Feature sponsored by Flow, level & control W
ith increasing fuel prices, all sectors of the transport industry, from small trucks to larger super-tankers, are taking an interest in
effective ways of measuring their vehicle’s fuel consumption and hydraulic oil flow. Unfortunately, it is not a simple task! Engine fuel consumption and efficiency generally goes unmonitored, the performance of a diesel power unit assessed from the manufacturer’s engine test figures and seldom checked again. Whilst vehicle manufacturers do spend a lot of time and effort checking the consumption figures for an engine, this is done on
dedicated dynamometers and, later, with very sophisticated mobile set ups. These test systems are very costly and unsuitable for general use; on the other hand, a dashboard fuel gauge simply measures the fuel left in the tank, rather than the fuel consumed by the engine.
FLOW METER TYPE IS CRITICAL Metering fuel consumption can be achieved using positive displacement flow measuring devices such as Oval Gear flow meters. However, differing results in flow meter accuracy and error rates can be seen depending on a number of factors. For example, an increase in the return fuel temperature will affect the density and viscosity of the diesel and, on a poorly maintained engine, it may even contain combustion gases or air. A small amount of this may make the engine look very efficient, as apparently more of the fuel is being returned to the tank and less used. The density
change alone could easily result in a one per cent change in volume if the return diesel is around 25°C higher than the supply from the fuel tank. The choice of flow computer is critical; it must
have flow meter linearisation on both channels as well as the ability to perform density corrections using pre-determined fuel characteristics, together with fuel flow and return temperature measurements. The flow sensors should, preferably, be positive displacement types and they must be very repeatable; certainly better than ±0.1 per cent. If the flow sensors have ±0.1 per cent repeatability and built-in temperature measurement, density and flow meter temperature correction are then performed to give a more precise result. As indicated previously, if the condition of the engine is not in good running order, the return line could contain spurious amounts of gas which would increase the apparent volume of the return flow. An engine flow measurement system installed and correctly monitored could be a useful indicator of engine decline or other anomalies in the fuel system. So, although installing a fuel flow measuring system is not simple, it can be achieved with careful consideration of all flow parameters.
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October 2022 Instrumentation Monthly
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