Air intake and exhaust volume and velocity - need help on calculations.

[JD Cross]

I'm trying to figure out how much air volume our C30 engines take in for each revolution.
I believe our C engines have 120 degree firing intervals.

Does this mean that for each revolution three cylinders are somewhere in the intake/firing mode while the other three are somewhere in the exhaust cycle?
If this is correct then our engines are needing 1.5 liters of air/fuel and expelling 1.5 liters of exhaust each revolution.

At 8k rpm then the engine needs 1.5 liters of intake X 8 k rpm or 12000 liters of intake per minute or 200 liters per second.

I think our intakes are about 3" in diameter so then each inch of intake equates to a 115 ml or 11.5 % of a liter

So in a 3" intake every 8.66 inches equates to one liter.

To fill a demand of 200 liters of air per second we multiply 8.66 x 200 equalling 1732 inches of air per second.
That's 144 ft. per second or about 98 mph.

So my math suggests at 8 k rpm the air intake velocity needs to be about 100 mph to fill the cylinders.

I guess the exhaust needs exit at higher speed as there is a larger volume of exhaust due to gas heat expansion.

Would someone please check my math here?
And can anyone calculate the extra volume of exhaust gas and therefore the extra velocity needed to expel the gases?

 

[Old Guy]

JD

I worked the numbers and the average intake velocity, over the 720 deg engine cycle at 8000 RPM, assuming a 3 " diameter intake came to 156 km/hr in the 3" section of the intake. Approximately the same as your 'about 100 mph'. The average air velocity in different parts of the intake manifold will be different depending on the cross section. This is average air velocity over the 720 deg engine cycle. Peak air velocities might be slightly higher. You would have to consider the intake overlap between cylinders, cylinder filling versus valve opening and the volume of the intake manifold. The larger the manifold volume the more it tends to smooth the fluctuations in velocity. This calculation also assumes that the volumetric efficiency of the engine is 100 % which it won't be and that the charge air does not change temperature of pressure as it goes through the intake system, which we know does occur.

Taking a first crack at estimated exhaust gas velocities would be really harry. You would do a first order estimate of the total energy released by the combustion of the gasoline. You would have to estimate the component that results in useful mechanical work. Probably something less than 30%. The rest goes into heat loss, some of which goes into the coolant; but, most of it probably ends up going into the hot compressed exhaust gas. You could probably model the exhaust valve, manifold and exhaust system as a series of expansion processes. With some general assumptions or some empirical data from an actual engine, you could come up with an estimate of the flow rates required to exhaust an engine. When you know the flow rates and the cross sectional areas, you can estimate the gross gas velocities. The fluid dynamics guys probably use some sort of finite element analysis software backed up by empirical data from a test engine to do these sorts of calculations.

Its been 42 years since I have done any calculations on thermodynamic cycles. Better find a mechanical engineer or a fluid dynamics specialist or somebody who has used the word adiabatic in a complete sentence in this century!

 

[JD Cross]

Old Guy, thanks for your reply.
I was curious about intake speeds as there are a lot of NSX intakes being offered these days.
As the intake speeds aren't that high I'm thinking our OEM intakes are probably not a bottleneck in the system.

Looking at the exhaust side where each 500 cc of spent gases expands dramatically, exit velocities must be very high, or at least need to be very high.
Reading about gas laws suggests the exhaust gas pressure will be highest just before the exhaust valve opens, but as gas exits velocity increases and pressure drops.
I've been told by a knowledgeable NSX engine builder that the constraint of building more NA power is exhaust restrictions.
This is in reference to the valves/exhaust ports not a low backpressure muffler.

I need to do more reading on the exhaust side of things.

 

[Old Guy]

Screwed up my post with an unintentional edit!

 

[JD Cross]

An excellent point on measuring the intake pressures.
That would pinpoint either knowing OEM does the job or more flow is needed.
There's a lot of aftermarket intake systems but none that I know of have done the measuring.
You're right it would be best to start with the intake side and find out if there are gains there first.
Thanks

 

[greenberet]

During a dyno day, some NSX owners in England T'd pressure sensors into the vacuum lines coming out of the intake manifold just downstream of the throttle body. They logged the results and the results were discussed in this thread: http://www.nsxprime.com/forum/showt...s-best/page5?p=1009727&viewfull=1#post1009727 (look at the next page of the thread as well).

I agree it would be great if vendors measured the pressures drops their intakes cause vs. the pressure drop caused by a stock intake. Maybe they have and don't want to publish the results!

 

[Old Guy]

During a dyno day, some NSX owners in England T'd pressure sensors into the vacuum lines coming out of the intake manifold just downstream of the throttle body. They logged the results and the results were discussed in this thread: http://www.nsxprime.com/forum/showt...s-best/page5?p=1009727&viewfull=1#post1009727 (look at the next page of the thread as well).

That dyno post is a really interesting result. Thanks for the link.

