Talk:Diesel cycle

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: [], [],[],[],[]. 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)

Not entirely wrong, just needs formatting
TOP DEAD CENTRE  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)

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)

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

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)

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)

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)

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.

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)

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)

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)

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)

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...

—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)

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)

Cut off ratio
Is the cut of ratio V3/V2? Can someone verify this? Lkleinjans 10:05, 6 June 2007 (UTC) Answer:- If your compression process on the p-V diagram( 'p' on x-axis) is represented by process 1-2 then definitely the cut off ratio is v3/v2.... — Preceding unsigned comment added by 115.242.110.194 (talk) 16:39, 13 August 2014 (UTC)

Pressure
Everyone knows that diesels are more efficient because the pressure ratio is higher. Is it worth noting specifically that the T3 is higher than the Otto cycle (due to ideal gas law based upon higher pressure ratio)? Most people know Diesels run colder. As far as I can tell, this is somewhat of a misconception. The actual thermodynamic cycle is hotter in a diesel. Coolant temperature may be lower due to more efficient cooling system (required to keep engine block colder due higher loads and lower material strength at high temperatures) and more efficient use of the fuel. Is this correct?

Also, is it worth noting here that intercoolers are in inefficient process (because heat is rejected to the atmosphere), but their loss is made up for due to an increase in power density of the engine?

Is it worth putting in some actual pressure and temperature values (such as those listed on Diesel_engine) to give the reader some real world numbers to put in perspective?

Metric america (talk) 05:09, 19 January 2009 (UTC)

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)

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)

??? diesel engine properties ???
???""The piston is already moving down by the time combustion begins. !!! [citation needed] Fuel injection can be limited to a short part, or continue to near the bottom of the power stroke  !!! [citation needed]. If combustion is incomplete when the piston reaches the bottom of its stroke, smoke is generated and fuel is wasted. [vague] [citation needed]

Diesel engines are nevertheless more efficient than Otto cycle engines overall, but only during partial load with fuel cut-off at part of the power stroke. !!!  [citation needed]""  ???

anybody agrees with the above?

Wdl1961 (talk) 02:51, 23 January 2009 (UTC)

Upper case and lower case
Why does the article say, that pressure is symbolized by upper-case P and specific volume by lower-case v, while on the picture it is vice versa? Which is right? Or does it mean that the letter-case does not matter? Thanks. Jan.Kamenicek (talk) 09:26, 6 September 2009 (UTC)

The Diesel cycle article is plain wrong
The article starts “The Diesel cycle is the thermodynamic cycle which approximates the pressure and volume of the combustion chamber of the Diesel engine, invented by Rudolph Diesel in 1897.” This statement is plain wrong.

The “Diesel Cycle” as described has no useful correspondence with any real “Diesel Engine”. Of course, real engines never correspond exactly with these theoretical thermodynamic models. However, the Otto Cycle p/v diagram is generally accepted as a usefully good explanation of a petrol (gasoline) engine.

If there is a real engine that corresponds to the “Diesel Cycle” p/v diagram, its nothing like any real diesel engine. In fact the theoretical p/v diagram that best matches a Diesel Engine is exactly the same overall shape as the Otto Cycle p/v diagram. What is different in the diagrams are only the typical values in the p-axis.

How do I justify these assertions? First, the mechanics of a 4-stroke petrol engine and a 4-stroke diesel engine are geometrically similar (in fact when Volkswagen introduced their first diesel in the 1970’s, they just adapted their 1600 cc petrol engine, mainly by increasing the compression ratio and of course fitting fuel injection). Second, consider the process of the combustion of the fuel. In both engines it happens around about (but not exactly) the point where the piston is near the top of the cylinder (TDC). Around this point the piston is moving relatively slowly, and therefore the volume contained within the cylinder is changing relatively slowly. In both cases (whether or not “knock” occurs) the burning of the fuel is fast compared with the speed of piston or volume change. What is different, and the nub of Rudolf Diesel’s invention, is the ignition of the fuel. In a petrol engine the fuel/air is mixed before it is fully compressed and though the temperature of the mixture increases as it is compressed, the compression ratio is such that it does not reach the temperature where the fuel ignites spontaneously. In contrast, in a diesel engine the air without fuel is compressed to such a pressure, and hence temperature, that as soon as the fuel is injected, it ignites and burns. In both cases the burning adds heat to the gaseous mixture and it being confined to a nearly fixed volume, dictates that it’s pressure increases. This is an isometric process as correctly stated for the “Otto Cycle”. Putting it another way, there is no mechanical feature of a normal diesel engine that corresponds with a isometric heat addition, or the horizontal red line between points 2 and 3 on the diagram.

