Fuel Consumption Measurement

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Fuel Consumption Measurement technically is not as simple as you may think… But before we get on to the technical sections, it pays to look at the reasons why we would want to measure it directly at the engine.

Firstly it is possible to simply wait until you fill the tanks and work out how much fuel was burnt. So looking into FC measurement means we’re looking for real time data.

So the next obvious question is Why? or more so in context in what can you do with the data?

Well, qualification of fuel burn totals is a big one, as is momentary fuel burn under certain loads. From that you can look at engine efficiency and make possible maintenance scheduling changes, driver behaviour, set contract KPIs and so on.

So why is there so much negativity surrounding Fuel Consumption Measurement (real time) on diesel engines? Simple: IT IS COMPLICATED.

Firstly, to get a reading, we need to measure the fuel transfer Into & Out of the Engine. Our Diesel engines generally have a circulation rate of fuel to cool the injection components and to ensure fuel cleanliness. The worst part is that this circulation rate is usually 3-4 times the engine’s consumption value at 100% load.

So, if we have a 1% accurate meter, we now have  4% error factor for the inlet meter, and an accumulated 3% error for the return. Therefore, our system accuracy is now 7% (or better inaccuracy) of the consumption value – the end game – and that is a lot.

Adding to this, the temperature is usually quite a bit hotter on the way out from the combustion process. Most Oil refined products have a basic volume change over heat rate of 7% per 100 degrees C. Therefore, if the fuel in the outlet is 50 degrees hotter than initially measured from the inlet side – common in common rail engines, – the introduced error factor is 3.5% (half of the measured heat rate).

So as you can see, so far we’ve covered two of the potential issues, and we’ve already introduced over 10% error factor. Now it is worth nothing here, that this error factor is based on 100% consumption(or real time fuel burn), so if we’re looking at 50% load, this error factor is now multiplied by 2 to give 21%! The error factor at idle is (on average) well over 100%.

Next week, we will be posting up solutions to these problems plus a bunch of others not even covered here! After all, providing solutions is what we do!

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