While it is apparent that the long-discussed hydrogen economy is not emerging as rapidly as some had anticipated or hoped, hydrogen fuel-cell vehicles—specifically using hydrogen produced via renewable resources—remain a long-term vision for sustainable transportation.
One of the key questions for the hydrogen vehicle industry is how to develop transitionary technologies that can deliver immediate benefit to users while still advancing work on solutions to some of the well-known critical challenges facing the longer-term use of hydrogen (production, storage, refueling infrastructure) and fuel-cells (cost, longevity, power).
An approach that is gaining more support is to focus in the short-term on hydrogen-fueled internal combustion engines (including engines fueled with a hydrogen-CNG blend) in both direct drive and hybrid applications.
WestStart-CALSTART and the Federal Transit Administration organized a one-day Hydrogen Internal Combustion Engine Symposium around that topic, with two purposes. The first was to try to better define the role the internal combustion engine might play in the near- and medium-term during the long-term transition to hydrogen as a transportation fuel.
The second was to foster communications among public agencies and the private sector surrounding projects and technology involving internal combustion engines (ICE) fueled by hydrogen or hydrogen blends.
To put some structure around that first discussion, Fred Silver, vice-president of WestStart-CALSTART and the host for the symposium, posed three questions at the beginning of the day:
Can HICE (Hydrogen Internal Combustion Engines) achieve efficiencies that exceed the best gasoline and diesel systems?
Can HICE support an ATPZEV or Near Zero Emission Bus application?
Can HICE provide the same or even improved power densities compared to conventional platforms?
The answer to all three is a provisional “yes.”
There are some immediate caveats that go into that answer. The first is that, on the principle of dealing with what’s real, the vast majority of hydrogen that will be used in vehicle systems in the short- and medium-term will be produced from natural gas by steam methane reforming—decidedly not optimal from a greenhouse gas perspective.
Tad Wysor, Senior Technical and Policy Advisor with the EPA Office of Transportation and Air Quality (OTAQ), noted that attention to the total fuel lifecycle is “critical.” “The overall degree of greenhouse gas emissions and fuel consumption will ultimately drive the commercialization of different hydrogen technologies.”
Another caveat is that the HICE industry—even the vendors with the most experience—is still learning. It’s not really even possible to discuss engine durability data in an HICE or HCNG scenario because, as several manufacturers pointed out, there just haven’t been enough HICE engines on the road for long enough to provide the data. (By contrast, DaimlerChrysler’s fuel cell fleet has racked up more than 1 million miles.)
That situation will begin to change, with more HICE prototypes emerging and larger-scale test projects underway, such as Ford’s planned deployment later this year of between 20 and 30 HICE shuttle buses, and with a growing number of medium- and heavy-duty HICE projects emerging.
Efficiency. HICE systems theoretically can deliver greater efficiencies than their gasoline counterparts. BMW currently is at 37% with its hydrogen-fueled 750h prototype, and has set an internal target of 50%. The Department of Energy has set a target of 45% BTE (brake thermal efficiency) with 0.07 g/mile of NOx.
Hybrid HICE systems deliver even more capability.
Emissions. The operational emissions issue with HICE (leaving aside the question of greenhouse gas production during the fuel cycle), is NOx. All the other criteria emissions are practically at zero. Engine designers have two basic approaches to take with HICE: they can opt for a lean-burn strategy, which minimizes NOx emissions but has an effect on power, or they can go for a richer fuel mixture, which ups the power, but also increases NOx.
Approaches reflected at the symposium seemed split, with some opting for lean-burn (lambda of about 2.0) and taking their hit on the power, with others opting for a richer charge, and using catalytic converters to handle the NOx.
As one example of the NOx issue, initial data from the Quantum HICE Prius models showed a reduction in all criteria pollutants compared to the conventional Prius—with the exception of NOx, which edged up slightly.
Power density. Compared to port-injected gasoline engines, port-injected hydrogen engines see a theoretical drop in relative power density (with a port-injected gasoline engine being 100%) to about 82%. Actual test data, however, as described by Sandeep Munshi from Westport Innovations, indicates only 65% efficiency for port-injected hydrogen compared to port-injected gasoline.
The use of direct injection, however, reverses that, with direct-injection hydrogen delivering 117% of the power density provided by port-injection gasoline systems, according to Munshi.
Supercharging for low-end torque and turbo-charging are also solutions being applied to improve the power density of the HICE systems. In the HICE Prius developed by Quantum, for example, the Quantum engineers turbocharged the engine.
HICE as a transition. Some speakers expressed some nervousness about positioning HICE as transitionary—what customer will want to buy into a solution that is defined to be short-term?
Andy Abele, Executive Director for Strategic Development at Quantum, was even more direct in his concern, seeing the recent surge in interest in E85 as a definite distraction, if not outright competitive threat.
The dynamic has changed in the last six months. We are now competing with E85 vehicles at zero differential cost. We can’t bury our heads in the sand...It is hard to argue against E85 and flexible fuel vehicles.
Is “transitional” good enough for a company to make a business case?