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Old 4th September 2012, 20:24   #76
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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Originally Posted by Off Roadie View Post
Ok sir what was your final compression temperature at the lower pressure? If the doubt continues I shall try posting something simpler and explanative once I am back in December.
Have to rework it again, but IIRC 753 K!

The calculations would be self-explanatory. Don't think anything needs to be added to it.

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Sutripta
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Old 4th September 2012, 21:29   #77
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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Originally Posted by SS-Traveller View Post
That is not the same as 'flash point', which may also be relevant in the present discussion - IIRC diesel has a flash point of 60*C-70*C. Would a hot glow plug provide the equivalent of a spark plug to start the first ignition? (The Safari DiCOR engine has glow plugs, the Scorpio mHawk CRDe engine does not).
A glow plug does go red hot when heated and aids in the combustion but is not the equivalent of a spark plug.
There is a very comprehensive explanation about it in wikipedia. Just search 'glowplug' I too learnt a few things I was not aware of.

@ sutripta
I shall post something in December for you, until then, let's just leave it at that, rather than prolonging this thread without adding value.
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Old 4th September 2012, 22:43   #78
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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Originally Posted by Sutripta View Post
Have to rework it again, but IIRC 753 K!
The calculations would be self-explanatory.
Sutripta-da, could you make the self-explanatory calculations a little more "simple as water" (jalabat taralang) for maths averse dummies like me? Let's actually see the back of that envelope! Your carrying on a 1-on-1 conversation with a marine engineer who understands diesel engines like I understand enamel, leaving some of us in the dark, is not fair. Perhaps Off Roadie can explain your calculations further - is that OK with you, Off Roadie?

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Originally Posted by Off Roadie View Post
A glow plug does go red hot when heated and aids in the combustion but is not the equivalent of a spark plug.
There is a very comprehensive explanation about it in wikipedia. Just search 'glowplug' I too learnt a few things I was not aware of.
Thanks - interesting. Wonder what is the temp (or range of temp) achieved by a glowplug.
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@ sutripta
I shall post something in December for you, until then, let's just leave it at that, rather than prolonging this thread without adding value.
Oh no! Waiting till December might just mean our engines won't start in the peak of winter!
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Old 4th September 2012, 23:19   #79
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My take on the problem is that, it is the inadequacy of ECU mapping which is the cause of the problem. It does not cater to the optimised operation of engine at the extreme conditions faced by you at the places mentioned.
Since the problem is not a function of temperature but only of the altitude the issue of freezing of diesel, wax formation etc is disregarded as well the issue of inadequate heating in the cc.
Blending of diesel will also not result in resolution of the issue. (anyway not even god will be able to decipher the blend of Indian diesel)

I assume your vehicle is a BS 3/4 standard.

My hypothesis also says that the problem is not due to common rail or vvt or any other such technological marvels.

It is most likely that ECU look up tables do not have the extreme conditions factored as 99.9% of vehicles do not operate in that area. Factoring those conditions may complicate the ECU algorithms beyond their obvious cost/benefit ratios.
As always in our country, there are two ways to overcome the problem. Firstly understand the ECU mapping for your vehicle and supplement the look up tables, maybe go for performance chip sets. We will have to disregard engine performance or emission standards in this case.
Second 'jugaad' option is when you face the mentioned conditions, flash your ECU. This wil reset your ECU to basic default values. Now your vehicle is of same standard as a normal DI engine. It should start accordingly.

Well, mine is a theoratic explanation to a practical issue. I am willing to elaborate or explain if it considered necessary. Also another factor which I have not considered is the effect of altitude and temperature on air. But then it tends to favour my case.

Lastly, as to why the vehicle starts after six to seven cranks? The engine cranks creates conditions which are better understood by the ECU resulting in startup. Most obvious are the purging of the exhaust system and better indication of mass flow.

Cheers

P.S. : Apologies if I have butted in the discussion between Sutripta and Offroadie.

Last edited by PGA : 4th September 2012 at 23:25.
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Old 5th September 2012, 02:51   #80
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

@SStraveller
Really wished I could have helped further but I am on limited resources. As Scorpio owners you and I have more to worry because we don't have glow plugs like the Tatas. Best thing to do would be:
1) adding 2 litres of kerosene or petrol to a tankful of diesel.
2) ensure that the battery is good and can give multiple cranks
3) pour hot/warm water over the intercooler while cranking.

