On SHELL 89 grade fuel - Improved mileage? Topic is solved

[jimmy57]

Advancing timing will make it knock worse. If you have the peak fuel burn occur when the combustion space is less then the pressure and thus temp is higher. The higher temp will make deposits in cylinder and the spark plug ground electrode hot enough to be ignition source without spark (pre-ignition) and the higher pressures reached in the moment of fuel explosion will push the fuel air mix into self ignition range (detonation). If you retard timing then the space for combustion to occur with spark is greater and therefore peak pressures and temp will be lower. Lower temps will have less effect to make deposits and spark plug ground electrode hot enough to light off fuel/air mix.

The coolant temp is affected inverse. Advanced timing will cool coolant temps as the cylinder head is so full of valves and spark plug that heat transfer to coolant is low. If an engine runs hot idling there is an almost universal strategy used that advances ignition timing when idling to cool engine temps down. Retarding timing on cold starts is an almost universal strategy for the opposite reason as the piston being down in bore when peak burn temp occurs gives the heat a good path to coolant through cylinder wall and will heat coolant more readily. The late timing also will result in higher exhaust gas temps due to the burn not being complete when exhaust valves open. Catalysts and oxygen sensor warming is sped up by the retarded timing in the first minute or so. The flame leaving exhaust port from late timing is also used as an ignition of the trailing end of the last cylinder's exhaust discharge that can and does often times have unburned fuel in it. The extra fuel injected into a cold engine has a significant component of fuel that condenses as a liquid onto piston and head surfaces. The ignition of the fuel air mix heats up the liquid and it evaporates but some of this is too late to be burned so it leaves as raw fuel (hydrocarbon emissions) down exhaust and is not something the catalyst can take care of. The air pumps used for some years were used to pump air into exhaust to handle the HC emissions in the first minute or two. Variable cam timing together with retarded ignition timing handle the HC emissions far more reliably than air pumps and their plumbing ever did. Exhaust CVVT was about this and was not much of a contributor to engine torque or HP when it was fitted beginning in 1999.

[okidiver]

Great stuff and yes, I do remember air pumps, but didn't realize they were only for cold starts. Pretty big piece of equipment sitting in the engine bay just for a couple minutes of a cold start.

[jtopiso]

As I understand it:
If it preignites, it is because the pressure&temp is so high that the fuel/air in there that it burns expontaneously.
So you have some solutions:

1.-Start a flame front before it preignites, to do a controlled burn. "Before" implyes advancing ignition. This means not all the energy of the combustion will be used to push the piston down (it will startt burning before the TDC), so you are wasting power (but avoiding pre-ignition).

2.-Use a richer mixture, to lower the temperature of the compressed mix. Fuel vaporization will cool the combustion chamber. You will be richer than the estequiometric A/F ratio, so some fuel will be unburnt (wasted, so worst mileage), and I guess emissions will be worse (better for NOx, but worse for unburnt hydrocarbures).

3.-Use higher octane fuel, so high pressure and temperatures can be tolerated without pre-ignition. This way, combustion can be initiated by the spark just in the right moment, making sure the combustion doesn't increase pressure in the chamber while the piston is still going up, to get the maximum energy from the combustion to the piston along all the down stroke.