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Power and torque curve dips
2

Power and torque curve dips

Power and torque curve dips

(OP)
I've just taken delivery of a modifyied i4 Duratech engine that's been taken out to 2.5lt The builder supplied a power and torque plot from the dyno and at 2200 and 4500 revs there is a pronounced dip of some 30-50lbs.ft I've asked him and he says this to some extent is always the case and is caused by resonance, would you agree?
I would add that I'm more than happy with his work and reputation but somewhat puzzled by this phenomenon.
Thanks, Colin.

RE: Power and torque curve dips

2
Hi, it's very engine type specific.
I have done extensive work on engine dynamometer and shadowed it with GT Power/Ricardo Wave 1 D cycle simulation codes for excellent correlation but more importantly until I got a "feel" for that type of engine. In my career I've worked mainly with Cruciform crank (conventional) V8s , 60 degree V6s, Boxer 6s and some straight sixes. There are definate trend with these types of engines almost regardless of make, such that when I see a V8 Torque curve with dips in the 1000 to 2500 rpm region I'll automatically look toward the exhaust system and then come toward the catalysts [this is assuming the engine does NOT have two stage variable length inlet manifolding].

Unfortunately my experience of in line fours is limited. I DO know that they are more sensitive to the inlet air box volumes and general geometries upstream of the plenum throttle, where as on the types of V8s and V6s I've worked on, quater wave resonators for NVH upstream of the throttle body made naff all difference to the measured torque curve.

A pronounced dip of the amount you say over such a big rev range sounds like alot more than just a "resonance" phenomenon- could be mismatched cam specs- to the inlet runner tuning lengths (lots of overlap with very short runner lengths?). I'm suspicious of the "resonance" explanation. Assuming it is a "resonance" effect, resonance can always be tuned out, if you have the freedom to redesign the inlet manifold/intake system- so I don't buy that it's "always the case". May be always the case if this engine is build in this kind of way.
Best most expedient/cost effective way to tackle/understand it would be to get someone to build a 1 D cycle simulation model of the engine. Even if the simulation doesn't correlate with the absolute measured numbers for the engine- I have enough confidence (and MORE confidence than a back street chasis dyno or inertia chasis dyno) that it would allow good understanding. Unfortunately, although cheaper than an extensive dyno test investigation would still not be cheap.

Good luck!

RE: Power and torque curve dips

Adding to the above, sometimes dips in torque mysteriously appear exactly where the engine operates during a pass-by noise test.  I wonder why.  You never know, it might even be intentional.

RE: Power and torque curve dips

(OP)
Your comments regarding inlet lengths and cams ring a bell. The commission for the engine was to maximise low end torque and to give no particular mind to peak power. That was to be whatever it turned out.
During our conversations the builder expected to be using longer inlet lengths and said when they were tested, with  the i4, he found they made very little difference. This surprised him as he specialises in 4 cylinder engines and had not seen this before.

RE: Power and torque curve dips

I'd wander over to the LARC archives and read up on intake manifold tuning.

Incidentally, it may not be a coincidence that 4500=2*2200, for sufficiently large values of 2

Cheers

Greg Locock

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

RE: Power and torque curve dips

Almost certainly its primery pipe lenths 2.5 duratec do not
responed the same way as small engined He.  Engine induction
lenth becomes more sensitive when correct but do not use same formula as small capacity

RE: Power and torque curve dips

Greetings, Marquis,

Just the man I have been looking for!  On many published torque curves for V8s I see in magazines and many of the torque curves I generate with the commercial DynoSim program, there is a noticable dip in the 2500-3200 RPM range.  I have adjusted most of the variables in the DynoSim program without causing the dip to smooth out.

Most of these are performance engines, with larger porting and longer cam timing, but still street engines.  Some of the dips remain even when switching from NA to supercharged, so it isn't primpary pipe length on the V8s.  Where should I look for the causes?

thnx, jv.

RE: Power and torque curve dips

(OP)
Thanks for comment so far. I've searched LaRCNASA but didn't fnd much I recognised as helpful. Just to be sure we're are talking about the same engine: mine is the 4cylinder Mazda/Ford Duratech originally a 2.3lt and bored out here in England to 2.5lt (I wasn't sure from some of the abbreviations).
As I've mentioned, the builder expected to be using longer inlet lengths than usual but when he tested it on his dyno he reported that he couldn't get much difference. It's some way off being ready to use the car and see how it feels. I can't see how to upload the graph aand see for your self.
Colin

RE: Power and torque curve dips

PackardV8, why should the dips disappear when switching from NA to supercharged?

