Why Diesels Smoke

[Sharky1]

I didn't look at the link but to save a lot of folks from becoming paranoid, I thought I'd shed some actual facts regarding this. I am an ex mechanic although not a diesel mechanic however I do own a patrol diesel & have had a landcruiser diesel before that. I'll try to explain this in a simple manner without getting technical but knowing me, I'll get sidetracked........

Most of that info in the 1st post is true however diesels WILL naturally smoke to some degree & I'll explain why. 1stly, they run off the compression of the motor, not from a spark as such, so not as much of the fuel gets burnt nor does it burn as efficiently as a petrol. Another reason, which goes hand in hand with what I just said, is that oil is a biproduct of petrol. It is extracted during the refining process ie petrol is more refined, hence thinner & has a lower flash point & higher combustion rate. These days they make diesel a little differently but for argument's sake we'll stick with what most people can make sense of.

Smoking diesel is NOT necessarily unburnt diesel. If it were unburnt, it wouldn't smoke. However it is probably more correct to say that it's not fully burnt. However even if it is, every fuel that burns has some sort of carbon output & diesel has a higher carbon output than petrol, therefore will smoke more, especially under load, whereby your accelerator pedal is further towards the floor but your engine isn't revving as high which means you're using more fuel than is being fully burnt. ALL fuels create carbon & the more refined the fuel, the less carbon output, hence less smoke. So yes, a diesel will naturally smoke under load to some extent. It's for this reason that poor fuel quality will make your engine smoke more.

An easy way to test this is to (VERY CAREFULLY), throw a little bit of petrol on a camp fire & see how quickly it flames up & how much smoke it emits. Then throw the same amount of diesel on the same part of the fire & observe again. Then throw the same amount of engine oil on the same part of the fire & see what happens.......more smoke, less flame & will sizzle as it takes time to burn, unlike the petrol which burnt immediately. Remember liquids DON'T burn. Heat turns liquid into gas, which burns. Hence why manufacturers try to get petrol to "atomize" before it hits the compression chamber. The smaller the atoms, the faster the burn, the bigger the explosion, the more power you can create. Diesel is different though & as I stated, not only is it an oilier fuel, the burn process is completely different. If you get some cold diesel & hold a lit match to it, it won't burn. I've heard many people say it will but I've done it & it never has. I'll come back to this point later. So you may ask, why don't I put petrol in my diesel & get more power? Well there are many other differences in the properties between diesel & petrol & I'm trying to keep this simple. The biggest reason though is that petrol is a lot more acidic & corrosive than diesel & doesn't have the lubricating properties that a diesel engine needs to be reliable & long lasting.

Yes, inadequate air intake will cause smoking. Any fuel needs oxygen to burn. Too much oxygen & it will over-rev ("a dieselling effect"). Too little & it will choke & smoke. Hence where the theory comes from of getting more power from a motor is not to give it more fuel as such, but to make it breathe. More fuel means more power ONLY provided it can breathe enough to burn 100% of that fuel which is what efficiency is.

I'll go a little further. A petrol engine gets it's timing by timing when the spark plug emits a spark. Hence why a petrol motor can also smoke if the timing is out although this is rare these days with the quality of fuels. So if a diesel runs off the compression, how is it timed & how does it shut down? A diesel gets its timing by allowing a metered amount of fuel into the engine at the right time. It stops by a shut off solenoid on the fuel/injector pump. If your injectors or pump are worn, your diesel will smoke as the fuel system will let too much diesel into the engine than what it can burn.

The reason diesels get a lot more torque in general but a lot more low down torque is mostly but not completely due to the fact that because diesel will burn purely off the compression & burn at a slower rate, a diesel will hold lower revs & do so under load. The other reason is due to the amount of compression a diesel motor has. Increase your compression ratio & you increase your torque & power outputs. However there is a balance in doing this & maintaining strength of the engine components, which is why when you build a motor to go fast, it must also become stronger ie better quality parts.

You may have heard of a petrol engine "dieseling". This can happen for a few reasons, not just too much air, which can also make your engine stall or flat spot depending on a few factors but we'll leave that for another time. A petrol engine can diesel if you put a lower flashpoint, higher octane fuel in. For example when I was a teenager, I had to mow the lawns one day before I was allowed to play with my mate & his remote control helicopter. As an experiment, we put some liquid nitro into the fuel tank of the lawn mower. About 20 secs later the motor started revving higher than ever & even though I closed the throttle & turned the fuel tap off, it kept running flat out until all the fuel was burnt. How my dad didn't hear it I'll never know but probably best that way for me lol.

