Team-BHP (https://www.team-bhp.com/forum/)

-   Technical Stuff (https://www.team-bhp.com/forum/technical-stuff/)

-   -   Common Rail : Why is cold start tougher at higher altitudes (https://www.team-bhp.com/forum/technical-stuff/85817-common-rail-why-cold-start-tougher-higher-altitudes-7.html)

Gentlemen, each one of us is beating the same bush, but not hitting the nail on the head.

It is obvious that a combination of factors is cause of the issue. Some of them are well known and understood such as atmospheric conditions of pressure and temp and the fuel. These are constant in this case. They are equally applicable to all vehicles available at a geographic location. Then as mentioned in the above post why does a skoda start in half crank and xylo suffers.

Only variable is engine technology.

Modern day fuel control systems are closed loop. ECU is heart of this system. Sensors feed all the relevant data to it and it controls the pulse of injected fuel to make it rich or lean as the situation demands. It works by correlating the input data against a prefed operational data. Thus effeciency of this system is dependent on the accuracy and detail of operational data.

Now comming back to our basic problem, in the existing case,intial state of the vehicle at the extreme conditions is not understood by the vehicle ECU. Cranking causes purging of the system and refeeding of inputs from critical sensors, in this case it is exhaust, intake and throttle valve which enables ECU to correct its pulse width thereby starting the vehicle.

As to why skoda starts and xylo doesn't even though they belong to same generation, it is all about the operational data loaded on to the look up tables of each vehicle and the algorithm of ECU, the reason why Skoda is considered more advanced than M&M and TATA.
Cheers

@PGA: Isn't it a little presumptuous to state that all Skoda diesels will fire at half crank at 15000' with (-)2*C ambient, while all Xylos will not? Do I find hints of Tata/M&M bashing in your post? How many Skodas have you observed being used at those altitudes, and how many Xylos have failed to start in front of you?

And one would really wish that the ECU codes & algorithms were the only determinants in firing up an engine in high-altitude cold-start situations, with no concern for the physics involved.

Sir, I think my post has been misunderstood both in its letter and spirit.

My discussion is only from the engineering and technological point of view.

I am an owner of both a TATA and an M&M product, and wear that aspect proudly on my sleeve, as I have proven them to my compatriots to be as worthy as any other vehicle in the Indian conditions. So no question of bashing those companies.

My reference to Skoda and TATA/M&M is only from the point of highlighting a specific technological gap existing between companies having generational difference. Hope it reduces fast enough by the acquisition of JLR and SS'yong by the Indian companies.

With regards to my experience with Skoda at 15000ft. I would like to share that I have handled FADEC managed engines at Leh for quite a few years and needless to say in the process would have handled some terrain bound transportation systems too. But specifically Skoda or Xylo, No.

Lastly as regards to ECU algorithms, if these are not based on fundamental physics, what else could they be based on? Same is the case with data, it is obtained over the years and years of operating an equipment in various conditions. In which Indian companies are still very nascent players.

Once again apologies for not being able to make my earlier post understandable, hope I have been able to clarify now and we can carry on with discussing stated aim of the thread.

(I'm a layman and have not studied diesels or engineering and this could be seriously wrong)

What prevents the use of compressed pressurized air to compensate for the lack of atmospheric pressure? It would need special valves/piping and compressor and storage. If a vehicle is designed to be used in high altitude, it could have these built in and compressed air can be stored during running the previous day and released at the required pressure till engine start. This would be akin to the idle done on heavy vehicles to fill air for the air brake.

Hi,
Thanks.
You should have added that (in ideal case)
a) final temp depends only on initial temp and CR (T2 = T1 * CR^(2/5))
b) final pressure depends only on initial pressure and CR (P2 = P1 * CR^(7/5))

Could you do me a favour and post the derivation for SS and others. Saves me a long post. And I do not see how I will write V1 by V2 to the power 7 by 5 minus 1 etc in text without confusing all and sundry.


Loss of compression will manifest itself as a lower CR. (actually worse) See equations above, and wait for Vina's derivation.

Regards
Sutripta

Your concept is absolutely in order. However converting it to practicality for ordinary vehicles may not be a great idea as benefits may not offset the penalty imposed by additional mechanical complications.
Anyway the problem is not of starting at all but only that off delayed lightup.

