I did back to back testing on a rolling road with and without my Pipercross PX600 airbox. It turned out with the airbox fitted the engine lost almost 20 rear wheel horses! I desperately need to find a new solution to kill intake noise, and in order to do that I need a better understanding of what made the Pipercross airbox a bad performer.
On the rolling road we tested three scenarios. With airbox and filter, airbox without filter, and without airbox without filter. The filter did no difference what so ever, but with the airbox the difference was huge above 4000 rpm and up. We had problems with wheel slip on the rollers so the accuracy of the performance loss is low, but it is safe to say at least 15 rear wheel horses and probably more.
So CFD to the rescue. Or is it? This is not my home turf, this is me on a journey. I now know that this kind of static analysis is close to worthless. The difference will be huge in real life with valves opening and closing causing pulses that interfere with each other. I suspected that from the beginning but now I know for sure. I hesitated before publishing this post.
Anyway, I find this fun and interesting. And cool! But that's me. :-)
How I did the simulations
I used Autodesk CFD 2017 with the "old" solver. Advection scheme 1.All simulations are made with a total flow of 420 cfm (ft^3/minute) on the supply port. (*)
An engine consume about 1.5 cfm per produced horse power [1] so a 280 hp engine requires 420 cfm of air.
I have set a pressure constraint of -28 in.H2O on all cylinder ports, and a flow volume constraint on the supply port. The idea is that all cylinders "suck" the same amount, and the cfm constraint will solve the pressure/velocity on the air supply port. Again, this is not my field of expertise, but I can't figure out a better way?
Then I run the simulation until convergence, run mesh adaption, and one more run until convergence. I used result planes with the bulk tool to calculate flow value results.
(*) I use these awkward units because that is what is commonly used in engine literature. At least the literature I've read.
PX600 airbox
First model is a rough model of my Pipercross PX600 airbox with a 90 deg silicon bend, as currently fitted to my car. As I wrote above, this box performed very badly on the rolling road test.
I started with a max flow simulation to see if the box suffocated the engine. It does not. It wouldn't surprise me if it could supply enough air for a Formula 1 engine. That is definitely not the problem.
Then I did the 420 cfm simulation as described above.
From left to right: 22%, 25%, 28%, 24% of air. If I recalculate this to air/fuel lambda it would be equal to: 0.74, 0.82, 0.93, 0.80, while the lambda sensor in the collector would read 0.82.
Here it is clear that the amount of air to each cylinder is not equal. If we don't use individual cylinder fuel trim the air to fuel ratio will be very different for each cylinder. That is not only killing performance, it could even be disastrous for a knock sensitive high compression engine! [2]
Second model is the same Pipercross PX600 airbox without the 90 deg silicon bend.
From left to right: 23%, 25%, 27%, 24% => lambda 0.77, 0.84, 0.89, 0.80.
A lot better. The silicon bend is not helping! But it is clear that the sharp turns into the trumpets are problematic.
R500 Caterham airbox
Third model is the R500 Caterham airbox. The measurements are just rough estimates taken from a few pictures I found on the internet and probably not very accurate.
From left to right: 32%, 23%, 22%, 23% => lambda 1.04, 0.76, 0.71, 0.77.
Own design #1
So I realized airbox design isn't easy. I figured I'd need more volume in order to slow the air down. I also realized that I could build a bigger air box if I was having the entry tube facing rearwards.
This is an attempt to slow the air down before the first trumpet, still withing the space constraints of my bonnet. I know, it isn't pretty.
From left to right: 27%, 25%, 23%, 24% => lambda 0.90, 0.83, 0.76, 0.79.
OK not bad, but still...
Own design #2
Next attempt was with smoother curves in an attempt to guide the air into first cylinder. Pretty nice looking if I may say!
From left to right: 29%, 25%, 24%, 22% => lambda 0.96, 0.82, 0.79, 0.72
That didn't work very well. Feeding an engine from the side is tricky business.
Conclusion
Again, a static flow analysis doesn't say anything. But one thing it shows very well is how difficult this is. In particular how hard it is to feed air to an engine from the side! I suspect that the only thing that works ok is individual fuel trim or air boxes with enormous volume.I did a quick test with a transient simulation. Lets see if I follow up on that or if I spend my time on better things.
Sources:
[1] Engine Airflow, Harold Bettes, HPBooks
[2] Four-Stroke Performance Tuning, A. Graham Bell, Haynes
