The present paper illustrates an investigation about the potentialities of injection rate shaping coupled with an after injection. A pilot shot can either be absent or present before the rate-shaped boot injection. The experimental tests have been performed on a partial PCCI Euro 5 diesel engine endowed with direct-acting piezoelectric injectors. Starting from optimized triple pilot-main-after injection strategies, boot injection was implemented by maintaining the direct-acting piezo injector needle open at part lift. The results of two steady state working conditions have been presented in terms of engine-out emissions, combustion noise and brake specific fuel consumption. In addition, in-cylinder analyses of the pressure, heat-release rate, temperature and emissions have been evaluated. Considering the in-cylinder pressure traces and the heat release rate curves, the injection rate shaping proved to influence combustion in the absence of a pilot injection to a great extent. A pilot shot, added before the boot injection, determined a mitigation of the influence of rate shaping. A simultaneous reduction in NOx and soot can be achieved for small quantities of fuel injected during boot injection, compared to the optimized triple injection strategies. However, a trade-off exists between emissions and combustion noise when boot injection is applied. On the other hand, the variations in bsfc were only minor. The advantages of the application of boot injection are greater for low to medium loads than for medium to high loads. However, it was not possible to find a working condition in which boot injection provided simultaneous advantages on emissions, noise and consumption, compared to the optimized triple injection.
