[Sutripta]

Originally Posted by GreaseMonk

Isn't torque also dependant on the stroke of the engine? Increase the stroke, but reduce the bore to maintain constant displacement. A longer stroke implies a longer crank, which implies greater torque at the crank shaft. Although a longer stroke usually implies the rotational speed of the crank shaft is reduced resulting in a lower peak power (if the increase in torque is not sufficient to counter the decrease in max RPM).

Piston area decreases.

Originally Posted by oxyzen

I can't in here.
PM me in case you really need it.

Was thinking of normalising the curves, and then comparing. Forget it, not really important.

Originally Posted by sgiitk

Also, I saw a comment that the the efficiency will be the the same at the same CR.

Same? Interesting! Where?

Regards
Sutripta

[vina]

Originally Posted by sgiitk

@vina; Please read my post again, I said, 'afaik almost all diesel locos are actually diesel electric locos....'. Quite likely the shunting and yard units are not having a DE part as the cost and complication have to be kept low.

You sound annoyed sir. I probably misread what you had written (or may be read someone else's comments, then yours, then something else, then mixed everything up). Sorry about that. In fact in a second reading too, I missed the word "almost"

Originally Posted by Sutripta

...
Piston area decreases.
...

I was going to write that when thinking about longer crank and stroke, don't forget about reduced force. But you beat me to it.

Originally Posted by SPIKE ARRESTOR

Dada, I think the "ideal Otto cycle" will be more efficient than a "ideal diesel cycle" with same compression ratio. I have illustrated the explanation through a comparison of cycle efficiency of both (see attached pic).

Attachment 836253

In the second expression, see the additional factor coming due to cut off ratio. An Otto cycle at higher compression ratio and a diesel cycle at lower compression ratio have their own set of problems (that is a different story altogether).

What do you think?

Spike

I have forgotten all thermo now the charts remind me of the romantic old times of first year of college, but I can't really make out most of the stuff, but I couldn't understand your drawing of the auto cycle. Do feel free to poke holes through what I write below, I would love to understand it from practicing MEs:


1->2 seems to be the compression stroke - ideally nearly adiabatic, but why would P-V be nearly linear? Also assuming similar CR, why would 1->2 be very different on the two plots? Similarly 2-3-4 is (I think) ignition stroke, but why would the slope of 3-4 be similar to that of 1-2 (i.e shouldn't dP be larger between 1-2 than between 3-4?).




Also, while ideal diesel and petrol cycles are understandably what you have plotted (I guess assumption is isochoric, 0 time of ignition in Otto and isobaric ignition in diesel), in reality, wouldn't petrol take time to burn (while the piston is still moving) causing the 2-3 phase to have non-zero dV? Similarly, in a diesel cycle the 2-3 phase should have non-zero dP (add to this the fact the multi-injection can and actually control 2-3). So any practical otto or diesel engine should have the 2-3 looking like an upward sloping line (rather a curve), the difference in efficiency should not be very big for real cycles (correct me on this one - I'm just speculating).

I don't think any practical implementation of either cycle will have any major difference in efficiency even at the same CR, as @sgiitk sir have mentioned in the quote below already (though I think same is not the word to be used, again I barely remember what I read in Thermo 101)

Originally Posted by sgiitk

...

Also, I saw a comment that the the efficiency will be the the same at the same CR. More or less, but unless you have sophisticated direct injection, preignition will get a petrol engine much below the CR required for the lowest rated diesel.

The fact that for practical engines, diesel obtain a higher CR much more easily does mean in practice diesels are more efficient. I think that is why @Aroy was so confident in his assertion that theoretical diesel cycle is more efficient,

he wrote

Originally Posted by Aroy

All things remaining equal, the diesel engine is more efficient. Why? The thermal efficiency; at enhanced combustion temperature coupled with higher operating pressure; in a diesel cycle is higher than in the petrol cycle.

Forcing air at a pressure increase the thermal efficiency further (by allowing more air, hence better utilization of fuel), but by practically the same proportion in both the engines.

Just like forcing air, evacuating air, by creating a negative pressure at the exhaust, also increases the combustion efficiency, primarily by evacuating stale air more thoroughly, thus letting more fresh air to enter and participate in combustion. The higher the engine speed, the more will be the increase in efficiency, as the evacuation decreases with speed.

