# Diesel engine operation
**URL:** https://boards.straightdope.com/t/diesel-engine-operation/265595
**Category:** Cecil's Columns/Staff Reports
**Created:** [September 23, 2004, 1:23am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595 "2004-09-23T01:23:25Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@tracer](https://boards.straightdope.com/u/tracer)
#### Post date: [September 23, 2004, 1:23am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/1 "2004-09-23T01:23:25Z")
In [Una’s diesel-fuel-in-a-gasoline-engine article](http://www.straightdope.com/mailbag/mdieselvsgas.html), she writes:
> [@](#):
>
> Diesel fuel is typically burned in compression-ignition internal combustion engines, which mix diesel fuel and air, then subject the mixture to very high compression–much higher than in a gasoline engine.
When I went through driver’s ed in 1980, we saw a film about how diesel engines operate. The film said that on the intake stroke, diesel engines draw in **just air** , with no fuel mixed in. This air is then compressed (on the compression stroke) at anywhere from a 12-to-1 to a 20-to-1 compression ratio, then, **right** when the piston reaches the top of the stroke (or thereabouts), a fuel injector at the top of the cylinder sprays diesel fuel into the compressed air, which spontaneously ignites and drives the piston down in the power stroke. Te timing of this fuel injector is just as crucial as the timing of the spark plugs in a gasoline engine.
Have diesel engines changed since the the early 1980s, so that now they suck in a diesel-fuel-and-air mixture on the intake stroke?
---
### Author:  [@GaryM](https://boards.straightdope.com/u/GaryM)
#### Post date: [September 23, 2004, 3:17am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/2 "2004-09-23T03:17:48Z")
Not an engine expert, nor do I play one on TV, but every diesel engine I’ve seen or read about for the last 40 years had an injector pump and injected fuel into the cylinder as you said.
Maybe we’ll learn something new! Always a treat.
---
### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [September 23, 2004, 3:24am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/3 "2004-09-23T03:24:37Z")
No, you both are correct (as is the person who e-mailed me about this early today…), diesels generally haven’t changed that much. That sentence was an editing mistake on my part, which resulted from combining two paragraphs together to reduce length. I’ll re-write that part so it can be corrected.
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### Author:  [@Dog80](https://boards.straightdope.com/u/Dog80)
#### Post date: [September 23, 2004, 5:22am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/4 "2004-09-23T05:22:33Z")
One of the biggest developments in diesel technology in the last years is the **common rail**. Some info here: [http://www.swedespeed.com/news/publish/Features/printer\_272.html](http://www.swedespeed.com/news/publish/Features/printer_272.html)
---
### Author:  [@chukhung](https://boards.straightdope.com/u/chukhung)
#### Post date: [September 24, 2004, 3:42am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/5 "2004-09-24T03:42:18Z")
The only diesel engines I know of that don’t use fuel injectors are [diesel model airplane engines](http://www.barnyard-buzzards.com/Engines/Diesel.htm). But because these are two-strokes, use oddball fuel mixtures and have tiny displacements, they’re very different from what most people think of when the term “diesel engine” is used.
