1. Introduction
Diesel engine plays a fundamental role in a variety of applications; its combustion process allows for the adoption of high volumetric compression ratios, leading to high thermodynamic efficiency and low CO
2 emissions [
1].
Due to the strong need to reduce CO
2 emissions, the overall efficiency of the engine [
2,
3] and the use of alternative fuels (renewable or synthetic) assume higher and higher importance in research activities [
4,
5,
6].
On the use of alternative fuels, the literature is rich; many contributions are devoted to investigating their influence on spray formation processes [
7,
8,
9], on combustion [
4,
10], on pollutant formation, emission, and performance [
7,
8,
9,
11], on wear and components’ life [
12,
13], and on after-treatment systems [
14].
Several other research efforts are devoted at improving the overall efficiency of the engine. Some of them focus on reducing friction losses (cylinder-piston pair, piston rings, bearings, and lubrication) [
15,
16,
17], on engine induction efficiency [
18], on reducing losses related to heat exchange during combustion-expansion phases [
19], and on the energy recovery at the exhaust [
20].
Keeping in mind the research lines mentioned above, it is useful to regard the injection system as a mechanical-hydraulic ancillary operating on fuel. Plainly, the overall performance of the engine reflects the fuel properties, through at least two factors in mutual dependence: on the one hand, the influence on combustion and on the other hand, the influence on the efficiency of injection system, requiring engine torque to be operated.
Referring to common rail high-pressure systems, a considerable fraction of engine power is required to drive the pump. In [
21] it is well highlighted that even a limited increase in pump efficiency, given the high power involved, can represent a significant contribution. Pump efficiency depends significantly both on the shaft speed and on the injection pressure. Thus, the overall efficiency of diesel engine depends not only on the absolute performance of the pump but also on its managing (in relation to the other components of the injection system).
From the energy viewpoint, whatever is the level of injection pressure, the common rail system is penalized by the excess fluid laminated by the pressure control valve. Therefore, the sizing of the pump for a given engine and for a given application should undergo careful consideration.
It is now evident that the exhaustive characterization (and modeling) of pump performance is the key element for an efficient management of the injection system, both for traditional and alternative fluids.
The scientific literature on high-pressure diesel injection pumps covers many topics, typically encountered when high-pressure hydraulic machines are investigated.
About pump structure, tribology, and noise, reference [
22] deals with the tribological aspects of the piston-cylinder pair, taking the characteristics of the contact surfaces into account. In [
23,
24], the effects of plunger micro-motion (offset and inclination) on pump operation are investigated by modeling. Reference [
24] reports the types of wear encountered in CR high-pressure pumps, highlighting the effects on their functionality. In [
25,
26], the role of wear in pump noise emission is analyzed, pointing out that the opening of the pump delivery valve is the greatest contributor to noise emission. In [
27], the possibility to reduce the gap between piston and cylinder is discussed and a hollow layout of the piston is introduced.
The role of pump efficiency on the injection system management has been recently highlighted in [
28]. The dynamical performance of the high-pressure pump and the impact of the rail pressure control strategy on instantaneous torque, energy saving, and flow rate ripple are thoroughly investigated in [
29].
The fundamental investigation on CR pump performance is reported in [
21], where the results of a theoretical-experimental activity aimed at characterizing the behavior of injection radial pumps are reported; it is worthy to be pointed out that pump performance is expressed by volumetric efficiency and torque efficiency. These parameters are the fundamental information to define the proper management of the injection system. The results reported in [
21] refer to the radial pumps, whose layout is based on three pumping elements arranged radially, displaced by 120 degrees.
The configuration of the current generation pumps is instead based on a single pumping element that performs two work cycles for each shaft revolution; pump shaft speed is also modified, being the same as that of engine crank shaft.
Experimental investigations on single piston pumps and results on performance in terms of volumetric and torque efficiency have not yet been reported in the literature. Aimed at filling this gap, the authors have recently devoted a research activity in this field to investigating a single piston pump; volumetric and torque efficiencies in a wide operation range of the pump are measured both with standard diesel fluid and with diesel-biodiesel blends [
30].
On one side, volumetric and torque efficiency of the pump certainly provide the crucial information discussed above. On the other hand, especially when alternative fluids (e.g., biodiesel and its blends) are considered, it is fundamental to investigate the pump work cycle by experiments.
The instantaneous measurement of pressure in piston working chamber allows one to identify suction, compression-expansion, and delivery phases of the fluid. The effects of fuel properties become appreciable on each pumping phase and comparative analyses are straightforward. Such information is also crucial to validate pump models that take the cylinder-piston pair into account, where leakages, deformations, and other non-idealities affect pump operation and its efficiency.
Although this experimental approach is notorious in the field of volumetric machines, it lacks in the literature on CR injection pumps. The contributions present in the literature, such as [
21,
31,
32] are limited to the modeling activities. In addition, reference [
33], that deals with an in-depth modeling activity of a three-piston radial pump, reports just a single indicator diagram, communicated by the pump manufacturer.
The severe stresses on pressure transducers and the limited accessibility for their installation on pump bodies represent critical issues for the measurements in piston working chamber. Nevertheless, this article reveals the suitability of such experimental approach on the current-production CR pumps. Once the details of the method and the experimental set-up are presented and discussed, a single-piston CR pump is characterized through the analysis of its pumping cycle diagrams. After the analysis is completed for the standard diesel fuel, the focus is moved on the use of alternative fuels (pure biodiesel B100 and biodiesel/diesel blends B20 and B40), highlighting how the fluids influence the behavior of the pump. The investigations are aimed at describing the operating characteristics of the pump, quantitatively. Among the various aspects of interest, here the attention is focused on those features playing a fundamental role on the global efficiency of the pump. Thus, the amplitudes of the pump work phases, the ranges of pressure fluctuations, and the pressure-rise rates are quantified and reported, providing crucial information in the field of current generation high-pressure injection pumps.
4. Conclusions
This article presents an approach for the experimental investigation of common rail pumps, that is based on the measurement of pressure in the working chamber of the piston. It represents an innovation in this area, since the past research activities focus on global performance (in terms of volumetric- and torque-efficiency), without investigating the internal processes of the pump. The provided experimental data allow one to effectively describe the pump operation, to separate and analyze the phases of suction, compression, delivery, and expansion, at quantitative level.
This contribution is particularly useful in characterizing the pump response both to the operating parameters (delivery pressure and shaft speed) and to the fuel type. It undoubtedly helps to unveil the processes inside the pump, which originate from the combination of relatively complex phenomena driven by leakage, by the dynamics of the valves, and by the fluid-structure deformation, on which the efficiency of the pump ultimately depends.
The results, expressed in terms of indicator diagrams and their key features, are completed through pressure measurements in the inlet and outlet environments, correlating the pump operation to the conditions of inlet and delivery chambers. Results highlight the role of the fundamental operating parameters (speed and delivery pressure) and the influence of fuel type, exploring diesel, biodiesel, and their blends.
Most importantly, the reported indicator diagrams together with the quantitative description of the key features of pump operation phases (pressure rise rate, pressure fluctuations, and phases extent) are essential to build and validate CR pump models based on piston-cylinder pair simulation, taking leakage, deformations, valves’ behavior, and fuel properties into account.