Fuel Burning

Efficiency is determined by the difference between the high temperature reservoir (combustion) and the low temperature reservoir (exhaust). With diesel, the compression ratio is =>20 to 1 where with gasoline it is about 8.5 to 1. This is necessary to burn lower grade fuel and efficiently extract most of the available energy. Ignition is determined by the fuel injection timing. If the fuel were in the cylinder during compression, it would pre-ignite due the higher heat from the higher compression ratio (compared to gasoline engines). In a jet engine you have essentially the same scheme except that compression and combustion are continuous.

Answer by rcapper:

1. If the fuel (diesel) is in cylinder during compression it would preignite due to higher compression.

Question: Then why adopt higher compression?. Adopt a lower compression (below preignition point) and then spark ignite as for gasoline.

2. In jets what fuel is used, diesel or kerosene?

The pre-ignition is not a direct result of compression but the heat that results from compression. This is basic thermodynamics. The reason for the higher temperature is to efficiently burn a fuel that has a higher flash point, that is, is less volatile yet has a high energy content. Diesel fuel would not burn adequately or at all at the lower temperatures in a gasoline engine spark or no spark. A diesel engine will run on "just about anything" within limits. Check out bio-diesel for example.

Fuel oil, heating oil, jet fuel, kerosene to the best of my understanding just various grades of the same thing. Gasoline is mostly benzene with other components the composition and proportion of which is mixed to be appropriate to the season in which it is supplied. Winter gas is different from summer gas.

The diesel is not in the cylinder during the compression stroke. It is squirted into the cylinder through an injector which atomises the diesel, during the last few degrees of crank shaft rotation before the piston reaches top dead centre. The temperature of the air in the cylinder has been raised to a temperature above the ignition temperature of diesel so it starts to burn very rapidly.

The efficiency of an engine is associated with the compression ratio. The higher the ratio the more efficient the cycle. For the same power output a diesel would use less than half the energy of a petrol engine. Climate warming and all that.

Aviation jets use aviation grade kerosene. Land based jets use diesel, Natural Gas almost anything which can be compressed or pumped at high pressure and atomised through an injector or is a gas already.

hate to decompress everyone; but modern diesel engines(car especially) have compression reduction for starting ease/on those cold winter days too!. and "gloplugs, not "sparkplugs(1200 degrees in 60 seconds makes that injected fuel go "bang!" much faster an cleaner!). somebody needs to checkout the newer GM diesel V8's. and if fuel cmpny's come out with fuel that burns cleaner(by EPA standards) new "D model cars" would be getting 30-40mpg, intown/openroad,an a lot cheeper too!. as for jets; kerosene burns with a bigger "flame", hotter too, making it the fuel of choice. fun note: world war II aircraft engines combined "high octane fuel injection and "gloplug ignition",for more power and higher altitude turbo charged combat operations". yours truly, "chuck finn" Love jus' a "midnight engineer"(no phd needed)

Discussing the second answer from Mr rcapper.

1.Preignition is a result of heat. Correct?

The heat is a result of compression. Correct?

Then why not 'preignition is a result of compression'?

2. What is preignition?

Ignition/burning .. before what?

Pre-ignition is the result of incorrect fuel timing.

To answer your questions in the order they appear:

1. No. Pre-ignition is not the result of heat. Admittedly it would not occur if the air in the cylinder was not hot enough to ignite the fuel but it is the timing which is all important.
2. Yes. Heat is the result of compression.
3. The spark in a petrol engine or the fuel being injected in a diesel engine is timed so that the fuel ignites in time to generate the highest pressure in the cylinder just as the crank has passed through top dead centre and is therefore available to push the piston down on the power stroke. The time from injection to peak pressure is almost constant so timing of the spark or injection varies with crank rotational speed. The faster the crank rotates the earlier the spark or injection. If the ignition occurs too early the peak pressure occures before the crank is at TDC and acts to try and stop the rotation of the crank. In a petrol engine it can be heard and in the UK we call it "pinking" or "pinging" because that is what it sounds like. That is pre-ignition. Ignition before the correct angular position of the crank.