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### Author:  [@Blown\_Injected](https://boards.straightdope.com/u/Blown_Injected)
#### Post date: [September 26, 2004, 9:04am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/6 "2004-09-26T09:04:17Z")
This is how I understand it:
\>\>derive from the same
source–crude oil. Although they share the same
parentage (just like motor oil, some plastics, and
Velveeta), they have very different properties
(2). The point is, gasoline
and diesel fuel aren’t the same. \<\<
**Same is true for copy paper and a 2x4’s, but I do not hear persons attempting to interchange 'em**
\>\>Gasoline is well suited to
this process, although sometimes either it or engine
operating conditions can cause “knock,” an unwanted
and destructive condition in which a portion of the
fuel ignites spontaneously due to compression alone,
without the aid of the spark.\<\<
**FALSE, Knock occurs AFTER the spark and is very different from pre-iginition - a condition where the fuel ignites before the spark (sounds more like marbles in a tin can)**
…Diesel fuel is typically burned in
compression-ignition internal combustion engines,
which mix diesel fuel and air, then subject the
mixture to very high compression–much higher than in
a gasoline engine. This increases the temperature of
the fuel and air mixture to the point where the
mixture self-ignites (autoignition). \<\<
**True, but often confused with mechanical CR.**
\>\>A couple of old
grease monkeys at the shop I used to work at claimed
that their “super high-compression” gasoline engines
of the late 1960s could easily switch between gasoline
and diesel fuel with no problems, but I tend to think
they spent too much time inhaling fumes from the
carburetor cleaning tank.\<\<
**This is pure BS because it is not the mechanical CR of the engine that causes the combustion. Today, it is not uncommon to find a race engine with over 14:1 CR, but it is the fuel injector pump that cranks the pressure to MASSIVE PSI that is the root of the massive compression of the fuel that causes the combustion.**
\>\>Some old-timers and shade-tree mechanics will add
small amounts of diesel fuel to the gasoline in their
cars in the belief that it will lubricate the valves,
in the same way that people will add such items as
“Marvel Mystery Oil”\<\<
**And then there are some that actually blend real mixes that do some good as is evidenced by solid measurable changes such as methanol injection and xylene/toulene mixes for gasoline engines. These combos require a lubricant such a MMO**
---
### Author:  [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)
#### Post date: [September 26, 2004, 1:50pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/7 "2004-09-26T13:50:47Z")
> [@Blown & Injected](#):
>
> **FALSE, Knock occurs AFTER the spark and is very different from pre-iginition - a condition where the fuel ignites before the spark (sounds more like marbles in a tin can)**
Not so. Although knock is distinguished from pre-ignition, and _can_ involve ignition after the spark, it is not defined so, and definitely includes both ignition before the spark and even ignition by the spark if the flame front advances faster than the spark timing allowed for. (The spark is not at the exact instant of maximum compression, but a little bit before, so that the flame front will reach the piston at the right moment.) Indeed, I am not sure that “knock” has a clear and universally accepted definition, beyond “when the engine is making a noise like _that_”. At any rate, that was the definition as late as 1946.
---
### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [September 26, 2004, 3:52pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/8 "2004-09-26T15:52:18Z")
> [@Blown & Injected](#):
>
> This is how I understand it:
>
> \>\>derive from the same
> source–crude oil. Although they share the same
> parentage (just like motor oil, some plastics, and
> Velveeta), they have very different properties
> (2). The point is, gasoline
> and diesel fuel aren’t the same. \<\<
>
> **Same is true for copy paper and a 2x4’s, but I do not hear persons attempting to interchange 'em**
I don’t understand the point you are trying make.
> [@](#):
>
> \>\>Gasoline is well suited to
> this process, although sometimes either it or engine
> operating conditions can cause “knock,” an unwanted
> and destructive condition in which a portion of the
> fuel ignites spontaneously due to compression alone,
> without the aid of the spark.\<\<
>
> **FALSE, Knock occurs AFTER the spark and is very different from pre-iginition - a condition where the fuel ignites before the spark (sounds more like marbles in a tin can)**
I’m uncertain of why you say “FALSE” in all caps to yell at me when I do not even address whether or not I’m talking about pre-spark or after spark, I’m talking about “without the aid of the spark”, which is kind of the whole key to the knock sound/fury. While I do typically use “knock” to refer to after-spark phenomenon, John W. Kennedy is correct in what he says as well. Maybe a closer reading of the Staff Report is called for.
> [@](#):
>
> …Diesel fuel is typically burned in
> compression-ignition internal combustion engines,
> which mix diesel fuel and air, then subject the
> mixture to very high compression–much higher than in
> a gasoline engine. This increases the temperature of
> the fuel and air mixture to the point where the
> mixture self-ignites (autoignition). \<\<
>
> **True, but often confused with mechanical CR.**
I don’t understand this comment either.
