This post again goes back to the EMS basics that i have been writing about in the previous posts.
Early systems when started to use the Gasoline Fuel injection ( Manifold injection) only controlled the Fuel pulse. Fuel pulse is the industry term that has evolved for the duration for which the Fuel Injector electrical signal is enabled. How does the Fuel Pulse help us to control the fuel?
The fuel line is maintained at a specific pressure. This pressure is usually created by a pump either in Fuel tank or outside of it. The Fuel is maintained at a fairly high pressure. The Injectors are solenoid based valves. A electric signal running through the solenoid opens the valve. The valve closed back with the help of a spring. This spring ensures that once the fuel pulse is cut off the valve closes immediately. However, this is not true and there is adjustment added in the EMS software to take care of this variation.
As in the earlier discussions the Fuel pulse directly depends on the engine load and the engine rpm. Now i explain in some bulleted points why some sensors are needed and how they contribute to calculation of the fuel pulse.
Intake Air Temperature Sensor : The density of air depends temperature hence the temperature sensor is used to indirectly calculate the density of air. Warm air is less dense while cold air is more dense.
Air Flow Sensor: In systems which use the Air-Flow to determine the intake air mass a Air Flow Sensor is used. This is basically done using a simple logic. 1) Calculate the volume of air that has passed the flow sensor. 2) Use a Lookup table based approach to get the density of the air based on the temperature 3) use the density and volume/sec to get the mass air flow into the manifold.
Manifold Absolute Pressure : Not all systems use the Air Flow Sensor, some ( like the Bosch D-Jetronic) use the pressure in the Manifold to calculate the Mass air flow into the manifold. This system has a drawback because as the engine revs up the pressure fluctuations inside the manifold are immense ( Because at the other end of the manifold we have the engine which is acting like a pump!!). Using the Gas equation it is easy to derive the Volume of the air using the Pressure and the temperature information.
Just a small deviation here.....Air Flow Sensors come in two type....1) Vane Type 2) Heating element type.
Early systems when started to use the Gasoline Fuel injection ( Manifold injection) only controlled the Fuel pulse. Fuel pulse is the industry term that has evolved for the duration for which the Fuel Injector electrical signal is enabled. How does the Fuel Pulse help us to control the fuel?
The fuel line is maintained at a specific pressure. This pressure is usually created by a pump either in Fuel tank or outside of it. The Fuel is maintained at a fairly high pressure. The Injectors are solenoid based valves. A electric signal running through the solenoid opens the valve. The valve closed back with the help of a spring. This spring ensures that once the fuel pulse is cut off the valve closes immediately. However, this is not true and there is adjustment added in the EMS software to take care of this variation.
As in the earlier discussions the Fuel pulse directly depends on the engine load and the engine rpm. Now i explain in some bulleted points why some sensors are needed and how they contribute to calculation of the fuel pulse.
Intake Air Temperature Sensor : The density of air depends temperature hence the temperature sensor is used to indirectly calculate the density of air. Warm air is less dense while cold air is more dense.
Air Flow Sensor: In systems which use the Air-Flow to determine the intake air mass a Air Flow Sensor is used. This is basically done using a simple logic. 1) Calculate the volume of air that has passed the flow sensor. 2) Use a Lookup table based approach to get the density of the air based on the temperature 3) use the density and volume/sec to get the mass air flow into the manifold.
Manifold Absolute Pressure : Not all systems use the Air Flow Sensor, some ( like the Bosch D-Jetronic) use the pressure in the Manifold to calculate the Mass air flow into the manifold. This system has a drawback because as the engine revs up the pressure fluctuations inside the manifold are immense ( Because at the other end of the manifold we have the engine which is acting like a pump!!). Using the Gas equation it is easy to derive the Volume of the air using the Pressure and the temperature information.
Just a small deviation here.....Air Flow Sensors come in two type....1) Vane Type 2) Heating element type.
- The vane type is pretty straight forward....you put a piece of thin metal in the path of the air flow....faster the flow more the vane will be pushed. The other end of the vane is connected to a potentiometer which will show variation of resistance to the ECU.
- The Heating element uses a small coil and maintains it at a particular temperature using a small current. The air flowing over this coil cools it down changing its resistance and thus varying the current.
The RPM of the engine is detected easily by a magnetic pick up and a gear. Note that early systems did not control the Ignition Timing. It was fully controlled using the Camshafts and a Distributor. However, LE- Jetronic systems from Bosch implemented Electronic spark distribution, However even in these systems there was very little control on the Ignition timing which still heavily depended on the Camshaft.
Corrections and Compensations
Though the base Fuel pulse width was calculated using Lookup-Table ( i.e FuelPulse = f(rpm,engineload)) there are some special diets for the engine based on some special conditions.
Corrections and Compensations
Though the base Fuel pulse width was calculated using Lookup-Table ( i.e FuelPulse = f(rpm,engineload)) there are some special diets for the engine based on some special conditions.
- Startup : The mixture leaning effect is seen in a cold engine. What does this mean? When the engine is cold ( manifold and other portions also) then the mixture formation is not at its best...Which means that all the fuel that is injected is not going to burn. Hence to achieve the same behaviour more fuel needs to be added. Conclusion : Increase in PW
- Low Battery: This means that the battery voltage has dropped below a specific value. This directly translates to insufficient current supplied to Injectors. Since there is lesser current on the injector solenoids the do not open so well hence we need to elongate the pulse to ensure that the same quantity of fuel is delivered as we intended to. Conclusion : Increase in PW
- Idling : The engine needs a certain RPM to be maintained to that in can overcome its self resistance and continue to function. However, if the throttle position supplied by the driver is taken into consideration....there is none...i.e the driver doesn't push accelerator to keep the engine idling. Hence the EMS should understand this condition and automatically provide a finite amount fuel. Usually a special switch is incorporated in the Throttle pedal assembly which detects a no throttle press condition.
- Acceleration Enrichment: This sudden demand for power occurs when the driver floors the throttle. This sudden flooring of the throttle can cause the mixture to lean out instantly making the engine under go a "lean stumble". This is avoided by providing a throttle floor switch which is activated when the throttle is floored and indicates the ECU about a sudden power demand. Conclusion : Increase in PW
In my current project there have been fierce discussion about which one is a correction and which one is a compensation....I say does it matter ? If you understand how we can classify these please let me know too !!
[Note: The above is just the beginning and talks only about early Manifold injection system, Next post will contain how other signals were added to this base system to improve the performance!!]
[Note: The above is just the beginning and talks only about early Manifold injection system, Next post will contain how other signals were added to this base system to improve the performance!!]
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