Airflow VS. Boost

[ronco]

For the purpose of discussion and learning, let's say my "95 automatic truck makes 15 pounds of boost max at any point. If I put in boost elbow and raise the boost 33 percent to 20 pounds, or 66 percent more boost to 25 pounds, will the airflow thru the filter increase proportionally with the percentages given? Is boost and airflow linear in my case, or not? If not linear, how much more airflow is required to produce the boost numbers listed?



I'm not making modifications to anything right now, this question came to mind recently and I thought I would post it here. TIA





Ronco

 

[curatchko]

I don't believe Airflow and Pressure are proportionate. The engine airflow requirements at 'said' rpm haven't changed with pressure. In other words, the engine will only consume so much cfm of air due to it's restrictions either through efficiency or governed.



Take an air hose on a compressor for example. An airtool operating at 5000 rpm at 120 psi isn't going to use more air than when operating at 5000 rpm at 40 psi... .

 

[ronco]

curatchko, my question isn't as much about engine requirements, as it is simple boost and airflow. As for as your analogy with the air compressor and air tool, those aren't apple to apple comparisons with a diesel engine. We all know that boost, among other things, is a product of load and fuel comsumption. Boost can only be produced under load. The air tool uses air under any running situation, loaded or unloaded.



Anyone else have an answer?





Ronco

 

[RustyJC]

I don't have time to run the calculations right now, but treat the engine as a fixed orifice, and make the calculations like you would for pressure drops across the orifice at different flow rates. This isn't 100% true because of the variable turbo exhaust side back pressure, wastegate effects, etc. , but it'll give you a decent approximation of the correlation between flow and upstream pressure. Just think of the system starting at the turbo discharge (higher psi) and ending at the exhaust tip (constant atmospheric psi) with everything between being the orifice or flow restriction. If you have a Daniel handbook or "orifice/flow slide rule", you should be home free. Treating the engine as having a fixed flow coefficient is commonplace in first-cut design of turbochargers.



Rusty

 

[ronco]

Rusty, your post left me without much hope



Is that a yes, no, maybe?





Ronco

 

[rbattelle]

Rusty is trying to say it's nonlinear, but you can come "close" with a linear approximation. And he's absolutely correct.

 

[ronco]

rbattelle, thanks for that translation and thanks everyone for your replies.





Ronco