RE: Power and torque curve dips

A little theory and a bit of experience:

1. Supercharging changes everything about the inlet track.  The same length manifold runners behave completely differently under pressure than they do under vacuum.
2. Compression ratios behave very differently when supercharged.
3. Observe the published torque curve for the turbocharged GM Ecotec four in the Saturn Sky.  It is perfectly flat at 258 lb-ft from 2500-5300 RPMS.  Not only no dips, anywhere, It goes nearly straight up, makes a near 90-degree turn and stays flat to the cut-off.  Tne NA version is the expected arc.

thnx, jv.

RE: Power and torque curve dips

Umm

I can see a little bit of experience, but where is the theory.

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: Power and torque curve dips

I have read in a relatively recent reference-type of text book relating to modern developments in IC engines that the turbo-charged engine is a self-regulating (feed-back) type of device.

For this reason it is stated that engine torque vs rpm curves are more or less the same, say, between sea-level and higher altitudes. The same is definitely not true for a NA engine - power tapers off pretty dramatically specially at higher altitudes.

Not had much luck to actually model a turbo-charged engine as such (self-regulating), although tapping into waste power (energy) that would normally exit uneventfully in the exhaust  and feeding back some of this power (energy) in the way of boost pressure does look like "power feed-back" .
 
So recalling some results from lectures and course-work (lab) on negative-feedback (NFB) as the latter applies to electronic amplifiers...... the curves of amplitude (power) response vs frequency get incredibly flatter when NFB is applied around the loop.

In adition, should the NFB loop include frequency-selective components, lo and behold you have bfore you a very fine tone-control !

Our lecturer used to emphasise "gain X bandwith product is constant", so if you emply NFB and reduce gain, the 3-db bandwidth goes up.

NFB also confers a great deal of immunity to extraneous factors such as for example power supply variations.

Could some - or all - or none (?)- of the above analogy be applicable to turbo-charged engines ?





RE: Power and torque curve dips

"Published" torque curves are a little suspect anyway...
However, with an electronically control turbocharged engine, these days, boost pressure is regulated during maximum operator demand so as to deliver a specified torque curve, so it is no longer governed by volumetric and mechanical efficiency.

Paulista,
ruling out electronic control using algorithms designed to maintain constant charge density with varying ambient pressure (i.e. at maximum operator demand), I don't see that a mechanically wastegated turbocharger by itself fully compensates for reduced barometric pressure.  All a conventional mechanical wastegate does is limit boost pressure to a calibrated delta above ambient, so a reduction in ambient pressure directly translates to a reduction in absolute boost pressure.  I suppose you could have an aneroid (absolute reference) wastegate, but I've never heard of one.  An even more noticable effect is seen in the spool-up portion of the torque curve, when the wastegate is still fully closed.  This is a "bootstrap" operation for the turbo, and with reduced ambient density (whether due to pressure or temperature), free-floating (i.e. non-wastegated) boost pressure is drastically affected, due to the physics of turbocharger operation.

RE: Power and torque curve dips

hemi,
     Agree that at low revs and load, with a prevailing lower ambient (barometric) pressure, boost pressure should  be correspondingly lower than when ambient pressure is at nominal value.

     The waste-gate mechanism I am familiar with is operated by bleeding off a sample of the boost from the compressor side of the turbo and pitting it against a calibrated spring/diaphragm combination, with a mechanical linkage to the waste-gate on the turbine side.

     If for whatever reason - including lower ambient pressure, or a punctured compressor-to-intake manifold hose - enough boost is not developed at higher revs and load, the waste-gate stays put(shut).

     Could it be that the "self-regulating" label assigned to the turbo-charged engine derives from this ?


      
      

RE: Power and torque curve dips

The wastegate is not having any effect until it is open, at least a crack.  Then it is acting like a simple mechanical pressure regulator, with more or less flatness vs flow rate according to the design.  But it is referenced to atmosphere, so it cannot compensate for changes in atmospheric pressure.

RE: Power and torque curve dips

hemi,
      
    The loaded-spring/diaphragm arrangement that comprises the wastegate should only budge if the pressure for which the former assembly has been adjusted/calibrated for is attained during operation.

    So unless the latter value of boost pressure is attained, the wastegate will be shut and boost will build up.

     To what degree the build-up of boost compensates for a less dense atmosphere, is a matter for speculation.

     At 5 k feet, the pressure/temp is some 12,24 psia/ 41,7 ºF, compared to 14,69 psia / 59ºF at sea-level.

     Max boost on my 4-cylinder turbo-diesels at full-load is some 1,3 bar (gauge) at sea-level.