@PGA
No comment on the ECU matter as I dont know everything that it factors. That said could you just share some info on how to flash an ECU, it just might come handy sometime.


Edit: just did a bit of reading.
While the ECU can assist by giving more fuel to the engine which will aid in combustion, if the temperature at the end of compression does not reach auto ignition point, the excess fuel won't be of any help. So multiple cranks or glow plug applications, to warm the engine up will be required.

Last edited by Off Roadie : 5th September 2012 at 03:08.
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Old 5th September 2012, 10:37   #81
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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@SStraveller
As Scorpio owners you and I have more to worry because we don't have glow plugs like the Tatas.
I have personally not faced any cold start issues, but I do use a diesel antigel product in high-altitude low-temp regions. Hate working too hard to start an engine, if preemptive measures can be taken to ease such efforts.
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BG 247...never faced cold start issues.
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Old 5th September 2012, 21:18   #82
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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Originally Posted by SS-Traveller View Post
Sutripta-da, could you make the self-explanatory calculations a little more "simple as water" (jalabat taralang) for maths averse dummies like me? Let's actually see the back of that envelope! Your carrying on a 1-on-1 conversation with a marine engineer who understands diesel engines like I understand enamel, leaving some of us in the dark, is not fair. Perhaps Off Roadie can explain your calculations further - is that OK with you, Off Roadie?
Hi SS,
[Whining boy hat on] But SS, I asked first. [Whining boy hat off]
[Naughty boy hat on] I'll show him mine if he show me his. [Naughty boy hat off]

Actually, will do. But it will be a long post. So I need both time and inclination. Give me a couple of days.

Marine Engineer: I think even their smallest engines (Daihatsus/ Yanmars etc) have compressed air starting. OffRoadie can confirm.

Regards
Sutripta
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Old 6th September 2012, 02:43   #83
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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Originally Posted by Off Roadie View Post
Sorry, but this is the best I can do on a cellphone. The link below should help clarifying things further.

en.m.wikipedia.org/wiki/Adiabatic_process

Just to reiterate my point, a lower pressure at the start of compression, (when the piston has sucked the gases into the cylinder) will result in a lower compression pressure hence lower temperature and difficulty for the diesel to ignite.
The cold diesel has it's own set of problems but we are just talking about altitudes here, so left it out.
Interesting discussion, I can't resist jumping in -

Based on the same wikipedia article - T*(V^0.4) = constant, since for air gamma=1.4 . This then gives T2/T1 = (CR)^0.4 where CR is compression ratio, where T2 is final temp. T1 is initial temp.

Basically, whether you start from low pressure or high pressure, as long as CR is same and initial temperature is same, you'll end up with the same final temperature.

The lower initial temp. does play a role (since T is in Kelvin, it is not as much as some would think, though it may be very significant), but pressure doesn't seem to. Assuming CR=16 and T1=300K (i.e. roughly room temp.) we get T2=900K (i.e. roughly 630C), but with a T1 of 270K (roughly freeqzing) you get about 90C less. In each case initial pressure doesn't really play a role for final temp. (it does decide final pressure)


All of the above for idea gases and reversible adiabatic process of course, in a real engine (especially a cold one) considerable heat will be lost to the engine block - this may effect lower initial pressure (for a given volume) systems more than higher initial pressure systems; However assuming your initial calculations were based on adiabatic process alone, either the above is a result of some simple misunderstanding (please point it out) or your calculation has some error.

Last edited by vina : 6th September 2012 at 02:46.
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Old 6th September 2012, 06:20   #84
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Quote:
Originally Posted by vina

Basically, whether you start from low pressure or high pressure, as long as CR is same and initial temperature is same, you'll end up with the same final temperature.

The lower initial temp. does play a role (since T is in Kelvin, it is not as much as some would think, though it may be very significant), but pressure doesn't seem to. Assuming CR=16 and T1=300K (i.e. roughly room temp.) we get T2=900K (i.e. roughly 630C), but with a T1 of 270K (roughly freeqzing) you get about 90C less. In each case initial pressure doesn't really play a role for final temp. (it does decide final pressure)
Pardon my understanding, the above two paras seem contradictory to me.