> [@](#):
>
> \>\>A couple of old
> grease monkeys at the shop I used to work at claimed
> that their “super high-compression” gasoline engines
> of the late 1960s could easily switch between gasoline
> and diesel fuel with no problems, but I tend to think
> they spent too much time inhaling fumes from the
> carburetor cleaning tank.\<\<
>
> **This is pure BS because it is not the mechanical CR of the engine that causes the combustion. Today, it is not uncommon to find a race engine with over 14:1 CR, but it is the fuel injector pump that cranks the pressure to MASSIVE PSI that is the root of the massive compression of the fuel that causes the combustion.**
Increasing the pressure of a generally incompressible fluid (such as water, or diesel fuel) does not raise its temperature very much at all, compared to a gas like air. Even the Ideal Gas Law we all learned about in Jr. High tells us this. The enthalpy added to the fuel by the pumping inefficiency is not going to raise the temperature of the fuel to the autoignition point. Just taking water as a convenient example of a generally incompressible working fluid, raising the pressure from saturated at 80F to 3000 psia only adds 9 Btu/lbm of enthalpy using an 80% efficient pump. According to the published heat balance of Riverside station Unit 1, a 3500 psi increase in boiler feedwater only adds 10 degress F (see Chapter 2 of here: [http://www.personal.utulsa.edu/~kenneth-weston/](http://www.personal.utulsa.edu/~kenneth-weston/) )
So what happens to the air?
From the Bosch Automotive Handbook:
“during the compression stroke, intake air is compressed to 30…55 bar. and its temperature thereby increases to 700…900 C. This temperature si sufficient to cause autoignition of the fuel…”
From “Internal Engine Combustion Fundamentals”, John B. Heywood, pg. 27:
“The compression ratio of diesels is much higher than typical SI values, and is in the range of 12 to 24, depending on the type of diesel…Air at close to atmospheric pressure is inducted during the intake stroke and then compressed to a pressure of about 4MPa and a temperature of about 800K. during the compression stroke. At about 20 degress before TC, fuel injection into the engine cylinder commences…The temperature and pressure are above the fuel’s ignition point. Therefore after a short delay period, spontaneous ignition (autoignition) of parts of the uniform fuel-air mixture initiates the combustion process”
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### Author:  [@Blown\_Injected](https://boards.straightdope.com/u/Blown_Injected)
#### Post date: [September 27, 2004, 3:54am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/9 "2004-09-27T03:54:42Z")
> [@Una Persson](#):
>
> I don’t understand the point you are trying make.
Just that because they are made from the same thing does not mean people are attempting to interchange the items. Both maybe made from crude oil, just like paper and a 2x4 is made from trees. - Maybe a closer reading of the Report is called for 🙂
> [@Una Persson](#):
>
> I’m uncertain of why you say “FALSE” in all caps to yell at me when I do not even address whether or not I’m talking about pre-spark or after spark, I’m talking about “without the aid of the spark”, which is kind of the whole key to the knock sound/fury. While I do typically use “knock” to refer to after-spark phenomenon, John W. Kennedy is correct in what he says as well. Maybe a closer reading of the Staff Report is called for.
Jezzz touchy - Sorry about the caps.
When the fuel ignites without the “aid of spark” it is a pre-ignition event. After the spark it is knock, especially with regard to hi-compression when the fuel detonates. It is why higher octane is needed in hi compression/turbocharged/supercharged engines. Knock is two flames colliding in the combustion chamber, one from the spark and the other from hi pressure on that air/fuel mix. It seems possible that in severe conditions it may be possible for the air/fuel mix to start before the spark in several different places causing a violent spike in cylinder pressure, but this would still be a pre ignition event.
> [@Una Persson](#):
>
> I don’t understand this comment either.
It is not just the compression of the air due to the mechanical compression ratio, an extremely high pressure super fine mist is also part of the process.
There are gasoline engines with more than 14:1 compression and the easier to ignite air/gasoline does not spontaneously ignite until the spark. Modern diesels may be 14:1 + compression, but it is not just the mechanical compression ratio that is going to cause the fuel to ignite. The fuel injector and pump system is pressurizing the fuel to thousands of PSI and that pressure it enough to cause the fuel to ignite. There are diesel injector demonstration devices that will fire diesel fuel in open air and that fuel will ignite in open air. It is a hand lever operated pump and diesel injector.
