×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Torque vs RPM carachteristic in diesel engines
2

Torque vs RPM carachteristic in diesel engines

Torque vs RPM carachteristic in diesel engines

(OP)
Can anyone oblige as to what are the predominant factors in engine design that determine the shape of the above in diesel engines, both for NA and turbocharged types ?


Or put in another way, if one wished to push the max. torque developed in a turbocharged diesel from the original 1600 RPM to say, 2000/2100 rpm, what would be the principal modifications required ?

RE: Torque vs RPM carachteristic in diesel engines

injection timing and boost map (if wastegated).  you'd have to redevelop for emissions, most likely.  I think driveability would be poor with peak torque so close to rated - usually drivers like to have a good torque rise as speed drops.


RE: Torque vs RPM carachteristic in diesel engines

(OP)
Let´s forego the emissions for the time being !

Can you be more explicit as to how ignition timing and the turbo boost map need to be altered ?

It would be expedient to do so for a pleasure boating application, viz., to have the max. torque developed close to cruising rpms (2000/2200 rpm).

Work boats could do with the standard road-vehicle torque peak at 1600 rpm.  

If you look up the torque vs rpm carachteristic curves for Cummins, Hino, Mercedes etc. you will notice that the marine engines have their max. torque developed at higher rpm than their road-vehicle counterparts.

A host of after-market shops tailor petrol-engines to the customer´s wants and needs, but diesels are till today shrouded in secrecy.

RE: Torque vs RPM carachteristic in diesel engines

for a marine application, you're limited by the torque that the prop can handle.  The prop curve causes the engines in question to have large portions of the mid-range torque curve "lopped off" (because they are unusable in a boat).  I don't know whether any engine control mods are required, or if the prop curve sets the engine torque curve by itself (engine&prop accelerate automatically if prop can't handle torque @ a given speed).



RE: Torque vs RPM carachteristic in diesel engines

(OP)
True, the mid-range torque vs. rpm goes largely unused in normal pleaure-boating applications, but may not be the case if the boat is facing heavy seas, or if it is a work-boat(tug, for example) on load with a 4:1 reduction gear-box.

That is seemingly why the engines in question have their torque vs rpm carachteristics "drawn out" to peak at higher rpms, as registered on a dyno.

In reality, the engine hp vs rpm curve is normally above the prop hp vs rpm, thereby enabling the vessel to accelerate if need be, when until finally the two meet at the rated power point (no reserve power left).

RE: Torque vs RPM carachteristic in diesel engines

Maybe I can add a pinpoint of light with this factoid:

In mechanically controlled engines, the HP/rpm curve slopes upward, and is convex upward, until it meets the prop power requirement curve, which in marine application data sheets is always drawn in as a cubic curve.

In electronically controlled engines, the HP/rpm curve slopes upward over the first ~half of the usable rpm range, at which point it becomes perfectly flat.  I.e. an electronically controlled marine engine will run at constant power against a dynamometer while above what would otherwise presumably be its torque peak.

I don't see how it makes a difference.  In a boat you can't actually get above the prop curve anyway.








Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

RE: Torque vs RPM carachteristic in diesel engines

(OP)
Having the engine deliver peak torque close to cruising rpm means that you are operating the engine where it is most efficient. (Take a look at the "sfc" curve and you will note that it is a minimum at the peak torque point).

Do you necessarily have to go the electronic injection route and discard mechanical injection, to push out the torque peak from 1600 to 2000/2200rpm for marine applications ?

Don´t know, but most current marine diesels are derived from their truck counterparts and achieve the desired result by retaining the mechanical injection (duly altered, of course).




RE: Torque vs RPM carachteristic in diesel engines

In all recip engines the torque drops off because of the
reduction of volumetric efficiency. The designers of that particular engine have set those parameters for a specific reason. If you want to raise the torque point you need to raise the volumetric efficiency, and do like Mike says.
In essence you are increasing the HP by doing such.

Say you have 100 lb.ft. at that 1600 rpm point. Thats 30 hp.    And now you wish to raise that torque point to
2000 rpm and for argument sake its still 100 lb.ft.
Now the hp is 38 hp. So its just a matter of more air and more fuel, and setting the governor for that rpm. The reason Mike mentioned timing is because as you speed up the mechanics, and since the chemical reactions don't follow, you need to advance the timing as you speed things up.
Most likely if you icrease air and fuel your power increase will be more than in the example. If the components in the engine have been optimized for the original power settings, you will be drasticly reducing the life of the engine.

