Talk:Diesel cycle

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Can anybody add links to see images of internal structure and working of diesel cycle and diesel engine ???. An image is more usefull than 1.000 words. Thanks in advance.

If the diesel cycle is "ideally suited" to aviation, why have I never heard of a modern aircraft using it?

Two reasons, First until recently there were no new engines certified for aviation use and almost all engines at the end of WW2 was Otto engines. Secondly, a Diesel engine is heawier than an Otto engine so the lower fuel consumption only works if total weight of engine and fuel is lower for the length of flight, which means fairly long flights. There was about a 50/50 split at the beginning of WW2 with Germans leaning more towards Diesel.
Today there are new Diesel engines certified (I think) and also new Otto engines, thank good. The fuel burn of the engines certified 10 years ago was horrible, about doubble that of an new car engine of the same power. Seniorsag 18:42, 27 February 2007 (UTC)

Some of the facts need to be checked out on this article. The Diesel cycle is MORE efficent than the Otto cycle. What kind of retards are writing this stuff.

Don't know if it is "ideally suited", but diesel engines are indeed used in aviation today. Mostly as replacement engines, but new planes can be fitted with a diesel, too. For examples, see: [[1]], [[2]],[[3]],[[4]],[[5]]. SGJ 11:02, 26 September 2005 (UTC)

Is this article really needed as a seperate piece to Diesel engine? It dupliates many of the facts of that other article. The other article is much more comprehensive and clearer. Perhaps merge the two? Kcordina 13:48, 7 December 2005 (UTC)

Contents

[edit] Not entirely wrong, just needs formatting

I think the fact that "the first trans-atlantic crossing" of a diesel powered aircraft just happened in 2004 clearly demonstrates why diesels are seldom, if ever used in aviation:

Due to higher compression ratios, most of the engine has to be able to withstand increased pressure, and temperatures. Cast iron is cheap, compared to aluminum, or newer (but not widely used either) magnesium alloys. It weighs alot. So in order to have a workable diesel for an aircraft, it has to be constructed out of specific, high strength, high temperature strength, and low weight alloys. This is prohibitively expensive.

I wouldn't call whoever wrote this a retard. The diesel cycle IS more efficient than the Otto cycle, but only when the engine is scaled down from its maximum rated power. I have a 90 hp Otto cycle engine in my car. When I need 45 hp, for say freeway travel, it does not use 1/2 of the gasoline that it would when it is producing 90 hp. It is more around 3/4. Otto engines do not scale their power well, because they always need a minimum amount of fuel so that it does not pre-combust. Diesel cycle engines can inject as little fuel as they want, since they use 'knock' to ingnite the fuel anyway. So they can scale their power alot more efficiently.

A little bit of information: The reason Diesel engins are of lower power than Otto engines in the same car is that if a Diesel engine runs for a long time at low power the temperature drops so low that there is a lot of soot produced in the cylinder and that can case engine sizing.
In Sweden with cold weather when lorrydrivers stop for a snooze with idling engine it is quite common that the engine is wrecked. (quite common means often enough to be a small problem, not as big as the driver falling asleep driving but still) Seniorsag 18:24, 27 February 2007 (UTC)

[edit] Is "Knock" the correct term to use?

Is Knock the corrert term to use in regards to a diesel engine? I'd consider it more of a preignition type of event, but since there are no sparkplugs it isn't really the best term. In a diesel engine the compression, at roughly TDC, of the air and fuel ignites the fuel-air charge, if this were to happen in a gasoline engine it would  be considered preignition. Knock is when unburned fuel-air mixture ignites after the piston is moving downward and the pressure wave slams into it.
Knock is the term for when the combustion goes from a flamefront (Explosion) to detonation ( usully caused by pressure but sometimes by heat radiation).
Explosion = flamefront burning, ie less than soundspeed propagation of flamefront. (Black powder)
Detonation = flamefront propagates at orfaster than soundspeed giving a shockwawe. (TNT)
shockwawe from detonation has a higher breaking force than the same pressure rise from explosion. Seniorsag 18:31, 27 February 2007 (UTC)

[edit] Knock

Knock is the sound, detonation is the process. In detonation some fraction of the fuel/air mixture goes off at once almost instantaneously. Detonation may or may not be audible. In a gas engine, normal combustion takes something on the order of a millisecond, which is much, much slower. Tobyw 14:01, 8 March 2006 (UTC)

