Torque vs RPM carachteristic in diesel engines 2

[Paulista]

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 ?

 

[GregLocock]

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.

 

[patprimmer]

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.

 

[Paulista]

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.

 

[Warpspeed]

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.

 

[dgallup]

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.

 

[Paulista]

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.

 

[Paulista]

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 :

http://www.tstproducts.com/

 

[Warpspeed]

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.

 

[TexasDieselGuy]

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.

 

[Paulista]

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.

 

[Warpspeed]

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.

 

[Paulista]

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

 

[Warpspeed]

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.

 

[kmb1949]

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

 

[Paulista]

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 ?