The Great Generational Divide

[PeterPolyol]

Originally Posted By: PimTac
I am far from an expert, just an interested consumer.

Same here, no formal accreditations here. I think if you're really interested in anything, you'll eventually know it. I love reading patents and technical papers because I can learn about not only the proposed solutions, but more importantly the original problems that they have to solve for. It's like reverse-engineering an education

 

[Shannow]

Originally Posted By: PeterPolyol
Originally Posted By: PimTac
I am far from an expert, just an interested consumer.

Same here, no formal accreditations here. I think if you're really interested in anything, you'll eventually know it. I love reading patents and technical papers because I can learn about not only the proposed solutions, but more importantly the original problems that they have to solve for. It's like reverse-engineering an education

To me, living in the Internet age and not diving down whatever rabbit hole takes your fancy is wasting your life.

I've got formal engineering qualifications, and at Uni had the entire SAE library on microfiche...it was great, but nothing like the 'net.

 

[bigj_16]

Originally Posted By: Shannow
Originally Posted By: PeterPolyol
Originally Posted By: PimTac
I am far from an expert, just an interested consumer.

Same here, no formal accreditations here. I think if you're really interested in anything, you'll eventually know it. I love reading patents and technical papers because I can learn about not only the proposed solutions, but more importantly the original problems that they have to solve for. It's like reverse-engineering an education

To me, living in the Internet age and not diving down whatever rabbit hole takes your fancy is wasting your life.

I've got formal engineering qualifications, and at Uni had the entire SAE library on microfiche...it was great, but nothing like the 'net.

I wish I had the entire SAE library. You know, Christmas is coming. If everyone pitched in...

 

[CT8]

Originally Posted By: BrocLuno
I may grow old and die before I actually buy a DI engined vehicle ...

I have a Ford F150 with the 2.7 engine . It is the the most enjoyable engine I have ever owned so far. . We on this web sight have seen pictures of sludged up engines and surmised conventional oil is really bad.

 

[wemay]

I must concur, I absolutely love our Direct Injection vehicles. We've never experienced any significant issues with any of them. The 2.0T is especially impressive.

 

[PeterPolyol]

Originally Posted By: Shannow

To me, living in the Internet age and not diving down whatever rabbit hole takes your fancy is wasting your life.

I've got formal engineering qualifications, and at Uni had the entire SAE library on microfiche...it was great, but nothing like the 'net.


Hear hear! It's truly a remarkable time to be alive. Nevertheless, for non-conspiracy and "official" mainstream things, I'd obviously prefer access to the SAE library and have educational certifications

 

[Shannow]

True,
but the internet being what it is, you have people claiming that you don't hold the quals anyway.

In hindsight the 4 years isn't about downloading the information (which they can't in 4 years), it's creating thought processes and the ability to find links, collect and assimilate (or question) data.

Too many of my peers walk out thinking that's it for the rest of their working life.

 

[SR5]

Originally Posted By: Shannow
In hindsight the 4 years isn't about downloading the information (which they can't in 4 years), it's creating thought processes ...

Too many of my peers walk out thinking that's it for the rest of their working life.


Laugh !!! Reminds me of a few (but not all and not the good ones) Engineers I know....

I finished 4 years of Physics and thought "wow....what was that all about ..... ?" But I agree, it's the logical process that is important and that is open to every person with the correct mindset.

A few years later I decided I needed to add some practical real world ability to my book knowledge, so I joined the army and became a Sapper (Combat Engineer). I remember back in Sapper School doing my first barbed wire defensive structure with the precision of a university trained physicist. The Sergeant in charge was walking down the line, inspecting work, correcting errors, and yelling at anybody who caught his eye. He paused behind me, said nothing and just watches me work for a little bit. Then he walked over to me and quietly said "We are not trying to build a work of art here to save the world. We are just trying to slow them f**kers down (pointing outside) before they run inside and try to kill us all. This gives us the chance to kill them first." Then in full volume "Now hurry up before they shoot you in the f**ken head !!!". That's when I discovered that the third decimal place doesn't always matter.

 

[Ducked]

Originally Posted By: SR5
Then in full volume "Now hurry up before they shoot you in the f**ken head !!!".


Yeh, I got quite a lot of that too.

 

[PeterPolyol]

+3
Lots of smarts around here and some topics discussed are very inspirational and have driven me to learn things I definitely wouldn't have otherwise. Sure, there's a lot of bollocks on the board too but that's worth enduring for the good stuff

Example, Shannow posted about SCCI before I even knew it was officially a thing- but then I became very curious about it and how it fit into my conventional tuning knowledge. I mean, what other forum does one casually find out about the existence of 'SCCI'?


Originally Posted By: SR5
That's when I discovered that the third decimal place doesn't always matter.

haha yeah!

 

[Ducked]

Originally Posted By: PeterPolyol
Hey guys. I'll try to spare you all the yapping and word soup so here goes

Originally Posted By: Ducked

The general drift of apparently informed comment seems to be that engines will often be quite tolerant of detonation, (as opposed to pre-ignition).


