Compression ignition

[NapierDeltic]

That's right .
But with a compression ratio of 12-1 and a temperature of 330 degrees Celsius, it is too high compared to the auto-ignition temperature of gasoline - 280 degrees Celsius.
And with that 330 degrees Celsius, 2-stroke and 4-stroke engines will not need an ignition system and we may not be able to use it.
Still saying: I don't believe it - 330 degrees Celsius

@Lloyd-ss But at least you gave me a formula to calculate...Thank you for that.
Personally, I don't know much about engines and engine-related calculations, so I'll stop here.

In fact, you are right, Minh-thanh; most high octane gasoline has antiknock additives. only those allow high compression ratios without issues.
Though, in many engine operating conditions - for a c.r. 12-1 engine-, compression does not start from 1 bar; engine being partly choked on its admission.

https://en.wikipedia.org/wiki/Tetraethyllead

 

[Lloyd-ss]

That's right .
But with a compression ratio of 12-1 and a temperature of 330 degrees Celsius, it is too high compared to the auto-ignition temperature of gasoline - 280 degrees Celsius.
And with that 330 degrees Celsius, 2-stroke and 4-stroke engines will not need an ignition system and we may not be able to use it.
Still saying: I don't believe it - 330 degrees Celsius

@Lloyd-ss But at least you gave me a formula to calculate...Thank you for that.
Personally, I don't know much about engines and engine-related calculations, so I'll stop here.

Ha ha, I guess we will just have to accept the fact that each of us is right, and the other person is wrong, haha. No hard feelings, of course. BTW, I have seen compression temperatures and auto ignition temperatures listed in many places with wildly varying numbers. A solid consensus seems hard to pin down. BUT, the engines do indeed work! And that is the most important part.

Personally, I have always found the speed at which modern IC engines can operate to be rather hard to comprehend, almost unbelievable, and a bit magical. A 4 stroke running at 10,000 rpm is firing each cylinder 83 times a second, and the full180 degrees of the power stroke only lasts 3 milliseconds from TDC to BDC. And that heavy, weird, unbalanced looking crankshaft spinning at 167 revs per second without shaking the entire engine to death??
Do you think Mr Otto ever dreamed of 10,000 rpm? I hope he did!
Lloyd

 

[GreenTwin]

As others have mentioned, there can be a great deal of difference between a calculated hypothetical maximum temperature and actual operating conditions, where many things can reduce the temperature inside the cylinder.

Hypothetical calculations are just that, hypothetical; ie: not real-world parameters.

.
Edit:
There are simple formulas for electrical transformers, where the theoretical voltage and current values are a function of the turns ratio.
In reality, there are all sorts of things that affect a transformer; core losses, eddy currents, core saturation, hysterysis, I square-R losses, operating temperature, etc.

General formulas get you in the ballpark, but reality is far more complex than a simple formula.

.

 

[Lloyd-ss]

Very interesting and enjoyable discussion, and @NapierDeltic Thank you for dropping into the middle of it.
@minh-thanh , I definitely hear what you are saying and that's why I called it a conundrum. With the use of different grades of gasoline, and antiknock additives, and knock-sensors to retard the timing, and racing gasoline formulated for high RPM and compression ratios, and some ambiguous specifications, I guess the answer is somewhere in the middle. I doubt I can add anything more to the conversation, but again, it was fun and educational.

​

 

[The Ignoble Troll]

Fellas, don't forget the difference between static vs dynamic compression

 

[Lloyd-ss]

Fellas, don't forget the difference between static vs dynamic compression

Ok... please help us out. Always ready to learn.
Lloyd

 

[GreenTwin]

Fellas, don't forget the difference between static vs dynamic compression

Yikes, just when I was starting to feel that I had some sort of casual understanding of it all.