Low speed engines will be more efficient and last longer, than the high speed engines
. Because there is less friction loss.
. There is more time to complete the full combustion cycle - Fill in Air, inject fuel, combustion and evacuate the spent gasses. Each stage gets more time hence is more thorough.
. There is less wear and tear, as the engine runs less number of revolutions for the same work, hence they last longer.

I think there were several inconsistencies in the above, (e.g. if all things are equal including CR and fuel, diesel should actually have less operating temp. rather than enhanced operating temp.), but I think that should be OK for a discussion here, after all many people like me would have no chance of learning these things unless people discuss freely. And people will not discuss freely if criticized severely for mere enthusiasm.

[Aroy]

Originally Posted by vina

I think there were several inconsistencies in the above, (e.g. if all things are equal including CR and fuel, diesel should actually have less operating temp. rather than enhanced operating temp.), but I think that should be OK for a discussion here, after all many people like me would have no chance of learning these things unless people discuss freely. And people will not discuss freely if criticized severely for mere enthusiasm.

Point noted. Criticism is welcome. I promise not to "Sulk" (or feel offended) if you criticize!

From what I remember of college thermodynamics, the higher the temperature, the better the combustion efficiency. The nominal temperature at which an Internal Combustion engine operates is limited by the material used. There have been a few research engines built (I could not find reference immediately), using ceramic components, which achieved much higher operating temperatures and resultant higher efficiency.

Another point to be noted is that force and its duration will determine the torque. So if there is a large piston area and a prolonged combustion time, we get more torque. Due to its massive size, a slow speed diesel has a large piston area and due to its slow speed enough time to have a long combustion time. This gives it more power and time to burn fuel efficiently, hence better efficiency.

[vina]

Originally Posted by Aroy

Point noted. Criticism is welcome. I promise not to "Sulk" (or feel offended) if you criticize!

From what I remember of college thermodynamics, the higher the temperature, the better the combustion efficiency. The nominal temperature at which an Internal Combustion engine operates is limited by the material used. There have been a few research engines built (I could not find reference immediately), using ceramic components, which achieved much higher operating temperatures and resultant higher efficiency.

Another point to be noted is that force and its duration will determine the torque. So if there is a large piston area and a prolonged combustion time, we get more torque. Due to its massive size, a slow speed diesel has a large piston area and due to its slow speed enough time to have a long combustion time. This gives it more power and time to burn fuel efficiently, hence better efficiency.

carnot cycle efficiency depends on max. and min. temp ratios - this is the theoretical limit for any cycle. Practical cycles will always achieve less, so when comparing two different cycles that should also be considered. Even when considering ideal Carnot cycle - not only the peak temperature obtained but also the exhaust temperatures should be considered (it is the ratio of the two that matters assuming heat is not lost to piston, cylinders etc.), I'm not sure there is guarantee that diesel engine exhausts are always similar temp. as petrol exhaust, let alone cooler.




Your discussion on torque is interesting - but let's discuss it another way that doesn't bring in force/time/stroke - length etc. directly but accounts for all of them together - Work done.

Work done over one complete revolution (or N completed revolutions) is directly proportional to average torque - no matter how much time is taken for the revolution, what the radius of the rotation (and hence crank dimensions) etc. were ... So max. work possible for a revolution (2 revolutions for a 4-stroke) is a direct proxy of torque produced, no matter what kind of engine or fuel we are talking about.

Looked this way, for a given amount of energy released by the fuel (and assuming same efficiency) there will not be difference in torque between two engines. For a given amount of displacement and VE the first factor above depends on the type of fuel to some extent (CNG gives more heat than petrol, which in turn gives more than diesel, for a given amount of O2 available) but bore/stroke and rpm have no role to play (rpm may have a small role in terms of degree of combustion achieved)

Basically for a given fuel and type of engine, the torque depends only on displacement in the first order. Different optimization can then cause second order effects to cause small but sometimes significant differences - I'm not sure about torque, but power is definitely traded with FE in high performance engines. And Jet engines and rockets for example maximize thrust (I guess a proxy for torque - in that application there are no tyres) while sacrificing FE.

