Reliability testing in Formula One
With the ban on in-season testing, one of the greatest increase in uses of any other testing equipment is that of the power train transient dynamometer. Costing millions of pounds to buy and even more to run, these are used to simulate precisely the events experienced by the engine and transmission as if it were installed in a vehicle circulating around the track. Controlling not only the engine speed and load but just about every other parameter you can think of – air temperature and humidity, oil temperature, fuel temperature, not to mention the shock loading directly as a result of changing gears, the dynamometer system also tries to simulate the airflow around and through the engine as if the vehicle was in motion. To simulate this induction, air has to be delivered into the engine intake at the same velocity and temperature as the real thing with a similar flow passing around and through the engine bay. Furthermore, since the heat flux passing out of the engine through the oil and water coolers will depend very much on the duty cycle and hence speed and load of the engine, the amount of cooling available has to be simulated depending upon the speed of the car.
But in any form of reliability testing it is essential to get the right balance between realism and repeatability.
In a Grand Prix car the engine duty cycle varies from lap to lap and depends on many factors; tyre temperatures and wear, are but only two. As the tyre heats up it produces more grip and with more grip comes more speed, and as a result the engine duty cycle changes. Towards the end of a driving stint the tyres wear, lose grip and the engine then may spend more time on the speed limiter. At this point the driver will then come off the throttle, become less aggressive and the duty cycle changes yet again. Over the course of a weekend we mustn’t forget either that the track will “rubber-in” progressively changing the duty cycle, and then perhaps an overnight shower of rain could wash everything away and so we will be back again where we started. A simple change in wind direction can also change the test parameters. A tail wind on the main straight may cause the team to run a taller gear to avoid the engine running on the rev limiter well before the end. With just a change in the wind, we would already be 200 rpm down on where we were earlier in the day.
If all that wasn’t enough, traffic is another variable, which has to be taken into account. When one car is following another, not only does the drag change, but the engine may spend much more time on the limiter. Cooling to the engine water and oil systems will also change significantly, producing spikes in the running temperature. And of course we can’t lose sight of the driver. He is probably the greatest variable of all! At one time he may be driving quite sensibly but when the ‘red mist’ comes down, his driving can become either totally irresponsible or very inspired. In any form of realistic reliability test cycle therefore, all of these variables have to be covered.
However, realism is one thing, repeatability is another. For reliability testing you need the test to be as repeatable as possible but if you want more of one then you almost certainly need to compromise on the other.
Written by John Coxon.