Print

Variance-Corrected Measured Values.

Temperature compensation.

Temperature difference between forward and return flows.
If the temperature difference between the fuel forward and return lines is ignored, the measured value will be incorrect. In most diesel engine systems, the low pressure fuel system is designed as a ring line. This ensures that even in the case of rapid changes of engine load, enough fuel is available. The lines are also completly flushed, to ensure that there is fuel for injection without outgassing. The kinetic energy of the returning fuel when the injection pump pistons are shut off, the friction of the injection pump pistons and the radiated heat of the engine are the reasons for a higher fuel temperature in the return flow.

In practice, the temperature differences are up to 60 °C. The fuel expands with increasing temperature. The volume changes.

KRAL fuel consumption measurement measures the temperatures of the forward and return flows. Temperature compensation ensures that the difference between the forward and return flow quantities is calculated at exactly the same temperature. The highest measurement precision can only be guaranteed in this way.

Density table of fuels (DIN 51757, method B).
TρLFO(T)ρHFO(T)rel. error LFOrel. error HFO
[°C][kg/m³][kg/m³][%][%]
20906.5976.30,03.2
40892.5961.3-1.51.6
60878.6946.2-3.10.0
80864.4930.9-4.6-1.6
100850.2915.6-6.2-3.2
120835.9900.2-7.8-4.9

Amazingly large effect.
The effect of temperature differences on the measurement precision of the whole system is unexpectedly large. The density table shows that fuel density falls by about 1.5 % per 20 °C temperature increase. In the case of an LFO system with a 40 °C temperature difference, that is about - 3.1 %. For differential measurement in the whole system, Gaussian error propagation indicates an error of 8.5 %. In the case of HFO systems, despite fuel preheating in the booster module, a temperature difference of 20 °C and a density difference of about -1.6% occur. The result is a system error of 5 %. Precise measurements require highly precise KRAL flowmeter with temperature compensation.

 
Temperature-based volume expansion.
Pressure pulse compensation.

Variations of flow rate.
The fuel does not flow evenly through the lines. It pulses, and may even change its flow direction briefly. If this effect is ignored, the measured value for consumption will be incorrect. The injection pumps cause pressure pulses. As the pump piston moves upward, it closes the volume above itself. The fuel is compressed in the pressure chamber to an injection pressure of about 1,500 bar. The end of injection is reached when the control edge of the pump piston releases the fuel line again. A fuel jet then shoots at high pressure back into the fuel line. A pressure wave in the forward and return lines is the result. The fuel is accelerated. The precisely manufactured measurement spindles of KRAL flowmeters quickly follow the changes of the fuel flow rate.

Reversal of flow direction.
In its downward movement, the pump piston creates suction on the fluid column. The result is a negative pressure. When the pump piston, in its downward movement, Releases the fuel line, the pressure chamber fills rapidly with fuel because of the negative pressure. The fuel which flows in makes the forward flow faster and reduces the return flow. In the return line, the direction of flow may be reversed. KRAL flowmeters in principle measure in both flow directions. Using a second sensor, the direction of rotation of the spindles and thus the direction of flow can be detected. KRAL subtracts the reverse flow quantities in the BEM 300 and 500 electronic units. For precise measurement of the small reverse flows, very precise flowmeters are necessary. KRAL flowmeters measure correctly and precisely.

Pump piston of the injection pump.
Pressure measurement in the low pressure fuel system.
 

KRAL AG | Bildgasse 40 | Industrie Nord | 6890 Lustenau | Österreich

Newsletter subscription