RE: Torque vs RPM carachteristic in diesel engines

(OP)
The vol. efficiency issue you raise above seems to have nailed it.

I am all for making more power at a lower per unit cost (max. torque point) in a region of the power band where the user is likely to spend most of the time.

NA engines see vol. efficiency drop off at higher engine speeds due to deterioration of engine-breathing dynamics.

Turbocharged engines can artificially locate the peak vol. efficiency at a desired rpm, by judicious choice of turbocharger and boost/fuel/timing map.

Umpteen aftermarket outfits perform such feats, day-in and day-out, with petrol engines but very few, if any, on diesels

RE: Torque vs RPM carachteristic in diesel engines

A turbocharger's turbine A/R ratio will have the biggest impact on VE and breathing charicteristics from the exhaust/intake pressure ratio. I fail to see the change that a denser intake charge will have on VE because the intake runner diameter, length, cam events and port volume are the principal governors of VE.

Paulista, changing the ignition map on a diesel won't get you very far at all :)

Diesels do not have the sensitivity that gas engines do regarding stoichiometric air/fuel ratios so the strategy, crude as it may be, is to run maximum efficient boost, and turn the fuel up perhaps with a shot of nitrous oxide to assist in charge temperatures and complete burning.

To move the torque peak higher in the RPM band, there's several ways to go about it but an extrude honed intake and some porting on the cylinder head would help.

RE: Torque vs RPM carachteristic in diesel engines

"""I fail to see the change that a denser intake charge will have on VE""""""THIS IS A QUOTE FROM ABOVE !

It will. Because its a positive pressure forcing
its way in. In an N/A application there is a time issue for cylinder fill. And that is why the racing cam industry
has thrived. Agreed a restrictive issue, valve timing, and size being one of them.
 
With a supercharger you are "packing" more air in. So what is happening is at a given speed where say VE was falling off with N/A, because the piston has to start moving air by sucking it, now there is a pressure head of air to help force the piston down and then pack some extra in. So
instead of being some number under 100% VE you now have some number OVER 100% VE. You need to compair the airflow
requirements of the N/A, to what the new airflow is to the
supercharged engine. If that airflow exceeds the cubic (what ever units) displacement, then you are over the 100% VE mark.
Anyway you may fail to see it, but the engine has no problem seeing it. Cause that is why any form of supercharge will produce more power than an N/A. Recip engines are air pumps, and the more they can get the more power they produce. Even with hp losses from a mechanical supercharger, it will still out produce a similar N/A engine. If it didn't the OEM's and drag guys wouldn't use
them.

RE: Torque vs RPM carachteristic in diesel engines

If you consider a supercharger to be part of the engine, it certainly increases the VE of the engine.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Torque vs RPM carachteristic in diesel engines

However, not everybody agrees that the VE should be measured based on the air intake. A lot of people make a lot of money improving the VE based on the conditions in the inlet port.

After all, claiming some huge increase in VE by doubling the power into a supercharger scarcely seems a level playing field unless that power is taken into account.

Cheers

Greg Locock

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

RE: Torque vs RPM carachteristic in diesel engines

(OP)
Ben Dutro wrote :

"Paulista, changing the ignition map on a diesel won't get you very far at all :)"

If you retard ignition timing on a diesel, it may overheat in a most violent fashion (I have personally witnessed this!)
True, excess air is always present for combustion, unlike petrol engines.

So where do I look (book ?) for reliable data on tailoring the torque vs rpm characteristic ?

GregLocock wrote:

"After all, claiming some huge increase in VE by doubling the power into a supercharger scarcely seems a level playing field unless that power is taken into account."

Granted, especially as the supercharger taps power from the useful engine  output.
This would not hold if a turbocharger that runs on spent engine gases were used, unless the thing was so badly designed as to restrict the exhaust process and thereby actually lower power output. Right ?  

RE: Torque vs RPM carachteristic in diesel engines

(OP)
But if the turbocharger is matched, the per unit cost of power output ("sfc") ACTUALLY DROPS.
There is a NET BENEFIT, if correctly done. The mechanical efficiency goes up.