---
### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [September 27, 2004, 11:55am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/10 "2004-09-27T11:55:34Z")
> [@Blown & Injected](#):
>
> Just that because they are made from the same thing does not mean people are attempting to interchange the items. Both maybe made from crude oil, just like paper and a 2x4 is made from trees. - Maybe a closer reading of the Report is called for 🙂
Well, the Staff Report says:
> [@](#):
>
> Aside from additives such as ethanol, which is made from corn, the gasoline and diesel fuel commonly used today derive from the same source–crude oil. Although they share the same parentage (just like motor oil, some plastics, and Velveeta), they have very different properties.
What you responded to was this line, back up in Post #6 here. Aside from my throwaway bit about “Velveeta” this is a true statement, and does not suggest in itself that anyone is considering that. The questioner, who posted
and the question, was considering it, when they wrote:
> [@](#):
>
> What would happen if I put diesel into my gas tank?
So I still don’t understand the context of your comment and I guess I can’t comment on it further.
> [@](#):
>
> When the fuel ignites without the “aid of spark” it is a pre-ignition event. After the spark it is knock, especially with regard to hi-compression when the fuel detonates. It is why higher octane is needed in hi compression/turbocharged/supercharged engines. Knock is two flames colliding in the combustion chamber, one from the spark and the other from hi pressure on that air/fuel mix. It seems possible that in severe conditions it may be possible for the air/fuel mix to start before the spark in several different places causing a violent spike in cylinder pressure, but this would still be a pre ignition event.
You’re mixing up your terms here, “knock” and “preignition”. Alright, where are you getting your definition of “knock” from? My definition of “knock” comes from my graduate textbooks on IC engines. From “Internal Combustion Engine Fundamentals”, John B. Heywood, Section 9.6.1:
> [@](#):
>
> Knock is the name given to the noise which is transmitted through the engine structure when essentially spontaneous autoignition of a portion of the end-gas occurs.
“Autoignition” is what he says, and that is **without the aid of a spark**. He goes on further to say in Figure 9.58 where he maps the different types of knock that knock is caused by surface ignition (pre-ignition) and is called by him “Knocking surface ignition”. Surface ignition can occur and not cause knock (called “non-knocking surface ignition”). Further, he later differentiates the two types of knock down the page in 9.6.1 by saying “spark-knock is controllable by the spark advance”, and “knocking surface ignition usually originates from preignition caused by glowing combustion chamber deposits: the severity of knock generally increases the earlier that preignition occurs”
Knock is not defined in any engineering work I can find as simply “two flames colliding in the combustion chamber”, it is due to the “spontaneous release of much of the end-gas fuel’s chemical energy” which “produces a substantial local increase in the gas pressure and temperature, thereby causing a shock wave to propagate away” and produce the resonant sound on the cylinder walls. I think it makes for an interesting mental image to think of two flames colliding to produce a sound but while that can happen during knock, that’s not the root cause nor the main effect (and it could be anywhere from 2-20 flame fronts, depending).
I am pretty much aware of the importance of octane. I worked in a lab for three weeks running engines on a dyno at various octane levels charting out what conditions caused knock, including equivalence ratio, spark advance, inlet air temperature, spark plug type, and of course throttle position and dyno load.
> [@](#):
>
> It is not just the compression of the air due to the mechanical compression ratio, an extremely high pressure super fine mist is also part of the process.
> There are gasoline engines with more than 14:1 compression and the easier to ignite air/gasoline does not spontaneously ignite until the spark. Modern diesels may be 14:1 + compression, but it is not just the mechanical compression ratio that is going to cause the fuel to ignite. The fuel injector and pump system is pressurizing the fuel to thousands of PSI and that pressure it enough to cause the fuel to ignite. There are diesel injector demonstration devices that will fire diesel fuel in open air and that fuel will ignite in open air. It is a hand lever operated pump and diesel injector.