As a rule a motor needs 4 things to run. Fuel, spark, air & compression. As previously mentioned diesel needs to be hot to burn due to it's low flash point (the point at which it will ignite). This is why you have glow plugs. Their role is to get the motor started. Your car will have a glow plug timer which heats the glow plugs so that they will ignite the diesel once your motor is running, the compression will be enough to keep it going. Unlike petrol cars, you will probably notice that your diesel runs better when it is hot. There is a myth that diesel engines run better on hot days. This is only somewhat true. Yes they will but the reason a petrol car runs better on cold days is due to the air density. I used to drag race V8's yrs ago & I'd recreate this by adding a stocking full of dry ice into my airbox. It's the same with diesels. They too like air density although not as much as petrol's so they will seemingly run better on hot days, or at least the difference between a diesel on a hot day & a cold day is not as different as a petrol on a hot day compared to a cold day. Anyway, as side tracked as I've got, that is the basics of diesels and the variables on why they can smoke.

[BIG-_JOHN]

@ Ben-e-boy, I have not read the article from the link provided. I however did read the thread.

Why did you bother to post in such a negative way? I have sent thousands of posts in other forums and this is the first time I have had to put up with crap like that. I thought I asked a reasonable question.
Rant over.
I will now read the article via the link provided and maybe I will ask the same question again.

[BIG-_JOHN]

OK, the link provided the same content as what was in the original post. I thought it may have been a more extensive read.
My original question stands.

[Sharky1]

My wife's Nissan is an 05 GU 3l turbo soon due for a 150,000 service. This vehicle blows black smoke but only at start up. No smoke is evident later or under load. Fuel consumption is within normal boundaries (about 10.5 litres/100 kms) and power once turbo kicks in is good.
I haven't read the article yet but does anyone know what may cause this.

Cheers, John

As I wrote in my post above mate, which was also covered a little in the original post, if your fuel isn't fully burning, the carbon emissions (in the form of smoke) will be evident. Given that diesel needs to be hot & pressurized to burn well, which it isn't at initial start up, it blows a cloud of smoke. Also, given that upon start up, all you're engine's lubricants will have dropped to the bottom of the sump, manufacturers often design injection systems to squirt a larger than normal amount of diesel into the engine at start up to help lubricate it until oil pressure builds up. This may or may not be the case with your car.

[nissannewby]

OK, the link provided the same content as what was in the original post. I thought it may have been a more extensive read.
My original question stands.

Get your glow plugs checked or even replaced. They should be done periodically on the di zd30.

[nissannewby]

I'll get sidetracked........

Fixed . Should have read the link. Some of your info is a little misleading but I can see you are trying to offer advice which we cannot get enough of. Diesels are in fact more efficient than petrols in their operation. I am a diesel fitter. The link is also a guide and can be quite helpful for those trying to to simple diagnostics by themselves before getting the mechanics involved.

[BIG-_JOHN]

Get your glow plugs checked or even replaced. They should be done periodically on the di zd30.

Thanks, I will have a look at this at next service.

[Sharky1]

Fixed . Should have read the link. Some of your info is a little misleading but I can see you are trying to offer advice which we cannot get enough of. Diesels are in fact more efficient than petrols in their operation. I am a diesel fitter. The link is also a guide and can be quite helpful for those trying to to simple diagnostics by themselves before getting the mechanics involved.

It depends on which way you look at it. We both agree that one of the reasons a diesel engine could smoke is because it isn't completely burning all the diesel, which is quite common especially under load. How is that efficient? A diesel will still happily run like this though whereas if a petrol has the same issue, it usually has other problems related & doesn't run so well. Also, the average N/A diesel 4x4 will get approx 700km/100L. The average N/A petrol car of the same engine capacity will get 800km/100L with the only difference being the weight of the car. That's where diesels become more efficient. Because of the torque they can make, they'll pull a lot of weight with minimum additional fuel usage. Add a turbo to this & you can magnify that by a lot as a turbo on a diesel will increase economy. I have no doubt that you're a lot more full-bottle on it than I am. The only reason I posted all that is that most of the posts following the 1st were from seemingly paranoid folk who are suddenly thinking their diesel has an issue because it smokes. How can anyone who doesn't understand the basic fundamentals of this, diagnose anything?

[Roland the Wolf]

Great post NISSANNEWBY . Since 3 months, I have a TD42T 1999 Patrol done 240000km. I have white smoke for about 10 seconds at cold starts and very rough idle. Idles ok, with no more smoke after 10 seconds at cold start and runs great once operating temp is reached. Any ideas, would be appreciated.
Thank you NISSANNEWBY. 13/7/2015

[Johnny R]

Oh yes... I put a fuel star canister on my old 4.2.==1km/litre improvement and slightly more power. It simply adds Tin( close to lead on the periodic table) to the fuel, which increases the octane rating of the fuel...Excellent for petrols>