.
Edit:
I have noticed in some industrial applications that you can get unexpected fluid and gas flow, stratification, etc. due to the momentum of the fluid or gas.
I have noticed uneven flow inside of my foundry furnace too, due to the velocity of the incoming fuel/air mixture being forced into the furnace with a leaf blower.
Dual-burner furnaces (two burners at 180 degrees) have much more even flame and heat distribution inside of a furnace.
.

 

[The Ignoble Troll]

Static compression is ideal and what you've been calculating.

The numbers seem optimistic just because you need to throw in is the messy real world which reduces those numbers.

https://en.m.wikipedia.org/wiki/Compression_ratio#:~:text=The dynamic compression ratio is higher with more conservative intake,than the static compression ratio.

 

[peterl95124]

Where did you get that information? I want to learn more....

Minh-Thanh, any text book on thermodynamics (I got my start designing a model turbo-jet engine made from automobile turbo-charger rotor parts, same approach as Kurt Schreckling and Thomas Kamps, so have a collection of turbine engine text books which also cover the thermodynamics)

you'll have to get familiar with these equations
first from high school
PV = nrT, or Pressure x Volume = constant x Temp for a given amount of gas
then the formulas they (sadly) don't teach you in high school
P x V^(gamma) = constant, for a given amount of gas
the exponent gamma depends on the molecular structure of the gas, 1.667 for monatomic gases like argon, 1.4 for diatomic gases like N2 and O2, 1.333 for triatomic gases like H20 steam and C02
finally there's the energy equations
E[compression] = T[in] x Cp x ( PR^((gamma-1)/gamma) - 1)
E[expansion] = T[in] x Cp x ( 1 - PR^((1-gamma)/gamma) )
here Cp is coefficient of heat (heat req per deg of temp change)
and from these equations you can derive these "ratio" equations
TR = PR ^ ((gamma-1)/gamma TR = VR ^ (1-gamma)
VR = PR ^ (-1/gamma) VR = TR ^ (1/(1-gamma))
PR = TR ^ (gamma/(gamma-1)) PR = VR ^ (-gamma)
Note that the "compression ratio" we describe our piston engines with is VR, the volume ratio, here in thermodynamics, "compression ratio" is not the pressure ratio, the actual pressure is much higher than the compression.

you'll probably want to consult a text book to be sure you get all the units right,
in addition all the "ratio" formulas and any that involves an exponent require
Temp units that start at "absolute zero" and Pressure units that start at "absolute vacuum". IE zero degrees is absolute zero and zero pressure is a vacuum.
I use degrees Kelvin (293 = room temp), Bar is convenient for pressure (1 = sea
level atmospheric pressure), and if you want the energy to come out in MKS
Joules then Cp = 1000.

welcome to the journey !!!

 

[peterl95124]

That's right .
But with a compression ratio of 12-1 and a temperature of 330 degrees Celsius, it is too high compared to the auto-ignition temperature of gasoline - 280 degrees Celsius.
And with that 330 degrees Celsius, 2-stroke and 4-stroke engines will not need an ignition system and we may not be able to use it.
Still saying: I don't believe it - 330 degrees Celsius

@Lloyd-ss But at least you gave me a formula to calculate...Thank you for that.
Personally, I don't know much about engines and engine-related calculations, so I'll stop here.

Minh-Thanh, the equations are real, the losses incurred by heat absorbed into cylinder walls is very small compared to the heat generated by compression, the problem is the definition of "auto-ignition", that was derived by an experiment that you don't really know the conditions of, that is primarily used as a guide for avoiding home and industrial fires where for example how long it takes to ignite isn't an issue, in a piston engine you have a small number of milli-seconds to ignite, and it really does happen with high compression gasoline engines with low octane fuel and its called "knock". bottom line, you don't actually know what "auto-ignition" means in a detailed technical sense in the context of "220C and 210C auto-ignition temp of diesel and kerosene", so this is a mis-leading bit of out of context information that is not relevant to piston engine design.

