Running engine (diesel) with the ignition turned off

[convey]

The only thing that affects compression ratio is the ratio of volume of the cylinder at BDC and the volume at TDC. The shape of the piston does however affect the efficiency of the combustion process. Big humped pistons were in vogue ...

And what did the big hump on the pistons do?

No, that's only how you calculate it. It's not a thing.


BTW, which petrol engines are you talking about, chainsaw engines or F1?

Do you want to do a head count of all petrol engines and come up with some averages?

It's sort of irrelevent as petrol engines aren't using compression for the sake of ignition. Without checking Wikipedia, do you have any idea how, how difficult, and how high octane fuel, it would take to manage 14:1 compression? It's like saying all humans run sub-10sec 100m.


What I wrote were perfectly fair and accurate for a starter, I'm still waiting for our resident expert to explain "The shape of the piston crown or cylinder head does not affect the compression ratio."

You'll learn very little to nothing going to school, even engineering school, or even to a consultant engineers like Ricardo, in comparison to what you'll learn going racing. One of the first interesting things you learn is that production engines rarely run at their designed or advertised compression rate due to manufacturing costs/processes limitations.

So you know how to increase it either to bring it up to its most efficient design parameter, or better?

(The wikipedia is not a reliable source of information)

 

[Beneteau381]

And what did the big hump on the pistons do?

No, that's only how you calculate it. It's not a thing.


BTW, which petrol engines are you talking about, chainsaw engines or F1?

Do you want to do a head count of all petrol engines and come up with some averages?

It's sort of irrelevent as petrol engines aren't using compression for the sake of ignition. Without checking Wikipedia, do you have any idea how, how difficult, and how high octane fuel, it would take to manage 14:1 compression? It's like saying all humans run sub-10sec 100m.


What I wrote were perfectly fair and accurate for a starter, I'm still waiting for our resident expert to explain "The shape of the piston crown or cylinder head does not affect the compression ratio."

You'll learn very little to nothing going to school, even engineering school, or even to a consultant engineers like Ricardo, in comparison to what you'll learn going racing. One of the first interesting things you learn is that production engines rarely run at their designed or advertised compression rate due to manufacturing costs/processes limitations.

So you know how to increase it either to bring it up to its most efficient design parameter, or better?

(The wikipedia is not a reliable source of information)

I used to spanner for Gary Bryan in the seventies, we were working with Weslake using their conversions for Triumph engines in sidecars. The latest iteration of that development is up to over 100bhp at 900 ish cc. It runs on aviation fuel. Ive built a few racing engines in my time. To say what you have said about manufacturing tolerance affecting compression ratios, sorry, but it would have to be a big intolerance to affect compression ratios significantly when we talk about the engines we deal with! My MD22 with a stroke of 3.5" and a compression ratio of 17:1 would have to have a significant defect to show any noticeable change in ratio.

 

[RichardS]

Look, I'm not sure I'm up for cock sword fighting over this but do you want to correct that first sentance?

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

I used to work for a cylinder head/race tuning specialist. Plenty of people in the trade and industry longer than you as thick as two short planks and no idea what they are talking about, like the flash point error immediately above.

Someone asked a question, I took it as sincere and was kindly enough to answer it.

Perhaps you need to use an irony or "sarcastic" emoji is that is your personality type.

Now, try again ... "The shape of the piston crown or cylinder head does not affect the compression ratio."

Really? I'm looking forward to your expertise on this one (I was actually looking forward to go on to discuss the toroidal effect of of piston bowl geometry with the apparently uninform member I was trying to help understand what was going on. "Swirl flows" were one of our things).

Paul is 100% correct I'm afraid. The shape of the piston crown or cylinder head does not produce the compression ratio.

If you think that it does, you cannot possibly have ever worked with cylinder heads.

For some of us on here, this is elementary stuff that we knew about decades ago.

Richard

 

[RichardS]

And what did the big hump on the pistons do?

The design of the piston crown does a great many things .... but what it does not do is produce a specific compression ratio. You can alter the compression ratio of a given engine by changing the piston/s to ones with a different crown design, just as you can produce the same result in many other ways whilst leaving the piston crown totally unchanged ..... but the compression ratio is solely derived from the swept volume.

Richard

 

[RichardS]

Tap or click on my tits (or more accurately, RichardS' profile pic), tap/click on ignore.