Pre-ignition refers to timing only. Normally, combustion begins at the firing of the spark plug.

Detonation OTOH refers to a combustion mode, and is the other of two main gasoline combustion modes (the first one being 'normal combustion'). Detonation can occur before or after spark timing.

Normally, we need the charge to burn in a progressive manner, starting with the flame kernel around the plug and expanding outward to the edges. I'll call this "flame-front" combustion and it is a separate kind of combustion than detonation. Conventional gasoline engines are meant to operate on flame-front combustion only.

Detonation decribes a condition where the active charge simultaneously experiences auto-ignition. That means the entire mixture detonates all at once, not progressively. So instead of a hot-spot triggering normal flame-front combustion prematurely as in Pre-ignition, the heat and pressure of compression "detonates" the entire thing all at once like dynamite, like an IDI diesel engine. As I mentioned eariler, gasoline in detonation "pops" much quicker and brutally than diesel in detonation. An old school HCCI IDI diesel engine can hold up to burning (detonating) diesel, but if you ran it on gasoline it would disintegrate. This is noteworthy when discussing the practical challenges of SCCI. Burning gasoline like diesel in ANY engine is no walk in the park.



OK. That all seems reasonable.

But I still don't like that "always"

 

[Ducked]

Late Edit

OK. That all seems reasonable.

But I still don't like that "always" in your earlier post

Plus the (admittedly unstated) implication above seems to be that detonation is more damaging than (normal progressive combustion due to) pre-ignition because its more violent, whereas the latter is purely (merely?) a matter of timing.

My understanding was that it was this "matter of timing" that makes (normal progressive combustion due to) pre-ignition so damaging, because it is opposing the upward motion of the piston. If (normal progressive combustion due to) pre-ignition starts at BDC it doesnt have to be explosive to be very rapidly fatal.

 

[PeterPolyol]

Originally Posted By: Ducked
pre-ignition so damaging, because it is opposing the upward motion of the piston

It is indeed opposing the travel of the piston, but it's the difference between compressing air in a cylinder as it moves upwards vs taking a sledge hammer to a piston, even at TDC.

The peak combustion pressure with detonation occurs virtually the instant it's initiated. It's a powerful pressure spike and is like hitting the piston with a hammer that makes it damaging. The temperature and trapped sound pressure waves of the detonation can skin the aluminum off the piston and push the top ring-land down so hard it cracks. Surely you'd agree that pushing a vehicle to move it is less damaging than firing cannon balls at the back of it.

Pre-ignition is almost always accompanied by damaging detonation but not always, depending on how close to the autoignition point that the charge is eg. an over-heating atmospheric engine running 7.5:1CR with high octane gas and a spark plug that's way too hot (a cause of pre-ignition) may experience pre-ignition without detonation. Otherwise in a modern engine, the premature combustion pressure rise of pre-ignition in a turbo and/or high CR engine can trigger detonation of the remaining mixture if the pressure rise and closing combustion chamber bring the charge over the autoignition point. If subequent detonation follows PI, then the charge would have entered two combustion modes when all's said and done. Even if the there is no pre-ignition and combustion ensues at the normal timing, if the mixure crosses it's autoignition temperature/pressure then there will also be two combustion modes (with accompanying engine noise/rattling/pinging), even though combustion began on time.

I stand by that any detonation in a conventional gasoline engine is potentially damaging. But some manufacturers (Nissan said it somewhere) have even advised that occasional faint knocking during light acceleration offers the best fuel economy. Faint knocking demonstrates detonation of remaining charge after normal flame ignition as described above, and it is indeed the ensuing detonation that enables the complete burn of the fuel (and associated fuel efficiency), thus the appeal of SCCI, but it comes with a risk on normal engines. Obviously Nissan of yore thought mild, light throttle "rattling" (detonation) was acceptable in the name of economy but with a denser AF charge, the risk of engine damage ramps up quickly.

Pre-igntion isn't the risk of damage so much as the detonation it can enable. When you try and start an engine with the ignition timing way out or wires crossed and you get a normal spark-initiated "pre-ignition", I mean way way early flame ignition, all it does is bring the cranking engine to a halt for that one stroke; it's much easier on the hardware than detonation.



**Also a correction to a previous post, where I boneheadedly put the explanation of HCCI under IDI diesel. IDIs are not homogeonous charge engines, they inject the fuel into a pre-chamber where the charge mixes up and is ignited by compression and then forced out into the main chamber having been decently mixed. Someone must have noticed that

 

[CR94]

Originally Posted By: PeterPolyol
... Obviously Nissan of yore thought mild, light throttle "rattling" (detonation) was acceptable in the name of economy but with a denser AF charge, the risk of engine damage ramps up quickly...
Back in the late 1970s, a GM executive was famously quoted as saying light knocking was "the sound of fuel economy."

