diesel engine flame propagation

Google "how flame propagation in diesel engine takes place"

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D'oh! RTFMA, or check the children's section of your local library for the wonderful set of books "How Things Work". Got my copy at age 9, passed it on to a nephew when he turned 9, he passed it on to my older son when he turned 9. Sheesh

The reason you are getting unsympathetic answers is that the lads who replied probably didn't realize you are an electronics engineer with very little knowledge of mechanical engineering.

Diesel engines are the simplest form of internal combustion engine.

They work by creating heat when the pistons squeeze the air in the engine cylinders to a high pressure during their upward travel movement in the cylinder towards the top, called the cylinder head.

Just before the piston reaches the top of its movement in the cylinder, the diesel fuel is sprayed by injectors into the chamber that is created by the space between the top of the piston and the cylinder head; and due to the ambient heat this fine spray of fuel --fine so that it quickly becomes the evaporated gas that actually does the igniting, since the liquid fuel itself does not ignite-- burns rapidly (does not explode like gasoline might), increasing the heat and air pressure in that small space, and assisting the piston to return down the cylinder with a pushy blast of hot air.

The hot air is prevented from getting past the piston by a series of rings fitted into the piston head in an alternate fashion. These piston rings brush the walls of the cylinder with a close tolerance measured in thousandths of an inch and are further assisted in their purpose of keeping the hot air on top the cylinder head by a wash of engine oil down the sides of the cylinder that lubricates the piston and its rings as it travels up and down in the cylinder.

Note that the ignition heat is caused solely by pressure. There is no igniter or spark plug that causes the burn in a diesel engine to commence. Even a glow plug's purpose is to preheat the cylinder, not to ignite the fuel.

As the piston continues in its downward plunge, the hinged rod connected to its bottom, called the piston rod, pushes a crankshaft; and the crankshaft's resulting rotating motion is used to supply power to some ultimate purpose, like spinning a generator or wheels or a propeller, and suchlike; often through a gearing system. The torque applied to the crankshaft as it does its job is often very high, so it has to be constructed of very tough materials.

The downward-moving piston also passes a hole in the side of the cylinder, and the hot gases rush out of the hole. This is the engine exhaust; and it is gathered together in the same way with all the cyliners' exhaust in a manifold and expelled away from the engine through the engine muffler and into its exhaust pipe. The piston head then passes another hole in the side of the cylinder where it picks up fresh air for a new compression run and subsequent ignition of the fuel up near the top of the piston's travel in its cylinder.

Just like in a gasoline engine, the pistons are positioned for efficiency so that they take turns being pushed in an order that best suits the number of cylinders in the engine. Pistons are not pushed just one after the other in a row. The order is staggered and the crankshaft is especially made and shaped to match the order of firing.

To get the process started, it is obviously necessary to assist the engine to turn over a few times. This is done using compressed air and/or preheating the cylinders with a glow plug, while employing a starting engine run on another energy source.

The amount of fuel injected, the geometric shape of the spray mist, the type of fuel used, the timing of the injection to achieve the most complete burn by taking the greatest advantage of the size, shape, and pressure in the small ignition space in the top of the cylinder, the accurate or faulty positioning of the piston rings, the load on the engine, the amount of air it gets and the ease with which it expells its exhaust are all parameters that determine how well and how smoothly the engine runs. If the engine is knocking, the fuel is not being injected into the cyliners at the same point or most efficient point which is generally calculated at being around 15^o before Top Dead Center (TDC) [think of the rotation of the crankshaft] of the cylinder positioning for all cylinders, and the injectors may need to be adjusted/cleaned; or the amount of pressure exerted by the burn is not affecting all cylinders in the same fashion, so the rings may need to be replaced or the piston head or rod or the cylinder head or cylinder cleaned, or gasketing or a piston rod might need to be replaced, etc., etc.

If the fuel is injected too soon or too late, or too much or too little, the amount of completeness of the burn will be affected; and in addition to a non-effective amount of 'push', the exhaust may show a white or black smoke instead of little-to-none at all because of the more or less useful condition of the 'flame' in the cylinder.

