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 ?

 

[ivymike]

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.

 

[Paulista]

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.

 

[ivymike]

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).

 

[Paulista]

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).

 

[MikeHalloran]

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

 

[Paulista]

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).

 

[icrman]

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.

 

[Paulista]

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

 

[BenDutro]

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.

 

[icrman]

"""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.

 

[patprimmer]

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.

 

[GregLocock]

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.

 

[Paulista]

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 ?

 

[Paulista]

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.

 

[GregLocock]

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

http://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.

 

[Paulista]

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)

 

[ivymike]

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?)

 

[BenDutro]

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?

 

[patprimmer]

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.