Power and torque curve dips 2

[colinmseries]

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.

 

[Paulista]

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 ?

 

[Lou Scannon]

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.

 

[Paulista]

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 ?

 

[colinmseries]

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.

 

[bigpaul]

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.

 

[Marquis]

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

 

[Lou Scannon]

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.

 

[gruder]

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

 

[Azmio]

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.

 

[BXRStang]

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.