As the energy prices continue to rise, the energy consumption becomes also more and more important to the economy of farms. To find ways to save energy, the energy use inside the system and its allocation in the sub-systems must be known. Energy analyses are used to receive this information. For this purpose, all the energy inputs and outputs crossing the system boundaries must be examined. One of the major energy inputs in arable farming is diesel fuel for field machinery.
Fuel consumption can be measured by several methods. The simplest way is manual bookkeeping of the consumed fuel and the work done. Even this simple method gives adequate accuracy in many cases, and it can be used as an information source for energy analysis. However, it requires extra effort and attention from the operator. Therefore, the most common way to fuel consumption measurement has been the use of flow meters. This method can provide a good accuracy and it can be used for wide variety of applications. When using flow meters, the return line from the injection pump has to be either measured separately or be returned to the inlet side of the transfer pump. This may require an external cooling for the fuel in the return line. For high accuracy measurements, the fuel temperature must also be recorded in order to calculate the correct mass flow.
Combining the fuel consumption measurements with GPS position information enables an effortless recording of the velocity and location of the machine. It can also be used to match the measured data to the correct field plots. This information is needed to calculate the fuel consumption in l ha-1. Since there are several low-cost GPS receivers with a data logging feature on a market, the position information was also included as a part of this study. Using the velocity information from the GPS data also makes it possible to determine the working efficiency during the field work.
Measuring the fuel consumption of agricultural machinery with fuel level sensor and GPS position information is a useful method with certain conditions. The continuous measuring periods have to be long enough to achieve reliable results. The fuel tank volume sets the final limit for the accuracy: the absolute error must be smaller than the desired relative accuracy multiplied by the fuel tank volume, or otherwise the desired accuracy cannot be reached. Under these conditions the accuracy is improved when the measured area or fuel consumption rate increases. The crucial factor in the measuring system is the accuracy of the fuel level sensor. This kind of measuring method can be used to provide energy consumption figures for energy analysis. It can also be used to find the critical working methods or processes. The accuracy depends on the type of the machine, length of the measuring period and the fuel consumption rate. The method is best suited for large farms with relatively large field plots. On the other hand, small farms have usually smaller machines with smaller fuel tanks, resulting better absolute accuracy. Separating the road transport from field works increase the workload of data analysis. If the transport distances are long, this method may not be suitable for fuel consumption measurement, especially if the field plots are small.