Infact, as brought out in the earlier posts by Offroadie, similar concepts are used to start bigger engines like the ones fitted on the ships and also those fitted on heavy earth moving machinery and aeroplanes. These are called turbostarters.
Cheers

Apologies for the misunderstanding. Now that I understand (just a little of) what you do, may I ask you a quick question? What was/is your assessment of the performance (both high-altitude cold-weather start, as well as performance under heavy running) of Mitsubishi Pajeros in high-altitude regions of Sikkim?

Pardon me for butting in, but are we seeing a loss in compression here? Its just that the final pressure is less owing to less initial pressure, doesn't the ratio remain the same? So IMO there is no loss of compression or CR. Or am I missing something here?

Compressed air starting on board ships is not to provide additional air for starting, it is for cranking the engine. Since it is impractical to have an electric motor to start. As already explained by him, newer generators nowadays use start air motors to crank the engine instead of compressed air being directly supplied inside the cylinder to push the piston down. But Main engines invariably use compressed air inlet to the engine to crank it.

Spot on on both counts.
The compression ratio, hence the compression temperature is the same wether the altitude is high or not, but this Is being theoretically demonstrated using the ideal theory. The reality is something else and I am not able to prove it scientifically/theoretically.

The starting system clarifications are brilliantly phrased.

Slightly OT but posting this for trivia.
The Ships main engines are anywhere between 2 to 4 storey's high (depending on the ships size), it's just not feasible to crank them externally.

Yes, that is quite expected. mHawk engine does not come with glow plugs.
It starts because 2.2 displacement is still medium-large.

In narkanda, a verna crdi (1.5L engine) could not start at all due to the fact that its a small displacement engine without glow plugs. Temperature was -2 overnight, and around 4-5 degree during day, with snow all around.


Vehicles without glow plugs will face more trouble starting that vehicles with glow plugs.

Reality is different, because the compression is not a pure adiabatic process. Heat is lost to the surroundings, colder the suroundings larger the heat loss. In cold temperatures, heat built up by compression is readily lost to the atmosphere. We shippies know that as well as any one. Go to Canada or Europe in winter and fail to maintain your generators jacket water temperatures, some engines will not start at all, no matter how much you crank them. Bring up your jacket temperature, you wont face trouble.

But why is this more evident at higher altitudes? I guess the answer lies in the density of air. I admit I dont know much about the ECU part hence am looking at this only from a mechanical/physics point of view.

The mechanism of combustion inside a combustion chamber of the engine involves atomisation of fuel and the areas of the injected fuel spray that is closer to the stoichiometric concentration ignites first. At colder climates, atomisation suffers, additionally the density of air being less means the spray is mostly rich. A richer mixture has a higher auto ignition temperature than a stochiometric one. So in this case where we are already facing loss of heat to atmosphere a deviation from stoichiometric region means problems in self ignition.

So additional means of heating up like glow plugs and all are required at higher altitudes.

The ECU should be designed to factor in these situations and may be the difference in various engine designs are the reason why some face more problems than others.

Dada, what derivation do you want, I would think both the equations you have here are trivial to derive (put T2, T1, V2, V1, write two equations and divide) from the ones that are written in the Wikipedia article.

If you want me to derive the state equation for Adiabatic process - the last time I dealt with Thermodynamics was a decade and a half ago. However a simple search yielded these links:

http://hyperphysics.phy-astr.gsu.edu...rmo/adiab.html
http://hyperphysics.phy-astr.gsu.edu...adiabc.html#c1




There has also been quite a bit of discussion on losses (real vs. ideal conditions), I would just add one more thought:

If the discussion is on why one engine has more problems than the other, the number of variables can quickly get out of hand. For example thermal losses depend on a whole lot of things and also in turbo engine it may not be a good idea to assume that the air pressure are intake is similar to (already low) outside air pressure during engine start. Plus engine material and geometry (size and shape both) along with valve timings will determine thermal losses.

It may make more sense to focus on one engine that shows this behaviour in a very pronounced fashion (e.g. Verna/Xylo or some other engine)

I have a feeling that the absence of a Glow Plug is the key factor. The 'hot spot' to start the ignition is not there.

Tanveer's Safari did have functional glow plugs, yet faced issues on cold start. My Scorpio does not have glow plugs, yet I've not had any issues.

I was at a higher altitude, with lower night temperature. (4400+), I guess at that altitude, even slight increases in altitude can have an effect