 

[The Ignoble Troll]

You can also absolutely get some small gasoline engines to run as diesels in the wrong circumstance.


Once I grabbed a 4 stroke string trimmer from the trash. Tried everything to get it to run, it wouldn't. I eventually tried shooting in some propane through the carb and it started, poorly. It continued to run very bogged down when I took the propane away.


I gave up and took it to bits to see if it was fixable, only thing wrong with it was that the ignition shutoff ground was off its mount and stuck in the grounded position... so Presumably when it ran it was acting as a diesel. No way there was any spark.

 

[Klrskies]

In the 80's, I bought a well-used VW diesel pickup. While driving it down the interstate on my way back home, the engine began running at high rpm without any touching the pedal. I shut the key off to close the fuel solenoid but it had no effect. A run-away diesel.
I held the brake with it in gear and pulled into the emergency lane where it stopped with me continuing to press the brake till it stalled. It's rings were leaking bad enough that engine crankcase oil was being drawn into the combustion space as well as blow-by of combustion gasses leaking past the rings and blowing oil-laden crankcase vapors up a vent hose into the air cleaner, eventually filling the bottom of the air cleaner housing until the oil was overflowing and being drawn into the intake and into the engine and combusted.
That engine didn't seem to be too finicky about the oil it ingested! After it cooled down a while I restarted and drove on home slowly ok. After it's rebuild it was ok. But it did show me that a diesel could run on oil without being injected... just not in a run-away state please!

 

[Nerd1000]

You can also absolutely get some small gasoline engines to run as diesels in the wrong circumstance.


Once I grabbed a 4 stroke string trimmer from the trash. Tried everything to get it to run, it wouldn't. I eventually tried shooting in some propane through the carb and it started, poorly. It continued to run very bogged down when I took the propane away.


I gave up and took it to bits to see if it was fixable, only thing wrong with it was that the ignition shutoff ground was off its mount and stuck in the grounded position... so Presumably when it ran it was acting as a diesel. No way there was any spark.

propane has a wonderfully high octane rating, it's not unheard of to inject it into the manifold of diesels as an auxiliary fuel, in which case it doesn't ignite until the diesel is injected and its ignition sets off the propane. What was probably happening in your trimmer was that some deposits of carbon or other crud in the cylinder were getting hot enough to act as a glow plug, and the engine was running as a (really bad) glow motor.

in terms of the general discussion:

The important thing for diesels and compression ratio is the ignition delay. To get this into a reasonable range for an engine the temperature has to be a lot higher than the autoignition temperature.

What happens when the fuel enters the cylinder of a diesel is surprisingly complicated. First the fuel droplets start to vapourise (each is surrounded by a thin cloud of diffusing fuel), then pre-combustion reactions start to happen in the vapourised fuel. That basically means breaking of the carbon-carbon bonds due to heat and formation of lighter, more volatile molecules, along with reactive free radicals. The radicals can then react with oxygen diffusing into the vapour cloud, producing heat and more radicals, which can also react with other fuel molecules to decompose them further. Eventually enough radicals are formed that there is a runaway chain reaction of more and more fuel molecules decomposing and the products reacting with oxygen, at which point ignition occurs. The remainder of the fuel droplet then gets burned by vapourising, partially decomposing and moving out as vapour to the flame front where oxygen and vapourised/decomposing fuel are mixing.

All chemical reactions are faster at higher temperature, so to get ignition delay low we need to make those vapourisation and pre-combustion reactions happen fast by heating well above the autoignition temperature. If the temperature is too low, the pre-combustion reactions won't get to the 'runaway chain reaction' phase in time and no ignition occurs before the pressure and temperature get too low (during the power stroke) to sustain the reactions.