In supercharging setups, an increase in sfc should be the normal outcome, aside from the augmented power output.In extreme cases, "the law of diminishing returns" may come into play.

RE: Torque vs RPM carachteristic in diesel engines

Turbos do affect the back pressure on the engine, so in general they do have a power cost associated with them.

However, for reasons that are no longer clear to me, turbos on diesels do tend to improve the overall efficiency of the system, whereas for SI engines they rarely help.

So, on a diesel you can get more than you lose from a turbo, but on an SI you'll still be less efficient than you were, although probably not as inefficient as when a crank driven supercharger is used. Here's some results for a production car

                     Supercharged    N/A

City (litres/100km)    16.1         11           

Highway (litres/100km) 12.2         6.6

Power    kW             235         155

Torque    Nm            465         305

s/c specs from http://autoweb.drive.com.au/cms/A_1590/article.html

                     Turbocharged    N/A

Average fuel economy
 (litres/100km)           12.9       11.5                   

Power    kW               240        182

Torque   Nm                 450        380

Figures from www.ford.foa.com

Since these are fixed driving cycles the extra performance of the s/c and t/c  models is not really a factor in the increased fuel consumption. There is a slight difference in weight, both the faster cars are higher spec.

Cheers

Greg Locock

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

RE: Torque vs RPM carachteristic in diesel engines

(OP)
Thanks for being so obliging.

Can they guarantee that fixed-driving cycles preclude the un-leashing of latent power by the test-drivers of the vehicles ?! Must choose very controlled types.

I have noted that t/c consumption figures are closer to n/a and that s/c is a long shot away.

One of the possible causes that springs to mind for the not so encouraging results of t/c in petrol engines, could be the higher probability of DETONATION ("knock") of the fuel/air charge at higher cylinder operating pressures.

Also, airflow and fuel flow are not independent parameters, as they are in diesels, although the available exhaust gas energy to drive the t/c is much higher in petrol engines than in diesels.

In fact, t/c caught on more after EFI established itself in the trade, as carburreted engines were tough to t/c.

Direct-injection diesels are immune from DETONATION(duel-fuel diesels that ingest air+natural gas and use pilot oil-injection are NOT)

RE: Torque vs RPM carachteristic in diesel engines

Paulista: If you retard ignition timing on a diesel, it may overheat in a most violent fashion (I have personally witnessed this!)

I'm pretty sure you meant to say injection timing, but you've made the same mistake a few times so now I'm starting to wonder...  (What kind of spark plugs do you use in your diesel?)

RE: Torque vs RPM carachteristic in diesel engines

I'm aware of the consequences of improper fuel timing, I was  referencing ignition timing, or lack thereof, on a CI engine. (That's a joke son!)

CI engine compression ratios are significantly higher than SI, there's more free energy available for reclaimation by a turbo.

Manufacturers' use spark-retard knock sensors and lowered compression ratios in turbo petrol engines to limit detonation, both of which hurt the BSFC.

Regarding Volumetric efficiency, I was under the impression that VE was a measure of breathing efficiency independant of density calculated by dividing the actual volume of air in the cylinder by the total displacement of the cylinder, e.g. (350cc)/(400cc)=87.5%

Even if you change the density of the intake charge, peak torque will still occur at the same RPM, VE hasn't changed.
The benefits of the denser intake charge are mainly dependant on the intake:exhaust pressure ratio, how much backpressure does it require to generate X-psi on the intake side?

RE: Torque vs RPM carachteristic in diesel engines

The actual volume of air in a cylinder is always identical to the volume of the cylinder at that point in time.

As the volume of air is always the same, it is actually the density or mass that changes with VE, or to be more precise, the volume that that mass of air in the cylinder would be at ambient temperature and pressure vs the volume of the displacement of the piston in the cylinder.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Torque vs RPM carachteristic in diesel engines

Paullista, when you drive a fuel consumption test you have to be within a certain error band of the desired speed/time profile at all times, I think it is +/- 1 mph.

Undoubtedly it is possible to drive in a more, or less, economical fashion whilst staying within that band. I don't know how that is regulated, in fact I doubt that it is.

Almost all objective testing of cars relies on the integrity of the people involved.



Cheers

Greg Locock

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

RE: Torque vs RPM carachteristic in diesel engines

Having run a testing lab many years ago, I would say that almost all objective testing relies on the integrity of the people involved.

