Experimental Evaluation of Lean-burn and EGR as Load Control Strategies for Methanol Engines 2012-01-1283

The use of light alcohols as SI engine fuels can help to increase energy security and offer the prospect of carbon neutral transport. These fuels enable improvements in engine performance and efficiency as several investigations have demonstrated. Further improvements in efficiency can be expected when switching from throttled stoichiometric operation to strategies using mixture richness or exhaust gas recirculation (EGR) to control load while maintaining wide open throttle (WOT).

In this work the viability of throttleless load control using EGR (WOT EGR) or mixture richness (WOT lean burn) as operating strategies for methanol engines was experimentally verified. Experiments performed on a single-cylinder engine confirmed that the EGR dilution and lean burn limit of methanol are significantly higher than for gasoline. On methanol, both alternative load control strategies enable relative indicated efficiency improvements of about 5% compared to throttled stoichiometric operation. This is mainly due to the absence of throttling losses, reduced cooling and dissociation losses caused by the lower in-cylinder temperatures obtained through dilution. On gasoline, the efficiency benefits are limited because of the slow burn rates and high cyclic variability associated with dilution. The reduced in-cylinder temperatures also allow a significant decrease in engine-out NO_x emissions, but this is not sufficient to justify the loss in three-way catalyst (TWC) efficiency associated with lean combustion.

Based on these results, it was concluded that the WOT EGR load control strategy seems more promising as it allows to significantly increase the part load efficiency without sacrificing in terms of tailpipe emissions. A second series of experiments was carried out on an engine more suitable for EGR dilution. This engine was derived from a four-cylinder turbocharged diesel engine and had a compression ratio of 19.5:1. This elevated compression ratio, in combination with turbocharging and high levels of in-cylinder turbulence allowed to raise the burning rates enough to use EGR in levels up to 50% with limited cycle-to-cycle variation. Throttleless load control was possible down to 3 bar BMEP in this way.

Thanks to the elevated compression ratio peak brake thermal efficiencies of up to 42% were obtained. At part loads, relative efficiency improvements up to 20% were achieved and the engine-out NO_x emissions were cut down to values that are negligible compared to throttled stoichiometric load control. These results confirm that WOT EGR operation is an interesting load control strategy for dedicated methanol engines, provided that the engine design lends itself to high dilution levels.