Smokey Diesels – why is it…?
Could be a million different things from a leaky gasket to a stuck EGR valve. The main reason is the driver. Mondeo owners are in middle age denial and Focus drivers like to make everyone think they’ve got an ST. As a result they both have bricks over the gas pedal.Posted 6 years ago
Me, I’ve got a diesel Vectra which I drive like a massey ferguson.NorthwindSubscriber
Junkyard – Member
Thankfully “boffins” can measure this invisible stuff and tax appropriately.
Well. It all goes off CO2 doesn’t it? Which isn’t a good measure of pollution.
That said, anyone who thinks diesels are dirtier than petrol, just because of the almost completely harmless soot, is a fool.Posted 6 years agoFunkyDuncMember
I have a Mondeo and when I bought it it smoked big time. Now it doesn’t so much as I make sure I drive it hard every so often. Even on the motorway at 80 your only doing 2000 rpm so doesn’t really get the soot out.
We also have a newer VAG diesel. On start up that always chucks out loads of soot, but doesn’t kick out as much as the Mondeo in normal driving.Posted 6 years agoglobaltiMember
Older Fords both diesel and petrol do smoke a lot but that’s grey smoke from oil being burned – I don’t know enough about Fords to tell you why it’s characteristic but it’s either down to worn valve guides or worn piston rings.
Older diesels like the 2007 Passat PD I’ve just handed back, smoke a lot on acceleration; a brown smoke of unburned hydrocarbons. Any fuel burned at too low a temperature or not fully burned will produce smoke as any stove owner knows. The new Passat I’ve just collected with the new common-rail diesel doesn’t seem to smoke at all, no matter how hard I try.
We used to have a 1986 Land Rover diesel, which had a very primitive fuel delivery system. If somebody was driving up your chuff you could declutch and floor the throttle to rev the balls off the engine and a massive cloud of greasy black smoke used to come out, which usually made people back off smartish. Not that tailgaters bothered us because when Mrs Gti got rear-ended by a Freelander it merely squashed our spare wheel carrier while the Freelander was absolutely flattened.Posted 6 years agospeed12Member
Diesel has a higher energy density than gasoline – so you need less fuel to get an equivalent amount of ‘bang’. The trick though is to get all the energy out of that fuel, which is why it has taken until reasonably recently for diesel engines to fully meet the performance (in terms of drivability, smoothness and power) of gasoline engines.Posted 6 years agosockpuppetSubscriber
it’s less down to the difference in energy density, and much more down to the operating temperature.
the efficiency of any heat engine is determined by the temperature drop over which the heat cycle can be run, in thermodynamic terms. for your internal combustion engine this is the temperature difference between cylinder temps and the ambient background.
diesels can operate hotter than petrol engines, since they don’t pre-detonate like petrols do at high operating temps (pinking, iirc). bigger temp drop for the heat cycle means better inherent efficiency.Posted 6 years agoandydickoSubscriber
Slightly off topic – what makes diesels so much more economical?
To make it simple, Diesel cars are more efficient, basically every £1.00 of fuel put into a Petrol about 30% goes to making the car go forward the rest is lost in heat. Every £1.00 put into a Diesel about 45% goes to making the car go forward, so less is lost to heat…. hence why Diesels take longer to warm up on colder days!!Posted 6 years agoDickyboyMember
diesels can operate hotter than petrol engines, since they don’t pre-detonate like petrols do at high operating temps (pinking, iirc). bigger temp drop for the heat cycle means better inherent efficiency
I think you have this the wrong way round? Diesel engines run about 200C lower than petrol engines but operate at higher compression ratios & are therfore able to convert more energy into forward motion, plus diesel whilst having the same calorific value of petrol (per kg) is about 15% denser, so more bang per litre of fuel.Posted 6 years ago
Another factor that makes diesels more efficient is direct injection. You only need to inject the fuel you need to burn, whereas in a petrol you have to fill the cylinder with air/fuel mix so that it ignites – then if this is too much power you have to throttle it back which wastes energy.
As for the smoke – it’s only bad when people tootle around town all the time. My diesels have always had lots of long trips and done some work, which cleans out any crud regularly. The Passat has never run so smoothly and cleanly as when I was in Germany doing lots of Autobahn.
My 1.9 Ibiza 105bhp wouldn’t smoke at all no matter what I tried; the Passat can do a bit if I do lots of town driving then floor it suddenly. Globalti’s Passat will have a DPF so no smoke.Posted 6 years agotonydMember
I’ve got a 7 year old BMW 320d (no I’m not a sales rep!) and notice it smokes when I accelerate hard. Not loads but if the car behind has headlights on you can see it sometimes. I just assumed this was a combination of ‘clearing it out’ and burning more fuel.