I think you’re missing a key point of the article. Gasoline is _not_ necessarily easier to ignite without a spark - that’s the whole point of octane measurement versus cetane measurement, and of the whole concept behind the diesel engine, and why when you put gasoline in the diesel engine it doesn’t do nice things. Second, even if those demonstrations you discuss are true (which I’ve never seen nor heard of), the air-fuel ratio when you spray diesel fuel into the air is not nearly the same as what you have in the combustion chamber, nor does the air contain the quantity of residual gas to pollute combustion that a running engine does. Third, I’ve seen plenty of Volvo and Saab diesel injectors tested by spraying them into the air (into a bucket, actually) to test them at the shop I worked at, and not a single one ever caught fire. Fourth, I presented two cites saying that it was the heated air from the compression that caused the autoignition of the fuel, not the compression of the fuel.
Not one of my books on diesel engines, nor my experience, nor the people I’ve spoken with, nor Google has shown me that by using pump diesel you can spray it into the air at the same pressures as a car diesel fuel pump and get it to become a flame thrower. I won’t say it’s impossible of course because in real life an awful lot of stuff can be made to happen depending on the conditions, but I would really like to see this from a reputable cite. And even if it is possible to make a test that does it, it still does not change at all the fact that what ignites the fuel is the heat from the compressed air, which is and always was one of the key aspects of the diesel engine, ever since its first inception and production.
---
### Author:  [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)
#### Post date: [September 27, 2004, 1:13pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/11 "2004-09-27T13:13:55Z")
> [@Una Persson](#):
>
> what ignites the fuel is the heat from the compressed air, which is and always was one of the key aspects of the diesel engine, ever since its first inception and production.
…especially when you consider that the _original_ diesel fuel was coal dust.
---
### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [September 27, 2004, 1:21pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/12 "2004-09-27T13:21:35Z")
> [@John W. Kennedy](#):
>
> …especially when you consider that the _original_ diesel fuel was coal dust.
Yeah, but people get tired of me always bringing up coal in everything…even my upcoming “deviled eggs” article talks about coal. 😉
(Not doubting you, but just stating that I was unaware that that was the original fuel for his engine was coal dust, although I did know that Diesel tried out a wide variety of different things - I think he even went down the path of the gunpowder engine like experimenters much earlier than him did. I think he even tried flour and micronized sawdust too, IIRC.)
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### Author:  [@paperbackwriter](https://boards.straightdope.com/u/paperbackwriter)
#### Post date: [September 27, 2004, 4:58pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/13 "2004-09-27T16:58:22Z")
[totally trivial comment]
Towards the end of WWII, the Japanese were also experimenting with other powdered organic materials in diesel-style engines. Including, IIRC, finely-ground dehydrated agricultural meterials.
[/totally trivial comment]
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### Author:  [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)
#### Post date: [September 27, 2004, 9:41pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/14 "2004-09-27T21:41:25Z")
I understand that injecting (hence making under pressure) the fuel does not add significantly to the temperature of the fuel itself. But wouldn’t the act of instroducing more ‘stuff’ (fuel) to the combustion chamber raise the overall pressure of the air in the combustion chambe thus helping with the fuel ignition? I am thinking along the lines that the max compression of the air by the piston is not sufficient to bring the temperature up to what is needed to ignite the fuel but the addition of the fuel causes the pressure in the combustion chamber to rise to the critical level.
I am just asking…I have no experience with engines worth mentioning.
---
### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [September 28, 2004, 12:47am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/15 "2004-09-28T00:47:08Z")
First, here’s a page that helps understand some of the thermodynamics behind an ideal and mostly-ideal diesel cycle.
[http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/diesel.html](http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/diesel.html)
Second, we need to look at what the actual autoignition temperature is of diesel. Sources online give many autoignition temperatures for diesel and diesel/fuel oil, but they generally seem to lie between 210 C and 385 C. NREL gives a figure of 316 C for diesel on a page for biodiesel, but oddly no temperature for the biodiesel…I found several MSDS sheets, which give values of 210 C to 257 C (diesel oil #3, not typically burned in automobile engines, has a higher autoiginition temperature, as one would expect).
