Why do diesels trains exhaust smokey when they pull off?

[thenorthern]

I have noticed whenever a diesel train pulls off from stationary or starts up it seems to let off a lot of grey smoke from the exhaust much more than a diesel car would do and I was wondering if there is any reason for this?

 

[Bornin1980s]

Which diesels?

 

[Goldie]

Turbo lag. More fuel is allowed into the engine as the throttle is opened, but it takes a while for the turbocharger - which is driven by exhaust gas - to spin up and start squeezing enough air into the engine for the fuel to burn cleanly. Alcos were / are particularly good at belchng massive black clouds of smoke when the throttle was opened:

 

[DelW]

Car diesel engines used to be smokier than current ones are. For a number of years now they have had to be fitted with particulate filters which are expensive, have a limited lifespan, and have to have the collected clag burnt out at intervals by running at high temperature - usually by squirting extra fuel into the exhaust. I doubt that many, if indeed any, rail vehicles have such a system.

 

[Master29]

If you want to see any good smoky diesels just type in Indian diesel locomotives and Robert is your uncle. Makes an old Valenta seem like a new electric.

 

[Mitchell Hurd]

This was a major problem with the Paxman Valenta engines once in HST's - from the 90's this was. That's where the Paxman VP185 and MTU 16V 4000 engines came in. Although I've a soft spot for VP185's, if I worked at a train company and wanted new diesel locomotives, I'd want the MTU engine or better!

 

[Goldie]

If you want to see any good smoky diesels just type in Indian diesel locomotives and Robert is your uncle. Makes an old Valenta seem like a new electric.

Fun fact: In an unlikely but wonderful case of industrial survival, those smoky Indian diesels are licence built Alcos, featuring variants of the same Alco 251 engine that allowed Delaware-Lackawanna's 2403 to belch out clouds of the black stuff in that video above. India's Diesel locomotive Works has been building them since the early sixties, and I think developed versions might still be in production now.

 

[WatcherZero]

Its less noticeable on modern diesel cars because the engines have better pipe geometry and often have a power assist to get the air circulating initially before turbos can spool up and take over.

 

[InOban]

They have an air mass sensor which restricts the fuel until the turbo has kicked in. Our local ferries have the same problem - a huge burdt of black smoke as they leave the pier.

 

[thejuggler]

A 30 year old diesel car will smoke as much as a 30 year old diesel train, its just you don't see many 30 year old diesel cars.

Modern car and HGV diesels are fitted with any number of systems to reduce pollution including DPF filters. These aren't retrofitted to 30 year old diesel trains.

 

[Railperf]

You certainly notice the difference between an MTU and VP185 engines power cars at idle and when starting off. Voyager and Meridian engines also seem to emit very little Clag but then again standing within the platform area at a station such as Birmingham New Street is not a pleasant place to be when a Voyager is standing there with all engines idling and no specific extraction for the diesel fumes.

 

[KevinTurvey]

Before the VP185s & MTU's came about I recall an article about 1983 to replace the Valentas with a Pielstick PA6 engine, but presumably the idea was abandoned.

Was this ever tried because some of their engines can be quite smoky!

 

[Kneedown]

Nothing beats a cold start from a 31 or 56 in Winter for clag! Toton depot used to disappear in a smoky fog until they warmed up a bit.

 

[infobleep]

A 30 year old diesel car will smoke as much as a 30 year old diesel train, its just you don't see many 30 year old diesel cars.

Modern car and HGV diesels are fitted with any number of systems to reduce pollution including DPF filters. These aren't retrofitted to 30 year old diesel trains.

Would it be possible to retrofit them to diesels and if it were, would it be prohibitively expensive to do?