So while we’re talking about diesel, why is it more expensive than petrol now? I seem to recall years ago it was a good amount cheaper than petrol, is this because more people are driving diesels so ‘they’ have decided they can wangle more money from us or does it really cost more to produce (or is now taxed higher)?
I thought I read somewhere once that diesel is a by product of the refining process. If that’s true (doubtful since I can’t remember where I read it) why is it more expensive now?Posted 6 years agorkk01Member
Smoking – largely down to the driver IMO. Mine smokes occasionally if you open the throttle sudenly. As someone posted above, diesels need to be worked hard from time to time to clear the soots out. It also helps to add some Redex diesel to the tank once in a while
Gasoline – as much as I dislike Americanisims, “petrol” is a misnomer used by the British public. Petrol is short for petroleum, which covers a whole range of petroleum products. Gasoline is a blended fuel comprising a variety of different light hydrocarbon fractions from C5 to about C10…. and of course one of those blend components, C8, Octane gives it’s name to the old octane rating
Cleanliness and efficiency – It took me a long time to get a diesel, and it was a purely fiscal choice. As someone who works in the environmental industry I remain sceptical about the benefits of diesel over petrol. Tax and profits are a significant factor as far as I see itPosted 6 years ago
There is always a light haze that you can see at night in the car behind’s headlights.. I woudln’t call this smoke really.
Petrol is short for petroleum
Have to pull you up on that one I’m afraid. Petrol is short for petroleum spirit ie something distilled from petroleum, which is not an unreasonable name. Next time you are at a garage look around you’ll see some warning signs somewhere that say ‘danger, petroleum spirit’ or similar.
Diesel is worse for some pollution (NOx and particulates) and better for others (CO2 and hydrocarbons, both greenhouse gasses). So it depends which you consider important. If I lived in Athens I would not drive one because NOx and sunlight create smog, which is a terrible local problem there.
This is why petrol hybrids were invented, really – for the US market. The fuel economy of a diesel but with far less tailpipe pollution. And before anyone starts banging on about batteries being terrible things, find some real actual data about it. I’d love to see some.Posted 6 years agocraigxxlMember
Most diesel smoke is down to poor servicing. Diesels require a good, clean flow of air. If the air filter is blocked up, leak in the turbo charger/pipework or blockage i.e. EGR then it will overfuel creating unburnt diesel in the exhaust and smoke.Posted 6 years ago
Diesel isn’t a clean fuel like petrol and injectors and pumps can collect sediment causing poor injector spray and weak combustion so the fuel system needs regular, at least annually, to maintian efficiency. Products like Redex are useless on diesel fuel systems and need stronger products such as BG44 or Forte.
I’ll look up BG44 and Forte, thanks. Definitely interested in keeping my diesel running nice and cleanly.
EDIT just looked at BG244, the first thing it says it helps with is ‘hesitation and flat spots’. Well I’m seeing hesistation a couple of times every time I drive off, so I’ll give it a whirl. Do you have to order it online?Posted 6 years agoEcky-ThumpMember
Have run nothing but diesels for a few years now in our house.
Yes they smoke a bit when you boot it. It’s just clearing out all the soot that’s built up in your exhaust as you’ve been pottering along for a bit.
Top tip immediately pre MOT, give it a good thrashing (eg red-line it up a big hill in second) to get everything out of the exhaust system before. 🙂Posted 6 years agotonydMember
There is always a light haze that you can see at night in the car behind’s headlights..
This is what I was trying to describe and is just what I see, although not always.
I’m amazed nobody has picked up on the use of the word ‘tailpipe’ already so here goes….
Tailpipe? TAILPIPE? It’s called an exhaust pipe over here you know. 😛Posted 6 years agoswamp_boyMember
The usual reason for black smoke is the engine running too rich, i.e. not enough air to burn all the fuel going through. It can be down to problems with the engine, but you can make almost any diesel do it by labouring the engine. Putting your foot down pumps more fuel through, if you’re in the right gear for the speed you’re doing, no problem, if the gear’s too high the engine will pump out black smoke until the revs catch up enough to send through enough air to burn all the fuel. Electronic engine management helps, but can’t always compensate for a heavy right foot.Posted 6 years agoowenfackrellMember
The modern ones shouldn’t do it if they have a fap (or particulate filter) as this is ment to trap the particules but its basically unburnt fuel. I can just about get my auto diesel to do it by giving it a real boot full but its only mild compared to others. Most manual diesel will do it noticable when floored or as above if they have ablocked air filter or spilt turbo pipe and the air/fuel mixture is rich.Posted 6 years ago
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