Running engine (diesel) with the ignition turned off

[rogerthebodger]

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.



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

The flat one makes the lower compression ratio, the raised one ... raises it.

Of cause changing the piston with a different crown shape will change the compression ratio but I would not call that "adjustable" in the same way as the valve clearance is adjustable or the engine timing is adjustable.

 

[black mercury]

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.

This 'adjustable' has simply confused everyone.

 

[JumbleDuck]

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.

At last!

 

[RichardS]

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

Errrrr ..... nope. That is not the question I asked as you have added additional data.

I will repeat the question I asked:

https://forums.ybw.com/index.php?attachments/95263/

https://forums.ybw.com/index.php?attachments/95264/

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

OK .... my above question is now irrelevant as you have now realised your error. I'm guessing that my above question is what finally made you realise that the shape of the piston crown, in isolation, does not tell you anything at all about the compression ratio because, as you now agree, these piston shapes "don't make the compression" , but their shapes and their match determines the compressed volume, that determines the compression ratio".

 

[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.

Slightly off tack the late and great Ricardo fiddled with VCR and is now being developed to enhance performance.

 

[RichardS]

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.

At last ...... you've finally got there!

Now, do you understand why your statement that "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" is total nonsense.

That is all that we have ever said and yet you have totally rejected our observation .... until now.

Richard

 

[rogerthebodger]

Slightly off tack the late and great Ricardo fiddled with VCR and is now being developed to enhance performance.

A variable compression ratio engine is quite easy to design but the cost to manufacture puts it out of reach for normal engine use.

Super charged of turbo charged is a better more economic way of increasing the air / fuel charge to gain more power

Even better is energy recovery systems of hybrid engines or even electric drive once properly developed

 

[PaulRainbow]

At last ...... you've finally got there!

Now, do you understand why your statement that "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" is total nonsense.

That is all that we have ever said and yet you have totally rejected our observation .... until now.

Richard

Got there in the end Richard, perhaps he'll put the spade away now.

 

[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"

Is obviously a load of nonsense, as everyone seems to agree. Changing pistons to alter the compression ratio was never mentioned, until later, although there are also references to factory designed components/compression ratios. If one wanted a ratio of 15:1 that can be achieved with any shape of piston and/or cylinder head. The claim that flat top pistons are usually fitted to diesels to increase compression ratio is clearly drivel, especially when one looks at the hundreds of piston images that i linked to (i see now that Convey is also posting images of dished pistons, pointing out that they are for a diesel engine) ?

Compression ratio is calculated from swept volume and combustion chamber volume (at TDC), so the only way to change the static (geometric) compression ratio is to change one of those values. There are several ways to do this, depending on the engine, the availability of alternative components and/or the viability of manufacturing/modifying alternative parts. Changes can be made to the bore, stroke or combustion chamber, con-rods can be changed, stroke can be changed by changing or modifying the crank shaft, cylinder heads and/or blocks can be machined etc. Often, changes are not made in isolation, modifying one component may require some changes to additional components.

There are additional ways to make changes to the compression while the engine is running that do not require changes to the above components, but they do not alter the geometric ratio.

 

[Beneteau381]

I thought so ... arguing at crossed purposes ........... load of rotary engines the lot of you

Bluddy heck Vic, you making funnies!

 

[Beneteau381]

You were assuming that he did not mean that through




Just polishing will as all polishing involved removing material to get a smooth/polished surface. The amount will be small it it will change the volume.

Of cause reshaping the cylinder head combustion chamber will change the volume thus the compression ratio. And while you are at it balancing the volume of the cylinder head combustion chamber on a multi cylinder engine will also change the volume by definition



Of cause with a totally flat cylinder head CR will not change by skimming. List the normal marine/vehicle engines that have a flat head.



The third design factor that determines the compression ratio is the volume of the cylinder. If you increase the volume of the cylinder without changing the volume of the head and the volume in the piston crown. If you seperate the cylinder diameter and the cylinder stroke the are 3rd and 4th design factor.



Yes if you look at the Kruger Dunning Syndrome that exactly what that says but most "well known members" have more than enough knowledge to fall into that syndrome as the know enough to understand how much they don't know.



I have a pretty good idea of JD's knowledge and experience that he knows what he is talking about and like I scottish farther is a straight talker hence and his hole digging post.

Richard, JD and myself are into rebuilding classic cars and I us to tune BMC A series engines when I lived near Longbridge in the UK.

It would be interesting to know where you are located

My MD22 has a flat head, skimming does not alter the CR as such

 

[STILL AFLOAT]

Well, that quickly disappeared into the realms of theory and technicalities ! Back in the 70s , I was allways told, that switching off the ignition, on a diesel engine, blew the diodes in the alternator. That applied to Diesel driven Trucks & Bulldozers , in the Army, as well as out sailing on other peoples boats .
So, Ok, perhaps things have evolved so much, that, that no longer happens, with modern engines, but what of older boats, with engines from the 70s ?
Surely a whole generation of army instructors, couldnt be wrong ? Nor seasoned captains of boats, back in the day ?

 

[rogerthebodger]

Well, that quickly disappeared into the realms of theory and technicalities ! Back in the 70s , I was allways told, that switching off the ignition, on a diesel engine, blew the diodes in the alternator. That applied to Diesel driven Trucks & Bulldozers , in the Army, as well as out sailing on other peoples boats .
So, Ok, perhaps things have evolved so much, that, that no longer happens, with modern engines, but what of older boats, with engines from the 70s ?
Surely a whole generation of army instructors, couldnt be wrong ? Nor seasoned captains of boats, back in the day ?

Nothing to do with modern engines, it was just that people did not understand the difference between disconnection the excitation circuit (charging light) and the battery connection.

In fact the overrun on a petrol engine does exactly the same the alternator still charges the battery for a second or to whild the petrol engine runs down and the excitation circuit is disconnected when the ignition switch is turned off.

You could fit a zap stop to your alternator if you wish

 

[JumbleDuck]

So, Ok, perhaps things have evolved so much, that, that no longer happens, with modern engines, but what of older boats, with engines from the 70s ?

It's an alternator thing rather than an engine one. Modern ones have internal protection.

 

[RichardS]

It's an alternator thing rather than an engine one. Modern ones have internal protection.

Although that only applies to disconnecting the alternator to battery connection. Disconnecting the ignition using the ignition switch has never been an issue. As Roger says, it is all based on a misunderstanding.

Richard

 

[179580]

Well guys, guess we have sorted the wheat from the chaff. You have all demonstrated knowledge or lack of. Personally I would not ask any of you on board to check my oil level

 

[RichardS]

Well guys, guess we have sorted the wheat from the chaff. You have all demonstrated knowledge or lack of. Personally I would not ask any of you on board to check my oil level

Go on then Tamarside, please tell us whether you think that "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" is a meaningful statement by Convey or is it total and utter nonsense?

Come on now, we're all awaiting your words of profound wisdom.

Richard

 

[JumbleDuck]

Well guys, guess we have sorted the wheat from the chaff. You have all demonstrated knowledge or lack of. Personally I would not ask any of you on board to check my oil level

And your opinion should bother us ... why?

 

[179580]

Well guys i am sorry I joined this forum voluntarily. I should have waited until I was invited to enter blindfolded. left trouser leg rolled up!

 

[Hadenough]

Go on then Tamarside, please tell us whether you think that "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" is a meaningful statement by Convey or is it total and utter nonsense?

Come on now, we're all awaiting your words of profound wisdom.

Richard

Did your leg ever get sufficiently better to take up that running challenge you threw out to some one a while back?