Converting a production 6-cylinder Heavy Duty Engine to Dual-fuel mode operation using supervisory calibration and manifold injection

抄録

Carbon dioxide emissions from heavy-duty vehicles can be reduced by converting, or retrofitting, conventional diesel engines into “dual fuel” mode operation using hydrogen and diesel fuel. Conversion can involve small and relatively cheap changes to design, thereby allowing a diesel engine to be fuelled by either conventionally, using diesel fuel (direct injection), or in conjunction with hydrogen (indirect injection – “fumigation”) in the intake manifold. We evaluate the conversion of a production, 6-cylinder 12.7L truck engine to run in dual fuel mode. Commercially viable conversions would permit, in the short term, the no-regrets ‘pump priming’ of a hydrogen pricing, production, storage and distribution infrastructure which, in the medium term, could also benefit the development of hydrogen-powered inshore marine vessels and trucks powered by, for example, fuel cells. The aim of this work was to establish the limitations and advantages of pursuing the conversion of an in-production heavy duty engine to dual fuel operation with the least changes to the engine.

The conversion used supervisory calibration, which consisted in maximizing the amount of hydrogen injection on top of diesel fuel, without any optimization of the other engine parameters. The strategy was, at a given operating point, to decrease the load seen by the diesel ECU while also increasing hydrogen injection until the torque at this operating point was recovered. As a result, the load ‘detected’ by the diesel ECU was lower when the engine was running on the dual fuel mode than when it was running on diesel fuel only. The displacement of diesel fuel by hydrogen directly resulted in a reduction in carbon dioxide emissions. Nevertheless, the variability of the hydrogen-air premixed mixture combustion was kept under control and, in particular, knock had to be avoided.

At low loads, up to 70% of the energy required to drive the truck could be provided by hydrogen. This percentage decreased quickly with increasing load and fell to about 25 − 35% at medium loads. Diesel fuel displacement increased with engine speed until a limit around 1800 rpm. In some areas, an increase in engine-out NO_x emissions (increase from 257 ppm to 523 ppm at most) were noted when the engine ran on dual fuel, although in other areas, a reduction in engine-out NO_x emissions was observed (decrease from 255 ppm to 139 ppm at most). These increases engine-out NO_x emissions were converted by the SCR aftertreatment system which maintained a > 99% conversion efficiency when the SCR temperature was above 275°C, so the tailpipe emissions remained within current legal limits. Also, there were some areas with poor combustion efficiency, with a corresponding increase in exhaust hydrogen concentrations, which were due, at least in part, to the use of a combination of deliberately unoptimized parameters such as injection timings, boost pressure and EGR ratio because these were derived the original, diesel-only, calibration. Consequently, these parameters were set by the diesel ECU for that ‘detected’ lower load, rather than being optimised for dual fuel combustion.

The supervisory calibration was then road-tested. For an unladen truck, the test resulted in a 36.8% reduction in carbon dioxide emissions. Because the displacement of the diesel fuel by hydrogen was largest at low loads, the reduction in carbon dioxide emissions fell to 29.0% for a laden truck. This promising result was achieved for an unoptimized calibration. Future work will consist of optimizing the other engine parameters which may allow a higher diesel fuel displacement, resulting in higher reduction in carbon dioxide emissions, recovery of the loss in efficiency and mitigation of the increase in NO_x emissions.