which ignites that fuel and creates very small pockets that detonate under the intense pressure and heat.

All said and done the combustion event is no less controlled(as there are no means for controlling the flame front).

theunchosen,I'm relying on memory going back to the 1970s,with my I.C.Engine Lab class.While "ping" was acceptable under load,"knock" was considered dangerous to the engine and is the reason for anti-knock additives in gasoline,as tetra-ethyl lead,Methyl Tertiary Butyl Ether,and Ethanol.Engine manufacturers will recommend an octane rating for a particular engine to protect it from the knock caused by detonation which is created by a non-uniform flame front propagating from the spark source.The octane helps guarantee "controlled" combustion at maximum spark advance,allowing the engine to achieve peak efficiency for its compression ratio.

[theunchosen]

Originally Posted by aerohead

theunchosen,I'm relying on memory going back to the 1970s,with my I.C.Engine Lab class.While "ping" was acceptable under load,"knock" was considered dangerous to the engine and is the reason for anti-knock additives in gasoline,as tetra-ethyl lead,Methyl Tertiary Butyl Ether,and Ethanol.Engine manufacturers will recommend an octane rating for a particular engine to protect it from the knock caused by detonation which is created by a non-uniform flame front propagating from the spark source.The octane helps guarantee "controlled" combustion at maximum spark advance,allowing the engine to achieve peak efficiency for its compression ratio.

Ping and knock are the same thing. Ping is one of its forms because it can be defined partially as a certain frequency that occurs in the engine. In that term it refers to a very short area with a very high output, the term also fits things like sonic-resonance where you ping the ground with a high frequency and listen for the ripple waves to come back. Lately the term more than most anything means a networking event in which a computer sends a packet that immediately returns after arriving to determine how much disconnect is between the two computers or however many machines or whatever array of machines.

Knock denotes what who was it . . . you also referred to as piston-slap for its tendencies to occur traveling out from the spark plug at the fringes of the system but almost always against the piston head itself.

knock/ping/piston-slap whatever is detonation. The knocking or the pinging don't cause the detonation. The jerk to the piston-driveshaft-trans-wheels-diff-whatever else comes because of that pulse emitted from the "instantaneous" conversion of fuel-air to CO2+heat+water+small amounts of other stuff.

All knock is detonation, but logic does not mandate that all detonation is knock. Such is the case here.

Fuel manufacturers are avoiding the fuel detonating during compression. Those compounds increase the amount of energy it takes to ignite the fuel, normally in open air this is bad. This is not open air though and the fuel-air is almost achieving its activation energy during the compression stroke so it just takes a little zap to make the ignition flame kernel start and expand. The activation energy would be a waste to burn it in open air because the fuel is then having to overcome that additional AE for no reason, but since its going to be compressed and we are going to add that enthalpy anyway, it doesn't hurt the cycle.

I believe the op is driving a Honda Jazz/Fit. Since the fit has anti-knocking ability its moot, but its not a very high power-weight engine so its not going to hurt his engine very much at all for ping to occur unless he runs it for say an hour under knocking. Then whats very likely to happen is something inside that engine is likely to outpace the cooling systems and pre-ignite destroying his engine. If that happens he will know because the engine's check light will come on and it won't accelerate properly at all and it will rev low at idle.

[Christ]

Piston slap is not always detonation. There are several cases of it happening on production vehicles which did not involve detonation at all. The culprit, in most cases, was either worn piston rings or worn wrist pins/rods. Less often, it was an out of round condition in the cylinder's bore.

[theunchosen]

duly-noted and thanks for the. . .edification. I was pretty unfamiliar with the term and associated it pretty strictly with knock.

[Christ]

Google "Ford Piston slap" and "Chrysler piston slap" and "Chevy Piston slap" etc... you'll see it numerous times in off-the-lot vehicles.

[aerohead]

Originally Posted by theunchosen

Ping and knock are the same thing. Ping is one of its forms because it can be defined partially as a certain frequency that occurs in the engine. In that term it refers to a very short area with a very high output, the term also fits things like sonic-resonance where you ping the ground with a high frequency and listen for the ripple waves to come back. Lately the term more than most anything means a networking event in which a computer sends a packet that immediately returns after arriving to determine how much disconnect is between the two computers or however many machines or whatever array of machines.

