Running engine (diesel) with the ignition turned off

[PaulRainbow]

It is combusted ... well, between some to most of it depending, as the plumes of black smoke they kick out testifies ... but by heat caused by compression. They operate as a much higher compression ratio than petrol engines. Some have glow plugs, heated by electricity, to give the initial heat boost to do so. old, elementary ones don't you just crank them up until they are going fast enough to generate enough themselves, and to stop them you can flip the decompressor lever. Some manual (cable), some solenoid operated.

I guess you missed the irony/sarcastic humour in my post

FYI, i rebuilt my first engine in 1972. I currently look after all sorts of boat engines, ranging from single cylinder engines to common rail V8 diesels.

It's very kind of you to explain to me how a diesel engine works, what the pistons and cylinder heads look like and how the shape of the piston dictates the compression and what the compression ratios might be. (Note the sarcastic humour here )

It would have been more helpful if you had been correct.

Yes, the fuel/air mix is combusted, but for that to happen it still has to be ignited. Combustion simply means "burned", you can't burn it without igniting it.

My only criticism of this video is that it shows the cylinder head to have a hemispherical shape, which would reduce compression, when most diesels would have a flat top piston, meeting a flat cylinder head, 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.

Your criticism is unwarranted. There are many diesel engines that use pistons exactly as depicted in the You Tube video. There are also many diesel engines that use various dished piston crowns, as well as cylinder heads that might be flat, or have an inverted dish surface.The shape of the piston crown or cylinder head does not affect the compression ratio. The compression ration is the ratio between the swept cylinder volume and the volume of the compression chamber with the piston at TDC

 

[TiggerToo]

Can I just make a general point here to help relative newcomers to the forum ..... when you see posters on this forum who have thousands of posts, it's probably best to assume that quoting them before typing long posts explaining how diesel or petrol engines work is somewhat superfluous to requirements.

Richard

OTOH, I find some of the posts useful "refreshers"...
And many of the "thousands of posts" are often filled with the rubbish some of us obsess about, something to do with X, T, R, B, I and E. But that is another story (and forum chamber) ;-)

 

[JumbleDuck]

Not strictly true. Most diesels (trucks, vans, cars, buses, etc) stop when you turn the "key switch" to the off position. Boat engines are different, because the stop solenoid has to be energised to stop the engine, not to allow the engine to run. See paragraph 1, post #14

There is a fuse in the cutoff circuit my Nanni N2.14 which, when it blows, stops the engine. Exactly how it does that I do not know; the cutoff solenoid is operated by a timed relay built into the "On/off" switch, so when you switch the engine off the solenoid operates for seven seconds.

It beats me how this works. If the solenoid had to be held open there would be no need for the timer and if it had to be shut a blown fuse shouldn't cause it to operate. Answers on a postcard, please, The narrows of Loch Sunart were not the best place to make this discovery ...

 

[JumbleDuck]

Combustion simply means "burned", you can't burn it without igniting it.

I refer the Right Honorable Gentleman to my mention of hypergolic rocket fuels above. In fact it is arguable that nothing "ignites" the fuel in a diesel - it is simply injected into air at a temperature above its flash point.

For anyone interested, by the way, the defining feature of a diesel engine is not the compression ignition. It's a relatively slow burn as the piston descends, nominally at a constant pressure. Injecting fuel relatively slowly into hot air is simply one convenient way of achieving that.

 

[PaulRainbow]

There is a fuse in the cutoff circuit my Nanni N2.14 which, when it blows, stops the engine. Exactly how it does that I do not know; the cutoff solenoid is operated by a timed relay built into the "On/off" switch, so when you switch the engine off the solenoid operates for seven seconds.

It beats me how this works. If the solenoid had to be held open there would be no need for the timer and if it had to be shut a blown fuse shouldn't cause it to operate. Answers on a postcard, please, The narrows of Loch Sunart were not the best place to make this discovery ...

Pages S26 and S27 in the manual give an insight how it works.

http://extranet.nannidiesel.com:808...4&catid=72:manuels-datelier&Itemid=97&lang=en

It is an energise to stop solenoid, but it has the added complexity of a relay to automatically stop the engine when the ignition key is turned off. This has the downside that you have discovered, a blown fuse has the same effect as turning the key off. Might be tempting to remove the timer relay and fit a stop button ?

 

[VicS]

I refer the Right Honorable Gentleman to my mention of hypergolic rocket fuels above. In fact it is arguable that nothing "ignites" the fuel in a diesel - it is simply injected into air at a temperature above its flash point.

For anyone interested, by the way, the defining feature of a diesel engine is not the compression ignition. It's a relatively slow burn as the piston descends, nominally at a constant pressure. Injecting fuel relatively slowly into hot air is simply one convenient way of achieving that.

Merely above the flash point is not enough. It must be above the autoignition temperature.

 

[JumbleDuck]

Merely above the flash point is not enough. It must be above the autoignition temperature.

You are quite right, of course. Thanks.

