Fuel and Engine Timing

[Rednaxs60]

Forum has been fairly quiet to date. Most of my posts to date have been more on the technical and theoretical side. This is because my '85 1200 FI model is at the "clean up" stage, reducing the amount of extra wires, locating components in more optimum spots and such.

This new thread about fuel and ignition timing is quite interesting, to me anyway. I can safely mention that most of us know about engine ignition timing, advancing-retarding as required, and have dabbled with different octane fuels whether the bike is N/A or has boost.

My research indicates that it is not that simple. I submit that for the most part it is that simple as most of us will not feel a noticeable difference in the engine performance or fuel economy, much like changing out the exhaust system for HD mufflers and such.

I have mentioned in various other threads that it is not the change that is an issue, although it can be, it is understanding what the change is doing. If you understand the technical aspects and still do a change, you are doing the change from an informed perspective.

I will include the usual caveat in that this thread is an opinion piece, and my opinion and understanding from my research. I did not create the information, but think it's nice to know. This type of information is helping with my ECU Replacement/upgrade project as well.

I have a document that is about this topic and will use it to explain my position on this issue.

Ignition timing is used to start the burning of the air-fuel mixture at the appropriate time to ensure the air-fuel mixture is fully combusted at or near approximately 15° after top dead centre (ATDC). The process is:

• The spark plug fires during the compression stroke at a set engine ignition timing value before top dead centre (BTDC).
• The air-fuel mixture in the combustion chamber is ignited.
• Pressure from the burning air-fuel mixture increases in the cylinder as the burning gases expand.
• Pressure from the burning air-fuel mixture should be maximized at approximately 10 degrees ATDC.
• Pressure from the burnt air-fuel mixture pushes down on the piston.
• The energy from the burnt air-fuel mixture is transferred to the cylinder piston increasing the downward motion of the cylinder piston improving engine power and fuel economy.

If the air-fuel mixture is ignited too soon BTDC – further away from TDC, ignition timing is said to be too far advanced, the air-fuel mixture will be completely burnt before the optimum ignition timing ATDC and engine power will be reduced, and fuel economy may suffer.

If the air-fuel mixture is ignited too late BTDC – closer to TDC, ignition timing is said to be retarded, the air-fuel mixture will not be completely burnt before the optimum ignition timing ATDC, but will continue to burn, or not, after the optimum ignition timing degree ATDC, reducing engine power output and fuel economy will suffer.

Ignition Timing Trends

• Advancing engine ignition timing results in a more fuel lean condition;
• Retarding engine ignition timing results in a more fuel rich condition;
• As engine RPM increases, engine ignition timing should be increased;
• As engine load increases, reduced engine MAP, engine ignition timing should be retarded;
• Riding profile will dictate a specific engine ignition timing in that the amount of engine timing will be less at cruise speeds compared to the engine timing needs during acceleration;
• For higher octane fuels, more timing is needed due to slower flame speed;
• For large combustion chambers, more ignition timing is needed;
• For forced induction, less timing is needed because of faster flame speed;
• For emission controls, less timing is used to reduce smog compounds;
• For richer fuel mixtures, more timing is needed due to slower flame speed;
• For alcohol fuels that are run richer than gasoline fuels, more timing is typical; and
• For nitro fuels that are richer than methanol fuels, even more timing is typical.

If the air-fuel mixture is ignited too late BTDC – closer to TDC, ignition timing is said to be retarded, the air-fuel mixture will not be completely burnt before the optimum ignition timing ATDC, but will continue to burn, or not, after the optimum ignition timing degree ATDC, reducing engine power output and fuel economy will suffer.

There are engine indications that the engine timing may be off. The most published engine indicators are:

Engine knocking
decreased fuel economy
Reduced engine power
Engine overheating

What differences are there between advancing iengine ignition timing and retarding engine ignition timing?

Advancing engine ignition timing is to compensate for fuel ignition delay. Fuels require a minimum amount of time to start the air-fuel mixture burning until it is in a full burn mode. Some fuels are more volatile and need less time, others more. A high-octane fuel will require additional time to come to a full burn than a fuel with less octane. The following diagram illustrates this:



The air-fuel mixture does not ignite immediately when an ignition event is triggered by the ECU. Advancing ignition timing may be needed to successfully start the air-fuel mixture burning, getting past the air-fuel mixture ignition delay, so that the air-fuel mixture is completely burnt and the pressure pulse from the burnt fuel is at maximum at the appropriate engine timing ATDC. The fuel of choice will also affect the engine ignition timing. Advancing the engine ignition timing should raise high-end power, but at the cost of reducing low-end power.