Knock denotes what who was it . . . you also referred to as piston-slap for its tendencies to occur traveling out from the spark plug at the fringes of the system but almost always against the piston head itself.

knock/ping/piston-slap whatever is detonation. The knocking or the pinging don't cause the detonation. The jerk to the piston-driveshaft-trans-wheels-diff-whatever else comes because of that pulse emitted from the "instantaneous" conversion of fuel-air to CO2+heat+water+small amounts of other stuff.

All knock is detonation, but logic does not mandate that all detonation is knock. Such is the case here.

Fuel manufacturers are avoiding the fuel detonating during compression. Those compounds increase the amount of energy it takes to ignite the fuel, normally in open air this is bad. This is not open air though and the fuel-air is almost achieving its activation energy during the compression stroke so it just takes a little zap to make the ignition flame kernel start and expand. The activation energy would be a waste to burn it in open air because the fuel is then having to overcome that additional AE for no reason, but since its going to be compressed and we are going to add that enthalpy anyway, it doesn't hurt the cycle.

I believe the op is driving a Honda Jazz/Fit. Since the fit has anti-knocking ability its moot, but its not a very high power-weight engine so its not going to hurt his engine very much at all for ping to occur unless he runs it for say an hour under knocking. Then whats very likely to happen is something inside that engine is likely to outpace the cooling systems and pre-ignite destroying his engine. If that happens he will know because the engine's check light will come on and it won't accelerate properly at all and it will rev low at idle.

theunchosen,I dug out my old text and it basically concurs with everything you've posted.There were over a dozen pages which dealt with "autoignition" which the text suggests is present in all "real" engines,from sub-audible high-frequencies,to the destructive "severe" detonation at the extreme end of the spectrum,where impulses from supersonic pressure pulses create stresses on the engine structure,enough to cause failure.---------- My thought,with respect to hummingbird's situation,was that if all other variables were held constant,that the lower octane fuel would prevent maximum timing advance,forcing the engine to operate at a lower efficiency,leading to the lower mpg.

[theunchosen]

Originally Posted by aerohead

theunchosen,I dug out my old text and it basically concurs with everything you've posted.There were over a dozen pages which dealt with "autoignition" which the text suggests is present in all "real" engines,from sub-audible high-frequencies,to the destructive "severe" detonation at the extreme end of the spectrum,where impulses from supersonic pressure pulses create stresses on the engine structure,enough to cause failure.---------- My thought,with respect to hummingbird's situation,was that if all other variables were held constant,that the lower octane fuel would prevent maximum timing advance,forcing the engine to operate at a lower efficiency,leading to the lower mpg.

I can agree with that. I don't think I ever disagreed with you on that. I think I just got over excited and wanted to tweak something just a little.

I'm not certain about the italics part. actually I agree. But with respect to overall savings I think the engine is definitely smart enough to avoid damaging itself and just run at lower hp output and get slightly less mpg, but in the long run cost less(premium here is I think 30 odd cents more than regular). Its hard to estimate this though without several tanks using premmy and several tanks using regular with maybe 3 tanks of one and then 3 of the other in succession to make sure the premmy fluid clears the 2-3 tank to get good results for the test.

My inclination is always to go with the cheapest possible fuel if it doesn't hurt the engine(or radically reduce FE), but I don't have all the data or maybe even enough data to get a feel for that(seasonal changes in op case).

[hummingbird]

Phew! Guys, that was some colorful analysis!

I am somewhat OK using premium fuel for the sake of FE as the difference in prices is actually closer to ~40c. The questions are, will full premium boost FE or simply increase chances of engine foul up? Will a blend be better? What ratio?

My ambient temp is on the rise gradually and would be anything between 90 to 120 deg. now for about 4 months. Having to haul kids and stuff in the heat of the summer for leisure activities is not helping either, and probably is going to pull me below 50 mpg for this tank. But one does not lose faith or enthusiasm so soon, after getting such spirited support! Keep it up guys!

[theunchosen]

Humming, I'm going to have to defer to someone else. My jury is out because I don't own a Jazz/Fit so it could be more advantageous for FE to upgrade or it might not. Its more a car specific thing than a general rule. Sorry it will increase FE of the engine but it might be more costly. You might get more MPG but it might cost more.

The only thing I can recommend is try a test of 2-3 tanks of each and compare keeping in mind any changes that you make either to the car or the driving.