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.

Si Vis Pacem Para Bellum

 

[Lou Scannon]

XXXLR8R, I agree with all your statements, but your data is a little confusing. Are you saying this turbo is getting 50psig boost with 25 psig TIP? That's much better than I've seen or would expect.

 

[XXXLR8R]

Goodness no!

'Tis the other way 'round - 1st coloumn is compressor boost range, 2nd coloumn is additional TIP required over & above 1:1 ratio.

Si Vis Pacem Para Bellum

 

[j2bprometheus]

I have some experimental data for a turbocharged non-EGR diesel engine.

At some engine conditions, intake pressure could be as much as 14 inches of mercury higher than exhaust pressure.

At "worst" engine conditions, intake pressure was 2 inches of mercury lower than exhaust pressure.


You can make the pressure ratio across the turbine very low by making turbine inlet temperature very high (provided nothing melts). If the pressure ratio across the turbine is low, then exhaust pressure is low.


j2bprometheus

 

[turbomotor]

I agree with j2bprometheus. I've run many turbocharged engine tests with numerous engine - turbocharger combinations and various ambient conditions. Engine delta P (P int man - P exh man) can vary significantly. Most notable I've seen is 30 in abs Hg intake manifold with 18 in abs Hg exhaust manifold and turbine inlet temp of 1900 F.

 

[Lou Scannon]

I've seen TIP's lower than boost pressure in modelling, actual data, and even on my own vehicle, under certain limited conditions. Nothing magical, it just depends on the turbo match, net turbocharger efficiency, and the operating point.

 

[SMOKEY44211]

XXXLR8R If the engine in question is using .35 lbs/hp/hr at 15psi boost expect to see .323 lbs/hp/hr at 5psi boost if you can achieve total combustion of fuel need at this power level. I have had some limited experience with diesel engines in the displacement range descrbed in earlier post to report this finding. I believe that decreased power draw needed to run compressor, ( decreased exhaust pressure) and decreased heat rejection within the combustion chamber are responsible for the net gain observed.------Phil