Optimized direct-injection hydrogen engine estimated to exceed 2016 CAFE fuel economy targets at Tier 2 Bin 2 emission levels
|The H2-DI engine was estimated to exceed 2016 CAFE targets with engine out Tier 2 Bin 2 (SULEV) emission levels. The team sees further potential for fuel economy improvement with engine downsizing. Source: Wallner 2011. Click to enlarge.|
A project led by Dr. Thomas Wallner from Argonne National Laboratory has optimized a spark-ignited direct injection combustion system for a hydrogen engine (H2-DI). The engine delivers peak BTE of 45.5% and 33.3% BTE at the world wide mapping point (WWMP), and 14.3 bar BMEP. These results exceed the US Department of Energy (DOE) 2010 efficiency goals (45% peak, 31% BTE at WWMP).
Estimated drive-cycle fuel economy and emissions—based on the single-cylinder 0.66L research engine efficiency and emissions maps—was 32.4 mpg US city, 51.5 mpg US highway, 38.9 mpg US combined (7.26 L/100km, 4.57 L/100km and 6.05 L/100km, respectively), with NOx emissions of 0.017 g/mile. At that level of performance, the H2-DI engine would exceed 2016 CAFE fuel economy targets, along with delivering Tier 2 Bin 2 (SULEV) emissions levels without aftertreatment. (Emissions other than NOx are negligible for hydrogen combustion.)
|Brake thermal efficiency results. Click to enlarge.||NOx emissions results. Click to enlarge.|
For the estimates, the vehicle was assumed to be a mid-size sedan weighing 1,553 kg (3,424 lbs), outfitted with a 3.0L H2-DI engine and a 5-speed automatic transmission. Simulating the same midsize sedan with a 2.0L H2-DI showed a combined cycle fuel economy improvement to 45.4 mpg US (5.18 L/100km) as the engine was pushed into a more efficient operating range. However, the corresponding NOx increased to 0.028 g/mile which falls outside the SULEV II range but is still well within a Tier 2 Bin 5 DOE target.
Using direct injection to stratify the fuel-air mixture properly stratify resulted in high engine performance coupled with low NOx emissions. The mixture stratification target was to deliver a hydrogen-rich mixture around the spark plug, with lean mixtures close to the combustion chamber walls.
3D-CFD simulation assisted the injection strategy development. Nozzle design significantly influenced jet penetration pattern and mixture formation. (The team ended up using a 4-hole design.) Start of injection (SOI) influences stratification, and later injection is desirable to reduce compression work.
Among the factors contributing to the performance of the engine were an optimized bore/stroke ratio of 89 mm/105.8 mm stroke—increased engine stroke enables higher flame speeds and reduced quenching distance—and an increased compression ratio (12.9:1). An upgraded injection system (provided by Westport Innovations) used fast-acting Piezo injectors.
Collaborators and partners in the project included Ford, Westport, Sandia and Lawrence Livermore National Laboratories; international team members came from BMW, Graz University of Technology, and Ghent University.
The DOE-funded project concluded in September. Wallner and his colleagues are working on two follow-up publications, one with SAE, the other to be published in an IMech journal.
Thomas Wallner (2011) Optimization of Direct-Injection H2 Combustion Engine Performance, Efficiency, and Emissions (2011 DOE Merit Review)