How does the temperature impact octane requirements?

[GoHeels1981]

In a recent auto magazine article the writer indicates that during very hot weather premium gasoline is the best remedy for "knocking".

I did not realize that the temperature had an effect on knocking, or octane requirements............does this mean we can use regular gasoline in cool/cold weather.........even if the vehicle requires premium fuel?

 

[Rickey]

Chemical reactions are generally made faster by increasing temperature. Combustion of gasoline is a chemical reaction.In simpified terms spark knock is caused by the fuel air mixture burning too rapidly. So cooler weather does decrease the octaine requirements of a gasoline engine. It's hard to say however if cold weather would allow regular to be used in a car speced for premium. Good Luck Rickey.

 

[Shaman]

Also remember that decreased temperatures increases density, and also drops the temperature of the incoming air. When you computer senses the lower temp, it adjusts by adding more fuel.

 

[eljefino]

If you're normally aspirated give it a shot.

If you have a turbo, don't, the effective cylinder pressure will be the same no matter the outside air temp/density.

Of course with knock sensors you can get away with cheaper gas most of the time if you drive lightly. You might find that your car is rated for 91 for example so if you buy 50/50 93 & 87 you're just a scoosh under the requirement and saving quite a bit of money.

 

[andyd]

quote:

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Originally posted by Shaman:
Also remember that decreased temperatures increases density, and also drops the temperature of the incoming air. When you computer senses the lower temp, it adjusts by adding more fuel.

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my 528e has a pretty good stock air intake. In the winter time would I get better MPG by rigging the intake to pick up air warmed by the engine?

 

[tensecondz1]

The reason is that IAT (inlet air temps) are higher in hot weather thus the charge is more prone to detonate... pre-ignition etc.

You see this in forced induction engines where you can run more timing if you manage to keep the inlet temps down.

The most efficient combination is to have an engine at operating temperature and the air/fuel charge cold.

 

[Shaman]

quote:

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Originally posted by andyd:

quote:

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Originally posted by Shaman:
Also remember that decreased temperatures increases density, and also drops the temperature of the incoming air. When you computer senses the lower temp, it adjusts by adding more fuel.

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my 528e has a pretty good stock air intake. In the winter time would I get better MPG by rigging the intake to pick up air warmed by the engine?

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warmer air would never be good for an engine. Keep the air as cool as possible.

 

[mechtech]

I have seen many cars knock when it's super hot out, and they don't normally knock.
I have also seen many cars knock when cold out, when they normally do not. The increased charge density may put the fuel over it's detonation resistance edge.

 

[Big Jim]

The general rules about octane requirement that I learned years ago are that octane requirement is increased with higher ambient temps, lower altitude, and lower humidity. The inverse is also true. Octane requirement goes down with lower ambient temperatures, higher altitude, and higher humidity.

As they used to say, a cool foggy evening at the beach was "racing" weather.

 

[Shaman]

High altitude means less air, humidity means less air. I have actually heard people complain about performance in SC applications when it got humid.

 

[Gurkha]

Along-with temperature and low octane fuel, the other cause of knock is carbon inside engine bay and lean mixture, mild knock with regular octane fuel and properly functioning cooling system is normal in extreme summer with a/c on and loaded vehicle, but heavy knock is a sign of trouble.

 

[Shannow]

The temperature at the end of the compression stroke to a high degree determines whether the fule burns progressively, or "pops".

The function is exponential with compression ratio.

Kerosene engines (not much octane) knocked at low compression ratios at ambient temps. Higher octane allowed higher compression ratios and higher temps.

Stand at a set of (long) traffic lights on even a slight rise, and watch the first and second cars "clack clack clack" away from the lights, as high inlet air temps (and high metal temps) cause the fuel to pass through the knock transition.

Shaman, High inlet air temps, help with economy at light throttle openings...guess that could be deemed beneficial.