[sgiitk]

@R2D2 & co.; afaik almost all diesel locos are actually diesel electric locos, and the traction is by electric motors, so gearboxes are not required. Electric motors have maximum torque at stall.

[Sutripta]

@oxyzen. Could you please post the torque/ rpm values for the engines in tabular format?

Originally Posted by Valkyrie

My two cents:
....
There are however trade offs.

Originally Posted by Valkyrie

@oxyzen:
That's why I said, its a fundamental difference.
....
what I pointed out was a basic difference on account of the definition of torque.

No, not really to a lot of points.
Think in terms of constant displacement.

Originally Posted by Aroy

If you go by the argument that more piston area gives more torque then the "over square" design will have better torque for the same displacement compared to long stroke engine, but most of the torquey engines have long strokes. So may be torque is a function of the pressure, piston area and the duration of the pressure, all factors maximized in slow speed diesels.

Controlled injection as in computerized engines can be used to prolong both the quantum and duration of power generating combustion stroke.

Again, think in terms of constant displacement.

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

Electric motors have maximum torque at stall.

Not all types of electric motors I'm sure.

Regards
Sutripta

[vina]

Originally Posted by sgiitk

@R2D2 & co.; afaik almost all diesel locos are actually diesel electric locos, and the traction is by electric motors, so gearboxes are not required. Electric motors have maximum torque at stall.

I'm sure that not all locos are diesel electric, though practically all of the long haul ones are - their diesel engines are heavily optimised to run at practically a constant rpm and the motor is designed to work over a very wide rpm range (and in case of engine trouble, the motor can get power from a different source and still pull).

I think the small shunting engines you see in yards do have mechanical arrangements - AFAIK (based on some books I read when I was a kid) these do have clutch-gearbox arrangement.


Also electric motors do not have max. torque at stall - not as a rule. Single phase induction motors (almost every single AC motor in a house) for example have no torque at all at stall and need a secondary coil to start them up. Electric motors can also be optimised for a particular rpm (synchronous motors almost always are) OR for a broad range of rpms (including stall).

Originally Posted by Sutripta

...

No, not really to a lot of points.
Think in terms of constant displacement.

Again, think in terms of constant displacement.

Dada, wouldn't constant displacement mean constant torque (assuming same VE and no turbo)?

Originally Posted by Sutripta

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

Regards
Sutripta

I don't know which one, but my guess - diesel cycle?

[vijaycool]

For outright acceleration from the startline - Petrol
For roll on acceleration - Diesel

This is the area where most of the people will get confused especially on highways. If a petrol car want to accelerate from 80 kmph if will certainly take time to move compared to the turbo diesel car even with lower bhp diesels such as figo, swift. This is because at 80~90 speed the rpm for petrol is around 2.5K certainly not in torque range but for diesel honeymoon just started. It will pull cleanly till 120 after that she lose out her breath. Now the game starts for petrol, dot at the max torque and you can see other cars in RVM. I have experienced this with my gtx over highways. I was frustrated initially but later I realised. Post 120 GTX is a beast it will make you scare.

Regards,
Vijay

[GreaseMonk]

Originally Posted by vina

Dada, wouldn't constant displacement mean constant torque (assuming same VE and no turbo)?

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).

[sgiitk]

Originally Posted by vina

I'm sure that not all locos are diesel electric

@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.

[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.

[Sutripta]

Originally Posted by Aroy

All things remaining equal, the diesel engine is more efficient.

Is this the answer to

Originally Posted by Sutripta

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

Regards
Sutripta

[Aroy]

Originally Posted by Sutripta

Is this the answer to


Regards
Sutripta

YES.
This is the answer (though a "YES" would be sufficient, the rules do not permit short posts, hence this addition!)

[sgiitk]

Originally Posted by Aroy

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 recall a discussion I had with an engineer trying to sell us a high speed (ralatively) genset that a low speed one.

