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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Engine basics - Knocking

This post talks a bit about a concept called "Knocking" which is very common in Spark Ignition Engines having very high compression ratio.
The question we need to answer first is what is Knocking?
"Knocking" is a metallic pinging sound that is caused inside the engine cylinder & leads to high intensity vibrations in the Engine block ( usually causing metal fatigue in the cylinder walls). If the engine knocks for very long duration then this might have serious impact on the usability of the engine and on the engine life.
What causes "Knocking"?
Flame front:
The power inside the cylinder which is generated when a spark ignites the air-fuel mixture leading to a flame front. The flame front is a high velocity pressure wave that travels, after originating near the spark plug, downwards to push the piston head and ensure that our engine keeps running.
Now, what happens is that based on the fuel quality among many other factors there are certain spots inside the cylinder with some carbon deposits.If the cylinder temperature keeps rising due to ignitions then at some point of time these hot spots reach a temperature where they are capable to ignite the air fuel mixture or the yet unburnt gases (also called End gas). Simply put there is sufficient heat accumulation at certain points that these act as potential spark plugs.
The Problem:
This causes a problem because we do not have any control over these spontaneously created spark plugs ( so to say). This means that they can ignite the end gas at any point of time. In homogeneous operation of engine ( more about this mode in future posts), the mixture is fairly rich and these hot spots have energy sufficient to create a ignition of the air-fuel mixture.  This phenomenon  is often known as "Detonation".This ignition causes a pressure wave to travel from the spot towards the periphery. This Flame-Front if collides with the flame front travelling from the spark plug will result  in wo very high pressure waves to bang into each other. This leads to very high pressure peaks which put extreme amounts of stress on the cylinder walls. Continuous Knocking might lead to permanent stresses getting formed in the cylinder walls & the piston head which eventually will lead to their failure.
How to Avoid it ?
A few years back when "leaded Petrol" used to rule the market the folks added something known as Anti-Knock in the petrol to ensure that it did not knock. This however, lead to higher amount of particulate matter in the exhaust and also was not good for the cylinder walls.The anti knock was a Lead compound which i cannot remember at this point of time.
These days we use more of "Unleaded Petrol" and there has to be  different mechanism to control the "Knocking". This is done these days by ensuring that the engine temperature doesn't reach very high ( then there is lesser chance of hot spots getting created). One of the methods employed to do this is EGR. Exhaust Gas Recirculation, ensures that he temperature of the engine come down apart from having benefits like enhanced fuel efficiency and reduced NOx in the exhaust. However, that is a different topic and will be dealt a little later. The other method used in conjunction with the EGR is spark retard. Spark retard will ensure that you ignite the air-fuel mixture late enough that there will be no pressure peaks. When we do not have pressure peaks the chances of knocking are lesser.
Finally, use of high octane fuel will result in better combustion of the fuel and lesser hot spot contribution because of poor fuel quality. The reduction of hot-spots will result in reduced Knocking even at higher engine temperatures.
Some links that give more info on how it is detected etc
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Which came first 2-stroke or 4-stroke?

We all know this and it has been told to us again and again...
4-stroke engines are better than 2-stroke engines. They are more efficient, less polluting etc. Also we know that 2 stroke engines make more power ( Because they have a power stroke every crank cycle!!). However, did any of you think why was there a 2-stroke engine in the first place if 4-stroke engines are so good.
Infact i just wikied this

Invention of the two-stroke cycle is attributed to Scottish engineer Dugald Clerk who in 1881 patented his design, his engine having a separate charging cylinder.

Now also wiki says

The four-stroke engine was first patented by Eugenio Barsanti and Felice Matteucci in 1854, followed by a first prototype in 1860. It was also conceptualized by French engineer, Alphonse Beau de Rochas in 1862.

As it is clear 1854 is before 1881, why did some one build an apparently less efficient version of the engine?
I believe the answer is as follows

• The 2-stroke engine is much lighter than its 4-stroke counterpart. It has no valves (intake or exhaust) hence no Camshafts & Cams. This also translates to lesser needs of lubrication. You can just add the lubricant to your fuel and you should be good.
• They could be constructed in lesser space because of lesser number of components, making them a good option for lawn mowers, power saw's etc.
  

Due to the above factors it made the 2-stoke engines much cheaper than the 4-stroke versions. Modern 2-stroke engines have started employing Gasoline direct Injection.
The 2-stroke has come a long way and still remain favourites among hard core bikers who drool at the power produced by these machines ( Remember the Rx-100). They have their issues when it comes to thermal handling ( they get heated faster and need to be cooled more) but that can be handled by having bigger engine cooling fins ( Greater the surface area of the fins better cooling!!). So next time you ride a RX...remember that they came after the 4-stroke versions.

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Fuel Injection - Evolution Journey-II

This is a fully automotive powertrain article.
Disclaimer: I am a novice in this area. If experts read this kindly correct...

