Abstract
Using hydrogen and methanol in spark-ignition engines are promising approaches to decarbonising transport and securing domestic energy supply. Hydrogen is a versatile fuel that enables high efficiencies and low emissions of NOx (oxides of nitrogen) throughout the load range. However, there are lasting issues concerning its storage and distribution. Liquid methanol is more compatible with the existing fuelling and distribution infrastructure and is easily stored in a vehicle. In addition, methanol can be synthesized from a wide variety of sources, including renewably produced hydrogen in combination with atmospheric CO2. Both fuels can be used in internal combustion engines with only minor adjustments and have the potential to increase the efficiency and decrease noxious emissions compared to gasoline engines. The present study compares the brake thermal efficiencies and engine-out emissions from a production-type four-cylinder SI engine that was converted for operation on hydrogen and methanol.
The experimental results indicate that for part load, hydrogen wide open throttle (WOT) operation enables the highest brake thermal efficiencies (BTE). Inspection of the cylinder pressure traces reveals that these elevated efficiencies are largely due to reduced throttling losses and the benefits of lean burning. The efficiency gains become most marked at low loads, where the application of the wide open throttle, lean burn strategy for hydrogen yields up to 30% relative increase of BTE compared to gasoline operation. The efficiency benefit of methanol is more modest (5-10% relative increase). Apart from reduced throttling losses and increased burning velocity, the use of methanol also brings along lower combustion temperatures, which slightly decreases cooling losses. For hydrogen, the NOx emission control at higher loads demands a switch from WOT to throttled stoichiometric operation, resulting in a considerable efficiency penalty. For methanol, the emissions of CO are comparable to those on gasoline operation. There is, however, a considerable reduction in engine-out NOx emissions (30% and beyond) due to the reduced combustion temperatures of methanol.
These promising results obtained on a converted gasoline engine suggest an even larger potential for increases in BTE and emission reductions for dedicated hydrogen and methanol engines.
KEYWORDS: spark-ignition engine, hydrogen, methanol, efficiency, emissions