Retarding engine ignition timing can be used to reduce engine detonation, commonly called engine knock. Using a fuel with a higher-octane rating can assist in reducing engine detonation, pinging.

There are many engine tuning strategies employed by engine tuners, and as such, there is no right/wrong approach to engine tuning. It is recognized that the VE (fuel) table is the first ECU table to be calibrated. The second ECU table to be calibrated is the Spark (ignition timing) table, and third is the AFR table.

The VE (fuel) table is generally calibrated first, need to have the appropriate amount of fuel injected into the engine to match the inflow of air to achieve an AFR of approximately14.7:1. Once the VE (fuel) table is calibrated, follow up with a Spark (ignition timing) table calibration.

Ignition Timing Control – Open Loop

For open loop systems commonly used in a carburetor or mechanical fuel injection:

Initial advance – typically 10 to 15 degrees before top dead centre (BTDC).

Electronic ignition timing is also added for smog control requirements. More recent electronic ignitions modulate spark advance for different driving conditions. This is typical in earlier mechanical fuel injection and lean burn carbureted engines since the late ’60s.

Ignition Timing Control – Closed loop

Timing in more recent ignition systems is computer controlled according to a closed loop ignition timing function. It may be varied for different engine temperatures, throttle positions, and engine loads. A knock sensor can be used to reduce timing when engine knock occurs.

Flame speeds are greater in alcohol fuels than for gasoline fuels in lean, highway fuel mixtures. In one combustion engineering test, methanol flame speed was compared to gasoline flame speed at lean mixtures for each respective fuel. Methanol combustion flame speed was 42% faster than the combustion flame speed in gasoline. Less ignition timing was needed for methanol.

Example: For blown gasoline engines at around 2 atmospheres, 28 degrees of ignition timing is common for best power. For the same blown engine on alcohol at a richer mixture, 32 degrees of timing is common.

Ignition Timing for Fixed Advance (Locked Distributor or Magneto)

Optimum timing from a fixed (locked) ignition advance occurs at only one engine speed. Ignition timing is too advanced at engine speeds below that and not advanced enough at engine speeds above that. Changing the timing value up or down changes the engine speed up or down for the optimum ignition timing. The engine speed operating range affects where the timing is the best. Increasing the timing advance raises the high-end power, reducing the low-end. Decreasing the timing advance raises low-end power, reducing high-end power.

Example: Magneto timing was reduced 6 degrees in our blown alcohol drag racer, and our low-end 60-foot times were quicker by 0.05 seconds from more low-end power. However, the quarter mile ET slowed down by 0.1 seconds, from less high-end power.

Centrifugal Timing Advance

Typically, a spark advance of 1 to 1.5 degrees increase per 1,000 RPM is characteristic of an engine demand. Bill Jenkins and Larry Schreib also reported that range of values in their popular Pro Stock drag racing engine-building book The Chevrolet Racing Engine.

The information presented here is from research and as such, I am not the author but the presenter. There is more to be learned regarding this topic, especially if you are using or contemplating an FI system.

Cheers

 

[Rednaxs60]

Ignition timing is occupying a lot of my thoughts. GW engine timing is fixed, in that there is very little wiggle room when it comes to adjusting the engine timing. The 1200 GWs have fixed timing, FI or carb. There may be some play with the 1000/1100 GWs, but very little if any.

I have mentioned individual cylinder fuel trim (ICFT) in other threads, and to compound the issue there is short term fuel trim (STFT) and long term fuel trim (LTFT). These three parameters affect engine timing because of the potential for the O2 sensor to indicate a lean/rich AFR ratio.

Where the O2 sensor is installed is quite important when considering these three parameters.

Narrow Band O2 (NBO2) sensors are limited in the AFR readings. These sensors generally read between 14.0 to 15.0 AFRs. Just enough to allow the user/tuner to calibrate near or around the stoichiometric ratio of 14.7. A wide band O2 (WBO2) sensor has a greater range from 1.1 AFR on the rich side to 18.0 AFR on the lean side.

The 1200 GW CFI system allow the user/maintainer to balance the right/left cylinder banks so that each cylinder bank is doing the same work within a 10% margin. ICFT requires the same precision, if all cylinders are within a 10% margin, all is good. I submit that this indicates that the OEM CFI system considered ICFT in the design without using an O2 sensor.