     Enough zoomp there to offset a 2,4 psia penalty at 5 k feet ?

RE: Power and torque curve dips

To the best of my knowledge, a wastegate works as follows.

Both atmospheric pressure and spring force holds the valve closed and boost pressure acts on the other side to blow it open. Exhaust pressure difference will also act against the valve face, but the valve is normally a lot smaller than the diaphragm.

The boost pressure will be maintained at ambient atmospheric which is variable, plus the spring which is constant while the valve is on the seat, unless deliberately reset, so due to this, a turbo engine will normally drop of power at reduced atmospheric density, but it will not fall off by as much as an NA engine will, so it is partly but not fully compensating.

The magnitude of the partial self correcting will be proportional to the ratio of atmospheric pressure to spring pressure.

There will also be a small variation due to exhaust pressure on the valve, but it should be much smaller than the other forces involved, and will tend to increase with speed and with increased air density as that will produce more exhaust gas. That will tend to further diminish the self regulating tendency.

To further complicate it, the increased exhaust gas will require the waste gate to open further, thus increasing effective spring pressure and so offsetting to some degree the effect of exhaust gas pressure on the valve.

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: Power and torque curve dips

patprimmer,
            You have emphasised that it is not only the spring pressure, but also the presure of the ambient air that acts on the diaphragm that holds the valve closed.
  
             The pressure exerted on the face of the valve seat in the exhaust assembly would seem to be a second-order effect.

             However, unable to generate insight into why the amount of self-regulation is proportional to the ratio of ambient atmospheric pressure to spring pressure....Obviously, in the limit (a vacuum), zero self-regulation.

              Now consider for sake of argument a standard turbocharged engine, less wastegate....would the self-regulation for lower ambient pressure be any better?

             Wonder if someone could come up with a simplified block diagram showing the control inputs, outputs and control loops in a turbocharged engine.             

RE: Power and torque curve dips

I have yet to see a piston engine run in a vacuum.

It should be quit obvious that the wastegate is the regulating device, so removing it should remove any regulation.

If you have a spring adjusted to say hold 30# boost before the wastegate valve cracks of it's seat plus atmospheric pressure acting on the wastegate valve. If atmospheric pressure is say 15 psi, you get 45# boost. If it is 10 psi you get 40# boost

The atmospheric pressure dropped by 1/3 and if NA, ignoring effective total compression ratio, the power would drop by about 1/3.

The drop from 45# to 40# is only a drop of 1/9 or 1/3 the drop of an NA motor.   

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: Power and torque curve dips

The reason for the dip in the torque curve may very well be a due to some kind of resonance in the intake or exhaust system. Variable intake or exhaust systems or adjustable cams may be the solution.

Some reasons behind dips in the torque curve and some possible cures may be found in the link below.

http://www.gtisoft.com/confarch/Torino.ZIP

RE: Power and torque curve dips

patprimmer,

          When zero self-regulation was stated,zero power output was implied too !

          The wastegate appears to act as a limiter - containing boost at high revs/load - on a turbocharged IC engine installation optimised for developing respectable boost at low-to-mid range revs/load.

           Do current state-of-the-art VGT turbos require a wastegate ....? Perhaps only as a last resort safety device ("crow-bar") should the VGT electronic controls be rendered inoperable for some reason .... ?

           Nevertheless, your output power derating examples smack of elegance, both for NA and turbocharged engines !  

           Can you guess what the power derating should look like for for a standard turbocharged engine - devoid of  wastegate and/or VGT - operating at lower ambient pressure ?  

RE: Power and torque curve dips

You are speaking of a "free floating" turbocharger, which I spoke of above.  The effect of reducing inlet density to a free-floating turbo is highly non-linear (because of the boot strapping effect - think about an engine at WOT at a given rpm, with compressor inlet density being the independent variable - the density reduction is multiplied by the pressure ratio, and this loss is visited on the turbine, which is the source of power for compression, compounding the loss).    I wouldn't want to guess the magnitude, but the appropriate thermo/fluid calculations or better still a computer model would be the way to converge on a solution for given boundary conditions.

RE: Power and torque curve dips

hemi,
     This is what I located in the text that I had referrred to in my previous posts on this thread:

Start of Quote:

Inlet air density reduces;
Hence, engine mass flow rate falls;
Since the diesel fuel pump still supplies the same quantity of fuel, air-fuel ratio falls and exhaust temp. rises;
The specific energy of the exhaust gas entering the turbine increases;
Turbine exit pressure (absolute) falls;
The combined result is to raise turbine specific power, hence turbocharger speed increases and presssure ratio rises
(absolute pressure at compressor inlet and exit has fallen).

