Fundamental explanation for different torque curve SI-CI 3

[Xplode]

Diesel engines reach their maximum torque at lower engine speeds than SI engines.
What is the fundamental explanation for this difference?
Is it just because a diesel is built heavier than a SI engine? Or is it caused by the different combustion?
What happened to the "SI vs CI" thread that is mentioned in other questions?

 

[SomptingGuy]

The OP mentioned "Torque Curve" (it was in the title of the OP). Hand waving arguments about instantaneous torques at various crank angles are OT. A torque cuve is measured by a dyno. Period.

- Steve

 

[ivymike]

The PV diagram shows the amount of work done on the piston, but not the timing of this work essentially (the diesel will be fatter towards the V2 end). The later this pressure occurs in the cylider, the more instantaneous torque is produced at the crankshaft due to the beneficial effects of the rod/crankshaft geometry

What, so cylinder volume varies independently of crankshaft position? Bull-oney. Assuming we all believe conservation of energy applies to internal combustion engines (speak up if you disagree), the the area under the P-V curve HAS to add up to the amount of work out of the crank over a cycle. The amount of work out of the crank over a cycle is torque integrated over a cycle. As stated above, If you build them to have the same area under the curve, they will in fact produce the same torque.

 

[JSteve2]

Warmington / Greg,
I think I've identified the source of your confusion.

This statement: "The work done on the piston by the gas is converted into the plotted torque line by multiplication of the distance of the acting force to the crank centreline"

illustrates that you are talking about P-torque still rather than e-torque. That is not how the plotted torque line is created. The plotted torque line is measured by a dyno, or you could conceptually imagine measuring the acceleration of a flywheel. The E-torque is measuring the sum of all cylinder effects dampened by the flexible crankshaft, and does not see individual combustion events occuring against the crankshaft.

E-torque is defined as the work produced by a given turning angle of the crankshaft. This is required to be proportional to the work performed by a combustion event because the same number of combustion events occur in a single turn of the crankshaft for gasoline and diesel (assuming 4-cycle, which I think we're not disagreeing about). Therefore, if you build me a gasoline engine with the same useful combustion work per event, same number of combustion events per turning angle, same torque.

The way in which the work gets to the crankshaft - i.e. in high p-torque bursts or low p-torque longer events - is irrelevant from the perspective of e-torque.

 

[al1]

ivymike,
“What, so cylinder volume varies independently of crankshaft position? Bull-oney.”

It’s not the cylinder volume per se, it’s the pressure from the combustion process that can be different i.e. CI verses Si. The same principal applies to why diesel have more low speed torque by having more of the pressure curve at a more favorable crank position. This is not challenging the laws of energy conservation as typically only 30 to 40% of the heat energy is only being used to turn the crank any way.
al1

 

[ivymike]

The P-V curve is pressure vs volume. The area "under" the curve allows one to calculate work done on the piston. Work done on the piston (for the purposes of this discussion) must equal work done on the crank. In real life, they're going to be a tiny bit different, due to frictional losses (hydrodynamic and otherwise). The P-V diagram includes the thermal efficiency you're alluding to (it's required to define the shape of the curve). What you're suggesting absolutely does violate conservation of energy.

 

[SomptingGuy]

Isaac, give up. You are in a perpeptual tennis match here. I will not be playing this rally any more. Not worth it, I have better things to do.

- Steve

 

[Warmington]

Steve,

I beg your pardon - am not confused! Please re-read my post that you replied to for the answer to your last post. I can clearly see where you guys have hit a brick wall in terms of relating the now used terms of p and e-torque. I understand you wish to stick to your guns in terms of conventions and current use of the term torque (or e-torque) when defining the characteristics of an engine.

The fact is that the 'dyno plot' is an average of the 'plotted line' for one cycle. Please forget about multiple cylinders and flywheels at the moment, that just clouds the water.

The plotted line (torque output) changes as a function of the angle of rod to crank and pressure. The 'dyno' graph takes a moving average of the torque over a cycle, i.e. the value displayed on a typical engine dyno will have the average torque over that sapce of time. Hence why I said it cancels out!