Second answer by Mr. Gasman:

1. Yes. pre-ignition is not the result of heat.

2.Pinging/pinking is also called knocking, also called 'detonation' (technical term)

Preignition is also called 'dieselling' , also called 'run on'.

You say preignition is same as pinging. Is it?

Refer 'Answers.com' and go into 'engine knocking'. There it is said as different, explained nicely. I also feel so.

Pre-ignition, or dieseling is not as common as it once was. At one time gasoline engines had a higher compression ratio, as high as 10:1 or more which required higher octane fuel. We used to get fuel that was 100 octane because they used tetra-ethel-lead to reduce the tendancy to pre-ignition. Pre-ignition occurs when the fuel air mixture is in the cylinder and reaches the flash point before the point of optimum timing so yes it is because of the heat of compression. Whet else would ingite it before the sparK, magic? It is from HEAT. If diesels did not use injection timing the point of ignition would vary and would also occur earlier than optimum. Your diesel would "diesel" ha-ha or ping or pink whatever.

Pre-ignition is the result of heat, in the sense that it cannot occur without heat. Preignition is entirely different than incorrect timing, and as the name suggests, means that, in a spark ignition engine, the mixture pre ignites -- in other words ignites before the spark "tells" it to. It can be caused by things such as a glowing piece of carbon in the combustion chamber, severe overheating, etc. It can lead to detonation, but it is not at all the same thing.

Detonation can occur in a perfectly timed well-maintained engine if the flame front travels too quickly, with the increase in pressure and temperature causing the rest of the mixture to explode with a bang (in other words, all at once) rather than progressively along the flame front. (Using low octane fuel would be a cause in a well maintained engine.) Most current otto (gasoline) engines are intended to run right at the threshold of detonation (pinging) and most have knock sensors to detect that threshold. Older engines were intolerant of pinging (knock, detonation) and the explosions and peak temperatures could blow wholes through pistons. In older highly- tuned racing engines, just a few seconds of intense pinging was enough to destroy the engine. Octane (or equivalent) is used to reduce detonation. In many modern engines low octane is not as destructive as it used to be, because the engine simply adjusts timing and mixture to compensate -- usually meaning slightly lower horsepower. (On the other hand, if the engine does not require high octane, then using high octane has no benificial effect.)

In both diesel and spark engines the fuel should burn smoothly. In a diesel, this is accomplished by smoothly injecting the fuel into a very hot, high pressure combustion chamber. Only the fuel that is there at any instant can burn. In spark ignition, the flame front proceeds from the spark plug, like a miniature and extremely fast forest fire. The temperature from this fire causes the pressure to rise dramatically, (which further increases temp) so if the fire is started too soon (pre-ignition or ignition timing set too early) you can see that the whole remaining mixture could get to the point at which it explodes.

Thus, early ignition timing can cause detonation. Preignition (which works as early unpredictable ignition timing) can also cause detonation, as can modifying an engine to increase its compression ratio.

So, for your original question, why not start diesel ignition with a spark: it's not necessary -- simply injecting the fuel into a hot, high pressure environment is enough. If the cylinder in a diesel were filled with mixture, and then compressed, the whole mixture would go off with a bang and destroy the engine. If you increase compression ratio enough on a gas engine you convert it into a crude diesel, in which preignition and detonation occur simultaneously -- and only the sturdiest of gasoline engines tolerate this for long. Thus 12:1 compression is about as high as you can go in a spark ignition engine.

Preignition in a gasoline engine can be caused by either incorrect spark timeing or latent heat. The latent heat usally stored in carbon buildup on the valves. The carbon deposit on the valves is not cooled via the coolant circulating in the engine block and head.

