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?

 

[franzh]

Several factors, but the most basic are as follows:

The engine operates unthrottled, no suction throttling losses;
Diesel fuel has a much higher btu rating than gasoline (roughly 145,000 versus 120,000 per gallon);
Higher compression ratio (ranging from about 16:1 to near 24:1);
The pressure curve of diesel has a longer duration than on an SI engine.

The weight has nothing to do with power, but to make the engine durable against the CI pressures.

Franz

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[SomptingGuy]

Franz, did you mis-read the question?

I would turn the question around to answer it. Why are SI engines able to maintain torque at higher speeds?

 

[Baldturbofreak]

faster flame propagation would be the biggest hurdle. SI fuels propagate combustion much faster thn diesels.

 

[BrianPetersen]

Well, not quite ...

With a spark ignition engine, you have a ready-to-fire mixture of air and fuel during the compression stroke. The mixing is (substantially) already done. When ignition occurs, the only thing limiting combustion is the flame speed.

With a diesel engine, since it is compressing air only, fuel injection and ignition and combustion all have to happen in a few degrees before and after top-dead-centre. After the fuel injection starts, there is a delay before ignition occurs. The delay is in the millisecond range, but it is nevertheless a delay, and a millisecond is 24 crank degrees at 4000 rpm. Then there is the time for adequate air and fuel mixing to occur, while combustion is taking place.

What's found with high speed (relatively) automotive diesels is that even though the engine may be mechanically capable of going 6000 rpm, any attempt to use such high revs results in combustion taking too long and not going to completion, so the result is too much smoke and high exhaust temperature but not much power.

There is an upper boundary to the amount of fuel that can be injected before the exhaust opacity ("blackness") goes beyond an acceptable limit, and that amount of fuel tends to drop off at higher and higher revs even if the boost pressure is kept the same.

 

[Xplode]

Thanks for the explanation.

I understand that the ignition delay of CI engines limits the amount of fuel that can be injected for a fully completed combustion at high engine speeds. This causes the torque curve of CI engine to drop faster than that of a SI engine at high engine speeds.

But what happens at low engine speeds? I thought that CI engines could produce a higher torque than SI engines at low speeds, in other words, that the torque curve of SI engines dropped faster there.

Is it the lower flame propagation speed that reduces the produced torque? From the explanation above, it seems that a CI engine doesn't have any trouble with the lower speed as there is more time available for a fully completed combustion and more fuel can be injected, causing a higher torque. Is this correct?

 

[thundair]

Brian touched on it "ignition and combustion all have to happen in a few degrees before and after top-dead-center."

The SI engine has to advance the timing to complete the burn. Some times 40* or more and that is what steals the torque as the piston has to overcome the approaching flame front. I have had 2 strokes in bad tune actually start and run backward because of the approaching pressures of advanced ignition timing.

The other point is the burn is more complete in DI especially at low RPM's and AFAIK franzh is correct about BTU values.


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

 

[Rjeffery]

And all this time I thought it was the greater stroke a Diesel Engine had, in order to produce the compression required to ignite the f/a mix...

 

[dcasto]

Get it more simple. Torque is Force on piston times the lever arm or stroke. Force on piston is area of piston times pressure in cylinder. Pressure in cylinder is called Brake Mean Effective Pressure.

The BMEP is a function of compression ratio. Diesels have a higher compression ratio.

 

[Lou Scannon]

"The BMEP is a function of compression ratio. Diesels have a higher compression ratio."
That'a an oversimplification. The main indicator of BMEP is energy release per unit volume; a secondary indicator is thermal efficiency, which is of cource influenced by compression ratio. Due to frictional and thermal losses during the compression stroke, there is little net efficiency gain above 13-14 CR, IIRC. Diesels typically run ratios of 16 or higher in order to support compression ignition under adverse conditions (namely cold starting & warmup).

 

[Warmington]

Yes the optimum CR for diesels for power is around 15:1. This has been shown over and over again. Most new DI diesels have a CR of 18:1 or even less. The higher 20:1 ish compressions were simply to get IDI engines to start, at the expense of some losses due to the reasons hemi mentioned - it is a compromise. Alot of the big truck diesels with starting aids have as low as 13-14:1 CR.

If we had identical SI and CI engines (bore, stroke, Vs) running at the same CR - say 14:1, would the diesel still produce more torque at WOT at low rpm? Yes.

