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Old 25th November 2010, 15:35   #1
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Question Exhaust in automobiles - Hill stations vs. planes

During a recent visit to Ooty, my son asked me this. "How come the vehicles in Ooty have black smoke coming out of their exhausts?" This I believe is true for most hill stations. I could think of a few reasons:
1. The wear and tear on these vehicles is more
2. With the O2 level being lower, the combustion might not be complete (stupid)
3. Since these vehicles run a good amount of their life in lower gears, the stress on the engine is greater leading to incomplete combustion and more carbon deposits
4. These vehicles run for longer periods before the engine reaches optimum running temperature thus contributing more to the wear and tear
5. In general the vehicles at hill stations are older

Please do add on.
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Old 25th November 2010, 18:01   #2
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SafeDrive, are you referring to all vehicles (including visiting vehicles) in Hill Stations or the ones that are always based out of there (Taxis etc..)?

Usually, the air in higher altitudes is thinner and hence vehicles that don't compensate for this will run a bit rich producing black smoke. However, the more recent diesels (controlled by ECU's) take this into consideration and compensate for this.

Hope the answer helps.
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Old 25th November 2010, 18:13   #3
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2 is bunkum.
1 doesnt hold. same vehicles in ladakh dont give as much black smoke enroute as they do when struggling up the crazy zoji la.

one theory i heard is that they are non-CRDI engines (electronic throttle control) so when accelerator is pressed in excess, the extra unburnt diesel comes out as black smoke. sort of #3 really.

I've accelerated hard in my swift vdi (55k km) but no black smoke ever till date so i'd sort of subcribe to it.
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Old 25th November 2010, 18:25   #4
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Your point 2 is true but it holds good for only 10,000 feet or above. Most Diesels (CRDI) running in the country are generally calibrated upto 4000 MSL and thereafter go on extrapolation.

Black smoke is primarily because of unburnt diesel and can caused due toeither choked air filter or increased nozzle size both of which can happen due to wear and tear. For most CV's its because of the above two reasons and not absence of oxygen that you get the black smoke.
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Old 25th November 2010, 18:28   #5
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Naturally aspirated petrol engines lose approximately 3 % power per 1000 ft, as far as I know, because of thinner air. If fuel injection, especially in petrol engines, does not allow for that, black smoke is possible. In diesel engines, there is lot of excess air used, so the height factor may not be that important. Turbo engines are supposed to be unaffected by height from sea level.
Water vapour is produced as a result of combustion process and could look like smoke in cold weather, like your breath.
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Old 25th November 2010, 20:25   #6
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Thanks for the responses. You're right, now that I think about it I don't think I've seen any of the newer generation vehicles emit this. So I guess that could be it.

Quote:
Originally Posted by kaushikr View Post
SafeDrive, are you referring to all vehicles (including visiting vehicles) in Hill Stations or the ones that are always based out of there (Taxis etc..)?
I was referring to the vehicles always based out there.
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Old 25th November 2010, 20:56   #7
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It shouldn't really make a diff in an MPFI engine. Prob the cars you saw were old or badly maintained.

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Old 25th November 2010, 21:00   #8
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@Safedrive - Any FI engine will compensate the altitude by running lean automatically.

However on higher altitude (may not be applicable for Ooty), people running on carb engine should lean it on reaching the top, which they may not be aware. So there're chances for smokes.

The black smokes, you may want to explain, which kind of vehicles you noticed? Were they Tata & AL buses or (abused) old Indicas or necessarily any petrol engines?
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Old 25th November 2010, 23:16   #9
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i think you might have seen diesel vehicles with black smoke.
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Old 26th November 2010, 12:14   #10
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Quote:
Originally Posted by aargee View Post
The black smokes, you may want to explain, which kind of vehicles you noticed? Were they Tata & AL buses or (abused) old Indicas or necessarily any petrol engines?
Quote:
Originally Posted by SirAlec View Post
i think you might have seen diesel vehicles with black smoke.
The black smoke were mainly from the tourist vehicles and commercial vehicles (buses and trucks). Yes, they should have been all diesel vehicles. I was driving a Swift and it did not have any visible exhaust.
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Old 26th November 2010, 12:25   #11
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Quote:
Originally Posted by aargee View Post
@Safedrive - Any FI engine will compensate the altitude by running lean automatically.

However on higher altitude (may not be applicable for Ooty), people running on carb engine should lean it on reaching the top, which they may not be aware. So there're chances for smokes.

The black smokes, you may want to explain, which kind of vehicles you noticed? Were they Tata & AL buses or (abused) old Indicas or necessarily any petrol engines?
The black smoke being talked about is for the Diesel engines.

I think you meant Fuel Injected and not Forced Induction. With increasing altitude the engine starts running richer and not lean (More fuel in the Fuel Air Mixture). So the air fuel mixture needs to be adjusted and certain cases even ignition timing. In ECU controlled petrol engine, the Oxygen / Lambda Sensor checks the air pressure and accordingly instructs the ECM to change the air - fuel ratio. In carb engines, you need to adjust the air screw.

In case of Diesel engines, this is slightly tricky. In case of Naturally aspirated Diesel Engine, with higher altitude, the black smoke will increase due unburned diesel coming out. Incase of prolonged use, the Fuel Injection Pump (FIP) needs to be recalibrated to reduced fuel follow and in certain cases a timing as well.

Incase of Forced Induction / Turbo Diesel engines, the boost pressure reduces but not that significantly and hence you'll see lesser black smoke than NA engines.

In case of CRDI engines, the atmospheric pressure is being monitored by the ECM, which regulates the fuel flow and rail pressure in tune with the drop of boost pressure for efficient combustion of fuel. So a CRDI engine is produce the least amount of black smoke.

The above comparison is based on the assumption that all the three engines are pretty well maintained with minimal wear and tear.

Hope this makes everything clear.

Cheers

Last edited by MileCruncher : 26th November 2010 at 12:28.
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Old 28th November 2010, 09:57   #12
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Thanks MileCruncher. Yes, that explains a lot. Makes perfect sense now.
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Old 28th November 2010, 14:39   #13
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That was a neat explanation MileCruncher, but is this the same reason why buses, lorries and other heavyduty (unmaintained) vehicles spew out black smoke?
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Old 28th November 2010, 16:14   #14
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Quote:
Originally Posted by MileCruncher View Post
Your point 2 is true but it holds good for only 10,000 feet or above. Most Diesels (CRDI) running in the country are generally calibrated upto 4000 MSL and thereafter go on extrapolation.

Black smoke is primarily because of unburnt diesel and can caused due toeither choked air filter or increased nozzle size both of which can happen due to wear and tear. For most CV's its because of the above two reasons and not absence of oxygen that you get the black smoke.
The unburnt Diesel escapes the combustion chamber because of lack of oxygen, whether by starvation of oxygen due to decreased flow through filter or worn/dripping nozzle.

The only exception is a cold engine.
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Old 28th November 2010, 16:31   #15
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Quote:
Originally Posted by CPH View Post
The unburnt Diesel escapes the combustion chamber because of lack of oxygen, whether by starvation of oxygen due to decreased flow through filter or worn/dripping nozzle.

The only exception is a cold engine.
Care to explain.

I thought in cold engines its primarily white/blue smoke.
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