The kilojoule energy production potential of the fuel is important to the type of engine being used, as are the eventual products of combustion of the various fuels and lubricants in the determination of the exhaust disposal methodology and recycling techniques employed as the engine is actually running. Diesel engines range from very small things to immense multi-cylindered monstrosities where just one cylinder might be the size of your bedroom. In the former, mainly because of the small parts, it is generally more advantageous to use a more highly refined diesel fuel; and quite often in the latter, the engine's components will make its own diesel fuel on the spot out of crude oil, called 'bunker fuel', through various stages of refining and heating the fuel to an appropriate cleanliness and viscosity before injection. Fuel ratings are to let the operator of the engine know which fuel is best for which engine, and the various fuels themselves are sometimes fine-tuned by the addition of (usually) petro-chemicals called 'additives'.

Some areas of mechanical engineering are dedicated to the design, operation, maintenance, repair, metallurgy, function, mathematical construction formulae, efficiency/thermodynamic calculations, loading and varieties of use, and different kinds of end uses motors and engines and their peripheral operating equipment produce.

Hope this helps; and when the mechanical guys ask you about the use of a zener diode or some such, I hope you aren't too tempted make fun of them.

Mark

Although I agree with most of your comments, they may not help our friend as it would appear that at one point at least, you have described a model Diesel engine running as a 2 stroke (are their large 2 stroke diesels around???).

These model engines actually pass the fuel/air mixture in a different manner from the crankcase up into the cylinder and compress the mixture there without usually using external injection systems though. I refer to your comment:-

The downward-moving piston also passes a hole in the side of the cylinder, and the hot gases rush out of the hole.

Surely most "normal" Diesel engines are 4 stroke and use valves.......? Please enlighten me on this subject, I am very interested in learning something new.

You also mentioned:-

Even a glow plug's purpose is to preheat the cylinder, not to ignite the fuel.

Which seems to differ from what I have read in many textbooks......eg. the glowplug is primarily designed to help with ignition of the diesel when starting and the engine is cold.

It is also not big enough to heat the cylinder head directly, though of course some heat transfer takes place, but the design of the glow plugs that I have seen show a unit that has a long nose that is placed in the fuel mixture and is only connected mechanically to the head, usually via a threaded hole....

Surely if the idea was to heat the head directly (as in some other systems, but they are not called Glowplugs, see below)) it would be preferable to have the hot part, physically lying against the head metal in some manner....

Some engines retain the glow plugs glowing until the cooling water has reached a certain minimum temperature. I know from personal experience that when the outside temperature gets below -25°C, when the timer turns off of the glow plug voltage, the engine of the Mitsubishi Diesel I had years ago would tick over very roughly at that point or even stop. Restarting the engine, also started the timer again and it then ran smoothly!!! Usually it managed to warm itself up enough by then to continue running at that point.

This demonstrated quite clearly to me that the glow plug was instrumental in firing the fuel air mixture when very low temperatures were experienced.

A Diesel heater before the filter was also a requirement in the very cold winters that we used to get.

I refer to Wikipedia with regard to these comments, please look at the following link:-

http://en.wikipedia.org/wiki/Diesel_engine

The following is taken directly from Wikipedia link:-

In cold weather, diesel engines can be difficult to start because the cold metal of the cylinder block and cylinder head draw out the heat created in the cylinder during the compression stroke, thus preventing ignition. Some diesel engines use small electric heaters called glow plugs inside the cylinder to help ignite fuel when starting. Some even use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Engine block heaters (electric resistive heaters in the engine block) connected to the utility grid are often used when an engine is turned off for extended periods (more than an hour) in cold weather to reduce startup time and engine wear.

I am very interested in learning more about these differences you mention, thanks for your time and trouble in this matter....

diesel engine to run requires high compression ( around 16:1) to ignite diesel fuel , 2 stroke cycle can not achieve such high ratios and also nature of engine that is different from gasoline where spark plugs are there to ignite the mixture(pre mixture) , in 2 stroke there is constant unburned fuel circulating within the engine cycle( due to displacement error ie offset of inlet and outlet ratio)

What has that got to do with it?

Did you read what Markthehandyman wrote? That is what I was replying to.

By the way, they may not be quite so popular today, but diesel 2 strokes have been around in the model market since before WW II.....of course there are Diesels that run in 2 stroke mode......!!!

If you're looking for 2-stroke cycle diesels, I suggest you check this out. www.bath.ac.uk/~ccsshb/12cyl/

Nice! That looks like a very reliable monster.

