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Old 2nd September 2012, 21:10   #61
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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
Hi,
insufficient heat = lower temp?
Could you give some illustrative values, (and if possible the basic equations) which lead to these results.

Regards
Sutripta
Oh boy, will keep it simple, hate complicated theories =p
If you check the compression ratios of diesel engined cars with their equivalent petrol counterparts you will see that the diesel engines have a higher compression ratio. This is because unlike petrol engines that have a spark plug to ignite the air fuel mixture, in a diesel engine the fuel is ignited by the heat generated while compressing the gas in the cylinder. A higher compression ratio develops a higher compression pressure and temperature which makes it easier for the fuel to ignite.

At high altitudes the atmospheric pressure is low so the pressure in the cylinder at the start of compression is low. Hence the final compression pressure is low if the ambient temperature is high it won't matter but when the ambient is low the heat produced / temperature attained at the end of compression is not enough to fire the fuel.

The low temperatures cause problems with the fuel itself which have been well discussed in this thread so won't go there.

Hope that helps I guess I am not too good at explaining in text.
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Old 2nd September 2012, 21:38   #62
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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I guess I am not too good at explaining in text.
Maybe we should start with simple (gas law) equations.

Regards
Sutripta

Last edited by Sutripta : 2nd September 2012 at 21:39.
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Old 2nd September 2012, 22:07   #63
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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.
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Old 2nd September 2012, 22:16   #64
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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
Thanks.

That link has a worked out example for temperature. What values do we get for temp if we change the initial pressure to a lower value?

Regards
Sutripta
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Old 2nd September 2012, 22:16   #65
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

Something interesting from a medical viewpoint, assessing availability of oxygen at given altitudes: http://www.altitude.org/air_pressure.php. Possible to correlate with why a compression ignition engine refuses to fire at times?
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Old 2nd September 2012, 22:48   #66
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

@SS-traveller, thanks for the help.
@sutripta,
If you just use the link given by SS-traveller, you will see that @ 10,000 ft the atmospheric pressure is 71kPa.
Using the same equation provided in the wiki link I provided.
When calculated at sea level the compression temperature is 750K
At 10,000 ft that drops to 540k and there is a difference of almost 210K
This of course is all textbook ideal conditions.
The realworld compression conditions will have a tough time firing the engine up. All the cranking we do builds up some heat in the engine which finally helps in firing it up.
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Old 2nd September 2012, 23:07   #67
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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
@SS-traveller, thanks for the help.
@sutripta,
If you just use the link given by SS-traveller, you will see that @ 10,000 ft the atmospheric pressure is 71kPa.
Using the same equation provided in the wiki link I provided.
When calculated at sea level the compression temperature is 750K
At 10,000 ft that drops to 540k and there is a difference of almost 210K
It would be nice if you could give the worked out example. Maybe as an excel file.

Regards
Sutripta
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Old 2nd September 2012, 23:53   #68
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

Sir I am on a cellphone only (until December) I wish i could help further. This is not rocket science really. Any college science student will be able to explain in details, I am sure you may be able to get hold of one. You could probably post it online for fellow members benefit.
Regards.

Last edited by Off Roadie : 3rd September 2012 at 00:17.
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Old 3rd September 2012, 01:27   #69
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

Okay now I understand.
Diesel engines are compression ignition engines.
So the downstroke, sucks in air.
When atmospheric pressure is low, sir sucked in is less.
At the compression cycle, when fuel is sprayed, it does not ignite. You have to give a longer crank.

I understand the theory behind it.

However, I am still confused because
1. The safari has glow plugs. I have verified they work. They should preheat the cylinder block. So when ambient is around 0-2 degree C, after multiple glow plug application, we should have a reasonably warm cylinder
2. At Hanle, when the vehicle did not start, I opened the bonnet, and primed the manual fuel pump bulb, also called hand primer. It was very soft. After pressing it for 3-4 times, it turned hard, indicating, fuel had come. Then I cranked the engine, and again it sputtered and died. I again repeated the hand priming thingy.
So not just ignition, but fuel delivery to the engine was an issue. After multiple cycles, I was able to start engine.

Is it possible, that at very low pressure, diesel waxification happens at a higher temperature, leading to clogging of sedimeter?
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Old 3rd September 2012, 02:18   #70
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

Thanks Tanveer, was wondering if my explanation was that bad, glad it made sense.
Do check out this BP document on diesel related problems.
http://global-4-lvs-turing.opera-min...ms_Summary.pdf

There are 2 questions that may arise if one mixes kerosene with diesel.
1) emission equipment related- can't comment on that
2) fuel injection related- the common concern is that kerosene has low lubricating properties and may damage the fuel pumping/injection equipment. Well, since, it's being done at lower temperatures where the deisel is going thick / waxing, the lubricity of the mixture is roughly the same as the lubricity of diesel in warmer conditions. So fuel equipment damage should not occur.

Can't confirm about the relation between pressure and waxing but there is a high likelihood that it may have a correlation as you mentioned.
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Old 3rd September 2012, 20:18   #71
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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
Sir I am on a cellphone only (until December) I wish i could help further. This is not rocket science really. Any college science student will be able to explain in details, I am sure you may be able to get hold of one. You could probably post it online for fellow members benefit.
Regards.
No, its not rocket science!
Your premise (that a lower initial pressure will result in a lower final temperature ) is pretty clearly stated by you in text. No ambiguity in that.

However, my back of envelope calculations are at variance with your premise. Thus my repeated (moronically so, I must add) requests asking for your calculations illustrating your premise. Esp the calculation where initial pressure at 70000 Pa results in final compression temp of 540 K.

Regards
Sutripta

PS - I too hate long posts!
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Old 3rd September 2012, 22:46   #72
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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

However, my back of envelope calculations are at variance with your premise. Thus my repeated (moronically so, I must add) requests asking for your calculations illustrating your premise. Esp the calculation where initial pressure at 70000 Pa results in final compression temp of 540 K.

Regards
Sutripta

PS - I too hate long posts!
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.
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Old 3rd September 2012, 23:33   #73
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

The actual document is here
http://www.bp.com/liveassets/bp_inte...ms_Summary.pdf

They recommend 5 liters of kerosene to 100 liters of diesel. I am thinking, but this logic, none of the diesel in India should have any low temperature problems . As it is its half kerosene.
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Old 4th September 2012, 19:21   #74
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

Just wanted to post this link here as it may add value. It's the temperatures at which fuels autoignite, without the need of a spark or flame.
Note:
Gasoline = petrol
Gas oil = diesel

http://www.engineeringtoolbox.com/fu...res-d_171.html
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Old 4th September 2012, 19:46   #75
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Default Re: Common Rail : Why is cold start tougher at higher altitudes

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...temperatures at which fuels autoignite, without the need of a spark or flame.
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).

Last edited by SS-Traveller : 4th September 2012 at 19:48.
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