3 3 F o c u s “At high engine loads,” says Rainer Schantl, “it is relatively simple to achieve high accuracy and good repeatability of fuel consumption measurements. But this also happens to be the end of the scale where it least relevant – most legislation, for instance, relates to low flow rates and so this is where it is most vital to provide high accuracy with excellent repeatability.” Dr. Michael Cernusca says that: “It is important for us to look at this challenge from the point of view of our customer – an engine developer working from a few different types of design. They measure with different settings then they compare which results in the lowest fuel consumption. Development cost is related to how much time is needed to do this engine calibration – and in order to try and reduce their costs, our customers try to reduce their development and test time. New, model based approaches help because they have the potential to dramatically reduce the number of measurements that are needed. However, those fewer measurements must be of highest accuracy and repeatability. This ensures quality data which is again the foundation for the achievable model quality. This is why there is a requirement on high repeatability on fuel measurement; and it is the combination of a measurement device with high performance and the proper setup on the testbed which enables you to reduce the actual time spent in development.” Influencing factors on the measuremant chain There are many influencing factors that have an impact on the entire measurement chain. So where do you start? According to Schantl: “The effort you have to spend to quantify these different influences are almost infinitesimal and this is the reason that we defined five major reasons – or parameters – that can be easily identified and can give a good element of prediction. In other words: we made a complex thing a little simpler.” These five parameters are: 1) Measurement uncertainty of the flow sensor 2) Measurement uncertainty based on fuel temperature stability 3) Measurement uncertainty based on measurement time 4) Measurement uncertainty of the torque measurement 5) Measurement uncertainty and control stability & measurement of the engine speed Schantl says that it is relatively simple possible to ensure (1) proper accuracy and repeatability of the sensors, but that AVL is the world leader in terms of providing (2) temperature stability of the fuel being measured (better than +/-0,02 °C), which results in a contribution of as little as 0.2 % of the overall measurement uncertainty at low engine part load. “No-one else in the world can match this to our knowledge, and already minor lower temperature stability results in much higher measurement uncertainty and much worse repeatability – especially in engine part load” says Schantl. Increased measurement time (3) can only partly compensate lower temperature stability but reduces the testing efficiency. Therefore, AVL’s fuel measurement systems are tuned for short measurement times too. However, Schantl identifies the complex torque measurement (4) as being a further crucial aspect of AVL’s differentiation compared to other measurement set-ups for specific fuel consumption. AVL’s accurate and reliable dyno and control systems mean that an exact reproduction of engine load conditions is ensured. A lot of application know-how All these different components of a measurement system also need to be put together in the right way in order for everything to work correctly and with the reproducability of +/- 1 g/kWh. According to Dr. Cernusca: “You need a lot of application know-how – in terms of the correct diameter of fuel pipes, the placement of the fuel filter, the distance between engine and fuel consumption measuring device etc. Crucially, we not only provide the equipment to our customers, but we support our customers at the installation too in order to make sure that no detail is overlooked. AVL is the only one that has more than 45 years experience of this and this is another crucial factor that allows us to achieve the high rates of accuracy and repeatability that are so important for our customers today.” < > Highly precise: AVL fuel consumption measurement systems
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