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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Throughput funda's

I have been quite inconsistent in updating the information presented on this blog. The one things that discourages me like any other new blogger is that very little traffic is seen here and since this is a technical blog, unless i get feedback it is difficult for me to judge if something is good or bad. However, for now i have considered that no feedback is negative feedback. Nevertheless i continue to write.
What is Throughput...This is the weeks question.
Wordweb tells me

Output relative to input; the amount passing through a system from input to output (especially of a computer program over a period of time)

The Throughput that i am talking is the one that embedded engineers often talk about. In simple terms this value is a measure of the CPU load under the worst conditions. What could be the worst conditions? These are basically situations which demand more processing power. For example, Image Stabilization, Red Eye Detection, Smile Detection, Ambient light detection along with Click detection or auto timer ( and image post processing) in a Camera perhaps put a great deal of performance demands on the CPU in a Digital camera and this also determines it worst case performance.
Unfortunately like many other engineering terms a higher value of throughput means that your system is more loaded. Often under some conditions the throughput reaches 100% in systems which means the CPU has almost no idle time and is being utilized all the time to its maximum capacity.
If i look at it from an EMS perspective then startup is the when there is very high CPU loads that are encountered. In an OS based system there are methods to determine for how much time the CPU is free. Usually, some background tasks which are not necessary to be done all the time are done in this free time. ( Example, CRC calculation or RAM Checks etc). In simple scheduler based systems the throughput is just the time remaining in the baseloop after all the tasks have been finished. One more parameter that influences the throughput to a great extent is the Interrupt Rate. A very high interrupt rate will ensure that the CPU loses a lot of time in context switches which are really an overhead and do not contribute in anyway to the functionality of the system.
[ This was my understanding of Throughput. After looking into wiki i think the origin of is term comes from the CPU bandwidth utilization. The term is primarily used for parametrization of Communication channel bandwidths]

How Do we ensure that we never reach 100% throughput?

• Plan your interrupts and their sources well. You should know the worst case rate of an interrupt beforehand. Example, the Door lock engage interrupt cannot come at say more than 1-2 times a second ( Due to the inertia of the lock).
• Write your code with throughput in mind. E.g if possible use binary search in place of linear search. Use macros instead of functions judiciously
• Overuns should be detected in software using some special variables or debug variables. Over-Runs occurs when the throughput goes more than 100%. Which means that you have asked the CPU to more than it is capable of handling in the given time. 

This is very brief and windowed perspective of Throughput. If you have some other inputs as to how this is related to your domain then do put a few lines of comments.
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