Airbox static flow CFD analysis

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.

Nice looking box though. I do think my cad model could be improved and that could result in different results especially into first trumpet.

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

3D printed cold air intake

Intake temperature has a direct correlation with engine power. For every 10°C rise in air intake temperature, engine power will be reduced by 2%. Last season I logged intake temperatures over 40°C on a 20°C day, which is really a waste of power.

So I need to duct cool air. I've been googling for different types of air scoopes and naca ducts but prefer not to cut a hole in either bonnet or nose cone. So I decided to try to get some air from in front of the radiator.

As I've written before, my 3D modelling skills suck. But where I work I have access to a 3D printer and wouldn't it be nice with an air duct that goes in the small space between the radiator and nose cone?

I started with a lot of measuring.  I used my kids' clay for the space between the nose cone and radiator, that I cut in pieces and measured. Then I modelled the constraints in the CAD program. I used the loft feature for a nice flow-friendly air duct and started printing.

3D printing is a slow process, and they can only print small objects. I had to split the part in five smaller parts and glue them together with epoxy. Each part took about 8-12 hours to print in medium quality! And it took a few tries before the outcome was good enough.

3D printing, first attempt.
I quickly learned that support stays should be avoided as much as possible.

I also did some CFD analysis to get a design that flowed all right.  I admit it is not perfect, but the first versions was worse...

CFD analysis of an early version

The theory is that the  higher air pressure in front of the radiator will force air into the duct. I've never had problems with high coolant temperatures and I hope it will still will be ok.

The parts glued together with epoxy.

Some filler and black spray can paint. As I didn't want to ruin the existing radiator alu frame I manufactured a duplicate and cut a hole for the duct. When I look at the result I'm amazed that I didn't put just a little more effort to make the end result better looking. But I just wanted to get it finished...

This duct has taken a lot of effort to produce. I've learned the hard way that 3D printing is not a mature technology and have a long way to go. The printer I used was far from a cheap entry level model.

Next step is a better suited air hose and how it will integrate with the filter. I have a temporary solution that works but could be much better. Also some back to back testing and see if there is any improvement.


References:
1. Comparison of Engine Power Correction Factors for Varying Atmospheric Conditions

Intake cone filter and air pressure

The Pipercross airbox I bough was supplied with a built in air-filter rated up to 200 hp. As don't want to strangle my engine I now have fitted a cone filter with larger filter area.

102mm to 80mm silicone reducing elbow from do88, a plastic tube and three hose clamps from the local hardware store. The filter is left overs from the CSR200 engine.

As I already have a MAP-sensor connected to ECU, I decided to do a road test and see if I could see any pressure drop at the intake. Ideally I would compare the three different filters against each other, both regarding pressure drop and maybe 100-200 km/h acceleration, but I rather not on public roads.

RPM vs air pressure (click to enlarge)

The lowest pressure seen was 0.9918 bar, which equals a drop of 8.2 millibars or 0.82%. In theory that would decrease power with the same amount, or in my case 2.2 hp*. The pressure increase seen on this chart is interesting. I have an hunch of what's happening but I don't have enough knowledge to either explain it or fully understand what the result is. Could it have something to do with air moving but the engine's volumetric efficiency is declining and that causes pressure build up?

I was worried the intake temps would suffer with the filter close to the exhaust manifold, but it appears that they are lower than they used to be with the big sausage type filter and never went above 28°C, with the ambient temperature being around 18°C.

* Source: Four-Stroke Performance Tuning page 26, A. Graham Bell.

Intake airbox

As I wrote in the previous post the track noise regulations in Sweden has become much harder lately. The current limit is 95 dBA drive-by measured from a 10 m distance.

I've fitted a Raceco silencer and now a Pipercross PX600 airbox. Now the car is much more quiet - huge difference! I can't even hear the revs because of the wind. A quick drive-by test with a cheap china noise meter gave 96-97 dBA but after glueing on some insulation material on the airbox the readings dropped to between 94 and 96 dbA on the most important intake side of the car.

On the exhaust side the noise level is 101 dbA and still too high. I'll try to mount a bend or something to point the exhaust to a different direction and see if that helps.

The airbox have an integrated filter that I suspect rob a bit of power. Next step will be an external filter with larger filtering area, and maybe some cold air ducts on the nose cone side or sides.

The good thing with this airbox is that it can be mounted within minutes if the occasion requires. Direct fit, but I had to remove the steel bonnet/nose cone support tube.