Why do some diesels smoke when cold?

[hayden]

Some large trucks and heavy equipment smoke profusely (white/blue) when started and while they warm up. I understand this is due to incomplete combustion until the engine comes up to operating temperature.

On the other hand, my kubota and many other diesels give one puff of black smoke when they first fire, but otherwise run visually clean from stone cold to full operating temp.

Why the difference? Is it direct vs indirect injection? Do some glow plugs provide residual heat to clean up combustion while warming the engine? Have engine designs just gotten that much better?

 

[GlueGuy]

Hayden,

I don't know for sure, but I would bet it's a combination of factors. One you mentioned is the direct vs indirect injection; I'm sure that makes some difference. Another would be the size of the engine. A little one or two liter diesel warms up a heck of a lot faster than the monster in some big rig. Another factor would be the age/condition of the engine, not to mention the age/condition of the emission controls (if any).

 

[Steve_in_Ont]

as GlueGuy mentions - It is a combination of factors. One more is:

Low emissions engines ironically will tend to produce more visible smoke at start up. This is due to higher injection pressures (cleaner burn when everything is warm, but almost "puts out the fire" when the engine is cold) and retared injection timing (reduces NOx, but leads to incomplete combustion when the engine is cold). Currently off-highway engines do not need to meet the same tough emissions standards of on-highway engines. Starting in 2006 (and phased in after that based on engine HP) off-highway emissions standards will be much more like on-highway (The EPA term is "Tier III").

One more thing about white smoke at startup - In moderation, it's not really anything to be concerned about... As long as it goes away as the engine gets warm. If an engine makes white smoke in operating conditions, then there is something wrong. In extremely cold climates, the intake air can be soooo cold that this is a problem. Generally, by re-diverting the intake air to use underhood (and slightly warmed) air, this problem is gone (There is a thread in this section on "warm intake air")

Blue smoke is burned oil. Also, at low levels not a huge issue on a cold engine, but it should go away almost right away on newer engines. Older diesel engines are generally designed around a slight continued consumption of oil (The stated oil consumption on 1965 Cat 977 is 1 gallon in 8 hours of use).

While we're on smoke - Black is partially burned fuel. Many people think of black smoke as a sign that a diesel is really putting out the power... However, it is really a sign that there is too much fuel for the amount of oxygen in the cylinder... And it's just wasted.

 

[ronjhall]

Peter
Many of today's larger engines have Turbo Chargers. At low RPM's and when first starting there is no boost from the Turbo. As the engine and Turbo warm up there is boost to improve combustion.
When our little <font color=orange>Kubota</font color=orange> engines first start, there is some fuel that is not burned until cylinders start igniting the fuel. This unburned fuel is the puff of black smoke that you see when starting.

 

[frank_f15]

steve: thanks for the great explinaton on smoke. and does the oil consuption as u mention apply to the newer , smaller diesels? such as on my b2400

 

[yaouk]

I have noticed white/blue smoke from my 1.5 litre Dong Feng diesel. It disappears almost immediately. However, when I allow the glow plugs to warm up properly there is almost no smoke at all.

 

[hayden]

I'm thinking of the white smoke that lasts for several minutes as some machines warm up. I've read a number of authoritative papers stating it's due to incomplete combustion as the engine comes up to operating temperature. It makes perfect sense, but I wonder why some engines run clean (at least visually) from a cold start. They have to warm up too, but seem to get more complete combustion while doing so.

 

[Steve_in_Ont]

Frank - Generally speaking, a modern diesel should only consume a slight amount of oil. Many will only be getting down toward the "add" line as the oil drain interval is reached (my TC21D did this on the first oil drain... I have not reached the second yet...).

As emissions standards get tighter, the oil consumption will fall even further (burning oil can be a large portion of total measurable smoke emissions).

For any engine, there should be a stated tolerance of "acceptable" oil consumption. I know that Cat states theirs as a factor of "X Gal. of fuel burned / Y Gal. of oil consumed" (However, I can't remember the actual number... but the key is that the harder you work it, the more oil consumption is considered "acceptable". Beyond just wear out, one primary reason for this tolerance is for dealers to determine if an engine actually has a warrantable problem.

 

[frank_f15]

hayden : u are right. my 2400 only gives a small shot to smoke when i first start it then it runs smokeless, why is that?

 

[gmason]

I have a 10hp Yanmar 2 V cyl (10hrs)that is for my generator. It's a pull start/electric, that for about 10 seconds puffs out blue/grey smoke while it's rpms are getting up.
I can let this set for 2 months and pull start it on the first pull...Awesome..