His counter arguments:
1. A low speed unit will require more displacement, so the parts will be far heavier. This will lead to more load on the bearings etc. Also, to compete on cost the low speed motor will have to make more design compromises, esp in terms of weight > durability.
2. At a low speed the swirl in the engine may be less, lading to a poorer air-fuel mix.

So I will leave it an open question.

Also, efficiency at the same CR is not a valid point. For ignition a diesel will require a far higher compression. Theory tells us that one can expect a Diesel to be more efficient than a petrol. The higher calorific value of Diesel as compared to Petrol will also help.

[Aroy]

Originally Posted by sgiitk

I recall a discussion I had with an engineer trying to sell us a high speed (ralatively) genset that a low speed one.

His counter arguments:
1. A low speed unit will require more displacement, so the parts will be far heavier. This will lead to more load on the bearings etc. Also, to compete on cost the low speed motor will have to make more design compromises, esp in terms of weight > durability.
2. At a low speed the swirl in the engine may be less, lading to a poorer air-fuel mix.

So I will leave it an open question.

Also, efficiency at the same CR is not a valid point. For ignition a diesel will require a far higher compression. Theory tells us that one can expect a Diesel to be more efficient than a petrol. The higher calorific value of Diesel as compared to Petrol will also help.

I hope I do not get an infraction for answering what is basically

. Low speed engine exchanges torque for speed, hence at a given HP, the low speed engine will have proportionately bigger displacement. Say 1000 HP of 300 vs 3000 RPM would result in the displacement of low speed engine 10 times that of high speed engine.
. Because of its much higher displacement, it will be heavier and proportionately costlier. Larger engine will require a larger installation space and a specialist operator.
. So why buy a much costlier piece of machinery. Because
a) The specific fuel consumption is less than half
b) The longevity is measured in decades verses years
c) Consumable costs are also less than half
Many low speed engines can run with full efficiency on lesser number of cylinders, a trick high speed engines cannot emulate.

Regarding combustion, the low speed engines as stated have much better combustion efficiency, hence they consume much less fuel compared to high speed engines.

some links
http://www.em-ea.org/Guide%20Books/b...t%20System.pdf
Advantages of Diesel Engines
Physics In an Automotive Engine

[SPIKE ARRESTOR]

Originally Posted by Sutripta

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

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).



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

[oxyzen]

Originally Posted by Sutripta

@oxyzen. Could you please post the torque/ rpm values for the engines in tabular format?

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

Originally Posted by vijaycool

For outright acceleration from the startline - Petrol
For roll on acceleration - Diesel

Depends on the power values. If the diesel has same max power than the petrol I guess the petrol will be beaten even in outright acceleration.

[Sutripta]

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.
....
What do you think?

Thanks Spike.
This matches what I also know.

But now one has to reconcile it with another experts forcefully stated view

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.

Over to you, Mr. Roy.

Regards
Sutripta

[sgiitk]

Originally Posted by Aroy

Low speed engine exchanges torque for speed, hence at a given HP, the low speed engine will have proportionately bigger displacement. Say 1000 HP of 300 vs 3000 RPM would result in the displacement of low speed engine 10 times that of high speed engine.
. Because of its much higher displacement, it will be heavier and proportionately costlier. Larger engine will require a larger installation space and a specialist operator.
. So why buy a much costlier piece of machinery. Because
a) The specific fuel consumption is less than half
b) The longevity is measured in decades verses years
c) Consumable costs are also less than half
Many low speed engines can run with full efficiency on lesser number of cylinders, a trick high speed engines cannot emulate.

Regarding combustion, the low speed engines as stated have much better combustion efficiency, hence they consume much less fuel compared to high speed engines.

I have been through the reference. They are essentially +2 level discussions.

a. Wrong since the displacement is higher. Obviously the efficiency cn not go up from the 30 odd percent to 60%!!
b. Yes. But replacing larger bearings, pistons, etc. also cost more.
c. No, since they require more of the fluids etc.

The only point I am in agreement is that low speed engines will be more suitable for continuous duty.

I think we cease discussions here, and go on the PM channel henceforth. I am also inform you that I happen to be a Physicist.

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.