Nice one!

ozz , its great

Hello Andy , I appolize for that stuff , i really was un aware untill i checked with wiki and others , that giant machine is really great, my profession runs within electronics & computers but little interest lies with mechanics and automobiles also , presently i am working(playing) with vehicles for efficiency , pollution control , hybrid , alternative fuel and so.

"Although I agree with most of your comments, they may not help our friend as it would appear that at one point at least, you have described a model Diesel engine running as a 2 stroke..."

Get off it, Andy!!

The purpose of the blog was to give our friend a really basic explanation of what's going on, not to get into specifics. I gave him that. I thought about including details and decided against it, as they didn't answer his needs. Go get picky with somebody else.

Mark

I don't see it as being "Picky" as most small (under 5 liter capacity) and a lot of larger ones too, are 4 stroke not 2 stroke......

I personally feel that we need to be as accurate and objective as possible when posting on a Blog, except when making some sort of joke of course.....otherwise we may mislead somebody for no reason whatsoever....I am sure that was not your intention either....

Have a great day.

OK, Andy, I retract the picky comment, since you don't see yourself in its frame. I apologize.

Here's this guy/gal who knows nothing at all about diesels. The basics were presented. Very basic, and not incorrect, but certainly not at an engineeringly debatable level. For a mechanical engineer, these things might have been basic, but to start introducing valving and strokes and different glow plugs, etc. would have been be over-information and not have filled the bloke's needs, do you see?

Remember the blog by Vermin on broadcast wave propagation where he had one single electron wiggling around on the antenna? My blog did not approach his brilliant simplicity, but the intention was --really obviously, Andy-- the same.

Mark

I see your point completely, because I have a good idea how diesels work (though I knew little about 2 strokes with a single valve, there I learnt something too!), but someone who hasn't got a clue, should be told exactly and then we wait for them to ask further questions about the bits they didn't understand......

I was a computer specialist for many, many years and part of my job was training people on the equipment that I specialized on. I had learnt IT in the RN, as everyone has to learn it, a good thing too, and I found that you need to give it to them the information at a level that may go over the heads of say 10-20% of the class and let them ask further questions. Pitching it too simple not only bored most of them, but it lost me the class as well.....

Like physical fitness, you had to keep pushing the morsel of knowledge so that it was just outside of their metaphorical reach, to keep them awake and actively participating.....

Teaching was a lot of fun for me and I never regretted the time I spent doing it both in the RN and later in the computer business.....

Mind you, I was teaching pros, you would not catch me in front of a class of teenagers in a normal school today, far too dangerous today!!!

Andy

"...someone who hasn't got a clue, should be told exactly and then we wait for them to ask further questions about the bits they didn't understand..."

I'm not sure I'm in complete agreement with this approach. The approach from the other end, which is my personal preference, is to give someone only the basic information, and if they want more, they'll ask for it. Same end result, two different approaches. I think that too much information can be confusing for someone who hasn't got the basics. That's why I got so excited when I saw how Vermin responded to the blogger who wanted to know about RF propagation.

A variation of my approach works well with teaching skills relating to the arts (music, graphic arts, creative writing, mathematics, physical exercise and games) to school students as well, where they are given nothing, only the opportunity and guidance with which to invent the basics of something they are acquainted with in the most general sense for themselves. The inventing process teaches them to become experts on the most fundamental levels; and makes them hungry to advance and not only speak to pros to learn the way the pros do it, but to learn it with a sympathetic understanding of its complexities. Pros learning new stuff from other, older pros, so to speak.

For the initial blogger in this thread, certain questions were asked, to which I limited my responses more or less directly, and with truly only the most basic information. Because he/she's an electronics engineer, I suspect he/she's following these threads and if he/she's interested, has picked up a lot more already from you and others on the subject...which would have been more understandable, hopefully, after those basic responses were supplied; although nobody thus far has said what single stroke and two-stroke actually refers to; and how IC valving actually works to affect scavenging (whatever that is! ) and exhaust.

Perhaps you'd like to supply a part two to the inquiry, taking it to the next level...but not TOO far technically! LOL

Mark

Oops! Forgot to mention 4-stroke. I just know somebody's going to catch me out on that.

Mark

In a way I would like to do that, no prob there Professor, but as the original poster has taken so little interest in this blog, it hardly seems worth the trouble really, especially if you are correct and he is already "over informed" so to say and totally "blown away" so to say.

If I am right, he will come back and ask further questions......

Wait up!

a lot of larger ones too, are 4 stroke not 2 stroke......