So while it’s true that adding more mass (the fuel) to the cylinder when it’s compressed to near TDC would raise the temperature somewhat, it’s not really why it autoignites, and just from compression of the air alone the air temperature gets well above the autoignition temperature. Now mind you, most autoignition temperatures are measured in atmospheric air; when the fuel is under the immense pressures of the combustion chamber it has different characteristics. Heywood gives values of autoignition temperature for many different types of diesel engine, operating at pressures of 10-60 atmospheres, and gives values ranging from a low of about 300 C to a high of about 400 C. I’ve also seen a common quoted figure of “385 C” for autoignition under pressure, but am unsure of the origin.
How high does the temperature get just from compressing air? If you consider it to be isentropic compression (contant entropy), assume an equivalence ratio of 1.0, and a value of 1.333 for k (typical for a fuel/air mixture), then the temperature increase can roughly be approximated by:
T[sub]final[/sub] = T[sub]initial[/sub] \* (CR)[sup]k - 1[/sup]
So assuming 40C air entering, and 20:1 compression, we get:
T[sub]final[/sub] = 313K \* (20)[sup]1.333 - 1[/sup] = 848K, or 575 C.
Of course, that’s only a simplification. If you want more accurate than that, you need to move to tables or a program.
(duh, actually, I just noticed the page I linked at the top has a Java calculator that will do this for you. Never mind…at least it got the same number as I did)
Some refs:
[http://www.nrel.gov/docs/fy00osti/28315.pdf](http://www.nrel.gov/docs/fy00osti/28315.pdf)
[http://www.usor.com/pdfs/msds/fuels/No2\_Diesel\_MSDS.pdf](http://www.usor.com/pdfs/msds/fuels/No2_Diesel_MSDS.pdf)
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### Author:  [@Blown\_Injected](https://boards.straightdope.com/u/Blown_Injected)
#### Post date: [September 28, 2004, 5:31am UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/16 "2004-09-28T05:31:08Z")
> [@Una Persson](#):
>
> Well, the Staff Report says:
>
> What you responded to was this line, back up in Post #6 here. Aside from my throwaway bit about “Velveeta” this is a true statement, and does not suggest in itself that anyone is considering that. The questioner, who posted
> and the question, was considering it, when they wrote:
>
> So I still don’t understand the context of your comment and I guess I can’t comment on it further.
Damn you like to argue! Maybe metoo but please don’t get all bent outta shape - my blurry preview told me I get nicer later in the post
If you don’t get it by now :rolleyes: and after all the bourbon at the Monday Night Football game I ain’t gonna even try to find post 6 and try to read what you think I’m responding to - I’m taking about the made from the same stuff but the fuel are quite different
> [@Una Persson](#):
>
> You’re mixing up your terms here, “knock” and “preignition”. Alright, where are you getting your definition of “knock” from? My definition of “knock” comes from my graduate textbooks on IC engines. From “Internal Combustion Engine Fundamentals”, John B. Heywood, Section 9.6.1:
Nice, even a gear head owner/driver might have a graduate degree too. At an always rated top 10 graduate school, I would have gotten by chops busted for not clearly defining my terms thus making it possible for confusion and even an attack on the comments.
I believe it is you that is conveniently pulling out the broad topic of “auto-ignition” when I have been talking about the word you chose in the original article \*\* KNOCK \*\* there is no mention of auto - ignition. Well, I don’t recall that anyway and I’m sure about the knock comment. The condition you described is closer to the other word I did use - pre-ignition, and I believe you even made a comment about the common, typical, or something used of knock vs pre ignition.
> [@Una Persson](#):
>
> I am pretty much aware of the importance of octane. I worked in a lab for three weeks running engines on a dyno at various octane levels charting out what conditions caused knock, including equivalence ratio, spark advance, inlet air temperature, spark plug type, and of course throttle position and dyno load.
I’m impressed. I would love to be part of something like that and I am curious as to what you might know about using Xylene and Toluene or if you could recommend a text that looks at modifying fuels.
Although I have spent the last 20 years going through thousands of gallons of VP C-16, and Sunoco GT100 / GT+ for the street grocery getter - can’t imagine how much time spent monitoring EGT, O2’s, and Knock Retard.
