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

Custom moulded ear plugs

I hate ear plugs. The sensation of being inside a bubble playing a video game has got me avoid them except when using power tools in the garage. I ride a motorbike to work everyday, and now when I'm getting older I can feel my ears has been taking a lot of beating over the years.

I've been testing different kind of ear plugs. From simple foam plugs to diy molded plugs. They either leak what can be described as a razor sharp noise, damp too much, or dampen the sound of the engine but not the noise from the wind.

So I decided to get a pair of custom moulded ear plugs and booked an appointment at Hörseltekniska Laboratoriet (Hearing Technology Laboratory) here in Stockholm, who often provide ear plugs for musicians and others with high demands of a good dampening curve. The plugs they recommended are from Bellman & Symfon, which sell a kind of plugs that are suitable for motorsport and that fits inside your helmet.

He filled my ear canal with some kind of foam and waited for it to cure. The silence was almost intimidating, nothing came through. Three weeks later I had the finished plugs in my mailbox.

The plugs are soft like silicone and goes in a long way in your ears. On the motorcycle they're great. I can clearly here the engine, but not the wind noise.

But in the Caterham on the other hand, the engine is just too quiet compared with the wind noise. When I drive with the plugs in at the motorway, I can't tell if the engine is running or not. I have to look at the rev counter to see if it's time to change gear.

As this year is a disaster from track day perspective, I haven't tried them on track, but I can imagine the wind noise is still so much louder than the engine noise, and no ear plug will help me there. I'll probably use them anyway, both for lowering the noise levels and for reducing the sensation of sensor overload.

UPDATE: They work great on track! I feel I even could use a bit more dampening.

TimeAttack - "King of Mantorp"

Valtonen Motorsport RX-8

Normally I don't like to compete but somehow I participated a Time Attack competition this weekend. Seven type cars are not allowed in their series, but this time they had a "no limits" class. Some of the competitors car's was completely insane, as the tube frame, all carbon RX8 to the right, accompanied by a complete race team.

Photo: Felix Eriksson

I managed to get the 4:th place. I'm really satisfied with that. Not so much for the final time (1.23,4), but that I kept my head together and managed to get a good run in the super final which was one lap only!

In fact, the qualifying lap and final lap was one second faster than all my practice laps during the whole day!


The official noise check reported 97.5 dbA drive-by which they said they allow today but not tomorrow. I'm getting there...

Everything else worked very good, except....

Gearbox problems!

Just when the day ended something happen with my gearbox on the cool-down lap. I works but makes some chirp noises and the gears are hard to engage. .... .... ghh

Update: I got a couple of seconds of fame in this movie:

Attempt to redirect exhaust noise

As I previously wrote my noise levels are now down to 94-96 dbA on the intake side and 101 dbA on the exhaust side.

In an attempt to reduce the latter I did a quick test to fit a 90° steel tube and point the exhaust towards the ground and see if it made any difference.

The result was a bit unexpected. The exhaust noise went down an half dbA, but the intake noise raised to 98 dbA.

I've been complaining about this before, but noise metering appears to be quite random.

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.

Raceco silencer

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. This limit is set by the SBF (Swedish motorsport federation) for all swedish circuits.

I know my car sounds a lot more than that, around 105 dBA. So far I've never been blacked flagged but I know it is a question of time. Also I wouldn't mind if the car was quieter as I'm not a big fan of loud noises.

The first step is to quieten the exhaust and that is now done with a Raceco 2.5" titan silencer built by Mike Riley. The high price made it a painful decision but it is very nice built and also a lot more quiet than the previous silencer. It take the edge of the bad part of the noise and what is left is a nice round sound. In fact the exhaust noise is now masked entirely by the (very nice!) induction noise.

It was a straight fit and no brackets needed to be fabricated.

It is also a bit lighter than my previous silencer - about 1.5 kg.

The car is much more quiet now but I think it still have a bit to go before I reach 95 dBA. So trying to reduce induction noise is next step.

Mantorp first time 2013

This years first trackday was a warm day with blue skies on Mantorp and a huge mix of different types of cars.

Things didn't go that well. It started with me being a bit late.

Then there was a Porsche that spilled oil on a long section of the track. (Those German cars....)
Then someone else went off and one corner was covered with gravel and the track was closed again.
When I finally got the tires warm there was too much traffic and no clear laps.

Engine problems

Lost top end power and the top speed on the straight was a bit too low. It got worse and worse, so I stopped driving and went home instead, worried that there was something very wrong with the engine.

When I got home I did some troubleshooting and found out the alternator warning light connector was broken, and that prevented the battery being charged. (Stupid design!) My speculation is that the battery's voltage went down during the day and caused the fuel pump not being able to keep up the pressure when the fuel demand was peaking, making the engine run lean. Running an engine like this lean is not good at all and could cause fatal pre-ignition.

I need some way to prevent this from happening again.

I would like to be able to log the fuel pressure, and have some warning system for a fuel or oil pressure drop.

Here is a video I found describing how voltage affects the fuel pump flow:
http://www.youtube.com/watch?v=RjykWzDr1Kk

Noise check

A couple of months ago I bought a cheap noise meter from Hong Kong, and today I asked someone to measure the drive-by noise of my car.

The guy told me it made 106.4 dbA!

In many race circuits in this country have from this season a drive-by noise limit of 95dbA.

I started measuring the other cars, and no-one, except a stock Corvette, was below that limit.

Then I moved a little, and all readings changed. Then the wind changed, and the readings changed as well. Then I realized I could not get the same reading twice for the same car.

It seems like measuring noise at a race track is not an exact science.

I refuse to think my car makes as much noise as 106db, but I'm not surprised I have to do something about it if I'd like to drive on other circuits. I just don't know what yet.

What about all the work done this winter?

All good!

The roll cage... stayed in place. 

The suspension bushes hold together. The car felt nice and precise.

The exhaust manifold was not leaking!

The silencer mounts did not break.

No oil spew from the catch tank.

Engine did not leak oil from the front cover

Overall the car feels wonderful. Too bad I was worrying about the engine instead of enjoying myself.

I did not have the aeroscreen mounted as I didn't think it would do much difference. At the end of the straight the force against the helmet was overwhelming. I must put the aeroscreen back on.