Diesel engines are built to take advantage of their slower running speeds and shorter rpm ranges. They have longer strokes and ports that promote high turbulance and swirl. These things promote low down torque production. There are components from the slighlty higher CR and also the energy in the fuel helping to produce better torque, but also the fact that the fuel can be kept burning for longer in the low rpm range - past the point that the piston is exerting the most force on the crankshaft. Like FranzH said, the pressure curve is of a longer duration. This actually helps at low rpm to produce torque.

Remember the Therm Eff for the diesel cycle includes beta- the cutoff ratio for injection, not just CR.

 

[al1]

Go into any tech book and look at the PV (pressure Volume) chart of a spark and compression engine. Spark engines are called constant volume because the combustion time and/or energy release time is quicker. Compression engines are called constant pressure because the combustion time is slower and the energy release is longer and is later in the cycle for it to be completed. You can plainly see that the compression engines has higher pressure later in the combustion cycle and there fore also has better mechanical leverage to apply the presser – thus why diesels have more torque. However, everyone knows that spark engines can typically make more power and since toque over time amd/or RPM is how you calculate power, toque by itself is not the total indicator. You have to remember that the diesel combustion process is not only slow, it is relatively constant in real time, which limits the efficiency at higher RPM due to the piston moving faster as the RPM increase and literally start to outruns the pressure --- thus making the diesel a low RPM engine. Yes there are higher RPM diesel engines but they don’t have the efficient advantage that they do at low RPM. Which takes us to the most efficient and powerful engine (2 cycle Sulzer used mostly in cargo ships) in the world yet today that work in the 100 RPM range. Though these engines are the most efficient at .26 BSFC and most powerful at 98,000+ HP, they also have the poorest power to displacement and power to weight in the industry as well, all due to these low RPM.
al1

 

[Warmington]

Just to illustrate the point:

As the piston descends in the combustion stroke, the angle of the connecting rod and big end relative to the crankshaft increases. Resolving forces shows that the push on the piston will generate more torque up to the point when the piston is half way down the bore and the big end is at 90 degrees crankshaft rotation. After that the 'leverage' is decreasing again. If the pressure in the cylinder can be kept as high as possible, later into the power stroke, this extra 'leverage' generates an increase in output torque. Diesels have a longer burn duration, allowing the pressure to be kept higher in the cylinder during combustion for longer, therby utilising more of this 'leverage' effect. The same longer burn duration is what also hampers the diesel engine in the higher rpm ranges that a conventional SI engine reaches.

There are of course other components that give diesels the low down torque, something that GDI engines have yet to match completely.

 

[SomptingGuy]

It always amuses me that so many engineers bang on about "leverage". Internal combustion engines operate on a cycle. Instantaneous torque varies through the cycle, but it's the cycle-averaged torque that's measured by a dyno.

- Steve

 

[GregLocock]

Engineers or 'engineers'?

I agree, laughter it makes.

Next they'll be telling us the engine goes round at constant speed.







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warmington]

Well done guys for answering the question! I knew you 'real' engineers would get it right away!

What is wrong with using the term 'leverage' to illustrate the condition at any one time window pray tell? Is the answer to the question fundamentally flawed?

And of course the dyno will cycle average the torque, you expect that I though that it gave a series of discrete measurements for one cycle?

And also, from the above posts WHY would you think that the extension of this would be that I thought the engine speed was constant through the cycle?

Intelligent input guys.

 

[SomptingGuy]

What is wrong with using the term 'leverage' to illustrate the condition at any one time window pray tell?

Because the magnitude of instantaneous torque peaks is largely irrelevant (unless you are concerned with stress calcs). It's the total PdV integral that's important.


- Steve

 

[drwebb]

Would it be correct to describe CI combustion as occurring closer to thermodynamically reversible conditions compared to SI combustion?

 

[dcasto]

If given the same engine geometry and a SI with 9 to 1 and a CI at 16 to 1, The SI has less torque? The only difference is compression ratio.

 

[ivymike]

Maybe I didn't scan this thread thoroughly enough (I've avoided reading it for days), but I didn't notice any mention of forced induction. I wonder whether the OP intended to compare apples-to-apples, or if the goal was to compare a typical (NA) gasoline engine's torque curve to that of a typical (turbocharged) diesel?

For the record, I disagree with the statement about the optimal compression ratio for a diesel. There is efficiency to be had at higher compression ratios, if you can handle higher cylinder pressure and still meet emissions. Nearly all of the 2007 on-hwy diesels seem to have CRs in the 16-18 range:

http://www.us.mahle.com/C125708F006...7925614C12570B90050FEE6/$FILE/07 hd chart.pdf

I don't think it was cold start that put them there; most of those are available with a preheater.