How is ignition timing controlled?

[cRiPpLe_rOoStEr]

Nowadays, besides MAP, MAF, TPS, and eventually knock sensors, the Lambda probe also has an important role for the control of ignition timing. Times were much simpler when vacuum was the most relevant parameter for ignition timing...

[Stubby79]

Magic!

[Ecky]

I can confirm the Honda K series (at least the A and Z engines) use fixed ignition timing. They use MAP rather than MAF, and ignition happens based on a set of tables.

Something I haven't yet figured out is how far ignition timing should be advanced at part throttle, and it's quite hard to test for even with a load dyno. More advance is not always better. Ignition advance refers to how many degrees *before* the piston reaches TDC it fires, meaning the piston is still moving toward the top of the compression stroke. Pressure is building and during that time it's actually doing negative work, trying to spin the engine backwards.

On most engines you can't over-advance ignition timing at WOT on pump gas. Before you have advanced timing enough to get peak pressure at the most efficient angle (which I've read is typically 14-16 degrees but may vary with flame speed), pressures get high enough early enough that the remainder of the charge in the cylinder ignites from compression ( = knock) while the piston is still rising. However at part throttle cylinder pressures are lower, and with lower compression engines (especially turbos) or those with great combustion chamber designs, it's almost certainly possible to over-advance ignition timing without running into knock.

[EcoCivic]

Originally Posted by Ecky

I can confirm the Honda K series (at least the A and Z engines) use fixed ignition timing. They use MAP rather than MAF, and ignition happens based on a set of tables.

Something I haven't yet figured out is how far ignition timing should be advanced at part throttle, and it's quite hard to test for even with a load dyno. More advance is not always better. Ignition advance refers to how many degrees *before* the piston reaches TDC it fires, meaning the piston is still moving toward the top of the compression stroke. Pressure is building and during that time it's actually doing negative work, trying to spin the engine backwards.

On most engines you can't over-advance ignition timing at WOT on pump gas. Before you have advanced timing enough to get peak pressure at the most efficient angle (which I've read is typically 14-16 degrees but may vary with flame speed), pressures get high enough early enough that the remainder of the charge in the cylinder ignites from compression ( = knock) while the piston is still rising. However at part throttle cylinder pressures are lower, and with lower compression engines (especially turbos) or those with great combustion chamber designs, it's almost certainly possible to over-advance ignition timing without running into knock.

That is very helpful, thank you for the information! I presume that the D series engines uses fixed timing tables too then. Another question- Since the timing tables are fixed, there would be no benefit whatsoever to using a higher octane fuel than recommended since the ECU won’t advance the timing anymore anyways. Is that correct? Also, under normal conditions, would there ever be any knock for the sensor to pick up to retard the timing?

Also, does the IAT affect the ignition timing? If it does, I presume maybe I could get a little more power by installing a resistor in the IAT wiring harness so the ECU thinks the air is colder than it is and advances the timing more if I use high octane fuel?

[Ecky]

Originally Posted by EcoCivic

That is very helpful, thank you for the information! I presume that the D series engines uses fixed timing tables too then. Another question- Since the timing tables are fixed, there would be no benefit whatsoever to using a higher octane fuel than recommended since the ECU won’t advance the timing anymore anyways. Is that correct? Also, under normal conditions, would there ever be any knock for the sensor to pick up to retard the timing.

No reason for higher octane fuel under normal conditions, no, since the ECU will only ever subtract (retard) timing, and never advance it.

Knock sensors are good to have especially on higher performance engines because they allow the manufacturer or tuner to tune advance at higher throttle much closer to the point of knock, since 1) you can't typically advance ignition enough on pump gas to get maximum economy and power at WOT, 2) under these conditions, knock can be very damaging to the engine, and 3) engine knock can vary based on conditions.

