The Arguments for Hydrogen Combustion Engines
26 September 2006
|BMW envisions the development of hydrogen combustion engines that eventually use charged, direct injection to deliver high efficiency. The current Hydrogen 7 is represented by the H2-PFI column. Click to enlarge. Source: BMW|
Speaking at the California Air Resources Board Zero Emissions Vehicle (ZEV) Symposium, representatives from Sandia National Laboratories and BMW laid out the rationale and technical strategies for a focus on hydrogen-fueled combustion engines (H2ICE).
Using hydrogen with a combustion engine, according to Dr. Andy Lutz from Sandia, is a pragmatic bridge to a hydrogen economy. The technology is available today and economically viable in the short term, with fewer constraints concerning hydrogen storage compared to fuel cells. Impurities, for example, are a non-issue for a combustion engine (“You burn them right up.”).
PHydrogen engines have demonstrated efficiencies (BTE) in excess of today’s gasoline engines, NOx is the only regulated tailpipe pollutant resulting from hydrogen combustion, and carbon dioxide is a non-issue, at least in terms of the driving (Tank-to-Wheels) component of the lifecycle.
Although current efforts by Ford and BMW reflect early stage development, both BMW and Sandia outlined technology approaches for subsequent generations of H2ICE systems that could deliver significant improvements in fuel economy and emissions reduction, while delivering additional power.
|BMW’s projected product pathway. Click to enlarge. Source: BMW.|
Dr. Edgar Berger from BMW, in particular, described a future generation H2ICE 4-cylinder engine that could deliver more than 140 kW (188 hp) of power with fuel consumption of 1.4 to 1.6 kg H2/100 km.
One can reach, in fact, 1kg/100km H2—but the price is to reduce vehicle properties and customer benefits.—Edgar Berger
In terms of its basic combustion properties, hydrogen offers certain benefits and certain challenges compared to gasoline. (See table below.)
|Combustion properties of gasoline, CNG and hydrogen. Favorable hydrogen properties are tagged in blue; unfavorable in red. Click to enlarge. Source: Sandia National Laboratories|
Its wide flammability range (Φ) supports a much leaner burn mixture—a factor that is important for emissions management strategies. The much higher laminar flame velocity produces stable flames under more extenuating circumstances, and, combined with the higher autoignition temperature, creates a higher research octane number that supports higher compression.
On the downside, hydrogen has a high stoichiometric volume fraction, which affects how much charge passes through the engine in a given displacement, and in turn affects the power of the engine.
It also has a lower minimum ignition energy and hence has a tendency to pre-ignite.
The researchers at Sandia have identified five possible approaches to dealing with the challenges posed by hydrogen combustion.
Continuous ultra-lean (Φ<0.45) operation with improved power densities. This, combined with turbo- or supercharging is the approach Ford is taking with its H2ICE Focus passenger car and E450 shuttle bus. For also has a H2ICE-hybrid research vehicle—the H2RV— that combines a 2.3-liter combustion engine with a 30 hp electric motor. All vehicles deliver SULEV emissions or better.
Operate at stoichiometric conditions (Φ=1) with aftertreatment. Possible routes within this strategy include the use of liquid fueling to prevent preignition if the fuel can be kept cold to the point of injection; direct injection, and the use of Exhaust Gas recirculation.
A multi-mode strategy. This is the approach BMW is taking with its Hydrogen 7, running ultra-lean under partial load to minimize engine-out NOx, and at stoichiometric condition under full load, coupled with the use of a three-way catalyst to handle the resulting NOx. (Earlier post.)
Another variation of the multi-mode strategy uses ultra-lean mixes at low load, pressure boost in the medium range, and then lean NOx traps at high load. Ford is looking into this for the H2ICE Focus.
Mixture stratification with direct injection. This approach would use a stratified and extremely lean mix at idle. At low-load, it would move to an ultra-lean homogeneous mixture. As load increases, the system would start using stratification with direct injection, and then rely on the lean NOx trap at high load. Sandia concludes that such an approach could theoretically deliver BTE of greater than 45%, with emissions significantly below SULEV.
Click to enlarge. Source: Sandia National Laboratories
It’s complicated, but with electronic controls there are a variety of things that can be done.—Andy Lutz
For its part, BMW outlined an ambitious development plan that it intends to result in mono-fuel hydrogen engines with greatly improved efficiency and reduced fuel consumption that it can apply across its entire model range, from luxury to compact.
|Advanced energy management. Click to enlarge. Source: BMW.|
Mirroring some of the Sandia work, BMW is ultimately looking toward a charged, direct-injection engine as a future generation platform. Berger also described a hybrid architecture that would combine a small fuel cell with the hydrogen combustion engine to augment electric power for vehicle subsystems and traction power.
A key enabler for this strategy is having sufficient hydrogen on-board to fuel the engine. BMW has already opted for liquid hydrogen storage, with its higher volumetric and gravimetric densities than offered by compressed hydrogen.
|Volume and weight of different methods of storing 10 kg of hydrogen, which is equivalent in energy to 38 liters of gasoline. Click to enlarge. Source: BMW.|
However, BMW believes that it needs to have 10kg on board hydrogen to met its performance and customer satisfaction objectives. Currently, the Hydrogen 7 stores 8 kg in a 150-liter container.
Accordingly, BMW has work underway to expand the storage density of its liquid hydrogen storage, to decrease the boil-off loss, and to increase the loss-free dormancy time.
Furthermore, for its 5 Series size cars, BMW is developing a shaped storage tank it calls the “double bubble”—a single-tank system providing central storage running down the midline of the car in the tunnel.
|BMW’s hydrogen storage roadmap. Click to enlarge. Source: BMW.|
Ultimately, it sees using liquid hydrogen in the larger classes (luxury and executive) with 7.5 to 10 kg in a given total package of 250-300 liters. For small to medium-class vehicles, BMW is looking at compressed hydrogen, and possibly some activity with cryo-compressed hydrogen.
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