Understanding anything at all about engines is clearly not your forte, SM.

Richard

 

[PaulRainbow]

What I wrote were perfectly fair and accurate for a starter, I'm still waiting for our resident expert to explain "The shape of the piston crown or cylinder head does not affect the compression ratio."

Already explained it, compression ratio is the ratio of the swept volume of the cylinder and the volume of the combustion chamber when the piston is at TDC.

That's it, end of story.

The design of the piston crown has nothing to do with compression ratio.

You'll learn very little to nothing going to school, even engineering school, or even to a consultant engineers like Ricardo, in comparison to what you'll learn going racing. One of the first interesting things you learn is that production engines rarely run at their designed or advertised compression rate due to manufacturing costs/processes limitations.

So you know how to increase it either to bring it up to its most efficient design parameter, or better?

(The wikipedia is not a reliable source of information)

I didn't learn engineering at school, i worked for an engine reconditioning company, in garages and spent 40 years working for myself. I need to use Wikipedia, thanks all the same.

I'm fully aware that production engines may not be entirely up to the optimum performance. There are manufacturing tolerances for all of the components in an engine, so you might get the shortest conrods, the shortest pistons, the biggest head clearances etc and be down to the minimum acceptable compression ratio. The process of adjusting those values is called "blueprinting". Manufacturers of mass produced, run of the mill, engines can't afford to do it and there is no need. Modern engines are built to finer tollerances than they used to be anyway.

 

[PaulRainbow]

Really?

As a new boy on the block, I recently commented on rudeness and bullying. I then researched certain peoples posts. I remember reading a post concerning Perkins 4108 and 4107 cylinder liners. The person concerned had not an iota of knowledge of wet liners v dry liners, and indeed was quite abusive to the poster.

No chance of a job on my boat.

Do you have anything useful to contribute to the forum or did you just join to moan and troll ?

 

[PaulRainbow]

My only criticism of this video is that it shows the cylinderhead to have a hemispherical shape, which would reduce compression, when most diesels would have a flat top piston, meeting a flat cylinderhead, which would increase compression. Notice how the pistons have indentations in the centre where the fuel/air mix would be compressed into and squirl to ensure a good mix and atomisation. The lip around the edge 'squishing' it in as it meets the flat face of the cylinder head.

As is all too often the case on internet forums, someone posts a complete and utter load of rubbish and when corrected uses the old "attack is the best form of defence" .

So, the poster here informs us that the piston above is incorrect for a diesel engine, because diesel engines have flat topped pistons. He also tells us the compression ration is dictated by the design of the piston crown, flat topped piston give more compression ratio. This is just nonsense, anyone interested in the truth can easily Google "compression ratio".

As for piston crowns, note the piston in the You Tube animation, the poster has kindly added a still image above. He tells us this is not appropriate for a diesel engine, diesel engines use flat topped pistons.

Let's try a few Google image searches. The first one is for Yanmar pistons, must be one of the top 3 or 4 used in marine engines.

yanmar diesel piston - Google Search

Compare some of those images to the one above.

How about Volvo Penta pistons : volvo penta diesel piston - Google Search

Perkins : perkins diesel piston - Google Search

Bukh : bukh diesel piston - Google Search

Mann: mann diesel piston - Google Search

Cummins: cummins diesel piston - Google Search

Mercedes: om636 diesel piston - Google Search

I could go on until i run out of manufacturers but you can very clearly see the lack of flat top pistons and a huge amount of pistons that look very much like the ones in the original post.

 

[black mercury]

Just to confuse things.
Are we talking static compression ratio or dynamic compression ratio?
For example Toyota uses a low dynamic compression ratio engine in its hybrid models because it runs on the Atkinson cycle. Not a true Atkinson cycle but a modern take on the principle. But the static compression ratio is between 10:1 and 13:1

 

[convey]

Engine guy, or chassis guy?

You said,

The only thing that affects compression ratio is the ratio of volume of the cylinder at BDC and the volume at TDC.

I asked,

And what did the big hump on top of pistons do?

You did not answer. So what does the the big hump on top of (some) pistons do?

*(The Nourish Weslake used flat top pistons with cut outs for the values).

 

[convey]

So, the poster here informs us that the piston above is incorrect for a diesel engine, because diesel engines have flat topped pistons. He also tells us the compression ration is dictated by the design of the piston crown, flat topped piston give more compression ratio.