[Sutripta]

Originally Posted by vina

I think that should be OK for a discussion here, after all many people like me would have no chance of learning these things unless people discuss freely. And people will not discuss freely if criticized severely for mere enthusiasm.

Hi,
I did not see any 'severe criticism' anywhere on this thread. But you have broached an important topic. I would suggest you use the 1-1 interaction with the Mod team to get (and perhaps appreciate) different viewpoints. Things which really can't be discussed in public.

Enthusiasm, experience, and expertise though sometimes related, are not the same. Synonyms these are not. Oftentimes this distinction is lost, leading to misunderstandings.

Regards
Sutripta

[SPIKE ARRESTOR]

Originally Posted by vina

1->2 seems to be the compression stroke - ideally nearly adiabatic, but why would P-V be nearly linear?

It is not exactly linear, it is a curve.

Also assuming similar CR, why would 1->2 be very different on the two plots? Similarly 2-3-4 is (I think) ignition stroke, but why would the slope of 3-4 be similar to that of 1-2 (i.e shouldn't dP be larger between 1-2 than between 3-4?).

Don't go by my sketch, firstly both are not in the same scale (area between the curve) . Jokes apart, we have first derived the cycle efficiencies and then arrived at a conclusion for same CR, not the other way.

Also, while ideal diesel and petrol cycles are understandably what you have plotted (I guess assumption is isochoric, 0 time of ignition in Otto and isobaric ignition in diesel), in reality, wouldn't petrol take time to burn (while the piston is still moving) causing the 2-3 phase to have non-zero dV? Similarly, in a diesel cycle the 2-3 phase should have non-zero dP (add to this the fact the multi-injection can and actually control 2-3). So any practical otto or diesel engine should have the 2-3 looking like an upward sloping line (rather a curve), the difference in efficiency should not be very big for real cycles (correct me on this one - I'm just speculating).

There are lot of assumptions during mathematical analysis but actual process does not obey them fully. In every process, 1-4 there are deviations like pumping losses, efficiency of heat transfer (addition / rejection), combustion efficiency etc so the graph looks more like a inflated balloon facing North West direction. Have a look at the T-S diagrams also and you will get more insights.

If you are more interested get hold of books by Heywood, Ganesan meant specifically for ICE.

Let the discussions continue.

Spike

[Sutripta]

^^^
Vina, really very very basic stuff. And you have the Heywood!?

Regards
Sutripta

[SPIKE ARRESTOR]

^^ Was that question for me? Yes, I have a copy of the book with me at Bhopal (soft), have you read it? Any other good books you have read and recommend?

Spike

[Sutripta]

^^^
Meant for vina re: your statement

If you are more interested get hold of books by Heywood, Ganesan meant specifically for ICE.

I know vina has a copy of Heywood!

Lost in all this, what I think is the most interesting part of my question is lost!

Between the Otto cycle and diesel cycle, which is more efficient for the same CR? And why?

I don't mean in terms of 'this is the equation. Plug in the values and see what you get', but in terms of feel for the subject.

Regards
Sutripta

[SPIKE ARRESTOR]

Ok, I thought it was intended for me. Yeah, it will be interesting to know from experts which cycle will be more efficient and why. Gas Dynamics + flame (combustion differences) is something which I feel plays a pivotal role among the two.

Spike

[Sutripta]

^^^
No, I still mean theoretical (ideal) conditions.

Regards
Sutripta

[SPIKE ARRESTOR]

^^ One is a constant pressure and the other a constant volume process (ideally), so the heat addition process becomes the differentiating factor, keeping rest others same. Is that what you are looking at?

Spike

[vina]

Originally Posted by Sutripta

Hi,
I did not see any 'severe criticism' anywhere on this thread. But you have broached an important topic. I would suggest you use the 1-1 interaction with the Mod team to get (and perhaps appreciate) different viewpoints. Things which really can't be discussed in public.

Enthusiasm, experience, and expertise though sometimes related, are not the same. Synonyms these are not. Oftentimes this distinction is lost, leading to misunderstandings.

Regards
Sutripta

Dada

I don't know what you are talking about here - either the above is a result of some misunderstanding or it is going to create a misunderstanding.