The above also helps to explain why different fuels have different octane ratings. Basically carbon-carbon single bonds are relatively weak, and break easily initiating the pre-combustion reactions at fairly low temperature, so molecules with lots of carbon-carbon single bonds in a row (like n-heptane) have low autoignition points. By contrast, molecules with few single bonds (e.g. toluene has only one, it has a very strong resonance stabilised ring structure including six of its seven carbons and only a single bond connecting the seventh to the ring) resist decomposition and so the pre-combustion reactions don't start until a higher temperature. Net result: n-heptane's octane rating is a big fat zero. Toluene's is about 120.

 

[The Ignoble Troll]

propane has a wonderfully high octane rating, it's not unheard of to inject it into the manifold of diesels as an auxiliary fuel, in which case it doesn't ignite until the diesel is injected and its ignition sets off the propane. What was probably happening in your trimmer was that some deposits of carbon or other crud in the cylinder were getting hot enough to act as a glow plug, and the engine was running as a (really bad) glow motor.

in terms of the general discussion:

The important thing for diesels and compression ratio is the ignition delay. To get this into a reasonable range for an engine the temperature has to be a lot higher than the autoignition temperature.

What happens when the fuel enters the cylinder of a diesel is surprisingly complicated. First the fuel droplets start to vapourise (each is surrounded by a thin cloud of diffusing fuel), then pre-combustion reactions start to happen in the vapourised fuel. That basically means breaking of the carbon-carbon bonds due to heat and formation of lighter, more volatile molecules, along with reactive free radicals. The radicals can then react with oxygen diffusing into the vapour cloud, producing heat and more radicals, which can also react with other fuel molecules to decompose them further. Eventually enough radicals are formed that there is a runaway chain reaction of more and more fuel molecules decomposing and the products reacting with oxygen, at which point ignition occurs. The remainder of the fuel droplet then gets burned by vapourising, partially decomposing and moving out as vapour to the flame front where oxygen and vapourised/decomposing fuel are mixing.

All chemical reactions are faster at higher temperature, so to get ignition delay low we need to make those vapourisation and pre-combustion reactions happen fast by heating well above the autoignition temperature. If the temperature is too low, the pre-combustion reactions won't get to the 'runaway chain reaction' phase in time and no ignition occurs before the pressure and temperature get too low (during the power stroke) to sustain the reactions.



The above also helps to explain why different fuels have different octane ratings. Basically carbon-carbon single bonds are relatively weak, and break easily initiating the pre-combustion reactions at fairly low temperature, so molecules with lots of carbon-carbon single bonds in a row (like n-heptane) have low autoignition points. By contrast, molecules with few single bonds (e.g. toluene has only one, it has a very strong resonance stabilised ring structure including six of its seven carbons and only a single bond connecting the seventh to the ring) resist decomposition and so the pre-combustion reactions don't start until a higher temperature. Net result: n-heptane's octane rating is a big fat zero. Toluene's is about 120.

That's more then possible.

 

[Jens Eirik Skogstad]

Long stroke engine vs short stroke in same ratio of compression does not have same air temperature due to the fact that gas molecules have more than enough time to collide with each other to heat up in a long-stroke engine than in a short-stroke engine.

 

[Roger B]

Some interesting and useful points in this thread. On that seems to have been missed is the problem of ignition delay and controlled rate of combustion. This is affected by the fuel itself, the atomisation from the injector and the air swirl/turbulence.

Diesels first trial with the injection of petroleum (I translate this as paraffin or kerosene) resulted in an uncontrolled combustion and the destruction or the indicator mounted on the cylinder head. This was the source of the commentary about the engine exploding. Diesel tried many different atomisation systems and finally settled on the air blast to produce the first successful commercial diesel engine. There were many other versions of solid injection developed but it was the Bosch system that became commercially successful.