Also why is the problem peculiar to a crdi iengine.

Lastly, why does the vehicle start after six to seven cranks.
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Old 6th September 2012, 07:35   #85
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

@ sutripta, specially vina (thanks)
My bad with the Calculation. I just redid the calculation on paper with a cool head and the compression temperatures are exactly the same in both cases.
However there is a big drop in compression pressure.
Now this is the tricky part after massive Internet searches, to help demonstrate my point in theory I am unable to get something credible.
The one thing that is very easily available is that a loss of compression pressure leads to a starting failure. In our case here we are facing a loss in compression pressure due to the higher altitude and there is no doubt about that.
I am unable to theoretically explain why, is the low compression pressure affecting the start, maybe you (vina) could shed some light on it. Now I am more curious to get a very clear picture on, why does the engine not fire. I think the mass/density of air at higher altitudes may come into play here, something which we have left out so far. Another factor that keeps cropping up is, pressure ratio but that seems more relevant to turbines.

Just adding a link to a PDF file that supports the point that loss of compression results in starting issues.
http://global-4-lvs-turing.opera-min...20problems.pdf

You may have to tweak the address a bit as I can't on the mobile browser.

@sutripta
Main engine starts by injecting compressed air into the cylinders to move the pistons in the downward stroke, thereby cranking the engine.

Newer ships have diesel generators with an air turbine that cranks the engine

Emergency generator and life boat engines are usually battery started like cars.
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Old 6th September 2012, 09:19   #86
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

Glow plugs never act as spark plugs.
They are used to warm the block.
In a small displacement engine, if engine block is cold, the compression cycle cannot warm the center of the chamber, as heat is quicly dissipated. Hence block warmer.
No such problems in large engine blocks
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Old 6th September 2012, 09:50   #87
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

I think the pressure plays an important role. The flame initiation and transmission are dependent on the pressure as well.
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Old 6th September 2012, 10:04   #88
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

Quote:
Originally Posted by Off Roadie View Post
Just adding a link to a PDF file that supports the point that loss of compression results in starting issues.
http://global-4-lvs-turing.opera-min...20problems.pdf

You may have to tweak the address a bit as I can't on the mobile browser.
Uploading the relevant PDF file as well as the link, for others' reference.

Diesel engine problems.pdf

http://www.thenuffieldandleylandtrac...20problems.pdf
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Old 6th September 2012, 10:41   #89
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

My Laura CRDI started at normal half crank at at Pangong Tso at 5am in the morning, car display showed 4 degree celcius. A Xylo parked close took a very long crank and started with a much cruder clatter.

The only effects i observed at high altitude were slight loss in peak power and increased idle speed. Starting and operation was all normal, engine after warm up was as smooth as in low altitude.
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Old 6th September 2012, 12:01   #90
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

The primary objective of having a high compression is to get a temperature high enough for the fuel to self ignite. Once you are getting that temperature, it seems the fuel should ignite.
Loss of compression in typical sense affects combustion because it fails to raise the temperature beyond the self ignition temperature of fuel. However, one more important thing I could come across was, the Auto ignition temperatures also is affected by pressure. A higher pressure, means a lower AIT. So a drop in the final compression pressure would necessitate a higher Auto ignition temperature.

Mass/density of air looks like another factor to me. At higher altitudes, due to less ambient pressure and less density of air the mass of air is too less thereby the mixture too rich to probably initiate the combustion.

Another thing I can think of that affects combustion at lower temperatures is the atomisation of the fuel by the injectors. Since the temperature is too low, its quite possible that proper atomisation is not happening as a result the large droplets of fuel instead of the fine spray require more air which is exactly what is not available at higher altitudes.

Why does the engine start after a few cranks?

As the mixture shifts from stochiometric to either leaner or richer side, the auto ignition temperature rises. At higher altitudes, lower ambient air density resulting in a leaner mixture would require a higher AIT than normal. Repeated cranking brings up the temperature and ignition is achieved.

As the engine is cranked a few times and the fuel starts flowing in the pipes, maybe you get a slightly better atomisation too. This with already increased temperature might be doing the trick.
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