I talked about severe criticism because of this post http://www.team-bhp.com/forum/techni...ml#post2565891 - I have known sgiitk sir and I know he probably didn't mean to criticise, but how he has framed the post can easily frighten the faint of the heart. In my experience very severe criticism to the face is usually easier to take then even mild disagreement in writing - we must be careful about how our comments will be read.

As for enthusiasm, I referred to Aroy sir's - I don't think I confused that with expertise (and I'm quite sure he has zero experience of thermo).

As for talking 1-1 with moderators both you and I have had some experience with some moderators. The less said about that the better.

Originally Posted by SPIKE ARRESTOR

^^ Was that question for me? Yes, I have a copy of the book with me at Bhopal (soft), have you read it? Any other good books you have read and recommend?

Spike

he was writing about me - I have a copy, and he knows .

Unfortunately it is very unlikely I'm going to get much time any time soon - there are 12hour shifts at work (equally interesting stuff, and that brings in the bread and butter too) and my 9 month old daughter makes demands on whatever time is left. Gone are the days when in-laws could take care of her.

Originally Posted by SPIKE ARRESTOR

^^ One is a constant pressure and the other a constant volume process (ideally), so the heat addition process becomes the differentiating factor, keeping rest others same. Is that what you are looking at?

Spike

Well, he is poking me there I think, I'm also quite certain he knows the answer

Well, I don't but let me make a guess Otto should be more efficient - In the diagrams SPIKE had made 1-2-3-4-1 cycles, the 1-2-3-4 parts do not really lose energy (or lose it to work) the 4-1 does lose energy. The inefficiency will be the ratio of dP.V in 4-1 to dP.V (for otto) or P.dV (for diesel) between 2-3.

Trying to draw the cycle such that 4-1 loss is fixed, starting (P,V) at 1 is the same and assuming adiabatic 1-2 and 3-4 (i.e. both hyperbolic) can be accomplished this way - first draw ideal diesel cycle with 1-2-3-4-1. Now take the line 4-3 and extend it (assuming adiabatic process) till a point 3' such that 3' is directly above 2 (i.e. same volume as at 2, but at a higher pressure). This mean 3'-3-4 is the new adiabatic line.

Now 1-2-3-4-1 is the diesel cycle with energy loss of 4-1, and 1-2-3'-(3)-4-1 is the otto cycle.


To find which one is more efficient all we have to find is for which one we added more heat (both wasted the same amount over the entire cycle). The answer to this should be obvious - between 2 and 3 on both graphs none of the cycles has lost heat (except to useful work), but Otto cycle has clearly done more work (higher average pressure for the same change in volume) - add to this the fact that at points 2 and 3 the working fluid had same state and this means between 2-3 Otto had added more heat than diesel added in that time (or it couldn't have done more work between 2-3).

Since for the same amount of heat lost, and same compression ratio, same working fluid etc. Otto added more heat, it was less inefficient i.e. more efficient.



How well did I do without Heywood?

[Aroy]

Originally Posted by vina

As for enthusiasm, I referred to Aroy sir's - I don't think I confused that with expertise (and I'm quite sure he has zero experience of thermo).

You are spot on. I have a lot of enthusiasm, but very little practical experience in thermodynamics (except writing code for CFD). Still I had developed an insatiable curiosity of internal combustion engines and read a lot (though most went above the head) in my college days. Some of the best and lucid books were authored by Russians. Unfortunately I lost most of my books to termites fifteen years ago. Now I collect soft copies only.

Google is a wonderful tool. Here are some relevant articles

Internal Combustion Engine Cycles - Thermodynamics - Engineering Reference with Worked Examples
1903 Engine Thermodynamic Analysis - Otto Cycle

[Sutripta]

Originally Posted by vina

Dada

I don't know what you are talking about here - either the above is a result of some misunderstanding or it is going to create a misunderstanding.
.....
As for talking 1-1 with moderators both you and I have had some experience with some moderators. The less said about that the better.

Some people get the hackles up of other people. Sometimes intentionally. Sometimes without knowing it.
It might be better if one knows why.

Re: moderators:- Its a hive mind. Believe me, at the individual level, they are just another member.

Regards
Sutripta