Running engine (diesel) with the ignition turned off

[PaulRainbow]

OK, like the other knowledgeable posters on here, I finally give up as the depth of your misunderstanding of i/c engineering as exemplified by your above statement is too fundamental to be corrected on here.

I'll keep an eye on the thread as it will be interesting to see if a single forumite ever comes along to support your statement .... and, contrary to your belief, VicS did not.

If I were to list what determines the compression ratio in the broadest sense, I would basically be writing a book called "The Fundamentals of Internal Combustion Engine Design" rather than listing your supposed three factors.

Keep learning and smiling.

Richard

Every once in a while we get some clown come along and spout utter nonsense about something or other. They won't listen, even if the whole forum tells them, even if 9999999999999999999 Google links to facts are posted, they just keep digging a deeper and deeper hole. They almost always use the old "attack is the best form of defence" and start throwing insults, to try and hide their mistakes. Fortunately, most of them slink off after a short time, usually telling what a bunch of tossers we all are and how we're not worthy of reading their posts.

All good fun ?

 

[convey]

Every once in a while we get some clown come along and spout utter nonsense

You really don't know the answer, do you? All this bluff, and you just don't know and can't admit it.

Isn't name calling someone "a clown", attacking as a defence?

If you know, just answer. Or are you just a fitter, rather than an engineer?

You said,

The shape of the piston crown or cylinder head does not affect the compression ratio.

OK, we have two pistons for the same engine. First is an 8.5:1, second is a 10 :1.

If shape of "the piston crown does not affect the compression ratio", as you wrote, why is the compression ratio 8.5:1 with one, and 10:1 with the second?

Once we've resolved this, we can move onto the third factor ... wan't to hazard a guess what it is yet, or are you willing to say you don't know? (you can list all design factors if you like/are able) ... and go back to the original nonsense you "spouted".

 

[Iliade]

Just to add another factor, the compression pressure rise rate, and hence combustion pressure rise rate is affected by the shape of piston, head and (pre-)combustion chamber if fitted.

Discuss...

 

[convey]

Yes, the compression ratio is affected by the shape of piston, head and pre-combustion chamber ... but it's not the third critical and adjustable factor I was thinking about. One that anyone who's worked on a Volvo Penta engine would know.

 

[RichardS]

View attachment 95263

View attachment 95264

I promised to give up but I'm enjoying this too much.

OK ..... above are photos of two pistons. Let's assume that we don't know anything else at all about either of the engines that these pistons are taken from, other than that the two engines have different compression ratios.

Now, the question is .... which piston is from the higher compression engine and which is from the lower compression engine?

Richard

 

[179580]

Yes, the combustion ratio is affected by the shape of piston, head and pre-combustion chamber ... but it's not the third critical and adjustable factor I was thinking about. One that anyone who's worked on a Volvo Penta engine would know.

PM sent

 

[179580]

I feel that a few vociferous members are going to regret their musings and indeed expose their lack of knowledge here.


All good fun

 

[rogerthebodger]

but it's not the third critical and adjustable factor I was thinking about.

One that anyone who's worked on a Volvo Penta engine would know.

Not having a Volvo Penta engine but many other both petrol and diesel engines, I may know know what you are referring to so would you please enlighten me

 

[179580]

Not having a Volvo Penta engine but many other both petrol and diesel engines, I may know know what you are referring to so would you please enlighten me

Then PM with your reply as I did, don't let the cat out of the bag. This is fun.

 

[rogerthebodger]

Then PM with your reply as I did, don't let the cat out of the bag. This is fun.

You don't you enlighten me then

PM me if it's considered a state secret

 

[convey]

Not having a Volvo Penta engine but many other both petrol and diesel engines, I may know know what you are referring to so would you please enlighten me

You probably do but let's wait to see if Mr "shape of the piston crown or cylinder head does not affect the compression ratio" PaulRainbow knows.

Oh ...

If the first piston is 8.5:1, and the second piston is 10:1, wouldn't a bowl shaped piston ... in the same, identical engine ... lower the compression ratio an equivalent amount? Say, to 7:1 ?

(This is a close approximate for sakes of illustrating the SHAPE of the piston )

Here's some diesel ones.

 

[JumbleDuck]

Not having a Volvo Penta engine but many other both petrol and diesel engines, I may know know what you are referring to so would you please enlighten me

I can think of four separate things which could be changed to vary the compression ratio beyond the cylinder crown and head shapes, but none are "adjustable" while the engine is running and one requires a complete rebuild.

 

[JumbleDuck]

... most diesels would have a flat top piston,

Here's some diesel ones.

 

[179580]

I can think of four separate things which could be changed to vary the compression ratio beyond the cylinder crown and head shapes, but none are "adjustable" while the engine is running and one requires a complete rebuild.

Cannot remember any claims of engine running.

 

[penberth3]

Yes, the combustion ratio is affected by the shape of piston, head and pre-combustion chamber ... but it's not the third critical and adjustable factor I was thinking about. One that anyone who's worked on a Volvo Penta engine would know.

There's a term I've never heard before. What is "combustion ratio"?

 

[convey]

There's a term I've never heard before. What is "combustion ratio"?

A typo, since corrected.

Tamarside knows his stuff and is humble to boot alongside.

 

[rogerthebodger]

I can think of four separate things which could be changed to vary the compression ratio beyond the cylinder crown and head shapes, but none are "adjustable" while the engine is running and one requires a complete rebuild.

So can I and as you say requires a complete rebuild and possible some machining.

I just want convey to coff up.

 

[JumbleDuck]

Cannot remember any claims of engine running.

He keeps saying "adjustable". Anything which requires the engine stopped or dismantled isn't really an adjustment.

 

[convey]

In all fairness, although correct, the "adjustable" element was either a hint or a little bit of a red herring, just to see if Paul knew his stuff.

OK ..... above are photos of two pistons.

Now, the question is .... which piston is from the higher compression engine and which is from the lower compression engine?

Both the pistons fit into the same engine without any modifications.

The flat one makes the lower compression ratio, the raised one ... raises it. Obviously a dished or bowled one would lover it. In this specific case, the cylinder head has a semi-hemi shape. A full hemi(spherical) might use a piston like this;

Same bore, same stroke ... different SHAPE of combustion chamber. The dome SHAPE require to create a sufficient (the same as the above, funnily enough) compression ratio. I suppose strictly speaking you could argue that the shapes "don't make the compression" or something, but their shapes and their match determines the compressed volume, that determines the compression ratio.

(Rough, I have to work ... these are not diesel, just used for sake of exaggeration to illustrate the effects)

 

[179580]

A typo, since corrected.

Tamarside knows his stuff and is humble to boot alongside.

Blushing!