The Early Fuel Pulse

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.

•  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.

1. 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
2. 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
3. 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.
4. 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!!]

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The torture that a Suspension undergoes!!

The pic should tell you why we need a good suspension in trucks. The pic is from a test track however the roads in India keep in store worst conditions for the truck. The suspension design for heavy duty trucks is a extremely challenging task. There are some very strict rules that need to be followed to ensure that the weight of the suspension is low ( so that in effect the Truck's unladen weight is less) and also provide the right amount of stiffness to ensure good drive-ability. The design should consider worst case scenario's to ensure that the truck suspension holds even under heavy load conditions. Usually a factor of safety of 2.5 to 3 is considered in designs.

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Glossary of EMS terms -part 1

This article just introduces a few terms that are very frequently used when we talk about Powertrain. I would say this is just some kind of a glossary of terms

Lean & Rich Mixtures: Both terms talk about air-fuel ratio. A Lean mixture is one in which the air (oxygen) is more than what is needed to burn the fuel. Obviously Rich mixture is the other way round ( though opposite of lean is fat!!). The stoichiometric ratio or the right ratio is about 14.7:1 ( for pure octane). Which means, to burn 1 part of fuel we would need about fifteen parts of air. Note, that when i talk of air it is actually the oxygen that i am really interested. One very big misconception is that all 14.7:1 is the universal ratio which is false because this depends on the fuel ( its octane number + if there are any additives like anti-knock).

Lambda : For a long time i could not understand this value and people told me all kinds of things. for starters "λ" is not the air-fuel ratio but it is a measure of the air-fuel ratio. The best of understanding λ is that it is "Excess air factor". So if λ is 1 that means there is no extra air and we are running at stoich ratio. If λ is greater than 1 that means we are running Lean because we have extra air in the mixture. Similarly, λ <1 means that our mixture is rich, there is less air than needed to burn the entire volume of fuel that we have.

Lambda Sensor: This is also called O2 sensor. Again this was a very confusing term for me ( still is!!). Why do we call this λ sensor? what does it measure? &Lamda sensor is actually a 0xygen sensor. This give out a voltage output based on the oxygen content in the stream. There is a platinum probe one side of which is exposed to exhaust gases while the other side is exposed to atmospheric gases. This works pretty much on the same principle as a electrolytic cell. The voltage output is used by the ECU to do other calculations about which we shall talk in future. In modern vehicle i would expect atleast 2 such sensors.

58x Signal: This is basically the term that has evolved for the cranktooth signal. For ECU controlled engines, with individual cylinder control it is necessary to know when to fire which cylinder and for this it should know which cylinder is at TDC and which at BDC. This information is available to the ECU via a toothed wheel which is connect to the crankrod of the engine. For a lot of reasons, including ease of software computations, 60 tooth where chose on the wheel with 2 teeth missing. The missing teeth helps the ECU recognize where the cylinders are. Some manufacturers are more comfortable using the 28x signal. i.e 30 teeth with 2 teeth missing.

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