Abstract
Cooled EGR system is focused on as one of the effective methods for knocking suppression in gasoline engines. On the other hand, hot EGR gas (internal EGR gas) is considered to have a negative effect on knocking. The objective of this study is to investigate the effect of the hot and cooled EGR on knocking phenomenon quantitatively, and find out knock suppression method in direct injection gasoline engines.
Firstly, in order to investigate the effect of cooled-EGR on knocking suppression, three different EGR systems are constructed for experiment. By adding the inert gas (N2/CO2) as simulated EGR gas upstream of intake pipe, the effect of cooled EGR is investigated under the condition that EGR and fresh air is mixed homogeneously. By introducing the actual EGR gas in two ways: low pressure loop EGR (LP-EGR) and high pressure loop EGR (HP-EGR), the effect of EGR mixing condition is investigated. Next, hot EGR effect on knocking is analyzed and the method for anti-knocking control is developed based on 3-dimensional fluid simulation (large-eddy simulation) and experimental approach.
As a result, regardless of mixing method of cooled EGR and fresh air, the combustion phasing at knocking limit is advanced by 0.6 [deg.CA] per 1.0% of cooled EGR ratio. The effects of cooled EGR on fuel consumption differ among cooled EGR systems because of the combustion speed change. No negative effects of cooled EGR on exhaust emission are observed. On the other hand, the hot spots due to the residual gas (hot EGR) exist in the combustion chamber after compression stroke, and contribute to the occurrence of auto-ignition that leads to a knocking. Multiple fuel injection, which includes the early injection in the intake stroke is able to decrease the temperature of the hot spots and makes the combustion phasing at knocking limit advanced by 2 [deg.CA] without increasing the smoke emission.
In conclusion, both systems for cooled EGR (HP- and LP-EGR) have almost same level of potential to suppress knocking under constant EGR ratio and constant intake gas temperature, and the combustion speed is the key factor for lower fuel consumption under higher EGR ratio. Cooling the hot spot (hot EGR) by multiple fuel injection is effective to minimize the negative effect of hot EGR on knocking.
KEYWORDS – EGR, Direct Injection, Gasoline Engine, Knocking, Combustion