Show me where I ever said a piston/that piston is "incorrect for a diesel engine".

Or that "a flat topped piston will [always] give more compression ratio".


You are twisting and misresprenting what I wrote, which makes you a very little person in my book.


I am happy to walk you through each step of this but I warn you, your personal and professional integrity is going to come out of this in tatters.

I f you don't come out of looking like a fool, I will, at the very least, fastituously demonstrate how you twisting and misresprenting others words to cover up where you are wrong.

Starting with what makes compression ratios ... which was hysterical to read.

 

[PaulRainbow]

Show me where I ever said a piston/that piston is "incorrect for a diesel engine".

Or that "a flat topped piston will [always] give more compression ratio".


You are twisting and misresprenting what I wrote, which makes you a very little person in my book.


I am happy to walk you through each step of this but I warn you, your personal and professional integrity is going to come out of this in tatters.

I f you don't come out of looking like a fool, I will, at the very least, fastituously demonstrate how you twisting and misresprenting others words to cover up where you are wrong.

Starting with what makes compression ratios ... which was hysterical to read.

I think everyone here can see what's what. The evidence is all there to be read.

 

[RichardS]

Show me where I ever said a piston/that piston is "incorrect for a diesel engine".

Or that "a flat topped piston will [always] give more compression ratio".


You are twisting and misresprenting what I wrote, which makes you a very little person in my book.


I am happy to walk you through each step of this but I warn you, your personal and professional integrity is going to come out of this in tatters.

I f you don't come out of looking like a fool, I will, at the very least, fastituously demonstrate how you twisting and misresprenting others words to cover up where you are wrong.

Starting with what makes compression ratios ... which was hysterical to read.

You said "My only criticism of this video is that it shows the cylinderhead to have a hemispherical shape, which would reduce compression, when most diesels would have a flat top piston, meeting a flat cylinderhead, which would increase compression"

I'm afraid that almost every word in this sentence is incorrect as demonstrated by the fact that engines with flat-topped pistons and flat cylinder heads can, and some do, have very low compression.

Richard

 

[rogerthebodger]

Compression ratio in any piston engine of even an air compressor is calculated as below.

The volume Vc is the volume of any volume in the crown of the piston plus the volume in the head no matter what shape both are.

With a flat top piston the the volume in the piston is of cause zero so in that case its only the volume in the head that counts.

This only applies to single piston cylinder engines an not to the atkinson or other variable stroke engines.

 

[Stemar]

So, just how many angels can dance on a diesel ignition switch?

Will all the hot air and raised blood pressure being generated, I'm surprised no one's compression ignited into complete combustion ?

 

[rogerthebodger]

So, just how many angels can dance on a diesel ignition switch?

Will all the hot air and raised blood pressure being generated, I'm surprised no one's compression ignited into complete combustion ?

An infinite number until you get spontaneous human combustion

Spontaneous human combustion - Wikipedia.

 

[jamie N]

Just to confuse things.
Are we talking static compression ratio or dynamic compression ratio?
For example Toyota uses a low dynamic compression ratio engine in its hybrid models because it runs on the Atkinson cycle. Not a true Atkinson cycle but a modern take on the principle. But the static compression ratio is between 10:1 and 13:1

Every day's a school day!
Thanks for this, in my mid 60's now and I'd never even heard of this, let alone that my wife owns!

 

[RichardS]

So, just how many angels can dance on a diesel ignition switch?

Will all the hot air and raised blood pressure being generated, I'm surprised no one's compression ignited into complete combustion ?

Come on, try and crack a smile, Stemar. It's all great fun and much better than watching soaps on the telly.

Richard

 

[Beneteau381]

Engine guy, or chassis guy?

You said,

I asked,


You did not answer. So what does the the big hump on top of (some) pistons do?

*(The Nourish Westlake used flat top pistons with cut outs for the values).

Weslake! you dont even know how to spell it let alone understand it!

 

[Beneteau381]

You said "My only criticism of this video is that it shows the cylinderhead to have a hemispherical shape, which would reduce compression, when most diesels would have a flat top piston, meeting a flat cylinderhead, which would increase compression"

I'm afraid that almost every word in this sentence is incorrect as demonstrated by the fact that engines with flat-topped pistons and flat cylinder heads can, and some do, have very low compression.

Richard

I wonder what he would make of old landie engines I over E ?