Guest NO. 1:

Latent heat? For me that is a new one...that I hope Guest No. 1 will further elaborate. Carbon (or metal or foreign object) presence or deposition is usually the cause of overcompression as a result of diminished cylinder capacity--the reduction being the volume displaced by the deposits.

Another cause, in otto engines, of detonation/ping can be a faulty HEGO or EGR function. During PTCruise (i.e., all but cold enrichment (start/idle) and WOT operation), the recirculation of unburned fuel (mixed with fresh fuel) is required to prevent too-hot combustion. This cooling also is necessary to reduce NOX emissions (a product of too-hot/mixture-misregulated fuel burning), and prevent premature destruction of catalytic converters.

Guest No. 2

1. Still nobody including rcapper is differentiating preignition and pinging/pinking/detonation.

Are both the same or different?

2. What is overcompression? Does it occur in piston engines?

3. Answer from rcapper:

(i) Preignition is result of heat of (a) compression or (b) combustiom or (ci) both?

(ii) the mixture can be ignited by (a) a carbon deposit on the wall of cylinder (b) an engine part e.g. a metal part of spark plug, which has reached the temperature of ignition point; definitely not by 'magic'.

Good grief. Pre-ignition, let's first establish the term. Pre, in other words, before the normal timing event, that is the spark in a gasoline engine. It is only possible when there is fuel in the cylinder during compression. This only occurs in a gasoline engine.

What causes pre-ignition? Heat. You must reach the flash point of the fuel air mixture. The flash point is a function of temperature and it is indicated in degrees and it is known for various substances in various states. What causes heat? I'm not sure about the latent heat in carbon buildup, or the consequent increase in compression there from, it seems plausible but I never heard that before.

Compression causes the temperature of the gases in the cylinder to rise. Look in your first year physics book under thermodynamics. The higher the compression the more heat. That is why when they banned the gasoline additive tetra-ethel-lead engine compression typically went down in production vehicles. Older cars that run on unleaded gas, especially higher performance engines (>8.5:1 compression ratio, have a tendency to experience pre-ignition, also known as knock, ping, pink whatever. The lower octane unleaded fuel has a lower flash point. In fact, if I don't use premium in my Lexus, when I suddenly accelerate I can hear the valves rattle a bit before the ECM can adequately adjust the ignition timing. Valve rattle, there's another term for you. Not as severe as ping which is not as severe as knock. They all mean the same just different levels of badness.

Pre-ignition can result in what was called dieseling but only in carbureted gasoline engines. When you shut of the ignition, especially when the engine was hot especially in the summer time, it would keep running, also called run-on. In this mode it is acting sort of like a diesel except the timing is ambiguous. In a diesel, ignition is timed by when the fuel is injected so diesels don't experience pre-ignition although both diesels and gasoline engines may have the timing set improperly (too early) which is not technically pre-ignition but just a maintenance issue. But yes, too early timing can cause ping or knock but the cause of the too early ignition is not the fuel pre-igniting but being ignited to early. Is this getting too confusing?

Gasoline engines with fuel injection do not experience run-on or dieseling because when you shut off the engine the fuel injectors stop operating and there is no fuel to ignite. Carbureted engines use a fuel pump. If it were a mechanical fuel pump that operated off the engine cam then the engine could run indefinitely. I saw this in a parking lot on an unattended vehicle once and it was not pretty. Carbureted engines with an electric fuel pump have enough gas in the float valve chamber to run for a while but will quit once this ounce or so of fuel is consumed.

I never heard of over-compression but referring back to an earlier comment I suppose it could occur with adequate carbon build-up but it sounds iffy. The compression ratio of an engine is fixed by the dimensional relationships between the piston, piston chamber, the head and the crank and is in all vehicles I know of invariant.

I should probably qualify my use of the term "the valves rattle" which is more a description of "what it sort of sounds like" but in fact it is a noise you get when the timing is not optimum and more of a mild ping or maybe that's what a pink is but I would describe it more as a tink than a pink.

Just realized I wrote much of this same stuff above, but later...

