01-23-2003, 03:47 PM
Join Date: Jul 2001
Originally posted by Jon [in CT]
Thanks for replying. I urge you to print out and read the following US patents, all filed by the same two guys at FHI at around the same time and all titled Method for controlling ignition timing of an internal combustion engine:
The first one lays out the basic strategy while the other two add embelishments to that basic strategy. These patents might alter your interpretation of the ignition and correction maps which, in turn might alter your tuning strategy.
Here are the essentials.
There is a base ignition timing table. Each entry (IGB) in that table represents a basic ignition timing advance and is indexed by load and RPM. The ECU will never retard ignition advance for a load/rpm point below the IGB value. Each value in this table represents the timing advance that produces maximum safe torque (i.e. without knock) when operating on the lowest supported octane fuel. I believe this table corresponds to what you call the "ignition timing table" and might explain why it seems so conservative -- FHI might have devloped this table using 87 octane gas.
There is a maximum advance table. Each entry (MBT) in that table represents a maximum value that can added to the basic ignition timing advance and is also indexed by load and RPM. The ECU will never add, for any load/RPM point, more than the MBT value to the basic ignition (IGB). Each value in this table represents, when added to its corresponding base table entry, the timing advance that produces maximum safe torque (i.e. without knock) when operating on the highest suppported octane fuel. I believe this table corresponds to what you call the "knock correction table" and the reason that its entries are zero in the low-load and low RPM regions is that the low octane fuel can successfully support the ideal (i.e. maximum brake torque) timing in those regions without knock and increasing octane in those regions won't change the ideal timing. And you mentioned that the "knock correction table" entries had the largest values in the high-load 5K RPM area. This is likely the area most prone to knock and therefore the area that benfits most from higher octane fuel.
I believe the "Ignition Advance Multiplier" which you mentioned in your first post in this thread is used to determine what the patents call a "rough" (i.e. coarse) correcting quantity. Think of the IAM as an indicator of percentage of the MBT entry to add to the IGB entry, with its lowest value meaning "don't add any advance to the base table entry and its highest value meaning "add 100 percent of the maximum advance to the base table entry." This concept is described in US patent 5,000,149.
Not mentioned in your description is what the patents call the "fine correction" table. It's a table stored in volatile memory (i.e. it's reset when the ECU is reset) with the same dimensions as the base ignition timing table and the maximum advance table. It contains advance corrections for each load point, to fine tune the advance after the coarse correction has been applied.
Incredible research Jon. My hats off to you on this post. I learned alot and for that I thank you.