I am not going to explain it all again, but it is frustrating that several people here seem to reach the point where they understand this principle and then fall back on the 'thats not torque' answer. IT IS.

One way to end this is to do a simulation on say GTPower for a one cylinder diesel and petrol and set the parameters so that you have the same BTU's of petrol and diesel injected at the same low rpm and have complete combustion at stoich. This should fix the area under the curve in terms of pressure. Then run the simulation and watch the diesel produce more torque. The program will calculate torque by multiplying through by the engine geometry.

I am certainly not going to to this simulation as I have been down this road before, and I happily convinced that the later pressure availiable in a diesel engine cycle provides a greater torque at low rpm compared to a gas engine due (in part) to the geometry of rod and crank.

That was the last post from me in this topic.

 

[Warmington]

Ok, that was my second to last.

Ivymike, you are just plain not reading the words on the screen.

V2 is the BDC piston position. If the pressure is higher toward that end of the diagram, then the combustion event has kept the pressure higher, later in the stroke. But thanks for totally colluding what I had said.

 

[thundair]

Same stroke same bore the SI engine 110% VE and perimeter spark ignites at rock-over ~6*BTDC now we are close to the DI torque.
In a SI engine you have ignition and burn before TDC you will not only loose that part of the energy but you will have to overcome it to get past TDC. Just calculate the force on the piston at 15* ATDC and now apply that same force at 20* ATDC.

Disregarding the BTU delta IMHO is the torque difference



Cheers

I don't know anything but the people that do.

 

[al1]

ivymike,
I apologize for misinterpreting the question of why CI has more torque then SI. Of course if you could have the same pressure under the curve and all other things are equal, they should be the same --- but in the real world they are not and that is why diesels have more torque. I also apologize for assuming gross heat energy verses net cylinder pressure on the debate of energy conservation.

This is one of my favorite subjects as I’m working on a true constant volume during combustion engine that has a different mechanical kinematics then a classic engine. There are major improvements to be had when you get away from what I call the 1.57 classic engine mentality. All engine are 1.57 no matter what size, meaning that the crankshaft travels 1.57 further than the piston does. Because of the 1.57, it also permits the PV charts to apply to all engines, however when you change that, it no longer applies.
al1

 

[FoMoCoMoFo]

Mmmm.. half a Pi.

 

[Lou Scannon]

"In a SI engine you have ignition and burn before TDC you will not only loose that part of the energy but you will have to overcome it to get past TDC"
Yes, but... in a well tuned engine this energy is not very significant.

The faster the burn rate (the more net energy release right after TDC, versus later in the cycle), the greater the net area under the PV curve. This is easily seen on a PV diagram. Think about effective expansion ratio (EER).
So maximizing torque at any engine speed is all about getting the fastest possible burn rate right after TDC (within the physical, material & NVH constraints). To accomplish this, diesel as well as SI engines do well to trade-off the small energy loss from initiating combustion slightly before TDC.

 

[JSteve2]

Warmington,
I assumed based on the OP that we were talking about the torque curve on a speed-torque plot. I agree with your comments in the context of a crank angle-torque plot.

 

[Warmington]

like a dog with an old bone, i just cant leave it alone...

We ARE talking about a speed vs torque plot.

If you agree with the fundamental reasoning behind the fact that the diesel will make more instantaneous torque at lower rpm, and that you agree with the fact that diesels will make more 'e-torque' at lower revs as an SI engine then you agree with the whole thing.

I have become totally disillusioned by this whole thread.

 

[SomptingGuy]

... I gave up when the hand-wavers failed to provide P-V diagrams when asked.

- Steve

 

[murpia]

...I think there's some confusion going on between a P-V diagram and a pressure-crank angle diagram...

Regards, Ian

 

[Lou Scannon]

Well, since volume is a function of crank angle, how much of a difference is there really?

 

[patprimmer]

The OP was answered and the originator has not commented since 11/01/08.

Maybe enough is enough.

Regards

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