 

[JumbleDuck]

Pages S26 and S27 in the manual give an insight how it works.

http://extranet.nannidiesel.com:8081/Joomla/index.php?option=com_content&view=article&id=291:manuels-datelier-n214&catid=72:manuels-datelier&Itemid=97&lang=en

It is an energise to stop solenoid, but it has the added complexity of a relay to automatically stop the engine when the ignition key is turned off. This has the downside that you have discovered, a blown fuse has the same effect as turning the key off. Might be tempting to remove the timer relay and fit a stop button ?

Thanks. That makes sense, but I still don't understand why the timer didn't de-energise the solenoid after the usual seven seconds. Perhaps it did, but every attempt to re-start the engine reset the timer. A top button is tempting, although having a single on-off switch is quite nice.

 

[PaulRainbow]

Thanks. That makes sense, but I still don't understand why the timer didn't de-energise the solenoid after the usual seven seconds. Perhaps it did, but every attempt to re-start the engine reset the timer. A top button is tempting, although having a single on-off switch is quite nice.

I suspect that might be the case. Could leave it as it is, but fit a normally closed switch in the cable to the solenoid, after the relay etc. If the fuse blew or anything else went wrong, flip the switch and the solenoid cannot be energised.

 

[RichardS]

OTOH, I find some of the posts useful "refreshers"...
And many of the "thousands of posts" are often filled with the rubbish some of us obsess about, something to do with X, T, R, B, I and E. But that is another story (and forum chamber) ;-)

That's fine .... but if people want to type a long post about how i/c engines work then please refrain from quoting the post of the forumite who knows a lot more about engines than you do. Just type your long post as a standalone educative post.

Richard

 

[AntarcticPilot]

I'm an academic. Petty, nit-picking pedantry is what I do.

And just to claim academic pedantry rights for myself, my diesel doesn't have an ignition key or switch - there's a push-button that enables the power supply to the electrical system and a different push-button that controls the starter motor. There is then a separate mechanical system for stopping the engine. There is no equivalent at all to the ignition switch of a car. And what is more, it's a standard setup on Volvo2003 engines; there is an alternative that is more like the car system, but mine has the simple one!

 

[179580]

Paul Rainbow has over 9000 posts. He's forgotten more about diesel engines than most people will ever know, which gives him the right to be somewhat tongue in cheek.

Read the thread if you want the answer to your second paragraph.

Richard

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.

 

[convey]

The shape of the piston crown or cylinder head does not affect the compression ratio. The compression ration is the ratio between the swept cylinder volume and the volume of the compression chamber with the piston at TDC.

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).

 

[penberth3]

Slightly off topic, but where can I find the 'ignore' button on this forum?

 

[Beneteau381]

I'm an academic. Petty, nit-picking pedantry is what I do.

Of course, but it's surprising how many people think that full-size diesels work like model aircraft engines which really do squeeze a fuel-air mixture until it goes bang. Not losing you with the technical terms, I hope.

and thats why ether, used in model aircraft engines when I was a lad, when used in Easystart to start diesels , can cause damage, it goes bang when it reaches a certain heat, rather than at the optimum timing point! 4' 6"

 

[Stingo]

Slightly off topic, but where can I find the 'ignore' button on this forum?

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

 

[Beneteau381]

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 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).

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 when combustion chambers were twin valved hemispheres as opposed to modern wedge shaped 4 valve combustion spaces in petrol engines. They had squish areas on the outside circumference of the hump and cutaways for the valves.

 

[Beneteau381]

And just to claim academic pedantry rights for myself, my diesel doesn't have an ignition key or switch - there's a push-button that enables the power supply to the electrical system and a different push-button that controls the starter motor. There is then a separate mechanical system for stopping the engine. There is no equivalent at all to the ignition switch of a car. And what is more, it's a standard setup on Volvo2003 engines; there is an alternative that is more like the car system, but mine has the simple one!

I would argue that a button is a switch!

 

[jamie N]

Tap or click on my tits...

Tits are they?! Honestly I thought that your avatar was a spiderman mask or something of that ilk!!
"Should've gone to spec savers etc..."

 

[Beneteau381]

It is combusted ... well, between some to most of it depending, as the plumes of black smoke they kick out testifies ... but by heat caused by compression. They operate as a much higher compression ratio than petrol engines. Some have glow plugs, heated by electricity, to give the initial heat boost to do so. old, elementary ones don't you just crank them up until they are going fast enough to generate enough themselves, and to stop them you can flip the decompressor lever. Some manual (cable), some solenoid operated.

Think of when you pumped up a bicycle pump as a kid holding the end, and they rapidly became hot (when a gas is compressed its temperature increases).

A diesel might have a compression ratios of between 15 to 23, while a petrol engine is between 8 to 10.

Petrol autoignites around 280°C, Diesel around 210 °C.

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.

some petrol engines nowadays are running at up to 14:1 compression ratio, some diesels only just above. My old fashioned MD22 only runs at 17:1 Petrol engine - Wikipedia The old Ford engines which were marinised only ran at 15.5:1 Ford 2715E | everythingaboutboats.org