The Effect of Post-Catalyst Exhaust Gas Recirculation on Combustion in Highly Rated SI Engines

Student thesis: Doctoral ThesisPhD


Despite the growing push by governments towards vehicle electrification, the SI engine will remain integral to automotive powertrain technology for the foreseeable future. With increasingly stringent emissions standards and testing protocols, further improvements in SI engine efficiency are required.

Engine downsizing and downspeeding are effective strategies for reducing CO2 emissions from GDI engines, but anticipated upcoming restrictions on CO emissions in the Euro 7 emissions standards have intensified the focus on knock control methods which can maintain stoichiometric combustion at high load.

Exhaust gas recirculation (EGR) has been proven to be a valuable technology for controlling knock whilst maintaining lambda one operation, and is also capable of providing efficiency gains at low load.
Whilst the evidence supporting the benefits of EGR is unanimous, there is no consensus on optimum EGR strategies amongst the wealth of various possible architectures.

Few studies in the literature address the question of EGR composition effects, namely whether the EGR gas is sourced from before or after the catalyst, and this remains an area which is often overlooked during assessments of EGR performance.

This thesis focuses on the combustion effects of EGR catalysis on high load operation using both experimental and modelling methods.
An artificial boosting system replaced the turbomachinery for the experimental work, which allowed the combustion effects to be assessed at constant load and engine boundary conditions.

Two experimental campaigns were carried out on two different high load GDI engines, with the experimental techniques evolved for the second iteration to improve the quality of the results.
The increased knock inhibiting effects of catalysed EGR were confirmed, with the catalysed EGR increasing knock limits by 0.5-1 CAD over equivalent non-catalysed conditions.
It became apparent that the presence of NO in the EGR is one of the most significant factors in this difference, which supports evidence from the literature on NO effects on autoignition.

The modelling study revealed more detail on how the different EGR species affect the combustion process whilst also providing an opportunity to assess the current state of the art in combustion simulation methods.
The modelling results further support the evidence of NO being a major autoignition promoter.

The relatively new simulation technique of stochastic reactor modelling was employed to carry out the modelling work.
Initial tests vastly underestimated the dilution effect of EGR, with high burn rates causing peak pressures to be over-predicted by up to 10 bar at 9% EGR.
Adjustments to the laminar flame speed correlation improved the accuracy of the predicted burn rate but the peak pressure was still up to 5% higher than experimental results with EGR. Further adjustments to the fractal flame propagation model provided more improvement to the model's EGR response within the speed/load condition tested.

With the increasing complexity of vehicle powertrains there is a growing dependence on simulation methods to reduce the time and cost of powertrain design and optimisation processes, and combustion modelling is one of the key obstacles to progress in this field.
This work highlights the importance in considering the chemical effects in determining optimal EGR architectures, and identifies where improvements are required for integration of more detailed combustion models into 1-D engine simulations.
Ultimately these methods could contribute to improved accuracy in simulation-based engine design validation and optimisation processes.
Date of Award15 Jan 2020
Original languageEnglish
Awarding Institution
  • University of Bath
SponsorsEngineering and Physical Sciences Research Council
SupervisorSam Akehurst (Supervisor), Andrew Lewis (Supervisor) & Chris Brace (Supervisor)

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