|An HECE test engine|
Work being done by ArvinMeritor, IAV (Ingenieursgesellshaft für Auto und Verkehr) and MIT on enhancing gasoline combustion with a small hydrogen gas stream is pointing toward a potential estimated improvement in gasoline fuel economy of 20% to 30%, depending upon the baseline engine.
Writing in the October issue of MTZ (Motortechnische Zeitschrift), Utz-Jens Beister from IAV and Rudy Smaling from ArvinMeritor describe their progress with the Hydrogen-Enhanced Combustion Engine (HECE) concept, as applied to an SUV-class 3.2-liter V6 test engine.
The premise of HECE, on which the research collaborative has been working for a number of years, is that the addition of a small amount of hydrogen to the cylinder charge can allow homogeneous charge ultra-lean-burn combustion engines to operate much leaner than otherwise possible.
That in itself is not a new discovery. What brings HECE closer to operational reality is the ArvinMeritor/MIT on-board, compact plasma reformer (earlier post) that can take a fraction of the conventional fuel, reform it in real-time and add the resulting hydrogen-rich gas to the remaining fuel-air mixture flowing into the engine.
In the plasma fuel reformer, air is metered into a plasma generator located upstream of a combustor. High voltage is applied to the air stream, forming high-temperature plasma. This high-temperature plasma torch flows into the combustor, initiating vigorous combustion of a rich fuel-air mixture. Within the plasma fuel reformer, partial oxidation reactions occur in the high-temperature gas phase created by the plasma, eliminating the need for a reforming catalyst.
|The shift of combustion limits with hydrogen-enhanced combustion.|
Adding hydrogen gas to the homogeneous fuel charge improves the ignitability of the mixture, and increases flame speed and combustion stability. In theory, the combination of a lean-burn engine with the plasma reformer could support an ultra-lean and turbocharged engine that would reduce NOx emissions to the point of not requiring aftertreatment in the exhaust stream.
Researchers at the Sloan Automotive Laboratory at MIT also discovered that both hydrogen and carbon monoxide (both products of the partial oxidation process of the reformer) act as octane enhancers. Adding the reformed fuel gas to the engine thus also supports a substantial increase in compression ratio.
Once an engine is developed that operates ultra-lean, is turbocharged—or super-charged—and is better able to withstand engine knock, engine downsizing while maintaining performance becomes a credible option that can lead to significant additional fuel economy and performance benefits.—Beister and Smaling
Such an ultra-lean-burn, high compression-ratio, turbocharged HECE could exhibit the following characteristics:
Extremely low engine out NOx emissions requiring little or no exhaust emissions control
Reduced pumping losses (~5-10% efficiency gain)
Improved thermodynamics (~10-12% efficiency gain)
Reduced friction (downsizing) (~5-8% efficiency gain)
ArvinMeritor is targeting release of the HECE for approximately 2008.