---
### Author:  [@zut](https://boards.straightdope.com/u/zut)
#### Post date: [September 28, 2004, 12:47pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/17 "2004-09-28T12:47:11Z")
> [@Blown & Injected](#):
>
> I ain’t gonna even try to find post 6 and try to read what you think I’m responding to - I’m taking about the made from the same stuff but the fuel are quite different
I think **Una** ’s comment and confusion stems from the fact that you’re essentially agreeing with her – gasoline and diesel fuel are not interchangeable. However, your analogy that the “same is true for copy paper and a 2x4’s, but I do not hear persons attempting to interchange 'em” overstates the case, since people _do_ ask about the interchangeability of gasoline and diesel. That’s what the whole staff report is about.
> [@](#):
>
> I believe it is you that is conveniently pulling out the broad topic of “auto-ignition” when I have been talking about the word you chose in the original article \*\* KNOCK \*\* there is no mention of auto - ignition. Well, I don’t recall that anyway and I’m sure about the knock comment. The condition you described is closer to the other word I did use - pre-ignition, and I believe you even made a comment about the common, typical, or something used of knock vs pre ignition.
I suspect you might be using a narrower, more specialized definition of knock than **Una** is, in order to distinguish pre- and post-ignition events. I’ve always understood knock to be the noise associated with autoignition – either before or after the spark. Heywood seems to agree. I don’t think you can fault **Una** for using fairly standard terminology.
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [September 28, 2004, 1:28pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/18 "2004-09-28T13:28:42Z")
> [@Blown & Injected](#):
>
> Damn you like to argue! Maybe metoo but please don’t get all bent outta shape - my blurry preview told me I get nicer later in the post
> If you don’t get it by now :rolleyes: and after all the bourbon at the Monday Night Football game I ain’t gonna even try to find post 6 and try to read what you think I’m responding to - I’m taking about the made from the same stuff but the fuel are quite different
I really don’t know what to say to that. But I will answer the argue comment just a bit later.
**zut** has explained better, I think, why I do not/did not understand your first point, or at least the wording used.
> [@](#):
>
> Nice, even a gear head owner/driver might have a graduate degree too. At an always rated top 10 graduate school, I would have gotten by chops busted for not clearly defining my terms thus making it possible for confusion and even an attack on the comments.
>
> I believe it is you that is conveniently pulling out the broad topic of “auto-ignition” when I have been talking about the word you chose in the original article \*\* KNOCK \*\* there is no mention of auto - ignition. Well, I don’t recall that anyway and I’m sure about the knock comment. The condition you described is closer to the other word I did use - pre-ignition, and I believe you even made a comment about the common, typical, or something used of knock vs pre ignition.
I mentioned “graduate” because I didn’t take the classes at an undergrad level to say that they were more detailed than basic overviews - undergrad thermo classes rarely go into the details of Otto-cycle and Diesel-cycle engines much. Also because the work I cited from is titled “Fundamentals…” when it actually is very clearly misnamed, and is not in fact fundamental. Even two of my PhD level courses [sub](I don’t have a PhD, but I did have to take a few shared courses)[/sub] had “Fundamentals” or “Basics” in their titles, something I found really freaking annoying from them…
Other than that I’m not going to respond to who went to what school where and how highly some unnamed school was rated in an unnamed program against another unnamed school and unnamed program. Next thing we’ll be comparing GPAs and the breeding and lineage of our professors, and that’s when we’re really in the Twilight Zone. Plus, all that is an Appeal to Authority, which AFAIK neither of us have on this subject.
I think your use of “Conveniently” seems actually quite snide; since you brought it up, maybe posting inebriated after MNF is not the best thing to do. Yes, I know MNF is so bad that one pretty much needs to be drunk to listen to the inane commentary (“Johnson went to Florida State…where he was in the drama club…performing in plays…you know…yep…I guess now he gets to wear tights too…I have a funny story about tights and the great Vince Lombardi…”), but still…
I’ll go back to the first argument here. Here is the entirety of what I said in the original article:
> [@](#):
>
> Gasoline is well suited to this process, although sometimes either it or engine operating conditions can cause “knock,” an unwanted and destructive condition **in which a portion of the fuel ignites spontaneously due to compression alone, without the aid of the spark.**
>
> Knock is a serious problem for gasoline engines. The resistance of a particular gasoline blend to knock is given by the octane number, with a higher value indicating greater resistance.