For the ideal diesel cycle the fuel should ignite almost instantaneously when it is injected into the compressed heated air and continue to burn in a controlled manner during the rest of the injection period. This is fairly easy to obtain in a large low speed marine or stationary engine. These typically have very limited air turbulence and rely on the atomisation from the injector nozzle. For smaller higher speed engines there is much less time available for intimate mixing of the fuel and air and a vigorous amount of swirl/turbulence is required. This was originally achieved using a pre or swirl chamber where the air was compressed through a small passage into a separate chamber with the fuel injector. This required less atomisation from the injector but had greater heat losses resulting in lower thermal efficiency and harder starting.

If some fuel is dropped on a surface heated above the self-ignition temperature it will vaporise and ignite, but probable not quickly enough for an engine.

A number of working model diesels have been made, some may achieve close to the diesel cycle, others may operate in a different cycle where the fuel is injected very early in the compressing stroke so it has time to vaporise before the ignition temperature is reached. It can be difficult to tell but if the injection timing is more than 10- 15° advance it is probably on the second cycle.

My first 20cc two stroke diesel did run but I think it was actually running on the fuel that was blown past the piston.

 

[Roger B]

Describing how to make an injector or injection pump is quite difficult so I don’t think Find Hansen is being difficult. At an amateur level it is not possible to measure and tolerance the various dimensions. I think I am achieving clearances of a few microns on a 2mm bore but can’t confirm that. My judgment of when a pin gauge will just enter a bore may be different to someone else’s. The angle of the poppet/mushroom valve on Hansen's injector is also difficult to measure. I guess that he knows how to set up his machines to achieve a working system but it will be different for others.

I have made some working injectors of this type. The cone angle is important, if it is too acute it will self-lock and if too obtuse the spray will hit the combustion chamber walls. The angle is initially set with the top slide on the lathe but deflections will make the actual angle different. Polishing the cone after hardening will change the angle again. Minh has also made this type of injector and I am sure has his own way of setting the angles etc.

 

[minh-thanh]

Ok, everything is relatively clear.
We confirm that the temperature is theoretical - and in perfect conditions - Everything should be perfect !
In reality, it depends a lot on other factors that we have mentioned, and it is impossible to achieve the calculated temperature.
Why do I want a clear explanation?? Because it's really important: The temperature of the compressed air in the cylinder. - If someone based on our information, designs and builds a diesel engine, after a lot of effort and time.. ..and as a result the whole project will go in the trash.
As I said, I know very few formulas...and I don't have time to learn them, so I usually choose the basic information that I know and personally like to practice : DO IT .

 

[Roger B]

In the 80's, I bought a well-used VW diesel pickup. While driving it down the interstate on my way back home, the engine began running at high rpm without any touching the pedal. I shut the key off to close the fuel solenoid but it had no effect. A run-away diesel.
I held the brake with it in gear and pulled into the emergency lane where it stopped with me continuing to press the brake till it stalled. It's rings were leaking bad enough that engine crankcase oil was being drawn into the combustion space as well as blow-by of combustion gasses leaking past the rings and blowing oil-laden crankcase vapors up a vent hose into the air cleaner, eventually filling the bottom of the air cleaner housing until the oil was overflowing and being drawn into the intake and into the engine and combusted.
That engine didn't seem to be too finicky about the oil it ingested! After it cooled down a while I restarted and drove on home slowly ok. After it's rebuild it was ok. But it did show me that a diesel could run on oil without being injected... just not in a run-away state please!

There was an 18cc carburetted compression ignition moped made some years ago:

https://onlinebicyclemuseum.co.uk/1951-lohmann-18cc-diesel-engine-new-old-stock-unused/

 

[minh-thanh]

I have made some working injectors of this type. The cone angle is important, if it is too acute it will self-lock and if too obtuse the spray will hit the combustion chamber walls. The angle is initially set with the top slide on the lathe but deflections will make the actual angle different. Polishing the cone after hardening will change the angle again. Minh has also made this type of injector and I am sure has his own way of setting the angles etc.

!
I have made quite a few injectors ( 10 or more ) and several different styles: As you said, I cannot determine the exact angle after finishing
The angle is too small and it will get stuck.