Using O2 sensor(s) to monitor and sample the exhaust emissions for ICFT/STFT/LTFT is a matter of O2 sensor placement. Most OEM car manufacturers only use a single downstream O2 sensor that monitors all cylinders, and estimates an AFR reading for the engine ECU. Works fairly well.

What does this have to do with engine timing you might ask. It has to do with the engine load. The AFR table is a correction table to bring the engine back to a normal operating state. When you accelerate, the engine requires additional fuel to operate short term so as not to operate in a lean fuel condition. The AFR reading will probably indicate a rich fuel condition. You let off on the throttle, settle into a stable operation and the AFR reading will stabilize. The VE, spark and AFR tables are now in sync, and the ECU uses the AFR table to fine tune the VE and spark tables for efficient engine operation. This scenario is the same for deceleration.

What happens when a fuel injector starts to fail, is partially plugged, or failing in the open position. My 1200 engine has one O2 sensor that uses the right cylinder bank for the O2 readings. The left cylinder bank is not used for the AFR readings. If all is operating well, good, but if not there can be some issues.

The ECU treats every fuel injector the same. If one fuel injector is failing or has started to fail, the ECU can adjust the engine VE and spark requirements to suit, but all cylinders are affected. The 1200 FI engine CFI system has a self-diagnostic program that monitors the CFI components and alerts the rider to a potential issue should both fuel injector be faulty on the same cylinder bank - ignition and fuel will be stopped, engine will shut down. The Supplement service manual is vague on what happens when a single fuel injector is faulty or fails, but I expect the indator lights for two failed/failing injectors would turn on, but the engine would not stop operating.

The O2 sensor installation for my 1200 FI model is using the right cylinder bank for O2 emission reading(s). This in effect provides STFT for the engine CFI system right cylinder bank only. LTFT would be if the O2 sensor was being used for both cylinder banks and installed further back in the exhaust system, possibly in the exhaust collector between right/left exhaust pipes.

This placement of the O2 sensor provides good information for the right side cylinders. If one file injector starts to fail or is faulty, the O2 sensor reads the exhaust gases for a lean/rich fuel condition. The ECU uses this information and adjusts the fuel injector pulse width (PW) to compensate; however, the issue with this is that all fuel injectors are affected, operating the engine in either a fuel lean or rich condition.

A fuel rich or lean condition affects the engine MAP (load) reading and as such, the ECU will adjust the engine timing and fuel requirements to suit. If the engine operates at 3000 RPM with an engine load of 60 kPa (load) and requires a VE reading of 45, timing of 32 degrees BTDC with an AFR reading of 14.8, changing the AFR reading significantly up or down will cause the ECU to adjust the VE and spark to change accordingly. The engine is no longer operating as expected, if the ECU inputs require a more fuel rich condition, spark plugs foul, fuel economy and power suffer. If the ECU inputs require a leaner fuel condition, engine pinging, power and fuel economy suffer as well.

Having only one O2 sensor on a specific cylinder bank is beneficial for troubleshooting that specific cylinder bank when the AFR readings start to change, but the cylinder bank not being monitored can be an issue.

If the cylinder bank not being monitored has faulty or failing fuel injectors, the only way to troubleshoot is to read the spark plugs, or instrument the injectors to determine if there is a lean/rich fuel condition misfire. This cylinder bank does not affect the fuel injector PW and as such, this cylinder bank could be operating in a fuel rich/lean condition and the operator may not know this.

Using two O2 sensors is probably the best solution to this issue. Most aftermarket ECUs cannot use two O2 sensors for engine operation, but it is possible to connect a second O2 sensor to monitor the second engine cylinder bank, collect data from this sensor. The user/tuner can use this information for troubleshooting and maintenance purposes.

I have queried the forums on the issue of two O2 sensors. Not a lot of information regarding using two O2 sensors on a motorcycle engine. OEM car manufacturers keep the number of O2 sensors to a minimum as well.

The number of aftermarket ECUs that can use two O2 sensors is minimal as well. The MaxxECU Pro seems to be the only one found to date that can use two O2 sensors. Overkill for my application.

Long dissertation on O2 sensors and how engine timing is affected. The issue here is that all aspects of engine tuning is interrelated and can affect other engine parameters.

Cheers