There is always some scope to optimise some conditions while staying within the test method parameters.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Torque vs RPM carachteristic in diesel engines

(OP)
IvyMike wrote:

"I'm pretty sure you meant to say injection timing, but you've made the same mistake a few times so now I'm starting to wonder...  (What kind of spark plugs do you use in your diesel?)"

Slip of the tongue.


BenDutro wrote:

"CI engine compression ratios are significantly higher than SI, there's more free energy available for reclaimation by a turbo"

Regardless, the enthalpy of exhaust gases in SI engines is way above that of CI engines. For the same reason, the t/c in SI engines often needs to be a ball-bearing type in order to survive the enormous heat.

Recall also that SI engines loose to CI engines when it comes to  mechanical efficiency, as more heat is lost to the surroundings.

GregLocock wrote:

"Almost all objective testing of cars relies on the integrity of the people involved."

Also seconded by Patprimmer. Not the integrity of the test drivers that I was getting at, but a natural overshoot-type phenomenon, viz., the tremendous difference in the responsiveness of the test vehicle and the time it takes to start driving it "normally". Jump out of the driving seat of a 4-banger into a V-8 and you will see for yourself! Nevertheless, the test figures that  were supplied actually corroborate with what is predicted by theory.

RE: Torque vs RPM carachteristic in diesel engines

Ve in your diesel may not change with a turbocharger, but mass flow sure will.

While exhaust back pressure rises, so does boost, and the whole mess then becomes a double compound system, with two stages of compression and two stages of expansion.

The trick with a turbo is to match the turbine Rpm, developed internal shaft power and pressure ratio as exactly as possible to the compressor requirement. The whole turbo will then reach an efficiency peak at some particular mass flow.

The turbo has absolutely no idea what the size or Rpm of the engine is, it just sees flow. So you size the turbo to suit the optimum flow at the desired torque peak.

That can skew the torque curve of an unblown engine, to move the raised torque curve either way depending on the application.

Turbocharged gasoline engines are usually hampered by a slightly reduced static compression ratio, and slightly increased exhaust back pressure, so light throttle fuel economy might not be so good as its n/a equivalent.

Diesels have no similar limitations. Provided there is efficient and complete combustion, and no over-fueling it should have a pretty good fuel specific either n/a or turbo.

RE: Torque vs RPM carachteristic in diesel engines

Getting back to the original question about the shape of the torque curve:

Diesel torque is primarily a function of the quantity of fuel injected until you reach the point where you have consumed all the intake charge air.  So the fuel injection system (mechanical or electronic) delivery curve can be tailored to give any desired shape up to the "smoke limit".  Many diesels are sold with their power delivery curve optimised for their particular application.  There may be other changes made to optimise efficieny such as turbo A/R, injection timing, etc. but these are fine tuning compared to fuel delivery.

When I worked at Mack Trucks engine developement laboratory, we used the same 6 cylinder block for anything from 200 to 375 hp ratings.  There were many differences in the tune of these engines including compression ratio, turbo size, pistons, ring packs, etc.  The higher power engines got piston cooling jets, main bearing tie plates, bigger oil coolers, etc.  So it is not just a matter of cranking up the fuel but that is the primary difference in the power.  You could buy the low power engine & crank up the fueling to get almost the same power as the high power version.  Of course, it wouldn't last very long.

RE: Torque vs RPM carachteristic in diesel engines

(OP)
Warpspeed wrote:

".....So you size the turbo to suit the optimum flow at the desired torque peak."

dgallup wrote:

"Getting back to the original question about the shape of the torque curve:
Diesel torque is primarily a function of the quantity of fuel injected until you reach the point where you have consumed all the intake charge air.  So the fuel injection system (mechanical or electronic) delivery curve can be tailored to give any desired shape up to the "smoke limit". "

Now the pieces fit, so it seems !
 
1. Choose a turbo that will give you the required air charge/mass at 2000/2100 rpm
2. Tailor your fuel delivery accordingly, keeping the latter below the smoke limit.

There is this ex-Cummins engineer who was employed at the factory for 30+years and has produced a cheap kit (US$ 200-300) for taking a 6BTA from 170 HP (in the older Dodge Rams) to 240 HP in just 2 hours of work ! I shall post his bearings on the Internet as soon as I am done with this post.

