IMPORTANCE of the CYLINDER BALANCING in CIS INJECTED TURBO ENGINES
In the ideal, theoretical world, the ratio of air to fuel is 14.7:1 (called the stoichiometric ratio). It is also referred to by way of the Greek letter "Lambda". Lambda reading of 1 is the ideal air/fuel mix. At an O2 sensor reading of 1, the O2 exhaust emission is theoretically ideal...meaning the air/fuel ratio is not too rich (lambda reading less than 1), and not too lean (lambda greater than 1). At Lambda=1, the air/fuel mixture is totally burned, exhaust levels of hydrocarbons, carbon monoxide, and nitrous oxide, are at a minimum.
Oxygen sensor measures the presence of O2 in the exhaust gases, (which under ideal conditions should be about 0.1 - 0.3%) and transmits the reading, thru a voltage signal, to the ECU. The ECU has the ability to interpret the voltage signals from the O2 sensor, and determine if total combustion is occurring. If total combustion does not occur, then more fuel, or less fuel, is delivered thru the injectors and into the cylinders, such that the O2 exhaust emission level stays at 0.1 - 0.3%. This cycling correction occurs several times per second, such that adjustments are being made continuously and the reading swings in a very tight band, on both sides of the Lambda reading of 1.
We have to remember that CIS mechanical injectors (if new/clean) always should flow the same amount of fuel, no matter how many there are, 4,6 or 8.
Thing to remember: CIS System pressure changes (dP/dT) are always the same across all injectors and always happen simultaneously. Individual injector flow can be adjusted/calibrated only at the Fuel Distributor head (by turning the set screws).
Air/fuel ratio corrections are constantly made by the CIS ECU (Lambda control box); by altering the duty cycle of the Frequency Valve. If Lambda readings are greater than 1 (a lean mix), the ECU increases the FV pulse width (adding more fuel). If lambda readings are less than 1 (a rich mix), the ECU decreases the FV pulse width (limiting the fuel supply). Air/fuel ratio corrections are constantly made by the CIS ECU (Lambda control box), by altering the duty cycle of the Frequency Valve.
–If lambda readings are greater than 1 (a lean mix), the ECU increases the FV pulse width bleeding out more fuel from the CP circuit which increases SP at the injectors (adding more fuel).
-If lambda readings are less than 1 (a rich mix), the ECU decreases the FV pulse width slowing the CP circuit bleed out rate. Higher CP causes drop in SP at the injectors (limiting the fuel supply).
NOTE: Fuel supply (System Pressure) adjustments in the CIS system are executed by a very precise metering device, the Fuel Distributor Head. FDH adjusts fuel delivery simultaneously, across all 4 injectors/cylinders. FDH is not capable to “compensate” or differentiate if one or more injector is clogged or faulty.
In a "perfect" engine... all injectors should flow the same exact cc volume of fuel,as shown below. Let's assume there are a 14.7:1 air/fuel ratio and a lambda exhaust reading of 1 (total combustion).
NOTE: The use of 100cc flow rate in the examples is for illustrative purposes only.
Let's assume the ideal fuel flow to burn all oxygen drawn into the cylinders is:
Injector #1=100cc; #2=100cc; #3=100cc; #4=100cc; Total 4cylinders =400cc
We expect the volume of fuel to be (almost) totally burned, and O2 exhaust content to be in a prescribed range of 0.1 to 0.3 %
a) Let's look what happens if 2 injectors are "dirty" and don't flow 100cc as the other two injectors.
Injector #1=90cc; # 2=90cc; # 3=100cc; # 4=100cc; Total=380cc. 20cc “short”!
In this example, the O2 sensor will read a lean mixture, as injectors #1 and 2 are under fueled, and there is excess O2 in the exhaust gases. ECU “wants” to add more fuel to restore cc flow rate to 400cc, as per below.
b) Injector #1=93.33cc; 2=93.33cc; 3=103.33cc; 4=103.33cc; Total=400cc
The ECU, has adjusted the fuel supply to 4 injectors simultaneously and by the same increment (CIS Fuel Distributor can not adjust flow to individual cylinders), such that the total cc flow volume returns to 400cc, and lambda returns to 1. While this appears to be a somewhat better than our condition a), it is NOT what “really” happens.
The O2 Sensor is reading cumulative median O2 exhaust content in the main exhaust stream (down pipe). Injectors (therefore cylinders) #1 and #2 are still under-fueled. The O2 sensor still reads a lean overall mixture, as there is still unburned O2 present in the exhaust from under- fueling of cylinders 1 and 2. The ECU continues to increase flow of the fuel, until the medianO2 content returns to 0.1-0.3% . That is when cylinders 1 and 2 are at stoichiometric. Injectors 3 & 4 become over fueled, as their O2 exhaust content has reached minimum and regardless how much additional fuel is added to #3 &4, no additional O2 will be burned, as there is no more available.
So, the actual end result of the adjustment performed by ECU is:
c) Injector #1=100cc; 2=100cc; 3=110cc; 4=110cc; Total flow=420cc FAT by 20cc!
While the O2 (cumulative!) content is now perfect, (according to the O2 sensor...all O2 has now been used), the HC (hydrocarbon) exhaust content is high due to unburned fuel in #3 and #4, and the carbon monoxide content is likewise high. The vehicle fails emission standards, suffers from poor performance, poor fuel economy.
This example illustrates why the air/fuel delivery to each cylinder has to be tested and balanced, (injector cleaning and CIS Fuel Distributor cylinder to cylinder calibration). Every cylinder must receive the same exact amount of air/fuel mix. If there is a cylinder to cylinder imbalance, the O2 sensor cannot detect it and ECU can not compensate flow to the individual injectors (drawback of the CIS injection system). Injectors have to be tested individually on a test bench, ultrasonically cleaned and flow tested. Only then O2 sensor/ECU readings can be trusted.
Now, let’s see what happens when we go into "OPEN LOOP" at WOT.
At WOT, more air is drawn into the engine (max air flow); therefore more fuel is required to meet 14.7:1 ratio. For the purpose of illustration, let’s assume 10% enrichment at WOT is presumed necessary.
At cruising speed, "prior" to WOT (still in closed loop)...we have the following:
a) Injector #1=100cc; 2=100cc; 3=100cc; 4=100cc; Total=400cc
At WOT the air flow increases to the max, engine goes "open loop", O2 sensor readings are ignored, and the ECU adds the pre-set amount of fuel across all injectors, equally and simultaneously, and we should get:
d) Injector #1=110cc; #2=110cc; #3=110cc; #4=110cc; Total Flow=440cc
Extra fuel had been added, all oxygen burnt, HP is maintained and engine temperature stays within limits.
Now...let's look at what happens if two injectors are "fouled". Remember, when system goes "open loop", no O2 sensor is used, and the fueling is "base mapped", which added a 10% across the board.
e) Injector #1=100cc; #2=100cc; #3=110cc; #4=110cc; Flow=420cc. 20cc short!
Injectors #1 and #2 are under fueled, even with the 10% additional fuel. Cylinders #1 and #2 are lean, detonating and overheating, which results in burnt exhaust valves or burnt pistons. Engine hand grenades!
This example illustrates why the air/fuel delivery to each cylinder has to be tested and balanced, (injector cleaning and CIS Fuel Distributor cylinder to cylinder calibration are the "must do"). Every cylinder shall receive the same exact amount of air/fuel mix. If there is a cylinder to cylinder imbalance, the O2 sensor cannot detect it and ECU can not compensate flow to the individual injectors (drawback of the CIS injection system). Injectors have to be replaced or tested individually on a test bench, cleaned and flow matched to within 5%. Only then O2 sensor/ECU readings can be trusted.
Last edited by MRDANGERUS; 10-20-2013 at 11:00 AM.