The spray momentum measurement can be applied also for GDI injectors in order to analyze the sprays generated by these components. Cleary it is possible to carry out the same analysis described above for common rail injectors in terms of spray momentum uniformity and spray axis measurement. However most of GDI injectors can be difficult identify the single spray structure because the sprays by multi-hole injectors tend to join together. For this reason the test system has been provided a particular function in which the measurement domain is unique for all the jets. A typical result of this measurement is showed in Figure: 9.

Based on this measurement it is possible to find the spray real axis trajectory and quantify the hole to hole un-uniformity in terms of spray momentum.

This ability to quantify the spray uniformity can be very interesting in order to optimize the fuel stratification in GDI engine and to avoid the spray impingement problem. In fact a spray with a low momentum can be very instable in air turbulent condition typical of combustion chamber; on the other hand if the momentum is very high it can impact on the wall of combustion chamber and cause high row emission.

Finally the spray momentum characterization seems to be crucial in the injector drawing and in the optimization of air flue mixing.


0.175 |

0.15 z 0.125 15 0.1 o


0.075 g 0.05 qj 0.025 g 0




Figure: 9 — Example of spray momentum distribution of GDI injector




In the present paper, the development of an experimental approach and a dedicated rig to measure the spray momentum flux distribution is presented. In order to develop the local momentum rig, a combined experimental and numerical approach was followed, using CFD for preliminary analysis of possible different configurations of the sensing device. Basically, the momentum distribution is measured by means of the same indirect approach used for the global measurement, i. e. measuring the impact of the spray onto an orthogonal surface (target). To this end, the target must be designed to orthogonally deflect the spray, which loses its axial velocity components: the application of the momentum conservation equation states that, in steady flow conditions, the impact force equates the axial spray momentum flux. In a previous paper by the same research group, it was verified that if the target is properly designed, this measuring principle can be considered correct also in transient injection conditions. The local momentum measurement is based on the same principle when only a small portion of the spray at a time is analyzed. To this end, a spatial filter must be adopted to select a small "flow tube" in the spray structure; this part of the jet is allowed to impact on a target transferring its momentum flux to a force sensor, while the other portions of the spray are excluded from the measurement. In the "flow tube" selected position, particular care must be devoted not to significantly alter spray structure; in this case, this result is obtained by the use of a conical adapter that softens the over-pressure field caused by the insertion of the measuring device. Moving the sensing arrangement (force sensor, adapter and target) inside the spray structure allows the determination of local momentum flux maps and their space and time integrals.

This research activity resulted in the development of a local spray momentum measuring bench, which it is possible to easily analyze the momentum flux for the different jets of the same injector, in pressurized conditions. The local spray momentum measure can give significant insight into the internal spray structure and evolution, allowing a direct evaluation of the injection process quality in terms of spray evolution, uniformity between jets and jet shape. Further, the spatial integration of the local data can give, with satisfactory accuracy, the same
information provided by the global momentum flux measurement in terms of in­nozzle cavitation intensity and mean flow velocity at the nozzle exit.

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