Hydrogen can be used directly in engines or in fuel cells. When used in conventional IC
engines, the combustion properties of hydrogen tend to cause irregular combustion and
backfires.^143 To prevent this, BMW has used very lean mixtures successfully, with the added
benefit of no measurable emissions of NOX and an improvement in peak energy efficiency of 12 to
14 percent. Because of hydrogen’s low density, however, operating lean results in a power
reduction of about 50 percent from the engine’s normal capacity. BMW uses superchargers to
restore some of the power loss,^144 but a larger engine is still required, and the added weight and
increased fiction losses could offset much of the energy efficiency gain. Mercedes Benz has
solved the low power problem by operating at stoichiometry or rich air fuel ratio at high loads,
coupled with water injection to reduce backfire and knocking potential. The Mercedes approach
results in significant NOX emissions, however, and the engine requires a three-way catalyst to
meet ULEV NOX standards. Overall engine efficiency is not much different from gasoline engine
efficiency owing to compromises in spark timing and compression ratio.^145
The use of hydrogen in a compression-ignition (diesel) engine has also been attempted by
directly injecting liquid hydrogen into the combustion chamber. Cryogenic injectors operating on
low lubricity liquid hydrogen poses difficult engineering problems, however, and
automanufacturers doubt whether a commercially viable system can ever be developed.
Gas Turbine Engines.
The gas turbine, or Brayton cycle, engine has largely replaced piston engines in
aircraft, and has been investigated extensively for use as an automotive powerplant
three decades. The engine of interest for automotive applications has a cycle that first
intake air, then mixes fuel with the air and ignites it, and finally expands the airmost small
for the last
compresses
to ambient
pressure. The hot, high velocity air turns a turbine that operates the compressor for the intake air.
Output power can also be taken directly from the same shaft as the compressor, or the engine’s
exhaust can be directed to another turbine to extract output power.As a replacement for the internal combustion piston engine, the gas turbine offers exceptional
smoothness, low emissions potential, and multifuel capability. It suffers, however, from other
serious problems that make it difficult to use as an automotive engine. The engine has very poor
part-load performance because the characteristics of turbomachinery are such that high
aerodynamic efficiencies are attained only in a narrow operating range. The simple “single shaft”
design, where the compressor and turbine and power takeoff are all on the same shaft, is not well
suited to automotive uses, where speeds and loads vary. The more complex two-shaft turbine
offers better performance in automobiles at significant increase in cost. Part-load efficiencies can
only be made high by a recuperator or regenerator that transfers heat from the exhaust to the
compressed intake air before combustion, which recaptures some of the energy remaining in the
exhaust. Overall engine efficiency increases with increasing combustion temperature, which is
limited by the materials used in the turbine. Since 1979, DOE has funded the development of