Black smoke / White smoke

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Originally Posted by YNOT

Diesels are the opposite of petrols when it comes to mixtures, an overfueled diesel will run hot.
It's interesting driving a turbo diesel with boost and pyro gauges for the first time and seeing what is happening in your engine. Lets say your cruising at 60km/h in 4th gear, about 1800rpm. Put your foot down to accelerate up to 100km/h, the boost gauge will come up very quickly (especially if you have a big exhaust fitted) but the pyro will lag behind and be a lot slower to rise. You will find you can move the needle on the pyro by lifting and pushing the accelerator. If your foot is flat to the floor the turbo will be on full boost (7psi standard) and the EGT's will rise to 500-550 if you hold you foot down for a while, but lift your foot slightly and the EGT's will start to drop without the boost gauge moving.
At other times under heavy load (climbing a long hill) you will often find that although you have the torque to maintain speed on that hill in 4th gear, the EGT's will be lower by dropping down to third and using less throttle to maintain the same speed.

Tony

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Thanks for the explanation of the relationship between EGTs, boost pressures and throttle openings.

If I have really understood what you said –

1. EGTs increase because of unburnt diesel and amounts of unburnt diesel are created by “more accelerator” relative to engine revs, instead of changing down a gear.
2. EGTs (i.e. unburnt diesel) are also increased by an injector pump which is incorrectly adjusted and supply too much diesel relative to throttle openings.
3. The result in each scenario is an overheated turbo, black smoke from the exhaust, a sooty exhaust and a very hot dump pipe.

Please let me know if I have got any of that wrong.

I don’t have a boost gauge or a thermocouple fitted, but found that if I leaned off the injector pump too much (which was about 1/10 of a turn of the adjustor screw) i got a lack of power, a hot engine (i.e. looking at the coolant temperature gauge - the turbo is oil cooled) and a hot dump pipe.
Anyway, to correct that “problem” I overcompensated so that I ended up with black smoke with acceleration, but coolant temperatures looked more comfortable, even though the dump pipe still seemed to get pretty hot.
With my third adjustment (about 1/20 of a turn the other way) I think I’ve got it almost right – less accelerator pedal for the same or more power, coolant temperature that rises when climbing hills (I’ll now be using 4th gear a little more) and a dump pipe that is less like a roasting spit.

I’m uncomfortable with the variation in coolant temperatures (rises from about 1/3 to 1/2) as my reading of it is that there is a peak in the engine operating temperature that is not fully reflected in the temperature gauge movement. That said, it does not appear to matter much whether it is a hot day or a cool day, the coolant temperatures seem to be roughly the same.

I would appreciate any additional info.

If I have really understood what you said –

1. EGTs increase because of unburnt diesel and amounts of unburnt diesel are created by “more accelerator” relative to engine revs, instead of changing down a gear. Diesels engines are an excess air engine, as far as airflow into the intake is concerned they are at full throttle all the time, the power is throttled by accelerator position and therefore quantity of fuel injected. The more fuel that is injected the hotter the combustion gets. As with any fire if you add more fuel than what the oxygen can support you will end up with unburnt fuel, in a diesel engine the unburnt fuel will continue to burn as it exits the cylinder past the exhaust valve and into the turbo (where fitted). Excessively high exhaust gas temperatures can melt pistons and valves, and damage turbos.

2. EGTs (i.e. unburnt diesel) are also increased by an injector pump which is incorrectly adjusted and supply too much diesel relative to throttle openings. A injector pump is incorrectly adjusted if it is supplying more fuel than what the oxygen in the cylider can support.

3. The result in each scenario is an overheated turbo, black smoke from the exhaust, a sooty exhaust and a very hot dump pipe. And possibly melted pistons and valves.
Please let me know if I have got any of that wrong.

Another thing to consider is your injectors. If they are not atomising the fuel properly the fuel will be slow burning and raising EGT's even though the actual quantity of fuel injected can be supported by the oxygen in the cylinder.It's important to make sure the basics are correct before you start doing any tuning, so valve clearances, injectors, injector pump, pump timing, boost etc.

I don’t have a boost gauge or a thermocouple fitted, but found that if I leaned off the injector pump too much (which was about 1/10 of a turn of the adjustor screw) i got a lack of power, a hot engine (i.e. looking at the coolant temperature gauge - the turbo is oil cooled) and a hot dump pipe.
Anyway, to correct that “problem” I overcompensated so that I ended up with black smoke with acceleration, but coolant temperatures looked more comfortable, even though the dump pipe still seemed to get pretty hot.
With my third adjustment (about 1/20 of a turn the other way) I think I’ve got it almost right – less accelerator pedal for the same or more power, coolant temperature that rises when climbing hills (I’ll now be using 4th gear a little more) and a dump pipe that is less like a roasting spit. I would not recommend setting the injector pump by engine temperature, the engine temp guage is far too slow reacting and no where near accurate enough. Do yourself a favour and fit boost and pyro gauges ASAP.

I’m uncomfortable with the variation in coolant temperatures (rises from about 1/3 to 1/2) as my reading of it is that there is a peak in the engine operating temperature that is not fully reflected in the temperature gauge movement. That said, it does not appear to matter much whether it is a hot day or a cool day, the coolant temperatures seem to be roughly the same. It might be time to do some cooling system maintanence.