Instead of just "turning up the fuel delivery" on the BOSCH P7100 pump as most mechanics do, he substitutes the original "fuel-plate" in the pump with his own proprietary design (supplied in kit). Another part that is also supplied and replaces the OEM part is a hose-connector on the fuel-injection pump which connects it to the turbo´s compressed air delivery, and which is used for metering fuel over the whole range of boost.

No doubt, the 6BTA is capable of 240 HP, a rating that was down-sized for early Dodge Rams, in order to keep the powerplant and drive-train compatible with each other. So taking it up from 170 to 240 HP is basically just tweaking it - correctly !

In practice, it may require at least changing the compressor and repeated trial-and error runs, to increase the power from a 4-cylinder turbo-charged diesel rated at 150 HP/2600 rpm , to say, 200 HP/2600 rpm.

Finally, why do electronic-injection diesels ("common-rail") make from 30-40 % more power than their prior mechanical counterparts ? Also, the torque-curves of the former are largely flat over a wide rpm range, unlike the mechanical that have a varying torque curve with a peak somewhere in the range.



RE: Torque vs RPM carachteristic in diesel engines

(OP)
Here is the web address of the company that sells the kit, with detailed installation info, for upping the power on the 170 HP Cummins 6BTA :

  www.tstproducts.com/

RE: Torque vs RPM carachteristic in diesel engines

Just one other point, if you want to tweak the turbo to change the boost characteristics, look first to the exhaust turbine.

The exhaust turbine more or less fixes the turbo Rpm versus flow, and changing the compressor size will only have a small effect on boost pressure and flow, unless the changes are really large.

On the other hand, a small change the exhaust turbine a/r will significantly change turbine Rpm for any given flow. That can have a more dramatic effect on the compressor operating conditions than just changing the compressor specification.

RE: Torque vs RPM carachteristic in diesel engines

Part of the reason some marine diesels reach max torque output at higher RPM is because they drop the compression ratio for marine apps.  A 'highway' rated 6.5 diesel has a CR of 21.5:1.  Where a 'marine' 6.5 has an 18:1 CR.  This creates a loss of volumetric efficiency, that is overcome by turbocharging @ ~2000 RPM and above.   

RE: Torque vs RPM carachteristic in diesel engines

(OP)
I would have thought that volumetric efficiency and CR are INDEPENDENT of each other.

But,yes, turbocharging - and force feeding an engine - can compensate for loss of vol. efficiency.

I think the reason for lowering the CR in marine diesels is to lower the mechanical stresses on engine components, granted that marine engines operate at higher loadings than vehicular ones.

RE: Torque vs RPM carachteristic in diesel engines

Compression ratio can have an effect Ve because of the presence of  hot, sometimes high pressure exhaust residuals remaining in the clearance space.

Imagine you could build in an infinitely high compression ratio with zero combustion chamber volume. All the exhaust would be pumped out mechanically, creating more space for the induced air on the next cycle.

RE: Torque vs RPM carachteristic in diesel engines

(OP)
In other words, operating at high CR may ensure better scavenging of exhaust gases and residuals ?


RE: Torque vs RPM carachteristic in diesel engines

Yes exactly, it just makes the whole pumping cycle more efficient which is really what Ve is.

It also effects a lot of other things once the valves are shut, but those are other issues.

RE: Torque vs RPM carachteristic in diesel engines

Torque is relative to cylinder pressure. If you move the torque closer to operating Rpm you will inevitably increase the engine operating temperature. Most marine diesels can only survive if they free up at operating rpm. Most marine failures are because of improper gearing or proping placing to much load on the engine. Maximum torque will mean max. heat.

Thanks,
Mike

RE: Torque vs RPM carachteristic in diesel engines

(OP)
kmb1949,
        Max. torque synonymous with  max. cylinder pressure seems OK.

        But not quite sure if I agree with max. torque coinciding with max. heat. In fact, I think the inverse should be true, that is, max. torque = min. heat being rejected, and this point generally coincides with the minimum of the sfc curve.

        Obviously, though, if you push out max. torque to a higher rpm, then you will be generating more power at higher rpm than you were prior to the modification.

        More output power being generated suggests more heat rejected to the surroundings, but nevertheless, proportionately less heat per unit of output power than at any other operating point.

Right or wrong ?
         

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources