Intake Boost vs. Compressive Power Requirement

[XXXLR8R]

In a compression-ignition Diesel cycle engine, what is the relationship between intake tract boost pressure and the required amount of crankshaft power to compress that amount of charge air in the cylinders?

I.E. What would the penalty be to BSFC if a turbodiesel engine operated steady-state at 15pisg instead of 5psig boost, assuming equal load, RPM, fueling, etc.

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[patprimmer]

Apart from friction, and depending a very small amount on unsymmetrical cam timing if present, you get it back as extra cylinder pressure on the power stroke.

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[XXXLR8R]

That was my instinctive opinion as well Pat; however, most in the diesel performance world have taken the opposite tack - insisting that more charge air = even more compression-stroke power requirement (causing power consumption to rise more quickly than potential power production).

Wouldn't the high(er) pressure charge air help fill the cylinders, providing more downward force on the pistons during the intake stroke? That would be something of an offset, but perhaps insignificant considering the disparity in magnitude with actual combustion chamber power stroke pressures.

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[dicer]

Yes and with more o2 available then you can add more fuel to the mix. And more power.

 

[pmrobert]

Think "spring"! Not the season...

 

[globi5]

The pressure on the turbine side is approx. double the pressure on the compressor side.

So, the work gained during the intake stroke is about half of the work lost during the expelling stroke (due to the higher pressure on the turbine).

 

[SMOKEY44211]

If there is sufficient oxygen available to consume all the fuel during the 5 psi. boost scenario you outlined expect to gain about 5% reduction in fuel consumption. This would be for an engine of 2-5liter displacement 4cycle. As the displacement is scaled upwards the gain is less significant. If the engine in question had the compression ratio raised to operate with a 5psi. ceiling then further gains would be realized.------Phil

 

[XXXLR8R]

Phil - are you saying that increasing boost (ALL else being equal!) in a given engine load situation will lower fuel consumption (raise BSFC)?

globi5 - how do you surmise TIP is double compressor boost?

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[patprimmer]

You are rolling two seperate functions into 1.

More pressure resisting the compression stroke results with more pressure on the power stroke, not considering extra fuel.

More boost from a turbo (which is not the OP) requires more energy from the exhaust which results in higher exhaust manifold pressure and therefore higher piston blow down pressure.



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[globi5]

xxxlr8r,
boost pressure = double manifold pressure is rather a rule of thumb.
It's not surprising though, both the compressor as well as the turbine have an adiabatic efficiency of roughly 70%. If one multiplies them, one ends up with an overall efficieny of 50% or less.

 

[XXXLR8R]

... then how would you explain typical empirical TIP/boost ratio observations in the .95-1.15 range? (excepting ratios approaching 1.6 when you run the turbo off the edge of it's map)



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[globi5]

Can you please provide a link to that typical empirical data?

 

[globi5]

Any given pump requires more power the higher the flow.

Why would a given engine pumping more air without burning it require less power as the exact same engine pumping less air but burning all?

 

[dcasto]

I don't know if I can buy the 70% time 70% gets 50% thing. I ran a turbo similation and at 2750 lb/hr air (636 cfm) it takes 50 HP to get 15 psig boost. With 3000 lbs/ hr exhaust at 900F in order to make 51 HP to drive the turbo, the inlet pressure is 16 psig. These all had .5 psi inlet and exhaust pressure drops for the piping.

 

[XXXLR8R]

globi5 - these are ubiquitous pressure readings from the post charge-air cooler intake tract and pre-turbine exhaust manifold on turbodiesels... confirmed many times over at every dyno session, dragstrip and sledpulling track.

I agree that as a compressor's pumping performance rises, the turbine's power requirement also increases. However, I don't understand where that "rule of thumb" came about... I don't think we could double TIP in relation to boost w/o destroying the charger!
As the compressor approaches an edge of it's efficiency map, every additonal psi of boost requires an ever-increasing amount of drive pressure... is that what you were getting at?

dcasto - what turbocharger parameters did you input?
Your simulation is fairly close to an observed 50% steady-state load.



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[dcasto]

I set the expander at 70% isentropic effiency and the compressor at 70% polytropic effiency.

 

[XXXLR8R]

Oh!

- I was thinking more in terms of housing A/Rs and wheel trims...

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[globi5]

Sorry, the rule of thumb came from a gasoline engine (much lower compression and roughly constant a/f ratio).

Can you plot TIP/boost-ratio in relation to the boost pressure?

 

[dcasto]

XXXLR8R, I only do the thermal calcs. The turbos I work with are industrial engines and in cryogenic service, thermodydamics are the same.

 

[XXXLR8R]

globi5 - yes, for specific turbochargers... i.e. Holset HE351 on 5.9L CTD:

0-5 psig boost = 3+ psig TIP
5-10 " " = 4+ " "
10-15 " " = 5+ " "
15-20 " " = 7+ " "
20-25 " " = 9+ " "
25-30 " " = 11+ " "
30-35 " " = 13+ " "
35-40 " " = 16+ " "
40-45 " " = 20+ " "
45-50 " " = 25+ " "

More fuel, more gear or more track would likely grenade the turbo on that curve's progressive rise.

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