Fuel Flow Meters
Diesel engines are becoming increasingly popular in a whole variety of power applications. For everything from generators, boats, trains through to hydraulic packs these power sources are becoming the unit of choice. Fuel consumption and efficiency are generally not monitored in Diesel Engines and the unit’s performance has to be gauged from the manufacturer’s test figures and often never checked again. With increases in fuel prices, the possibility of theft from remote installations along with many other factors. Consequently the monitoring the fuel consumption of diesel engines has become increasingly important with the transport industries in particular taking a lot more interest in their vehicles fuel consumption.
Vehicle manufacturers spend a lot of time and effort checking the fuel consumption figures for the engine on dedicated dynamometers and later with very sophisticated mobile set ups. These fuel systems are very costly and are unsuitable for general use.
On a typical diesel engine the fuel is supplied to the injectors at pressure from a “lift pump”, the injectors use the amount of fuel required for the engine load and return the rest to the fuel tank. If you wish to measure the amount of fuel used you simply need to place one fuel flow meter in the supply line and a second in the return line and use electronics to subtract the difference. Unfortunately the reality of making such a measurement is not so simple.
The table and graph below illustrates a hydraulic power pack running at fairly constant speed. This makes the metering somewhat easier as the ratio of maximum to minimum fuel consumption is quite small. A commercial road vehicle however would typically have a much larger ratio compounding the metering problems.
Engine power kWm
“Lift pump” Flow
Return fuel flow
If now we take an example of two low accuracy fuel flow meters in the flow and return lines of a diesel engine. If these devices each have an accuracy of ±2% of reading and the instrument doing the subtraction is absolute, the figures are shown in the chart below with the possible reading bands within the accuracy of the fuel flow meters. This illustration shows the effect quite graphically.
Engine power kWm
“Lift pump” Flow
“Lift pump” Plus 2%
“Lift pump” Minus2%
Return fuel flow
Return fuel Plus 2%
Return fuel Minus2%
Maximum calculated consumption
Minimum calculated consumption
At full flow the recorded flow reading could be out by over 8% and at low engine consumption the error could be out by nearly 40% from two relatively accurate 2% flow meters. A 0.5% accurate meter would improve the results dramatically but the errors could still be 2% and nearly 10% respectively.
If the engine installation is under fairly constant load and very little time is spent on light duty or tick over then the recorded errors at the low end may be insignificant. However this would not be true for a refrigerated lorry delivering produce in a city where a large proportion of the duty cycle is at low power and hence measurement of flow in this situation would be prone to greater errors.
Engineers are not sending this excess diesel fuel around the engine without good reason. It is used to lubricate and cool the injection system. This increases the return fuel temperature, and this in turn affects 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 which may make the engine look very efficient as apparently more of the fuel is being returned to the tank and less used. This density change on its own could easily result in a 1% change in volume if the return diesel is around 25°C higher in temperature than the supply from the fuel tank. This is of course compounded with the flow measurement errors shown above and would increase the overall error figures to perhaps 12% and 60% – although this will most likely to be biased in one direction. Pulsations in the flow may also affect the performance with some types of flowmeter.
Fuel Flow Meters – A more acceptable measurement solution
All is not totally lost in this diesel flow measurement application as an acceptable metering system can be achieved in one of 2 ways. The first involves a complete fuel system re-build and therefore is not usually practical. With this method a second tank (see schematic below) is installed to receive the return fuel and no attempt is made to measure it. The engine draws fuel from this secondary tank and it is supplied fresh metered fuel from a main reservoir. The single flowmeter is only measuring the fuel supplied to the secondary tank and has therefore has to have the ability to meter the consumption over the whole range of the engine. Great care has to taken to ensure that the fuel temperature does not rise excessively hence a return flow intercooler is required as is a substantial tank volume to ensure that there is no foaming and the fuel is not too hot for the engine fuel system as this will reduce horsepower. This can be an accurate method of measurement but it is not usually practical.
The second metering system (see schematic below) uses two flowmeters and a great deal of care in set-up, even so the results still may not be acceptable in every case. The choice of flow computer is critical. The chosen flow computer must have flowmeter linearization on both channels as well as the ability to perform density corrections using pre-determined fuel characteristics and fuel flow and return temperature measurements. The flow sensors should, preferably, be positive displacement and they must be very repeatable certainly better than ±0.1% and if possible ±0.05%. Their calibration should have the minimum uncertainty and the temperature coefficient for the meters should also be known. The temperature sensors should also be ±0.1% devices although this measurement is somewhat less significant in the end result.
The results of the previous error calculations suggest that, for this system, where the performance graph is very linear, the overall accuracy at full flow is 4 times the sensor accuracy and at low flow it is also 4 times the sensor accuracy but multiplied by the flow range turndown, which in this case approximately 5:1. If the sensors have ±0.1% repeatability variation and built-in temperature measurement, density and flowmeter temperature correction are then performed by the instrument which itself has a finite accuracy ( ±0.01%) we end up with roughly ±0.44% at high flow and ±2.2% at low fuel consumption. This is a much more satisfactory diesel flow measurement result.
An extra factor is the uncertainty of the calibration and this will vary from flowmeter supplier to supplier. To give guidance many national flow calibration facilities claim ±0.05% uncertainty but most supplied flow meters are unlikely to be this good and ±0.1% to 0.25% is a more typical range. Unfortunately some suppliers flowmeters offer uncertainty that is even outside this range. The condition of the engine is also critical as indicated previously if it is not in first class running order the return line could contain spurious amounts of gas increasing the apparent volume of the return flow. An engine flow measurement system installed and correctly monitored could be a useful indicator for such engine decline or other anomalies in the fuel system.
Different fuels will have different characteristics and changing the blend of bio-diesel will affect the results. Any recording instruments should ideally be able to be easily changed to reflect the alternative mixing ratios. Many of the characteristics of these blends are unknown as the properties of every oil will depend on the growing conditions and the processing and whether it comes from the local fast food outlet shop.