Timing Retard / Gasoline Octane

[Yellow Rose]

For the next two questions assume that the multitude of engine tuning parameters are held constant - RPM, fuel mixture, ambient air temperature, engine load, throttle position, etc.

If the timing is retarded (ignore octane) will the resulting exhaust gas have more or less polluting emissions?

If the octane is increased (ignore timing) will the resulting exhaust gas have more or less polluting emissions?

 

[sjs]

Beats the octane out of me. I suspect that the answer on at least #2 is "it depends". For example, what is being used to boost octane? Are you more concerned about hydrocarbons, carbon monoxide, oxides of nitrogen, or particulate matter? I'll assume that your questions relate to load levels relevant to emissions testing and that at those loads there is no detonation at the base octane and timing. Add alcohol and tune accordingly and it may be cleaner, but something stronger like toluene may do the opposite.

If I were to guess, I'd think more advance = cleaner at least up to a point. But as we know, some changes such as AFR can shift one reading up and another down.

 

[satan_srv]

If the timing is retarded (ignore octane) will the resulting exhaust gas have more or less polluting emissions?

More pollution. Retarding ignition time means less time to burn, you're looking at more wasted fuel into the cats, more incomplete combustion and more pollutants in the exhaust gas.

If the octane is increased (ignore timing) will the resulting exhaust gas have more or less polluting emissions?

More pollution. Increased octane resists knock and thus is harder to ignite. For any given level of timing and compression a higher octane is harder to burn and will do the same things I mentioned above in regards to fuel economy and emissions.


Basically...

For any given level of octane, an engine will perform most efficiently when timing is advance as much as possible without detonating.

For any given level of ignition timing, an engine will perform most efficiently when octane is as low as possible without inducing pre-ignition (combustion before spark from heat and compression)

 

[sjs]

Well, I was right about timing, but are you sure that the octane question is that clear-cut. I'm not saying that your statement is inaccurate, just that I'm not sure it directly answers the question under all conditions. Perhaps with available pump gas and the way in which they boost octane it holds true. Am I correct in believing that technically alcohol will increase octane, although at the expense of "power" available? And am I also correct in thinking that the now typical mix of 10% ethanol generally burns cleaner than the same gasoline without it? If so, then I'm back to my original answer.

 

[satan_srv]

Well, I was right about timing, but are you sure that the octane question is that clear-cut. I'm not saying that your statement is inaccurate, just that I'm not sure it directly answers the question under all conditions. Perhaps with available pump gas and the way in which they boost octane it holds true. Am I correct in believing that technically alcohol will increase octane, although at the expense of "power" available? And am I also correct in thinking that the now typical mix of 10% ethanol generally burns cleaner than the same gasoline without it? If so, then I'm back to my original answer.

Ethanol is an oxygenate and does burn cleaner as it has a more complete combusiton, it does have lower energy per gallon than gasoline so it cause a slight decrease in gas mileage blah blah. I wasn't aware running ethanol would decrease engine performance whatsover though.

But you're comparing apples to oranges. Andy was holding everything else constant which includes the type of fuel. You add that in and you're getting on a slippery slope. Depends on what Andy wants to know.

 

[sjs]

If adding Ethanol increases octane (it may not) then I don't think it is necessarily an apples & oranges thing. I have no idea what the source of additional octane is in typical premium pump gas but I would be surprised if it doesn't have exactly the results you first stated, which made sense. I just wonder if it is really a "given" regardless of octane boost method.

 

[satan_srv]

straight ethanol is rated at 113 octane, so yup it increases octane when you blend it.

Actually regardless of how you boost the octane if you don't advance the timing accordingly to compensate for the slower burn you're going to create more pollution. While the numbers will change depending on what fuel you may use, the general trend will be the same, as far as I know.

 

[Yellow Rose]

I'll assume that your questions relate to load levels relevant to emissions testing and that at those loads there is no detonation at the base octane and timing.

Correct.

Retarding ignition time means less time to burn.....

Technically, retarding timing means igniting the spark plugs later (as in closer to TDC) in the power stroke. The burn time (duration) is a function of engine speed not degrees BTDC. However we split these hairs, I do see how more unburned hydrocarbons are expelled via the exhaust valves.

Increased octane resists knock and thus is harder to ignite.

Somewhere I read that higher octane fuel will burn slower (maybe even cooler?) resulting in more complete combustion, thus less emissions. So while higher octane gasoline is “more difficult” to ignite, it still does ignite and does so more thoroughly.

This is where I’m headed. I know a guy that has a mega-HP turbo’ed Supra. He says that when it is time for his annual inspection, he loads up with leaded 104 octane race gas and he has no problems passing; whereas, pump gas results in borderline emissions. If this is true and if race gas is not available, could the timing be advanced to reduce emissions? The load conditions for the test are 15 MPH and 25 MPH under steady-state throttle, high vacuum. The NSX has an external timing advance / retard that provides +/-3° (exact value escapes me and my service manual is across town) but if one has the means to adjust timing via laptop, that would be much easier. Additionally, to the point of light pinging, one could advance the timing more than the above 3° to reduce emissions.

 

[Sulley]

I'm pretty sure retarding the timing reduces peak cylinder pressures and temps and reduces NOx. I can't remember the effects on CO and HC right now.
In California on the dyno smog tests they also check timing and will not pass your car if the timing is out of spec.
I thought the only fuels still sold with lead in them were aviation fuel.
Sulley
white '93

 

[Yellow Rose]

I thought the only fuels still sold with lead in them were aviation fuel.

Here you can purchase leaded 114 octane race gas at a nearby performance shop. Assuming the engine warrants such octane and the cats have been removed, the only thing "wrong" with running leaded gas is that it may foul the O2 sensors which are not calibrated (not expecting to see) for tetra-ethyl lead.

 

[orange606]

This may help.

"Other goals, like minimizing emissions, take priority when maximum power is not required. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Lowering temperatures helps reduce the formation of nitrogen oxides (NOx), which are a regulated pollutant. Retarding the timing may also eliminate knocking; some cars that have knock sensors will do this automatically."
howstuffworks.com