Q: What are the different fuel injection methods used?
Fuel injection systems evolved over the years from being a simple Manifold injection to DI ( Direct injection). The injection of the fuel into the Intake Manifold based on the Air Mass that is currently sucked into the cylinder is controlled either electronically or by mechanical methods. This is typically a function of the engine load. The engine load can roughly be determined by the throttle opening. The terms "Full-Load" ( i.e. Throttle valve is fully open) and "Idle" (i.e. when the Throttle valve is closed &amp; the vehicle is stationary) are very frequently used in the discussions further.
After the SPFI ( Single point fuel injection) where the fuel was sprayed into the manifold the next thing that became very popular was multi point fule injection (MPFI). Here multiple injectors were placed at suitable positions in the intake manifold to get finer control over the air-fuel mixture. Also having multiple injectors ensured that the injector closest to the cylinder going into suction stroke could be activated. In some cases this is called Port Fuel Injection (PFI). This provided also the following benefits:

• Fuel going into each cylinder could be precicely controlled.
• Air & fuel would get mixed just before inlet into the cylinder. This ensured that the atomised fuel did not have condensation problems. i.e. in manifold injection with single injector the air-fuel mixture when suddenly expanded in the cylinder under cold start conditions the fuel would condense & form larger particles inside the cylinder which led to higher emissions

The MPFI injection systems came in two variants, though this is not really something great !!..

• Batch Fired : In this all the injectors for one bank of cylinders where activated in one shot. i.e. for a 4 cylinder engine, injectors for cylinder 1 & 4 were activated together similarly 2 &3.
• Sequentially fired: Here the injectors where activated sequentially based on which cylinder was going into suction stroke.

Finally came the Direct injection. The direction injection opened up new domains of controlling the air fuel mixture because now only air would enter the cylinder and it could be the ECU that would control the fuel being injected into the cylinder. This ensured that by injecting at different points inside the cylinder & by different cylinder head profile one would achive higher spread of the atomized fuel. This lead to better flame front propagation, ensuring higher power output.
Why did we not go for Direct Injection to start with ?
The manufacturing processes in earlier days did not provide us with methods to have injectors that could bear the very high pressures generated inside the cylinder. Also, the fuel injector tip has to bear extreme temperatures which are generated inside the cylinder. Also, the pump which pumps fuel into the injector needs to generate very high pressures because if fuel is injected in the later stages of compression ( like it is done in Stratified mode of operation). The injector nozzel design also has undergone many design changes to ensure that a extremly fine spray with precise control on droplet size could be created.

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Fuel Injection - Evolution Journey

This is a fully automotive powertrain article.
Disclaimer: I am a novice in this area. If experts read this kindly correct...

Q: Why & How did FI ( Fuel Injection) systems evolve?
For a long time carburetor based engines dominated the automotive world. The were good and could make your car run smoothly. However, increasing emission norms & better systems being used by the air planes eventually led to their extinction  in the four-wheeler world ( Though my XCD-135 bike still uses a carburetor for managing the air-fuel mixture). The major things that led to the death of carburetor engines are :

• Too many compensations where needed making the design look like spaghetti code. The were compensations for idling, for cold start among other things.
• Carburetor icing had plagued high altitude flying machines, this led to development of what we today know as EFI systems ( Electronic Fuel Injection). They started off as SPFI  (Single Point Fuel Injection) & then evolved into MPFI ( Multi Point Fuel Injection)

With the new FI systems it was necessary to have electronics to control the fuel injection. This was done by having solenoid controlled fuel injectors. These injectors could be actuated by providing what is generally called the "Fuel Pulse" and the timing & the duration (pulse width) of the pulse could control the amount of fuel sprayed into the manifold. The Advantages of  the new Fuel Injection system ( Now of course its quite old) were :

• Better efficiency due to the fact that the fuel-air mixture could be made LEAN or RICH ( not fat for god sake !!).
  
• No carburetor icing wherever it was applicable. Just in case you are wondering what is Carburetor icing, it is the basically formation of ice in the carb's venturi due to condensation of humid cold air (due to further drop in the venturi). This is often accompanied re-conversion of the fuel from its atomized format to more liquid format.
• EFI systems will also help in reduction of NOx emissions due to which it is better complaince with emission norms is possible.  

The story is more compilicated than what has happened so far because soon a need was felt to control the ignition, Cams, valves and what not. There was also emergence of the EGR concept ( which was present even the carb engine days). The compensations & corrections that were needed to the various parameters based on vehicle parameters like Engine RPM, AC ( air conditioning), vehicle speed  ABS etc has transformed the electronic control systems into a work of art.
In the next post on this topic, I will be going into the various systems that influence a engine & how the EMS ( Engine management system) handles them.
more on....
carb icing
EFI


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