Understanding diesel knock

MikeT · Unread post by **MikeT** » 06 Jul 2010, 12:36

Can anyone decipher or simplify for me what the following means so I can understand and absorb it please?

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It says this can increase diesel knock tendency.

1. Increasing Air turbulence of compressed air. It strips the outer surface of fuel droplets to form a homogenous mixture.

2. Increasing speed of engine. A higher cam speed injects more fuel during the delay period which promotes knocking. If suppressed, it is due to the increased turbulence with increased engine speed.

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#1 - I always thought "swirl" was desirable but this is specifically talking about the compressed air after the valve has closed. I can't understand what difference it makes? I'm guessing, a stripped fuel droplet will detonate rather than burn?

#2 - I'm getting a complete brain-freeze on this one. How can fuel injected during the delay period increase in volume just because the cycles are happening faster? It should be bloody obvious I guess; faster engine speed requires more fuel?
The last sentence appears (to me, anyway) to contradict #1 about turbulence or is it referring to pre-compression swirl this time?

Until I can understand those descriptions, I probably won't be able to understand the following chapters fully so I'm sort of "stuck" at this point.

jgra1 · Unread post by **jgra1** » 06 Jul 2010, 12:38

I deleted my answer

MikeT · Unread post by **MikeT** » 06 Jul 2010, 12:43

Second thoughts? Would still like to know your first thoughts though John - although it might be wrong, it might set a train of thought I hadn't considered yet.

Unless it was "Get a petrol instead"

I have to go out for a while but will respond asap, thanks.

jgra1 · Unread post by **jgra1** » 06 Jul 2010, 12:58

well I wasn't too sure what outer surface was being referred to.. the drops or the cylinder/comb chamber or something else..

Unread post by **CitroJim** » 06 Jul 2010, 13:38

Mike,

I have a book on the theory of Diesel Engine Theory at home.

It really is a rivetting read, no really! It is a very interesting work, especially the chapters that discuss combustion.

It is on a PDF and fits onto a CD.

It'll tell you all you need to know about diesel knock in very full-on intimate detail. Read it and you will know more than you really wanted on the subject.

Would you like a copy?

Unread post by **RichardW** » 06 Jul 2010, 14:15

Could all be bollox

but....

Diesel knock is caused by the fuel detonating, causing a sudden increase in cylinder pressure. Key difference between diesel and petrol is that in dieselthe fuel burns as liquid droplets, whereas in petrols it vaporises and burns as a gas (one of the reasons why engine speed is limited on diesels - on petrols the evaporating fuel helps to cool the pistons, whereas on a diesel it doesn't, so high engine speeds tend to melt piston crowns!). My thoughts on your points would be:

1. Introducing compressed air, eg via a turbo, causes more turbulence in the cylinder which breaks up the fuel droplets, increasing the tendency for the fuel to detonate, rather than burn in a 'controlled' manner.

2. The delay period is, by my understanding, the time between the beginning of injection and the fuel igniting. If this is relatively independent of engine speed, then at higher engine speeds the rate in injection is higher (you only have a limited time to get the fuel in there!), and hence you can inject more fuel before it starts to burn, and if the conditions are right for detonation to occur, then there is more fuel there to do so and hence a bigger bang = more knock.

When you find out the real answer, let me know so I can be put right!

MikeT · Unread post by **MikeT** » 06 Jul 2010, 15:24

jgra1 - droplets of fuel are stripped.

Jim, yes please though, if it's like the exerpts of this book I've found, I might be back with more misunderstandings but I'm up for it.

RichardW - I'm in agreement with all except not sure if turbulence is occuring when the fuel is injected?

Unread post by **CitroJim** » 06 Jul 2010, 20:54

RichardW wrote:Could all be bollox but....

Not at all Richard, spot on in all respects

That's my understanding of it precisely..

Mike, next time I have my CD Burner fired up I'll do you a copy and drop it in the post. I guarantee you'll love every word of it. You can read that diesel has to be broken into new chemical species before it'll burn and how a lot of these are acetylenes and are the principle cause of soot and smoke.

Advise me by PM of your address....

citrov6 · Unread post by **citrov6** » 07 Jul 2010, 15:32

With turbulence if you think about what it actually is; swirling air currents and friction of those currents on the surrounding solid surfaces, it will be those that will be more the faster air is going through an engine.

Specifically through the inlet port past the valves and on the combustion chamber and piston.

That is just a guess. I also have a book on it somewhere that I have been waiting for some time to sit down and read through properly for a number of years now.

MikeT · Unread post by **MikeT** » 07 Jul 2010, 17:03

CitroJim wrote:Advise me by PM of your address....

PM sent, Jim. Thanks in advance.

MikeT · Unread post by **MikeT** » 07 Jul 2010, 17:12

Regarding turbulence, I am convinced the description is referencing the air in the combustion chamber when the inlet valve has closed and the fuel injection has begun. It seems to be saying that any air turbulence at that point, will reduce droplet size, resulting in more of a bang (knock) than a burn. My assumption is backed up by the pre-injection used in an HDI engine, which starts a less potent burn before the main injection in order to reduce knock.

As for #2, I think I need to refresh my memory on how the fuel pump works as revs increase.

citrov6 · Unread post by **citrov6** » 07 Jul 2010, 18:18

I think you are right. Now that I think back on it I think it is not during the inlet phase that the pre injection happens its during the compression, though turbulence happens when there is movement so I guess it could only be the combustion chamber swirling it.

It is a while since I did anything to do with engines and they were petrol engines. I am trying to recall something about lean burn in those, I think it was something to do with stratified layers. If only I had time and it were useful to go back and look into these it was one of my favorite subjects before hybrids made it not worthwhile in the longterm.

I may be again wrong on this but nothing ventured. Fuel I think is limited quite early in the rev range it is advance that is used to accelerate; I don't see why it should be different from petrol in this way and would need more advance in a diesel up the rev range.

Anyway I'm sure you'll figure it out