Here is what you said:
> [@](#):
>
> “FALSE, Knock occurs AFTER the spark and is very different from pre-iginition - a condition where the fuel ignites before the spark (sounds more like marbles in a tin can)”
Nowhere did I mention before or after the spark, nor go into specifics there, because the point of the report was not on knock.
I provided cites that I believe back up everything I said, especially about knock being due to spontaneous ignition of the fuel. I cited the pieces earlier; I’m not copying them again to take up space.
Now. I’m not arguing because I like arguing. I’m arguing because these Staff Reports are supposed to be a search for fact or at least the best stab at facts based on logic. What I want to know is whether there’s a mistake here, or there’s a misreading, or a misunderstanding. If there’s a mistake, it’s my problem and I need to fix it. If it’s one of the other two then all I can do is try to explain better, use examples, cite, and see why there is a disagreement.
I do not believe I am in error, and feel I have explained why and provided cites. You feel that I am in error but have not provided detailed cites as to why this is, or at least as support for your position. This does not mean you are wrong necessarily, but it also does not mean that I have a sound basis to feel I am wrong.
I do not think I can explain it any better so unfortunately there is not going to be a meeting of the minds on this.
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### Author:  [@zut](https://boards.straightdope.com/u/zut)
#### Post date: [September 28, 2004, 3:12pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/19 "2004-09-28T15:12:40Z")
Ah; here we go, **Una**. I think I see what **B&I** is getting at. Take a look at [this article](http://encyclopedia.thefreedictionary.com/Engine%20knock) which differentiates between knock and preignition. In particular, it states that
> [@](#):
>
> Knocking (also called pinking or pinging) in internal combustion engines occurs when fuel in the cylinder is **ignited by the firing of the spark plug** but burns too quickly…
>
> Knocking is a different phenomenon from pre-ignition, which occurs when the air-fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is caused by heat buildup in engine components or overheating of the air-fuel mixture during compression, and cannot be prevented by delaying spark plug firing. As such, if pre-ignition is allowed to continue for any length of time, severe engine damage can result. [my bold]
Which is part of what **B&I** is saying – ignition before the spark is pre-ignition, not knock.
However, there seems to be some disagreement as to that diffentiation. For example, [in this article](http://www.superchargersonline.com/content.asp?ID=104), _which is linked to by the previous article_, they say
> [@](#):
>
> As you probably have already figured out, detonation (aka “knock”) is a big issue in the world of forced induction…
>
> Detonation occurs when air and fuel that is ahead of the flame front ignites before the flame front arrives because it becomes overheated…
>
> If there is enough heat and pressure in the combustion chamber, detonation can **begin to occur before the spark plug even fires** , which would normally initiate the combustion. Under these circumstances, **known as “pre-ignition”** , the piston may be travelling up towards a wave of compressed, exploding gas. [my bold]
So knock, or detonation, _can_ occur before the spark.
Anyway, that out of the way, I think that **B&I** is objecting to the “without the aid of a spark” phrase. You’re saying knock is autoignition of the fuel, without the spark igniting the mixture. True. However, **B&I** is saying that the knock phenomenon requires one part of the mixture to be ignited _first_, which then compresses another part of the mixture, which then autoignites. So in that sense, knock _does_ “require a spark,” because without the spark, knock would not occur (as contrasted with pre-ignition).
At least, I think I parsed the argument correctly.
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [September 28, 2004, 4:30pm UTC](https://boards.straightdope.com/t/diesel-engine-operation/265595/20 "2004-09-28T16:30:40Z")
Well **zut** , you could be correct at why we’re looking at it from different sides. If that is the case then I think we’re both using somewhat different definitions (looking at the single-cycle conditions versus looking at the multi-cycle conditions). My textbooks and references are definitely looking at the single cycle, but it’s also valid to consider the multi-cycle viewpoint in the definition as a possible one. I’m guessing you’re likely correct here in your analysis.
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