Brake pressure sensor

Logging driver inputs is a way to get quicker, they say. I don't doubt that for a second. For me though, more frequent driving would probably be more effective. But that didn't stop me from putting in another sensor in the car.

When I actually do drive my car on a track I often find the lack of consistency very frustrating. I want to know why I'm faster in the morning than in the afternoon. I could look for excuses such as track temperature etc. Or I could stop fooling myself and start looking closer to the steering wheel. Can my braking be improved? (yes of course) How and where? That's what I want to find out. Also, in order to make sense of the other data collected from the car's sensors I need to know what the driver was doing.

"Aggressive application of the brakes is something that separates good drivers from average drivers. Most driver inputs to the car should be smooth, [...] but in big braking zones a driver should be very aggressive with the brakes." - Optimum G.

"Some of the most useful information to be had concerns driver performance and coaching. Braking is the one aspect of car control that goes against the accepted wisdom of smooth driver inputs."- Competition Systems.

Pressure sensor

For measuring brake pressure you need a pressure sensor in the range to about 200 bars, preferably with a DIN/ISO brake fitting. As there are plenty of production vehicles with brake pressure sensors you can find them at a decent price on eBay. I bought a second hand Bosch 0265005303 for about €25. The tech sheet was a google away and the electrical connector was available from Conrad.com or Farnell with part numbers:


Tyco PSA 2x3 pol
967 642-1 housing
965 907-1 contact pins
967 067-1 seals

1. GND
2. Output
3. Supply voltage +5V

The sensor was connected to one of the analogue inputs on my DL1 data logger.

Brake lines and flares

I installed the sensor on the front brake circuit right after the master cylinder. I made a new brake line with a tee coupling for the sensor. It was not hard, but a bit messy with the brake fluid dripping.
 
The standard Caterham brake lines are 3/16" with an ISO bubble flare. The nut on the master cylinder side is M10x1. On the front left/right tee split, Caterham thought it would be nice to use a UNF 3/8" thread instead.

There are at least three common types of flares and it seems to be massive confusion about ISO/DIN flares. Note that the DIN flare is flat on the underside with a matching nut. With some patience, you can make flares yourself using a simple €20 tool available from almost everywhere. Youtube is your friend!

Front suspension travel sensors

For a while I've had suspension travel sensors on my rears. Now finally I have sensors on the fronts as well.

The rears with 3D printer mounts.

Variohm PZ12A 75mm linear position sensors. (100mm on the rears)

This time I didn't have access to a 3D printer so the brackets were made of alu sheets.

The lower mount is a 90 deg bracket fitted to some existing unused threaded studs on the frame, right behind the steering rack. I have no idea why the studs are there in the first place. 

The upper mount is hold in place by the roll bar linkage ball joint.

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

Cool spark plugs

From recommendations I've always used NGK BR7EFS spark plugs - with heat range 7.

After a rolling road session I was told to use a cooler spark plug as the insulator nose was chalky white all the way up after a full throttle sweep.

A too 'hot' spark plug has less cooling, causing the plug to run hotter. On a road engine in traffic this is preferred but on full throttle it can get too hot and become a glow plug instead of a spark plug. Car manufacturers compensate for this with more fuel that cools the plug. On a race engine we can prioritise differently.

As 'cooler' plugs transfers more heat to the engine top it instead lowers the temperature of the tip of the spark plug and allows a more optimum air/fuel ratio on wot without overheating, and a overheating spark plug can cause pre-ignition which is disastrous for an engine. On the flip side are possible troubles starting when cold and carbon fouling when the engine is used at low speed. Very extreme engines even use two plugs with different heat ranges. If any of this will be a problem for me we'll see, but too cold is better than too hot in order to avoid engine damage.

As NGK doesn't manufacture a similar but cooler BR8EFS plug, I had to switch to Iridium. As a bonus iridium plugs require less voltage to fire and therefore allow wider plug gaps, which in turn gives a more stable combustion and less risk of miss fire.

Denso IT24 was cheaper than NGK TR8IX, and Denso is also what SBDev recommends. So I ordered a set from eBay.

Sources:

• Four-Stroke Performance Tuning, 4th edition, A. Graham Bell
• http://www.sbdev.co.uk/Duratec/Electrical%20components.htm
• https://www.ngk.com/Spark-Plugs-NGK-c593.aspx