I would quibble with your using the terms pre-ignition and knock or ping interchangeably as in "experience pre-ignition, also known as knock, ping, pink whatever." Pre-ignition is entirely different than knock, ping, and detonation, which are synonyms. See my post above.

Valve rattle is not really valve rattle, in the sense that it has nothing to do with the valves. It is a confusing misnomer. Mild (knock,ping,detonation) is mild because it occurs later in the combustion process (there is less fuel left to explode) and actually further from the valves, which have long since been firmly against their seats. (There is valve ticking, clatter, rattle, etc from tappet clearance -- but that is an entirely different matter.)

Over-compression is not a common term, but carbon buildup can, in fact, increase compression ratio, in the same way that milling a head by just a millimeter can increase compression meaningfully, while at the same time increasing squish area and turbulence. Carbon buildup has the double effect of serving as an insulator, so that mixture temperature will be higher earlier (promoting possible detonation). A third effect of carbon buildup is the likelyhood that edges of this buildup can start to glow, causing pre-ignition. But while preignition and detonation share causes, they are entirely different. One means explosion (detonation); one means simply that ignition has occurred too early by some cause other than spark timing.

detailed answer to the question was lost due to a flaw in site programming design. Sorry.

Discussing the last answer of Mr rcapper:

1. Para 1: (a) So, preignition is ignition of air-gasoline mixture before spark ignites. Correct? And the preignition is due to heat of compression. Correct?

Compression occurs once in 4 strokes in a 4 stroke engine. Then if preignition is due to compression preignition must occur once in 4 strokes. Correct? Does it? Then you wont need a spark for any engine? Remember compression ratio, so heat released in every stroke, is a constant in a given engine.

2. Is preignition same or different from 'self/auto-ignition?

3. 3rd para: ... have a tendency to experience preignition, also known as knock..

No, preignition is not the same as knock. Both are different. Go into the website I have given earlier. If you can't differentiate then I will.

4. 'Flash point' is a temperature. It is the lowest temperature at which.... Then how can it be a function of temperature?

5. 4th para. Last sentence. 'too early timing' , 'igniting too early' ; surely comfusing.

6. Are there gasoline engines with 'fuel injector system of burning' ?

1. Ignition of the fuel must be timed correctly for efficient operation. Not sure what you mean by "no spark" but spontaneous combustion due to pressure/heat is variable cannot be controlled precisely. Diesels do it by injecting the fuel into compressed hot air that is above the flashpoint at a precisely controlled point in the piston travel.

2. Yes or no depending on what you mean by self/auto-ignition.

3. Couldn't find your reference to website. Maybe you mean knock from loose wrist pins or crank bearings but again you could certainly use the term to describe lots of sounds and I don't know that there is only one correct definition.

4. OK flash point is a "function" of the composition of the fuel, the pressure, the temperature and the ratio of fuel to air mixture and not merely heat. Picky.

5. Pre-ignition: spontaneous combustion of fuel before ignition spark occurs. Igniting too early: Timing is not correct, fuel is ignited by spark but spark occurs at wrong time due to mis-adjustment or system malfunction, two ways to have ignition too early in the travel of the piston into the power stroke. When there is ignition too far before TDC (top dead center) then the engine makes noise because it is compressing against the burning fuel air mixture, regardless of why it has occurred.

6. I don't know what you mean by 'fuel injector system of burning'. There are fuel injected gasoline engines. Some inject into the cylinder some ahead of the valves I believe they call that port injection. I'm not sure of the exact timing of cylinder injection but port injection must occur on the intake stroke. If you used the injection of the fuel to initiate ignition in a gas engine then you would have a diesel that used gas and why they don't do that is beyond the scope of my knowledge on the subject but it probably is not advantageous or they would.

Are we done yet?

1. I am not talking of efficient operation.

2. Now we are discussing whether 'preigniton and knocking are the same or different?

You say they are same? But they are different.