End of quote

On the compressor map - with pressure ratio plotted on the y-axis and mass flow on the x-axis, for sea-level there is a line with a positive slope of some 30º that traverses the map, bound by the surge line limit on the left and compressor speed limit on the right of the same.

At altitude, there is a line paralell to the first, distanced from the latter by the aforementioned increase in compressor ratio.

The only point in the above argument that has me thinking is that it states that the diesel pump fuel rate does not change, which comprises the very mechanism for compensating the mass flow of air at altitude. Some danger of over-fuelling  ?

RE: Power and torque curve dips

(OP)
Thanks stenis, my builder felt very sure it was. He said it is there on most 4cyl engines, which are his field, and that it moved about from type to type. If anyone wants to see the plot I will send it on.
Thanks, Colin.

RE: Power and torque curve dips

my early reply was based on hours of dyno testing with
your type of engine most four valve and five valve engines suffer
from this problem you cannot remove by cam timing or induction lenth when you have that much of a dip, the dip can occor when the lenth of the primry is short or long.  The He engine induction lenth will depend on Throttle body type, the three main types used on your engine all set up compleatly diffrently once you have the exhaust sorted.

RE: Power and torque curve dips

Greetings, Marquis,

Just the man I have been looking for!  On many published torque curves for V8s I see in magazines and many of the torque curves I generate with the commercial DynoSim program, there is a noticable dip in the 2500-3200 RPM range.  I have adjusted most of the variables in the DynoSim program without causing the dip to smooth out.

Most of these are performance engines, with larger porting and longer cam timing, but still street engines.  Some of the dips remain even when switching from NA to supercharged, so it isn't primpary pipe length on the V8s.  Where should I look for the causes?

thnx, jv.

Hi, youll have to excuse the brevity of my answer as Im currently on Christmas break in Finland!

Is it a Volumetric Efficiency dip or torque curve dip?
Just for understanding and examining sensitivities- what if you tried to shorten cam duration or minimise the amount of overlap?
I wonder if there is some "anti tuning" going on from the exhaust manifold/down pipe side which is hindering good cylinder filling via the overlap?

I also think it unusual that the phenonemon is present on the boosted variant- which is why I asked if it is there on the VE curves and/or torque curves.

In boosted engines the "boost driver" be it a Roots blower, centrifugal blower, turbo charger etc etc is what predomaiantly drives the Vol. Eff curve and hence torque curve shape. This can be the dynamics of the boosting system be it waste gate, the supercharger pulley ratio driving off a particular area of the adiabatic isolines of the isentropic efficiency map. Im not saying conventional tuning effects arent important just much less apparent

RE: Power and torque curve dips

Paulista,
In the reference you quoted, I fail to see how the turbocharger is responsible for maintaining the torque curve regardless of inlet air density, when the fuel pump itself is unresponsive to the change.

Your last paragraph indicates your apprehension that the quoted argument does not tell the whole story.

Until you introduced this diesel engine example, I had been thinking in terms of a spark-ignited engine running with no excess air.  I will grant that a diesel engine, turbocharged or not, may be less affected by reduced inlet air density, depending on the fuel control system.

RE: Power and torque curve dips

Guys,

Don't over look the basic things first like A/F ratio's or advance curves, very often you can chase your tale with all the technical mumbo jumbo & get nowhere. Go back to basics first, we have come across many engines with similar problems only to find the A/F ratio is not right for the engine at certain rpm levels.
Engines don't care for technical jargon & often it gets in the way!

By the way you might want to listen to Pat on some of the above.

Gruder

RE: Power and torque curve dips

Colinmseries

I assumed that the original duratech engine was with 2.3 liter and the builder rebored it to 2.5L.

If this is the case, the original combination of intake runner length+diameter, exhaust pipe length+diameter, intake/exhaust camshafts need to be slightly modified.

By simply reboring the engine without making any modification, I am making a wild guess that there is not much improvement in term of horsepower. However, there will be around 20Nm torque increase at earlier than original max torque peak point.

Somehow, the amount of maximum airflow going into the engine is limited by the intake system and this will limit the engine max horsepower.  

RE: Power and torque curve dips

Hey guys,
Kinda new to this forum, but after reading this thread I had to reply. I didn't read any mention of valvetrain harmonics. I know I have read a couple of articles and have spoken with a couple of cam designers about this phenomenum. Apparently valve bounce or "float" can be responsible for this, and it doesn't always occur at high rpm, but can appear at several different rpm's, resulting in "unexplainable" torque dips.
Good luck.

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