Andy, I am afraid that that statement is incorrect. ALL large diesel engines operate on the 2 stroke cycle. All container ships, tankers, LNG carriers, etc,etc., are all powered by 2 stroke diesel engines........so let us not mislead anyone!!!!

Research was done on this at Penn State in the eighty's by Pinson. Check their archive of PhD dissertations.

Thousands of "Detroit Diesel" engines were manufactured as 2 stroke and have been plying the US roads for years. I wonder if this style of engine is still widely in use in the transportation industry in view of efficiency and emission guidelines.

Electro Motive still offers 2 or 4 stroke engines on their locomotives.

www.emdiesels.com

Just out of interest, which are the "modern" or "later" versions?

I believe EMD originally ran 2 cycles from the 1930's to 1990's. In the late 1980's they began to develop a 4 cycle to replace the 2 cycle. However, things did not exactly work as planned because both are offered. As I understand things the 710 cu. in. 2 cycle has been improve to meet EPA emissions requirements.

Both are currently in production. I do not know which has higher market appeal. Check there website www.emdiesels.com.

Personally, of all the diesels (though I am not a Diesel mechanic by trade!!)I have worked on from 25MW DGs and cars, small boats & yachts etc. (no Lorries or trucks!!) I have never seen a larger than model aircraft 2 stroke Diesel personally, all the rest were 4 stroke!

I was amazed at the size of the marine Diesel that someone posted yesterday I believe, simply fantastic!

I have really learnt something quite new for me and I thank you all for it!

As to whether they are more economical or not than 4 stroke Diesels I cannot say personally, but as the car market is always seeking to make diesels more and more economical, but has not gone to 2 stroke (yet!), I may hesitantly suggest that probably not, at least not in the 1 to 5 liter capacity engine sizes that dominate cars....

Using the infos supplied on CR4, maybe the a 2 stroke diesel is only better in much larger motors? Trucks and upwards?

The good point from 2 strokes is generally the fact that they are lighter in weight for the same HP as a 4 stroke, less complicated etc.....does that also play a role here?

Many thanks for such a Post that spawned so much interesting infos, I thank you all most kindly!

Just an interesting side point. Saab distributed 2 stroke gasoline engine powered passenger cars at least into the 1960,s in the US. I don't know how long they continued making them after that. One thing I learned from building racing motorcycle 2 stroke engines is that their dynamic scavenging/refill process made for an inefficient and dirty (by comparison) engine that was powerful and light weight. Of coarse the overlap can be decreased at the expense of power but less overlap tends to improve efficiency and decrease emissions. There were plenty of unique improvements (anybody remember the Yamaha RZ 350?) but the partial mixing of the exhaust and incoming charge is still an issue. At least in a diesel the incoming air doesn't get the fuel introduced until later by the injectors.

I should also add that some (maybe most) 2 stroke Detroit Diesel engines did not rely on crankcase scavengering like most gasoline 2 strokes. They had valves in the head and superchargers and yet still were 2 (not 4) stroke. There power cycle is weird and wonderful.

Just to add confusion, the modern passenger car diesel has a proportion of exhaust gas mixed into the inlet to improve emmissions (mainly?) on part throttle, at a helluvan increase in expense and complication - it has to be cooled for a start off.

Which automobile diesel engine uses the exhaust gas recirculation you mention in your recent post?

It was a feature of the Ford diesel range I was working on a few years back. Many of their units are bought from/common to Peugeot these days but I was of the belief that it is common practice.

http://www.carpages.co.uk/audi/audi-q7-16-09-06.asp was about the first I came across (which is a bit extreem!).

or http://delphi.com/manufacturers/cv/powertrain/egr/ for a someone in the market

Very interesting development especially since most diesel engine designs want to cram as much cool, oxygen laden air as possible into the cylinders. To have some of that air replaced with warm exhaust gasses is a remarkable development.

Only for part load of course (where most road vehicle's power plants spend most of their time). I think it is one of these things that has come about as a result of digital control technology coming of age/cost. Seems incongruous to me, now you have diesels with throttle bodies (or egr metering bodies perhaps more aptly)!

I did not read the full description of that monster. It looks as though the block is machined from a single billet of aluminum. Am I seeing that correctly?

So I see it says cast. WOW. I went to a shop here in tx once where they were machining a bearing race that was about 12 feet in diameter. It boggles my mind to think of mfg a part this size.