Conflicting evidence does not support making a firm conclusion on the fuel consumption benefit of catalysed Exhaust Gas Recirculation (EGR), according to a review of current studies by a team at the University of Bath (UK). In catalyzed EGR, a catalyst alters the chemical composition of the exhaust gas mix before its reintroduction to the engine. As an example, one study found a decrease in fuel consumption of up to 2%, while another found an increase of 1.5%-3.5%.
According to the review, the conversion of HCs, CO, and NO in the exhaust gas by the catalyst can result in up to a 4.5% reduction (in extreme cases) in the calorific value of the charge for catalyzed EGR when compared to equivalent operation with un-catalyzed EGR; this reduction in calorific value has a negative impact on the achievable BSFC. An open access paper on the study (an update of an earlier version published late last year) appears in the International Journal of Engine Research.
Downsized engines can use increased compression ratios and boost pressures to improve their volumetric efficiency, in order to achieve comparable performance to their larger counterparts. However, the level by which these can be increased is knock limited, leading to the need for knock control strategies to be employed.
… one of the most promising technologies for this purpose is widely believed to be exhaust gas recirculation (EGR)—the introduction of a proportion of the burned exhaust gases back to the inlet charge, which can allow an engine to run at stoichiometric conditions throughout its operating range with high compression ratios and spark timing closer to optimal.—Parsons et al.
Knock (autoignition) is heavily influenced by the end gas temperature– and pressure–time histories, which in turn are influenced by three factors: heat transfer; compression; and chemical reactions.
EGR increases the mass of the in-cylinder mixture, thereby reducing the temperature rise via heat transfer and slowing the combustion rate to reduce peak pressures. This is counteracted to some extent by a reduced specific heat capacity of the charge since the mass fraction of fuel in the charge is reduced by dilution resulting in a higher specific heat ratio.
Further, the chemistry of combustion is also affected by some of the components present in the exhaust gases, the most potent of these being NO, CO, and unburnt hydrocarbons (HCs).
EGR also can provide significant economy benefits under part-load operation in which optimal combustion phasing can be achieved without EGR—EGR at these conditions dethrottles the engine by displacing some of the inlet air, reducing the need for physical throttling to limit airflow into the cylinders.
A series of studies in the mid-2000s by Alasdair Cairns and colleagues at MAHLE Powertrain found that that EGR was a significantly more effective knock suppressant than either excess air or fuel, with large fuel consumption benefits of up to 17% at high load coming from the elimination of the need for overfuelling. Significant CO, CO2, and HC emissions improvements were also observed.
However, the Bath team noted in its paper:
The evidence for the fuel consumption benefits of catalysed EGR is not conclusive, primarily due to the potential error introduced via EGR rate measurement with a catalysed source. The existing literature concerning the use of catalysed EGR does not fully address this issue and therefore cannot give a comprehensive conclusion. However, there is compelling evidence, through the consideration of the species present, that catalysed EGR can improve the knock limit at high load. Whether this increased knock limit is offset by other losses within the system is as yet unclear.
At low loads, it would not be expected that catalysed EGR would show much advantage over un-catalysed EGR due to pressure losses across the catalyst. Slower combustion rates with catalysed EGR, resulting from the decrease in CO content and increase in CO2, may also have implications for low-load points, where combustion stability may be affected by lower flame speeds at higher EGR rates.—Parsons et al.
Further investigation on the impact of catalyzed EGR is required, they suggested, with specific questions being:
Would higher levels of instrumentation such as catalyst inlet/outlet temperatures and detailed Fourier transform infrared (FTIR) spectroscopy exhaust gas measurements shed more light on the mechanisms involved?
Does the trade-off between the extended knock limit for a given EGR mass flow and the lower calorific value of recirculated exhaust gases provide a potential benefit to engine efficiency?
If the maximum achievable EGR rate is reduced by catalyzing the exhaust gases, would the extended knock limit compensate for this so that benefits are still seen?
Would the extended knock limit reduce the cooling demands on the EGR loop, or would exothermic reactions involved in catalysis override this to increase cooling demands?
At higher loads, will the knock control benefit of catalyzed EGR become more apparent?
Dominic Parsons, Sam Akehurst, Chris Brace (2015) “The potential of catalysed exhaust gas recirculation to improve high-load operation in spark ignition engines” International Journal of Engine Research vol. 16 no. 4 592-605 doi: 10.1177/1468087414554628