24.224.214.173

[edit] DIESEL ENGINES DON'T HAVE 'KNOCK'

Simply because the fuel is only injected at the point where it's supposed to burn. Hot air alone doesn't ignite. cbraga 18:43, 27 December 2005 (UTC)

[edit] Oh yes they do

Diesel engines "knock" when fuel is injected before there is enough heat to ignite it. Fuel/air accumulates in the chamber until there is enough heat for ignition, then the mixture present explodes, rather than burning progressively as in normal combustion. My recollection is that fuel is injected over 20 degrees or more of crankshaft rotation (for engines using mechanical pumps - pump pistons having roughly sinusoidal motion). Fuel delivery rates are probably due to the need to limit pressures and temperature plus the physical limits of pumping the fuel into the CC. It seems that the stereotypical diesel knock is due to the engines being timed for maximum efficiency. I once timed a replacement pump way too retarded and the engine, which started with great difficulty, was as silent as a gas engine, but smoked terribly due to unburned fuel. A cloud the size of a house, as I recall! Engines with dirty injectors can also knock more than normal due to the larger droplets of fuel being harder to ignite initially, then going off later when there is enough heat and turbulence. Modern diesels are transitioning to electronically controlled injection which doesn't have the pressure and timing limitations of mechanical pumps. They are almost silent, and more odorless.

There are fascinating high speed movies of the gasoline combustion process done in the 1950's which show various forms of preignition and detonation. I think they are on the site for the man who pioneered ultra-high high speed photography - you know the ones, bullets going through apples, milk splashes, and so forth. Can't recall his name right now. Tobyw 13:36, 8 March 2006 (UTC)

[edit] Otto/Diesel cycle and diesel engine page errors

There are serious factual and conceptual errors on the following pages:

-Otto Cycle

-Diesel Cycle

-Diesel Engine ( I will add statements to the discussion there)

The Otto and Diesel cycles describe idealized thermodynamic air-standard cycles. The details of the real-world application of these cycles in internal combustion piston engines are discussed, but the theoretical ideal processes - particularly with regard to the heat release profile, the main trait that differentiates the otto and diesel cycles - are not described.

A quick peak at a thermodynamics textbook (I'm currently looking at Van Wylen and Sonntag, "Fundamentals of Classical Thermodynamics," 3rd ed.) reveals the following:

-Otto and Diesel cycles do not specify 2 or 4 stroke. The heat rejection at the end of both cycles is simply understood (on a P-V diagram) to take place at constant volume, i.e. any exhaust and intake process is assumed to require zero work and therefore does not need to be described in detail in the theoretical heat rejection step.

-Both (ideal) cycles include adiabatic compression and expansion; real piston engines proceed via polytropic compression/expansion, involving some heat transfer in each step.

-The theoretical ideal Otto cycle specifies constant-volume heat addition at TDC. In a real gasoline engine, it's not truly constant volume, but it's pretty close.

-The theoretical ideal diesel cycle specifies constant-pressure heat addition, beginning at TDC and proceeding through the early portion of the expansion stroke. In real diesel engines, combustion starts before TDC and is mostly complete soon after TDC.

-for a given compression ratio and heat addition, yes, the ideal Otto cycle theoretically is more efficient than the ideal Diesel cycle. In reality, a diesel engine has a much higher compression ratio than a gasoline spark-ignited engine; even theory will show this produces higher efficiency. A gasoline engine is also throttled, so that at part-load, the intake stroke requires a non-neglible amount of work, reducing efficiency. A diesel also always operates with a lean mixture, which keeps peak combustion temps down, reducing heat loss to the cylinder and head, and so will be more efficient, even at comparable power outputs.

Oh Yes! should: BUT since when there is some unused air you can get more ower by injecting to much fuel to compleatly use the air it is quite common that the driver (probably) removes the stop on the injection pump so it can overinject. Result a smoking engine and plenty of unused fuel giving bad economy. The one paying usually tries to satop it and in some countries it is illegal but it happens even so.
Also, awhen injecting fuel you at first have wey rich mixture (100%) and it goes to lean during evaporation and combustion. that is also the reason that you can run a diesel engine at low power and high efficiency since there is autoignition and the fuel passes trough carburated. Seniorsag 18:05, 27 February 2007 (UTC)

-In a gasoline SI engine, knock is indeed influenced by A/F ratio, but knock is not what limits it. With late timing and/or lower compression ratio, the A/F/ ratio can be lowered - but the mixture must always be rich enough to sustain a continuous flame front throughout the entire combustion chamber. The "lean limit" of an engine describes an arbitrary point at which there are an unacceptable number of misfires and partial combustion events. WIth careful attention to mixture preparation and spark plug properties, gasoline engines have been run with very low cycle-to-cycle variability at A/F ratios as high as 22.