 

[PimTac]

Originally Posted By: CR94
Originally Posted By: PeterPolyol
... Obviously Nissan of yore thought mild, light throttle "rattling" (detonation) was acceptable in the name of economy but with a denser AF charge, the risk of engine damage ramps up quickly...
Back in the late 1970s, a GM executive was famously quoted as saying light knocking was "the sound of fuel economy."





I remember something along those lines with Chrysler as well during their lean burn era.

 

[SR5]

It's a bit confusing out there, well at least it is for me, maybe others are doing better.

So Mazda is developing a new engine, Homogeneous Charge Compression Ignition (HCCI) that runs a much higher compression ratio and causes autoignition of the gasoline (petrol) fuel. It produces a lean burn, with compression ignition at multiple combustion points.

They can't get properly controlled compression ignition when the engine is cold, so during the warm up cycle they use a spark plug to start the process with Spark Controlled Compression Ignition (SCCI). Same engine, different modes of operation.

Now Sandia National Labs do research into Stratified Charge Compression Ignition (SCCI) which is auto ignition with a non-homogenous charge. Again as a very high compression (HC) gas engine. The overall burn is lean, but there is a rich charge to start the combustion.

They are all closely related, but slightly different I believe. And the fact that SCCI has two different meanings is down right confusing.

Stratified Charge basically means there is a fuel rich region to help start the ignition process, by whatever means. The rich region is usually surrounded by a lean region.

Homogeneous Charge is all one air-fuel (AF) ratio and usually close to stoichiometric (ideal) or lean for compression ignition.

Anyway the common theme when reading about all these engines is that they still work on combustion and are not designing in an actual detonation (as far as I can tell). The HCCI engine is designed to have multiple ignition points within the charge, and this will produce a much faster rise in pressure than a conventional flame front engine. But it is still not a detonation in the strict sense of the word, it's a multiple combustion.

So what is detonation? Well a deflagration is a rapid burn or combustion and propagates via a flame front. A detonation is a supersonic exothermic shock wave, the shock wave initiates the chemical reaction. The big difference is speed and deflagration is subsonic while detonation is supersonic. The first has a pushing effect while the second has a shattering effect.

 

[Ducked]

Originally Posted By: PeterPolyol
Originally Posted By: Ducked
pre-ignition so damaging, because it is opposing the upward motion of the piston


It is indeed opposing the travel of the piston, but it's the difference between compressing air in a cylinder as it moves upwards vs taking a sledge hammer to a piston, even at TDC.


Hardly. Its compressing a full charge which is doing its normal combustion thing and pushing in the opposite direction.

To describe that as "compressing air" seems a bit disengenuous.

I don't doubt that detonation is a bad thing, but I THINK (I don't have any basis for certainty, and havn't delved into it as deeply as you apparently have) you may be over-stating its general awfulness and under-stating the potential awfulness of progressive combustion due to pre-ignition, when the latter is timed badly.

I base this almost entirely on widely available "classic" descriptions of pre-ignition and detonation, which tend to consider them as separate phenomena with detonation happening exclusively after spark ignition, and with pre-ignition not involving detonation. These classic descriptions nevertheless consider pre-ignition to be much more dangerous. Here's one

http://www.contactmagazine.com/Issue54/EngineBasics.html

Before I'd heard of LSPI I wondered if the classic description was incomplete, since I couldn't see why pre-ignition could'nt cause detonation. It now appears that in LSPI, at least sometimes, it does, and that detonation may happen close to TDC with the piston on the upstroke, worst of both worlds stylee.

I'm not sure to what extent this is a new phenomeno. I suspect the classic description was always incomplete or an oversimplification, but I doubt that it was always completely wrong, which would seem to be implied by saying that pre-ignition almost always causes detonation.

Since detonation is detectable, it would have been detected, and there wouldn't have been so many descriptions of pistons holed by pre-ignition in the abscence of detonation.

As you acknowledge, modern (DI boosted) car engines (which are of only academic interest to me, since I don't expect, or particularly want, to have one) are especially vulnerable to detonation, so its a happening, current thing with terms like "super-knock" to describe the bigger bangs and bills they bring us, but this may not apply to the same extent to all engines past or present.

To get anecdotal, when I bought my current (1986) car I THINK it was detonating intermittantly at idle (sharp taps from somewhere close to the cylinder head) and had probably been doing so for some time. I water decoked it and it stopped doing it. IF detonation was always as damaging as you suggest I doubt I'd still be driving it 7 years later.

Originally Posted By: PeterPolyol

Pre-igntion isn't the risk of damage so much as the detonation it can enable. When you try and start an engine with the ignition timing way out or wires crossed and you get a normal spark-initiated "pre-ignition", I mean way way early flame ignition, all it does is bring the cranking engine to a halt for that one stroke; it's much easier on the hardware than detonation.


Well yeh. Its much easier on the hardware because its barely turning over, and then it stops.

This does NOT seem the same as 6000 rpm with one cylinder generating full power DOWN while the other 3, 5, 7, or 11 cylinders and all the rotating mass are pushing UP.

That'll stop too, but probably less temporarily.