Just as an example, the TSX motor in my car calls for 91 octane. It has tables for ignition advance at 6 different cam angles which it interpolates, as well as tables which add or subtract timing from those other tables based on coolant temperature, intake air temperature, and other factors. But, they can't make a table for everything. What if the intake air sensor goes bad? Or, I get a bad batch of fuel? A knock sensor both allows them to be less conservative in their ignition tables and provides protection in case anything goes wrong.

Originally Posted by EcoCivic

Also, does the IAT affect the ignition timing? If it does, I presume maybe I could get a little more power by installing a resistor in the IAT wiring harness so the ECU thinks the air is colder than it is and advances the timing more if I use high octane fuel?

It does, and what you suggest might work, but chances are good that part throttle ignition timing is already very close to perfect and more advance is not necessarily a good thing. Cooler air is more dense, and a higher density charge burns more quickly, so at part throttle it may actually pull timing for cold air, and it might add or subtract at WOT (or have tables for both) depending on whether the faster burn or higher cylinder pressures are more relevant.

I'll take a look at the factors I can modify on my Hondata ECU and get back to you.

[EcoCivic]

Originally Posted by Ecky

No reason for higher octane fuel under normal conditions, no, since the ECU will only ever subtract (retard) timing, and never advance it.

Knock sensors are good to have especially on higher performance engines because they allow the manufacturer or tuner to tune advance at higher throttle much closer to the point of knock, since 1) you can't typically advance ignition enough on pump gas to get maximum economy and power at WOT, 2) under these conditions, knock can be very damaging to the engine, and 3) engine knock can vary based on conditions.

Just as an example, the TSX motor in my car calls for 91 octane. It has tables for ignition advance at 6 different cam angles which it interpolates, as well as tables which add or subtract timing from those other tables based on coolant temperature, intake air temperature, and other factors. But, they can't make a table for everything. What if the intake air sensor goes bad? Or, I get a bad batch of fuel? A knock sensor both allows them to be less conservative in their ignition tables and provides protection in case anything goes wrong.




It does, and what you suggest might work, but chances are good that part throttle ignition timing is already very close to perfect and more advance is not necessarily a good thing. Cooler air is more dense, and a higher density charge burns more quickly, so at part throttle it may actually pull timing for cold air, and it might add or subtract at WOT (or have tables for both) depending on whether the faster burn or higher cylinder pressures are more relevant.

I'll take a look at the factors I can modify on my Hondata ECU and get back to you.

Thank you for your help! That is a lot more complicated than I thought. I knew ignition timing was complicated, but I had no idea it is this complicated. If the ECU could be tuned, what kind of power/MPG gains could I possibly get by tuning it to run on 93 octane fuel? Maybe nothing if the timing is already ideal from the factory?

Also, since my ECU is from a JDM Honda Stream, is the timing likely to be any different than the stock US ECU because of Japan’s supposedly higher octane fuel?

[Ecky]

Part throttle, probably little to nothing. For 3/4 throttle or greater you might see some decent increases in both horsepower and efficiency; often timing in these areas is compromised for knock resistance.

[Ecky]

Came across a great writeup on this topic as pertains to Honda ECUs:

Honda knock control is designed to cope with different quality fuel in a stock engine. It is important to remember these design parameters when tuning the knock control to suit your engine.

The knock control method Honda uses can seem complicated, but it helps to understand how an engine is tuned from the factory. All tuning is performed on an engine dyno using an automated process which tunes every location in each table. For VTC engines this includes each cam angle table as well.

1. Using a (very) high octane fuel, the engine is tuned to find the ignition timing which makes the most power. This is called 'mean best torque' (MBT) ignition timing.

2. Using 100 octane (RON) fuel, the engine is tuned to find the maximum spark advance before any knock is likely. This is called the 'knock ignition limit'.

3. Using 90 octane (RON) fuel, the engine is tuned to find the maximum spark advance at this octane. This gives the low octane knock limit ignition advance. The reason 90 and 100 octane is used (approx 85 and 95 octane r+m/2 method) is because this is the likely range of fuel octane the engine will used with.

An example graph showing ignition timing for a single rpm point:



The normal ignition tables contain MBT ignition advance values (black line above). An additional table contains the knock ignition limit values (blue line above). It is possible for the knock ignition limit to be lower than the MBT timing. In this case, the ECU will use the lower of the MBT timing and knock ignition limit timing so that the engine will make the most power possible, but will not knock. eg at 80 kPa manifold pressure the ignition table contains 30º, but the most timing the engine can run is 28º (from the knock ignition limit table). Thus the ECU may run less timing than the ignition tables at certain load/rpm points, even if it does not detect noise from the knock sensor. Note that this will not show as 'knock retard', as the ECU considers this to be a 'no knock' condition.

Actual ignition timing used (green line):



The ECU has the ability to retard the ignition further if it determines that knock is occurring. From the amount of knock sensor noise, and knock sensitivity, the ECU adjusts a 'knock control' ( or 'K.Control') variable. Nominally the 'knock control' contains the estimated fuel octane, as a percentage of the difference between 100 and 90 octane. eg a value of 0% means the ECU considers the fuel 100 octane or above, a value of 25% means the ECU considers the fuel to be 97.5 octane. The K.Control is dynamically adjusted at a fairly slow rate while driving.

If K.Control is greater than 0% and the maximum ignition timing is less than the MBT timing then the difference between the knock ignition limit and maximum ignition timing will show as a knock retard value.

Actual ignition timing used (green line) with a knock control of 50%:



The ignition timing calculation is:

Ignition advance = minimum(MBT Ignition, MBT Ignition + knock ignition limit - (knock retard x knock control))

NOTE: This is not taking other compensation factors into account, such as temperature. Graphs that I've found I can adjust that affect ignition timing are:

-Idle ignition control (idle is controlled by changing ignition timing in many cars)
-Air temperature idle to light engine load retard
-Air temperature medium to high engine load retard
-Low water temperature idle and light engine load advance
-Low water temperature medium to high engine load advance
-High water temperature idle and light engine load retard
-Low water temperature medium to high engine load retard
-Ignition dwell angle (based on RPM)
-Battery voltage compensation

The main tables are typically found experimentally from a load dyno, and will depend mostly on cylinder pressure (which there's no way to measure directly), RPM and AFR. Higher cylinder pressure requires less advanced timing since the burn is quicker, and higher RPM requiring more advanced timing. Because cylinder pressure can't be measured, there are additional tables for air fuel ratio and cam angles, with X and Y axes typically RPM and manifold pressure.


Whew.

~

My takeaway? You can probably advance timing a couple of degrees and get slightly better fuel economy (and power). But how to do this without a programmable ECU?

Looking at the tables, by default my ECU is not advancing or retarding timing at light engine load for air temperature, so spoofing that won't help during typical cruising conditions, and to make matters worse it DOES compensate by adding more fuel. It also only advances ignition timing when water temperature is below (in my case) 9 degrees celcius, and I don't think you want the ECU thinking the engine is always running cold. If you spoofed the battery voltage somehow it would also affect tons of other things, such as the O2 sensor and fuel injectors.

[Piotrsko]

Typically, if you add some reactive component to the timing sensor circuit, you could shift the signal to the ECU. Probably about 20 mfd across ground and signal wires. Might round the leading edge a bit too much, I dunno. Could be inductive instead, I am not fully functional this am. Net result is the signal arrived at the ECU before required so the timing advances by default. Haven't seen a obd2 car where you can still move the sensor. Sometimes the sensor is on the crank, so making a new bracket works.

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

No reason for higher octane fuel under normal conditions, no, since the ECU will only ever subtract (retard) timing, and never advance it.

Once a former neighbor of my grandparents from mother's side got a 5th-gen Civic VTi and attempted to convert it to ethanol believing it would respond better to forced induction. When he told me about the electronics of that engine I inquired him if it wasn't actually rendering his car with flexfuel ability, and it actually kept running better on gasoline...