-Combustion is incomplete in both gasoline and diesel engines, but not to any significant degree. In a healthy gasoline or diesel engine, the combustion efficiency is typically in excess of 98 percent (see Heywood, "Internal Combustion Engine Fundamentals," F3.9). It is misleading to differentiate the diesel and gasoline engines on this basis, and is probably not appropriate to include this information on a page that is supposed to be primarily describing the idealized thermodynamic cycle.

Joe Frickin Friday 13:58, 27 January 2006 (UTC)




== Headline text ==EYE ON IT

i have read your(joe's)explanation for diesel cycle and diesel engines. but its written that "in diesel cycle fuel burns at a much faster rate than in otto cycle" now what's that? from where has the fuel ignition come in? As these cycles are graphical so everything concerned should be clarified according to the graphs. in the article a term "otto cycle engines" and "diesel cycle engine" has been mentioned what's this? do they mean real engines? one thingmore is it true that diesel engines inject fuel in hot compressed air so proper fuel combustion takes place? is the fuel consumption more efficient that in SI engines? you said that sometimes in diesel engines fuel is injected earlier, so it gets collectes in hte chember and then knocking ocuurs. when does it really happen?(is it that it only happenes due to wrong injection time of the pump or when its timing has been disturbed).


AMAN(Mechanical Engineer)

[edit] Basic error..

Quote that the Otto cycle generates more power than a diesel cycle because ir revs faster - this is correct. It is not correct therefore to say that less fuel has to be carried. Diesel engines are inherently more efficient as they extract more heat from the combusted fuel due to a higher compression ratio - a diesel cycle will therefore require less fuel than a petrol engine for a given amount of work delivered.

[edit] The Diesel Cycle is not more efficient.

"Diesel engines are inherently more efficient as they extract more heat from the combusted fuel due to a higher compression ratio - - a diesel cycle will therefore require less fuel than a petrol engine for a given amount of work delivered."

Again, we are seeing the confusion of terminology. "Diesel engine" and "Diesel cycle" are two different things. A real Diesel engine is more efficient than a real gasoline, spark-ignited engine due to higher compression ratio and other real-world issues such as throttling and heat rejection. the Diesel cycle is an idealized air-standard thermodynamic cycle, and for a given compression ratio and power output is less efficient than the Otto cycle. The difference between the two idealized cycles is in the heat addition: for the Diesel cycle the heat addition is assumed to take place at constant pressure (a model of the slow combustion in a real Diesel engine), and for the Otto cycle the heat addition is assumed to take place at constant volume (a model of the relatively rapid combustion in a pre-mixed spark-ignited engine).

For full-load operation, a real Diesel engine operated at a reduced compression ratio (on a very high cetane # fuel) will in fact have lower efficiency than a real spark-ignited engine operated at that same compression ratio using a high-octane fuel (such as methanol).

The article badly needs a rewrite to remove its focus from real Diesel engines and place it on the ideal thermodynamic cycle.

Joe Frickin Friday 16:00, 28 February 2006 (UTC)

True for full load, but diesel engine really wins on partial load since it can rum idling on almost no fuel. Otto engine needs a carburetted mixture to burn. Otto engine can only lower power output by 1: Lover operating speed. 2: Leaning out fuel mixture until it burns badly (with rising proportion of unburnt fuel). 3: (Usual method in cars) Choking inflow of air, giving rise to loss of power in the choking and lower efficiency trough lower working pressure. Seniorsag 13:19, 20 January 2007 (UTC)

[edit] Efficiency

Some typical and maximum efficiency levels would be useful. Tobyw 12:36, 8 March 2006 (UTC)

An Ideal Diesel Cycle would be isobaric, meaning it would have a constant pressure at the end of the compression stroke before the heat addition process (combustion). However in real systems, the pressure is not really constant, just as during the compression stroke the process is not isentropic. However, during diesel engine analysis many times it is assumed isobaric, in order to resemble a thermodynamically ideal cycle. (Andres Duarte- Aerospace Engineering Student - University of Florida)