The individual providing the test results did not provide the details of their intake system (full disclosure - I didn't read all the previous posts!). If that is for a stock intake system, I am surprised at how little pressure drop is taking place. On the next page in the thread you discuss the potential lost horsepower due to the pressure drop. I assume that this is based upon the mass flow rate difference associated with the ambient pressure of 101.16 kPa versus the manifold pressure of 97.8 kPa which would yield about 10 hp on a stock engine?

Completely eliminating the pressure drop is impossible. Air doesn't flow without a pressure differential. I don't think that intake system modifications would yield any material improvement in horsepower (due to pressure drop) on a stock engine. Those results leave me pretty comfortable embracing the OEM intake system on my unmodified 2000 NSX.

In the discussion that follows the post of the dyno results, the VVIS and intake tuning get thrown into the mix. That is a bit of a separate kettle of fish. Before somebody takes umbrage with my comments, my statements on the effect of static manifold pressure on horsepower don't really bear on the tuning of the intake manifold to create resonance conditions that facilitate stuffing more air mass into the cylinder. I did find it odd that the presenter of the dyno results said that the dyno workshop attributed the pressure drop that was measured between ambient and the test point 'to the TB and/or the VVIS'. The TB presumably being upstream of the measurement point I could see being a contributing factor to static pressure drop. I am struggling to see how the VVIS would contribute to static pressure drop.

While we are on the intake manifold thing, and since you have clearly paid a lot more attention to the dyno results that have been posted, are you aware as to whether anybody has logged intake manifold temperature during a dyno test? I have another car that is instrumented. I know that under extended idle conditions, if the hood is closed the intake manifold air temperature can rise about 30C relative to ambient. The fresh air intake is up front in the grill adjacent to the radiator. At highway speeds the manifold air temperature drops precipitously, I recall rises of something like less than 10 C. I have never had the opportunity to test at wide open throttle so I have no sense as to whether the temp gradient goes up or down at sustained wide open throttle compared to operation at highway speeds. My sense is that the temperature gradient might drop. The 'other' car is an in-line four with the manifold off to the side are relatively far removed from the engine block relative to the NSX. The NSX being a V6 might have a greater temperature rise issue; however, the air flow rates might be sufficient to make temperature rise a non issue at wide open throttle.

 

[JD Cross]

Greenberet
Many thanks for that link.
What a fascinating read.

I have a question on markc's post of his dyno run showing a drop in pressure from about 1 bar to .98 bar.
If the throttle body diameter was designed to increase intake velocity wouldn't that alone explain the drop in pressure?
Higher velocity equals lower pressure.
Would a Pitot tube, if one could be fitted, show the air velocity and from that determine if indicated pressure is a function of a bottleneck or solely an increase in velocity?

The Honda engineers would understand that a too small throttle body would create a bottleneck unless they decided to trade off throttle body diameter for increased intake velocity.
The post also suggests that just changing the airbox size/shape may not do anything for performance.
What do you think?

 

[Old Guy]

Greenberet

I have a question on markc's post of his dyno run showing a drop in pressure from about 1 bar to .98 bar.
If the throttle body diameter was designed to increase intake velocity wouldn't that alone explain the drop in pressure?
Higher velocity equals lower pressure.
Would a Pitot tube, if one could be fitted, show the air velocity and from that determine if indicated pressure is a function of a bottleneck or solely an increase in velocity?

I don't think that anybody would ever design a throttle body to increase air velocity. You might design the throttle body to allow higher air velocities (air mass flow rates) with a lower pressure drop across the throttle body thus supporting a higher mass flow rate. As a note, going to big throttle bodies can have some pretty nasty side effects. With a larger throttle body, the reduction in pressure drop across the throttle plate versus the opening angle occurs much quicker than it will with a smaller throttle body. This makes for a very touchy throttle and can make part throttle operation (driving in stop and go traffic) a real pain.

As to the pitot tube question yes, theoretically! I have always been a bit curious as to why Honda picked that location for the MAP sensor port. My knee jerk reaction would be to tap the main plenum as a more reliable indicator of average manifold pressure for the fuel injection system. However, the Honda engineers may have determined that there is not a material difference in pressure between the two points (as in the change in air velocity is not significant). Or, perhaps the manifold resonance tuning that Honda has done which includes the VVIS creates some nifty standing waves in the intake manifold proper that makes getting a reliable pressure measurement difficult at that point.

 

[JD Cross]

I don't think that anybody would ever design a throttle body to increase air velocity. You might design the throttle body to allow higher air velocities (air mass flow rates) with a lower pressure drop across the throttle body thus supporting a higher mass flow rate. As a note, going to big throttle bodies can have some pretty nasty side effects. With a larger throttle body, the reduction in pressure drop across the throttle plate versus the opening angle occurs much quicker than it will with a smaller throttle body. This makes for a very touchy throttle and can make part throttle operation (driving in stop and go traffic) a real pain.

