3.2.2. Intake temperature sensitivity and controllability: primary reference fuels
The three strategies displayed significant combustion sensitivity to changes in the intake temperature. This was expected consid- ering that the strategies are sufficiently premixed and predomi- nantly controlled by chemical kinetics, which is driven by the mixture temperature. Fig. 16 shows the intake temperature sensi- tivity for the primary reference fuels. An intake temperature sensitivity correction for dual fuel HCCI was done by varying the fuel reactivity using the two port injectors, and the baseline com- bustion phasing was easily recovered for both hotter and colder intake temperatures through slight modifications of the global fuel reactivity the DI timing was kept constant). The major difference with dual fuel HCCI and RCCI was the resulting PPRR. RCCI yielded significantly lower PPRR than HCCI.
In the case of single fuel PPC with PRF 94 gasoline, for the hotter intake temperature the baseline combustion phasing was unre- coverable, despite increasing the premixed fraction from 79.1% to 95.2%. As mentioned before, the baseline operating condition for single fuel PPC yielded approximately the same combustion
duration as fully premixed HCCI. This suggested that the equiva- lence ratio distribution created by the baseline injection strategy was not broad enough to produce a range of ignition delays. Thus, it was expected that if changes were made to the injection strategy to create a more premixed charge, the ignition delay will be relatively unaffected, as observed for the increased intake temperature case.
On the contrary, for the colder intake temperature, by using a combination of more direct injected fuel and a retarded timing, the baseline combustion phasing was reasonably recovered. However, this came at the cost of increased NOx emissions.
3.2.3. Intake pressure sensitivity and controllability: primary reference fuels
In addition to intake temperature variations, the sensitivity of the combustion process to the intake pressure also was investigated by Dempsey et al. [5]. This is extremely important for the devel- opment of transient operation of a turbocharged engine, because under a speed and load transient, the intake pressure changes more rapidly than the intake temperature. Fig. 17 shows the results of changing the intake pressure by approximately ±0.1 bar for all three strategies. The same order of sensitivity to the intake pressure was observed for HCCI and PPC as there was to the intake tem- perature. Interestingly, when the intake pressure was reduced, the combustion phasing retard for PPC operation was greater than that
observed for HCCI. In addition, RCCI is not as sensitive to the intake pressure as the other strategies.
For the fully premixed HCCI combustion case, the intake pres- sure sensitivity correction was again done, just as with the intake temperature compensation, by simply varying the port fuel injec- tion percentage with a fixed DI timing. The baseline combustion phasing was recovered and NOx emissions remained well below the target level. The results of intake pressure sensitivity for single fuel PPC with PRF 94 were also extremely consistent with the findings of intake temperature perturbations.