So where has this “Diesel Cycle” as described come from if it has nothing to do with the engines?

Martinharryking (talk) 18:19, 20 January 2010 (UTC)


 * This article (and petrol is worse) as it doesn't clearly state the significant difference in an understandable manner. Ask around, how many people (mechanics included) can actually state this and get it right?
 * However your point here is simply wrong. The Diesel engine is an engine with a Diesel cycle. By definition, and by practice too. The curves aren't as square-cornered as those idealised Carnots in the textbooks, but it is the fundamental difference: constant volume during combustion (i.e. Otto cycle, from a short "explosion") vs constant pressure (i.e. Diesel, from sustained injection of fuel and a longer burn). If you look at a UK military vehicle engine of the 1960s (the great era of multifuel piston engines) you'll see a mixture of fuels, sparks, injection and scavenging or supercharging, but it's this fundamental difference of continued burning whilst the piston is already moving downwards to expand the combustion volume (and thus approximating the Diesel cycle's constnat pressure cycle) that distinguishes them. Andy Dingley (talk) 18:59, 20 January 2010 (UTC)

Try looking at e.g. http://www.scipub.org/fulltext/ajas/ajas55540-547.pdf figures 6, 7, and 8 show combustion chamber pressures peaking at 70-80 bar around TDC. Given typical diesel compression ratios of the order of 20:1 to point 2 in the p/v diagram how do you account for these if its not point 3 being more like Otto than "Diesel Cycle". Martinharryking (talk) 20:16, 20 January 2010 (UTC)


 * "there is no mechanical feature of a normal diesel engine that corresponds with a isometric heat addition, or the horizontal red line between points 2 and 3 on the diagram."
 * There is. It's fuel injection, past the point of ignition.
 * You seem to be advocating here a renaming of "Diesel engine" as "compression ignition engine". Now that's not a bad point, diesel engines usually are (that's why I noted the '60s multi-fuels) and also what a "diesel engine" has become is certainly more fluid than rigid adherence to the same cycles followed by the original slow-speed heavy-oil engines familiar to Rudolf Diesel himself. However this article is on the Diesel cycle, and that surely hasn't changed.
 * By all means clarify the relationship (in the diesel engine article) between the Diesel cycle, the early Diesel oil engines, and the later high-speed diesel engines - the article certainly needs clarification. As to your point, which we might state in a manner so that we can both agree it, "Modern high-speed diesel engines approximate the Otto cycle", then not only would I not argue too much, but there's even precedent for just that statement from the uber-reference of the lot, Ricardo:
 * "Modern developments in high-speed Diesel engines have led to a wide departure from the slow-speed and a much closer approach to the petrol engine, not only in mechanical design but also as regards the thermodynamic cycle on which they operate. That is to say, the idea of working on a constant-pressure cycle, never strictly adhered to even in the slow-speed engine, has in the high-speed engine gone entirely by the board, and instead a cycle is used which more nearly resembles the constant-volume cycle of the petrol engine."
 * That said though, that's applicable to the high-speed diesel engine (i.e., everything mobile or post-war). It's not true of Diesel's original heavy oil engines, nor is it true of this article, on the Diesel cycle. Andy Dingley (talk) 22:01, 20 January 2010 (UTC)
 * That said though, that's applicable to the high-speed diesel engine (i.e., everything mobile or post-war). It's not true of Diesel's original heavy oil engines, nor is it true of this article, on the Diesel cycle. Andy Dingley (talk) 22:01, 20 January 2010 (UTC)


 * The original diesel with air injection is more like the theoretical model with a more controlled combustion.  It is educational and  is not perfect.Wdl1961 (talk) 23:03, 20 January 2010 (UTC)
 * I don't understand that either. Air blast injection is one thing, but it doesn't (AFAIK) affect the shape of the cycle by a noticeable amount? Andy Dingley (talk) 00:20, 23 January 2010 (UTC)


 * So what's to do?
 * Yes, there's a lack of clarity here. How about we do something like this:


 * Diesel cycle is unchanged. It's right. Not commonly used, but the cycle isn't wrong.
 * diesel engine becomes a BOLD redirect to the canonical article under Compression ignition engine.
 * Compression ignition engine becomes the main article, moved from the old diesel engine. It expands to describe "high-speed diesel engines"
 * Homogeneous charge compression ignition (where the Compression ignition engine redir points at the moment) expands to describe Ackroyd cycle, hot bulb engines etc.
 * Heavy oil engine tries to cover early diesel engines, when they really approximated the Diesel cycle
 * Thoughts? Andy Dingley (talk) 00:29, 23 January 2010 (UTC)
 * Separately, we then make two stroke engine and four stroke engine as lightweight as possible, splitting most of their content into four articles: 2 petrol, 2 diesel, 4 petrol, 4 diesel.
 * I have no idea how to handle the politics of petrol engine vs. gasoline engine. Also Otto engine should probably move most Otto content to Otto cycle, and the bulk of the small detail to the relevant engine article. 00:29, 23 January 2010 (UTC)

PV curves are wrong surely?
Hi. I can't see why between points 2 and 3, there is no rise in pressure. Surely the whole point is to release heat energy to increase pressure, which will then drop during the power stroke, and then continue to drop through the exhaust stroke. The PV graph on the 4 Stroke page, looks more realistic.PJHB (talk) 09:19, 13 June 2018 (UTC)


 * It's a Diesel cycle. That should have been explained in the article and although correct to state this, the article might not be making it as clear as necessary.
 * This is the Diesel cycle, not the cycle used by diesel engines. It's idealised, so has sharp corners and level lines which the practical cycle might not. Also the modern high-speed diesel engine (since 1930) deliberately uses a different cycle, more of a combined cycle between Otto and Diesel.
 * The theoretical, and original, Diesel cycle uses combustion at constant pressure. I don't like the diagram  here, the section 2–3 should be longer and cover a wider range of volumes. Achieving such a thing in practice is (of course) harder, and typically the section  2–3 is only constant pressure for part of it, then starts to fall away slightly. If you can find a copy of the (rare and hugely expensive) Ricardo's 1922 The Internal Combustion Engine. Vol I: Slow-Speed Engines (1st ed.), there's the best discussion of this you'll find in English.
 * Diesel followed on from the gas engine and the Otto engine. He knew that the gas engine (without compression) was limited in efficiency, both thermodynamically and in power-to-weight. He also knew that the Otto engine (the rapid, constant-volume combustion engine) would be the choice for lightweight engines on cars, but that it needed a complex petrol fuel in order to permit such a rapid combustion in a controlled fashion. Diesel wanted to invent an efficient engine as a generating set (etc.) for factories and ships, running on a cheap slow-burning fuel.  His decision was to burn this fuel at a constant pressure, thus simplifying the combustion chemistry.  This slow burn would take place whilst the piston was travelling down the cylinder and expanding the volume, and the rate of combustion would be controlled by the precisely controlled rate of fuel injection, into an excess of air, such that the pressure remained constant.  Any drop in pressure from expansion would (ideally) be compensated for exactly by the combustion of more fuel. Point 3 (and how late in the cycle it can be) is limited by the piston expansion after this that is still required to have an efficient isentropic expansion before reach point 4, thus leaving the exhaust gases with minimal residual energy.  As I said, the diagram might be clearer (and more realistic) if the distance from 2–3 was increased. There is no separate "ignition phase" and "power stroke" on a diesel, they happen simultaneously. Andy Dingley (talk) 09:40, 13 June 2018 (UTC)
 * Some past discussion at User_talk:Yiba Andy Dingley (talk) 11:16, 13 June 2018 (UTC)
 * I am new to trying to help on an article. I have read many of the comments here.  I have a few  things to say.  To me, the article on the Diesel cycle is in a terrible state.  It might be better to start over with something different.  The Diesel cycle is of tremendous historical importance.  My understanding is that Diesel made engine design into a science.  That said, currently, there are no real engines that use the Diesel cycle.  Diesel engines follow the principles of the dual cycle--which is a "marriage" of two concepts--the Otto cycle (heat at constant volume) and the Diesel cycle (heat at constant pressure).
 * As I said, I am very new at this. I think there is a need for a clear discussion on the thermodynamic cycles of internal combustion engines. ErnsDesk (talk) 02:06, 10 December 2023 (UTC)