As to the pitot tube question yes, theoretically! I have always been a bit curious as to why Honda picked that location for the MAP sensor port. My knee jerk reaction would be to tap the main plenum as a more reliable indicator of average manifold pressure for the fuel injection system. However, the Honda engineers may have determined that there is not a material difference in pressure between the two points (as in the change in air velocity is not significant). Or, perhaps the manifold resonance tuning that Honda has done which includes the VVIS creates some nifty standing waves in the intake manifold proper that makes getting a reliable pressure measurement difficult at that point.

My comment wasn't well phrased on the throttle body.
As the throttle body has a tapered shape the venturi effect should result in an increase in velocity and decrease in pressure through the throttle body.
I'm wondering if this drop in pressure/increase in velocity explains the pressure drop in markc's dyno run or is showing a real bottleneck.

Our OEM intake system is quite complex and somehow I just don't think the Honda engineers would have built a bad design for what, at the time, was a very advanced engine.
I'm also wondering if changing one part of the system can interfere with the flows, pressures, and sonics of the whole thing

 

[greenberet]

The individual posting the results - markc - had an unmodified NA2 and the other data set is from another unmodified NA2 present at that dyno day.

According to Kaz, stock and lightly modified NA1s don't show gains with larger throttle bodies but stock NA2s do start to show some gains (see here). That indicates to me that Honda's engineers sized the throttle very well for an NSX engine putting out about 270 crank hp. The throttle is just big enough so that it does not noticeably choke the engine. Not making it any larger than that allows the driver to modulate the throttle well.

The horsepower loss due to the pressure drop was calculated using SAE standard dyno correction factors developed because, as you said, less pressure equals less mass. I agree that the pressure logs from stock NA2 engines show that the stock intakes - intake snorkel, airbox, air filter, throttle body - are well designed. I also agree that the dyno operator's comment about the VVIS potentially being responsible for pressure drop upstream of it doesn't seem particularly well informed.

I can't remember anyone testing the intake temperatures in NSX engines but I think Autospeed.com carried out some tests on other naturally-aspirated cars. If I remember correctly, significant temperature increases were measured at idle, when the air was flowing slowly. At full throttle, when the air passed through the intake system very quickly, it didn't have time to warm up more than a little bit.


Edit: I'm sure Honda's Formula 1 engineers designed the NSX's intake system very well given 1) the technology available at the time, and 2) the 280 PS gentlemen's agreement in effect in Japan during the NSX's production run.

Papers have been written about how Porsche developed the intake manifold for the 964 generation of 911s. The 964 came out a few years before the NSX and it also had a variable intake system. The lengths, volumes, and valves were calculated to set up resonances using the 1D CFD software package PROMO. At the time, there were no commercially-available ways to couple 1D CFD simulations, which modeled gas exchanges at least per degree of crankshaft rotation, to 3D CFD computations of components with complex shapes such as intake manifolds. Coupled 1D-3D simulations have really only been possible since the late 1990's. Only since then can you really optimize the shape of the intake manifold to get the resonances and flow even for all cylinders in dynamic conditions.

I assume the NSX's intake manifold was developed similarly - based on a 1D simulation of the pipes and volumes but without a full 3D optimization since that was, to the best of my knowledge, not technically possible at the time. Using today's technology, you could design a better intake manifold, especially if you didn't have to worry about any 280 PS gentlemen's agreement.

That impacts things downstream of the point where markc's measurements were taken. The things upstream (throttle body, airbox, air filter, intake snorkel) don't seem to pose much of a restriction. The intake snorkel may be a bit of a bottleneck and your engine might benefit from a gently enlarged throttle body if you've got more than I/H/E modifications on an NA1. There was a discussion about intakes in this thread as well.

 

[JD Cross]

greenberet
The other thread you refer to also covers a lot of ground and I'd forgotten I'd read it
This is a complicated science to be sure and until some hard data materializes I'll stick with my OEM intake and pass on the latest offerings.
Thank you

 

[greenberet]

I agree that it is a complicated science. The horsepower per liter naturally-aspirated engines are capable of producing has increased significantly in the past 100 years as we have come to understand and optimize the gas dynamics in the system.

You can design a decent intake system using rules of thumb regarding the length and diameter of the intake runners, the volume of the plenum, etc. However, you will not optimize how the waves from one cylinder affect the other cylinders, ensure that each cylinder gets the exact same volume of air, minimize losses due to turbulence, etc. using simple rules of thumb. There was a series of articles in Race Tech magazine last year on intake design, written by the former heads of aerodynamics and engines at Ferrari F1 and BMW F1. Regarding those rules of thumb they wrote, "These formulas are not repeated here as, to the authors' knowledge, they are no longer used in current design practices; these approximated theories were abandoned for more modern investigation tools."

That being said, you can optimize an existing intake system if you don't agree with the trade-offs chosen during the original design phase or if mistakes were made. Empirical testing can show whether your modifications "improve" the intake in the manner you want and that testing should generate data.

When modifying the intake of an NSX, I agree with your strategy. If a vendor can't explain the design process and doesn't provide hard data to back up his claims, I'll pass.