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

Airfoil behind roll cage CFD analysis

Does an airfoil generate downforce when placed behind a roll cage? Since I'm starting to get along with the CFD analysis software I did some more simulations.

The airfoil is a NACA 2312 at 160 km/h with 20° angle of attack. Length ~170 cm, width ~20 cm. As I suck on 3d modelling, none of the models correspond much with the real world. Neither the results probably, but can give an hint of what you could expect.

Without roll cage

Scenario 1 - No roll cage, airfoil 40 cm above trunk
Airfoil downforce: 791 N
Airfoil drag: 282 N
Total downforce: 1810 N
Total drag: 1230 N

40 cm

Scenario 2 - Airfoil 40 cm above trunk
Airfoil downforce: 542 N
Airfoil drag: 174 N
Total downforce: 1482 N
Total drag: 1296 N

50 cm

Scenario 3 - Airfoil 50 cm above trunk
Airfoil downforce: 857 N
Airfoil drag: 268 N
Total downforce: 1742 N
Total drag: 1438 N

Scenario 4 - No airfoil
Total downforce: 738 N
Total drag: 1051 N

Conclusion:

A wide airfoil behind the roll cage does generate downforce, but raising it just a bit increase the effect dramatically. To no surprise, the outer parts of the wing are the most effective regions. Real world experiments are necessary to find the optimal location.

Engine bay undertray aerodynamics

Some time ago I started to do some CFD analysis to see if I could do any simple changes that improved the aerodynamic properties of the car, without altering its appearance too much. It turned out that I had too much faith (or more probably - too little knowledge) of what the CFD software could provide me, and after putting in a great deal of work I finally gave up.

It is said that from an aerodynamic point of view, one of the most important features of a car's body design is how it looks underneath, and even if my previous CFD adventures failed it pointed me in the same direction. To fit an engine bay undertray is not very complicated and doesn't affect appearance much, but before I started I wanted to confirm if it would improve things or just be a waste. This time I made a much simpler 3d-model of the car, and had much lower expectations of what the CFD software could help me with.

Standard under body

The results should be taken with lots and lots of salt!

Standard
Drag: 879N
Downforce: 427N

Undertray only
Drag: 793N
Downforce: 455N

Undertray and skirts
Drag: 809N
Downforce: 590N

Undertray, skirt and splitter:
Drag: 858N
Downforce: 604N

All calculations are in an air speed of 160 km/h. The standard design does generate a bit of downforce, because the car body is at a small angle compared to the direction of the air flow. The ground clearance is 15 cm, minus the engine oil sump. The design measurements are just guesses as I didn't take time to go out to the garage and measure. (it does look a bit short). No rounded edges. The body is solid on the upper side.

An underdray reduces drag but doesn't give much improvement in downforce. On the other hand, extending the undertray with a 5cm skirt improves downforce quite a lot. I also tried to add a small splitter in the front and see if that made a difference (see top banner image) but the small gain in downforce is not worth the effort.

One apparent issue with fitting an undertray is of course heat. I have a plan for lowering intake temperatures (more about that later), but if the heat from the exhaust primaries will be just too much and start to melt things I don't know.

Caterham has a ready made undertray on their website, but I've already bought a sheet of 1 mm aluminium and started to cut out the under tray. I'll post more when (and if) I finish it.

Aerodynamics - CFD simulations

After some vacation reading of the book "Race car aerodynamics" I have had lots of thoughts of how to improve the car's aerodynamics, especially the lack of down force.

The common approach on this subject is that the seven is a hopeless case and don't even bother. That might be true, or could it be the other way around? Since it does have the aerodynamic properties of a brick (or worse) - even small mods can make a great difference?

One problem is that you don't want to change the classic look of the seven. That is also true for a majority of seven owners. But I believed something like a flat underside could make a big difference. You can also add removable elements that you only have on the car when you're really going for it.

The question was, how much can be gained, and is it worth the effort?

Autocar did an article a long time ago (?) about wind tunnel testing of a Caterham. You can find it here:
#1 #2 #3

Some time ago I did some simple simulations of how much some downforce would affect lap time. The result was that 160N more downforce @100km/h would shorten the lap time with over a second! As a reference, F1 cars generate more than ten times of that, at the same speed. (source: the book mentioned.)

Wind tunnels are not accessible for most of us. At first I thought I would do road testing with string potentiometers measuring the suspension compression and connect it to the data logger. Straight roads are not that easy to find where I live, and going 200+ km/h on public roads are not that great either. The number of variables are endless and if something works or don't work I probably won't know why.

The book mentioned above briefly talks a bit about CFD - Computational fluid dynamics - and concludes that it is very expensive, complicated and not something for others than high end race teams. Well, time has passed since the writing of that book, and now the CFD software is not that hard to use and 30-days trials can be downloaded over the internet from many different software companies.

It turned out that the book was quite right. After experiments I now understand why top race teams still use wind tunnels. CFD simulation is very complicated, and it takes lots of effort to get accurate results. But for rough estimates it could still be very useful!

Baseline

So I made a rough simplified CAD model of my car. It may sound easy, but for a complete 3D modelling newbie it took quite a lot of time learning.

I started with high ambitions, but after a while I lowered them just to get some results at all. In these simulations the wheels are not rotating and the ground is not moving.

@160 km/h
257N front downforce

1935N drag

Note, these are rough numbers. 

I use to complain about front end lift when I drive, but this is pretty much the opposite. 

Anyway I plan to do some changes to the model and see how it affects the output. I also plan to investigate some individual parts in detail with finer meshes and higher accuracy.