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Exhaust gas recirculation can be achieved by means of two different routes: the high-pressure route (high-pressure exhaust gas recirculation), where exhaust gas is conducted from upstream of the turbine to downstream of the compressor, and the low-pressure one (low-pressure exhaust gas recirculation), where exhaust gas is recirculated from downstream of the turbine and of the aftertreatment system to upstream of the compressor. In this study, the effectiveness of both exhaust gas recirculation systems on the improvement of the NOx-particulate matter emission trade-off has been compared on a Euro 6 turbocharged diesel engine equipped with a diesel oxidation catalyst, a lean-NOx trap, and a diesel particulate filter. Emissions were measured both upstream and downstream of the aftertreatment system, at different combinations of engine speed and torque (corresponding to different vehicle speeds), at transient and steady conditions, and at different coolant temperatures as switch points to change from high-pressure exhaust gas recirculation to low-pressure exhaust gas recirculation. It was shown that low-pressure exhaust gas recirculation was more efficient than high-pressure exhaust gas recirculation to reduce NOx emissions, mainly due to the higher recirculation potential and the lower temperature of the recirculated gas. However, such a differential benefit decreased as the coolant temperature decreased, which suggests the use of high-pressure exhaust gas recirculation during the engine warm-up. It was also shown that the lean-NOx trap storage efficiency decreased more rapidly at high engine load than at medium load and that such reduction in efficiency was much faster when high-pressure exhaust gas recirculation was used than when low-pressure exhaust gas recirculation was used.

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Tests with diesel/n-butanol blends (up to 20% by volume) were carried out in a Euro 6 engine following the New European Driving Cycle in a test bench. No decrease in engine efficiency was observed when n-butanol is used and fuel consumption increased proportionally to its lower heating value. Regarding emissions, total hydrocarbon and carbon monoxide increased when n-butanol is used. On the other hand, the use of n-butanol reduces the particle emissions down to a minimum value (for around 16% of n-butanol content) and then increases particle emissions again. This was observed in both mass and particle number. This trend occurs as a consequence of the compromise between the increase in oxygen content, which inhibits soot formation, and the increase in hydrocarbon emissions, especially under cold-engine conditions, which promotes liquid nucleation. Finally, NO X emissions remained unchanged as a consequence of the compensation between larger ignition delays and shorter combustion durations.

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