[edit] Dubious assertion re constant pressure

"constant pressure"? I don't think so. Paul Beardsell 14:06, 28 September 2006 (UTC)

This is a theoretical cycle. In this model, while the fuel is burning the pressure is constant. Read the above comments. But, I think it was poor wording that made you make this dubious assertaion claim so I removed it after trying to reword. If you disagree with the wording, please just change it. If you disagree with the centeral claim, please add a comment below. 70.17.92.51 20:23, 24 January 2007 (UTC)

[edit] comment on the two previous sections

According to the original patent and Rudolf Diesels ideas the COMBUSTION should be under constant pressure, IE inject the fuel at such a rate that the pressure remained constant.

Practice is not quite such BUT the semi-constant pressure means that you avoid the pressure spike in Otto engines and can thus use higher average pressure during the working stroke. The high pressure also means that usually the fuel autoignites so you have copmpleate combustion even at very low fuel/air ratios at low power. Seniorsag 18:15, 27 February 2007 (UTC)

[edit] New graph!

I've converted the graph to SVG (vector graphics) but I'm not sure it's ok. I am not sure the two curves to be arcs... while they are on the new version. Could someone please validate it? Thank you.

Note the new vector graphics scales better with resolution. Also, I've opted for a dark yellow / orange-ish color rather than pure yellow.
Unluckly, Wiki's SVG renderer (or my browser's I dont know) does not seem to support correct arrows at the end of the axis...
Image:DieselCycle PV.svg —The preceding unsigned comment was added by MaxDZ8 (talk • contribs) 13:05, 11 April 2007 (UTC).

This graph is excellent, the only problem is that it should be pressure (P) over specific volume (v) for that shape of diesel cycle. Specific volume is denoted by a small v, and means volume/weight e.g. m3/kg. Lkleinjans 13:15, 21 May 2007 (UTC)

Ok, I'll change all the 'V' to 'v'. It looked uppercase in the actual version. Anyway, I compared the curvatures and I observed a difference wrt the "original" on the page. Should I correct this as well or it's just ok anyway? The "new" version uses two circles, the previous one didn't use two circles. One is, the other is ... some kind of curve I'm not sure about.
MaxDZ8 talk 20:33, 21 May 2007 (UTC)

(Uploaded new version to the commons, MaxDZ8 talk 17:40, 22 May 2007 (UTC))

(Uploaded new version to the commons. Special thanks to Lkleinjans for the feedback!) MaxDZ8 talk 17:20, 23 May 2007 (UTC)

[edit] Reason for hiding text

Could I ask why so much text is hidden (only visible in edit mode) within the article? Shouldn't it just be removed or shown? Lkleinjans 16:04, 3 June 2007 (UTC)

[edit] Cut off ratio

Is the cut of ratio V3/V2? Can someone verify this? Lkleinjans 10:05, 6 June 2007 (UTC)

[edit] Octane rating of diesel

I remove the claim that diesel allows higher compression ratio because it doesn't self-ignite as easily. That's backwards--diesel is designed to self ignite easily and would have a very low octane rating. (http://www.madsci.org/posts/archives/2000-10/973012182.Ch.r.html). The diesel engine doesn't have knock problems because the fuel isn't injected until it's time. I'd explain more about that in the article but it belongs in the engine article not here.Ccrrccrr 02:00, 19 October 2007 (UTC)

Yeah, Octane rating is irrelevant in Diesels. The fuel is injected directly into the cylinder during the power stroke and self ignites near instantly. The fuel is supposed to self ignite and burn off as quickly as possible while being injected into the cylinder.--129.69.25.217 (talk) 18:01, 15 January 2008 (UTC)

A friend of mine told me there's a quality measure involving the easyness to self-ignite. Just the opposite of the Octane. I don't remember how this is called (Detanes?), maybe it's worth a quick note.
MaxDZ8 talk 08:57, 16 January 2008 (UTC)

Cetane rating is what you are looking for. Some should link it from this article, and maybe the engine article too.Ccrrccrr (talk) 13:34, 16 January 2008 (UTC)

[edit] Section General information is unnecessary

The section General information provides no relevant information on this topic and is misleading, because of that it could be deleted with no harm. Llewelyn MT (talk) 18:33, 14 April 2008 (UTC)