3. And now: 'spontaneous combustion'. Is it same as self/auto ignition? What are they?

4. Are ignition and combustion same or different?

5. I DONT mean anything for 'self/auto ignition; scientists have meant/defined it precisely.

6. Still no answer for: why dont we prepare an air-diesel micture, compress it and spark ignite it as for gasoline?

1. One essential difference between spark ignition engines and compression ignition engines, in practice, is that compression ignition engines must be fuel injected, so that the flame front propagates smoothly (although noisily, as you have no doubt observed.) The fuel is always injected into the cylinder after the compression stroke is almost finished (the intake valve has long since closed). The mixture can be called heterogenous. In a spark ignition engine the mixture is homogeneous, and is inducted into the cylinder in that state (approximately -- it actually continues to homoginize for awhile). If you ignore efficiency entirely, then a spark ignition engine can be made to "diesel" or "run on". Also, you could create an engine that runs on diesel fuel that could be ignited by a spark. You would not call it a Diesel engine however, because compression ignition is an essential part of the definition of same. Consider that both engine types will run quite well on natural gas. It's not the fuel that distinguishes one from the other, its the ignition method: spark in one case, compression in the other. Further, for the one that uses compression, it must also use fuel injection, because a homogeneous mixture would go off with a bang.

2. Preignition and Knocking are entirely different. See the Bosch automotive handbook. Causes of preignition: too hot a spark plug range, glowing carbon, glowing sharp edge in the combustion chamber, overheated engine causing autoignition (which is not synonymous with either preignition or detonation), etc. Detonation = bang.

3. The term "spontaneous combustion" is almost never used in conjunction with engines. If it were, it would not be the same as pre-ignition, which can be triggered by installing too hot a spark plug. Autoignition, per Bosch: "the mixture is ignited when the ignition temperature is achieved or exceeded during the compression of the mixture" (bolding mine). In a diesel, the ignition temperature is present, due to compression, before there is a mixture. So one could say that diesels do not, strictly speaking, ignite their mixture through (Bosch's) autoignition. But most engineers would use the term, as in "autoginition temperature" to describe diesel engine ignition. Clearly, they don't ignite the mixture via pre-ignition.

4. Ignition and combustion are quite different in the automotive world. Ignition is the event that begins combustion. They are of course intertwined. Ignition too early will generally cause combustion difficulties later on, such as detonation.

5. I'm getting lost: if I were to respond to rc's number 5, I'd say that noise is not caused by "compressing against the burning fuel air mixture". At high rpm, the ignition occurs well before top dead center, and the flame propagates without causing unusual noise. If there is an unusual noise, it is from detonation (explosion) which may be caused by two early ignition timing, but is not the same thing as too early ignition timing.

6. Because the octane rating of diesel fuel is too low for efficient operation of a homogeneous mixture engine. The compression ratio would have to be very low (5:1 to 6:1, I'd guess) which would translate to very low efficiency. Also cold weather starting would be tough, because the flash point (the real flash point -- not the imaginary use of the term RC came up with) of diesel fuel is 60 degrees c: you'd have to crank a cold engine for quite a while before you could get the mixture to ignite by spark.

Wikipedia has a pretty simple summary of some essential differences in the two fuels. I lifted it, and pasted it below. It is from http://en.wikipedia.org/wiki/Flash_point

Petrol (gasoline) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature.

Diesel is designed for use in a high-compression engine. Air is compressed until it has been heated above the autoignition temperature of diesel; then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have a high flash point and a low autoignition temperature.

• Petrol:
• Flash point: >-45 °C
• Autoignition temperature: 246 °C
• Diesel:
• Flash point: >62 °C
• Autoignition temperature: 210 °C

I think we've beat this to death.

Thanks Ken. I had to throw up my hands, it got way beyond the original question and it is clearly outside my field of expertise.

the reason desiel fuel is used in compression ignition engine due to its octane number , the petrolium fuels are classified based on the octane number.

Regards

Moin