Gasoline compression ignition—i.e., igniting gasoline purely by compression, as with a diesel, rather by using a spark—is a promising, high-efficiency, low-temperature combustion mode that offers low engine-out NOx and soot. (Earlier post.) GCI, however, is challenged by stable idle- to low-load operation (i.e., 0-2 bar BMEP) because it is challenging to ignite the low-reactivity gasoline purely through compression.
One way to address that challenge is through optimizing the injection system and injection strategy to ensure that the air-fuel mixture maintains a high level of reactivity. A team from Argonne National Laboratory now reports in a paper published in the International Journal of Engine Research on the effects of injector nozzle inclusion angle, injection pressure, boost, and swirl ratio on gasoline compression ignition combustion.
In our previous work, we found that there is an optimum start of injection (SOI) timing under low-load conditions, which results in greatest reactivity of the mixture, by being late enough in the compression stroke to avoid spraying fuel into the squish region (and thus reducing its reactivity through heat loss), and at the same time being early enough to provide enough residence time for chemical reactions leading to auto-ignition. Ra et al. performed a numerical investigation into the effect of injection strategy, charge composition, boost, and swirl on GCI performance and emissions.
In the current simulation study, we investigate the impact of injector nozzle inclusion angle and injection pressure on ensuring sufficient reactivity of the charge. We also explore the impact of boosting the intake on increasing reactivity and promoting stable ignition. Additionally, we study the effect of varying the swirl ratio on in-cylinder conditions, and consequent impact on auto-ignition.—Kodavasal et al.
The team performed closed-cycle computational fluid dynamics simulations representing a single cylinder of a four-cylinder 1.9-liter engine, operated in gasoline compression ignition mode with 87 anti-knock index (AKI) gasoline. (The researchers used “Fusion,” a 320-node computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory.)
The researchers studied two different operating conditions:
850 rpm, 4 mg fuel/cylinder/cycle, representative of a 0 bar BMEP idle condition; and
1500 rpm, 10 mg fuel/cylinder/cycle, representative of a 2 bar BMEP low-load condition.
They analyzed the mixture preparation and reaction space from the simulations to gain insights into the effects of the different parameters on achieving stable low-load to idle gasoline compression ignition operation.
In their earlier study, the Argonne team had shown that the stock injector with an inclusion angle of 148° resulted in fuel entering the squish region, resulting in loss of reactivity and poor combustion. In that work, they concluded that a narrower, 120° nozzle inclusion angle kept the fuel vapor within the bowl region, resulting in more stable combustion.
Among the findings of this study were:
Narrower nozzle inclusion angles allow for more reactivity or propensity to ignition (determined qualitatively by computing constant volume ignition delays) and are suitable over a wider range of injection timings.
Under idle conditions, lower injection pressures helped to reduce overmixing of the fuel, resulting in greater reactivity and ignitability of the gasoline. However, under low-load, lower injection pressures did not increase ignitability. The team suggested that this is due to reduced chemical residence time resulting from longer injection durations.
Reduced swirl maintained higher in-cylinder temperatures through compression, resulting in better ignitability. Increasing the swirl ratio results in retarded ignition under both operating conditions. This is not due to the overmixing of the fuel with higher swirl; rather, it is from the increased heat losses, resulting in lower near-TDC temperatures with enhanced swirl.
Boost advanced ignition under both operating conditions, by enhancing reactivity, through higher in-cylinder pressures.
Janardhan Kodavasal, Christopher P Kolodziej, Stephen A Ciatti and Sibendu Som (2016) “Effects of injection parameters, boost, and swirl ratio on gasoline compression ignition operation at idle and low-load conditions” International Journal of Engine Research doi: 10.1177/1468087416675709