03-01-2013, 03:03 PM
Operating strategy for a hydrogen engine for improved drive-cycle efficiency and emissions behavior
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ABSTRACT
Due to their advanced state of development and almost immediate availability, hydrogen
internal combustion engines could act as a bridging technology toward a wide-spread
hydrogen infrastructure. Extensive research, development and steady-state testing of
hydrogen internal combustion engines has been conducted to improve efficiency, emissions
behavior and performance. This paper summarizes the steady-state test results of
the supercharged hydrogen-powered four-cylinder engine operated on an engine dynamometer.
Based on these results a shift strategy for optimized fuel economy is established
and engine control strategies for various levels of hybridization are being discussed. The
strategies are evaluated on the Urban drive cycle, differences in engine behavior are
investigated and the estimated fuel economy and NOx emissions are calculated. Future
work will include dynamic testing of these strategies and powertrain configurations as well
as individual powertrain components on a vehicle platform, called ‘Mobile Advanced
Technology Testbed’ (MATT), that was developed and built at Argonne National
Laboratory.
Introduction
Due to their immediate availability, hydrogen internal
combustion engines are considered to be a bridging technology
toward a wide-spread hydrogen infrastructure [1]. It is
shown that the properties of hydrogen, especially the wide
flammability limits, allow a hydrogen engine to run very
efficiently [2–4]. Moreover, the lack of carbon in the feed fuel
means that nitric oxides (NOx) are the only emissions
component relevant in hydrogen operation.
Operating a hydrogen engine in lean combustion regimes
allows keepingNOx emissions low. Lean constant air/fuel ratio
operation in combination with hydrogen port injection is
currently the most prominent strategy used. Running at
different air/fuel ratios impacts the engine efficiency as well
as the engine power output [5]. In fact, the emissions and
efficiency improvement are a trade-off against the engine
power output. It is possible to combine the different constant
air/fuel ratios to run a variable air/fuel ratio strategy thus
overall improving the efficiency and emissions over the
engine torque–speed map while maintaining the engine
power output.
Setup and test plan
The four-cylinder engine was set up in a designated hydrogen
engine test cell that also contains a single-cylinder research
engine. The cell features a hydrogen delivery and extensive
safety system, as well as hydrogen-specific test equipment
(Fig. 1). A coriolis meter was used to accurately measure the
hydrogen fuel consumption. Details about the hydrogen test
cell can be found in [6].
Engine emissions results
The air/fuel ratio also has a crucial influence on the emissions
behavior (in particular the NOx emissions) of the engine. Thus,
the enrichment at high loads is limited by the acceptable
amount of NOx produced during combustion. The only critical
emissions component during hydrogen operation is NOx. The
NOx emissions target set by the U.S. Department of Energy
(DOE) for hydrogen engines is as low as 0.07 g/mi (Tier II Bin 5)
[8]. Similar trends concerning air/fuel ratio and NOx emissions
have been published by several authors (e.g., [9–11]). According
to those publications, the critical air/fuel ratio at which
NOx emissions increase significantly is around lw2. Fig. 4
shows the NOx emissions maps for the four different air/fuel
ratios tested in this study.
Optimized engine operating strategy
An efficiency-optimized operating strategy for the supercharged
hydrogen engine with port fuel injection that, at the
same time, allows for maximum power output could consist
of the following regimes:
- Throttled lean operation at l ¼ 3 at low and medium loads
and idle (constant l ¼ 3)
- Fuel enrichment at wide open throttle (WOT) at high loads
(variable l)
Development of ‘stand-alone’ capabilities
Unlike in dynamometer operation, where the dyno is used to
motor the engine, operation on a vehicle platform requires
that the engine can be operated ‘stand-alone’. To meet this
requirement an electric starter had to be integrated and an
idle control strategy had to be established. As mentioned
earlier, the engine is operated at a constant air fuel/ratio at
low engine loads and idle. The conventional idle controller
using throttle position for controlling the idle speed is used.
The idle controller was set to a target speed of 950 RPM. The
target air fuel ratio was set to l ¼ 3. Fig. 7 shows engine speed,
throttle position, air/fuel ratio and injection duration as
a function of time for an engine startup event.
Model assumptions and calculations
Based on the steady-state results the expected values for fuel
consumption and NOx emissions are estimated. Due to the
limitation in transferability of steady-state results to quasistatic
engine behavior, these values can only be used to
identify trends. Once the engine is integrated in a conventional
vehicle with a set ratio transmission, the engine operation
is constrained by the wheel speed, the selected gear and
its ratio. The vehicle power demand and losses then determine
the engine torque. Factoring in the engine speed and
torque, the engine efficiency is determined from the experimental
data. A vehicle can achieve the same operating point
in different gears by shifting the engine operating points and
potential engine efficiency. An optimum gear can be determined
for any vehicle speed and power demand condition.
Combining a hydrogen engine and a hybrid vehicle environment
enables new potential for powertrain cycle efficiency
improvements. Using a power model these benefits were
estimated and discussed.
Drive cycle
The U.S. FTP-72 (Federal Test Procedure) cycle, also called
Urban Dynamometer Driving Schedule (UDDS), was selected
to compare the quasi-static efficiency. The cycle simulates an
urban route of 12.07 km (7.5 mi) with frequent stops. The
maximum speed is 91.2 km/h (56.7 mi/h) and the average
speed is 31.5 km/h (19.6 mi/h).
The comparison of fuel economy numbers is based on
miles per kilogram (mpkg) values. A kilogram of hydrogen has
about the same energy content as a gallon of gasoline (Lower
heating value (LHV) for H2 ¼ 120 mJ/kg LHV for gasoline
approximately 115 mJ/gal); therefore mpkg was selected to
allow direct comparison to the widely used miles per gallon
standard.
Regenerative braking
A hybrid vehicle can store braking energy on board and reuse
that energy minus the storage losses to propel the vehicle
again. The simulation uses the regenerative braking energy to
electrically launch the vehicle after the last stop. Once the
stored energy is used, the engine propels the vehicle again.
The engine start–stop feature is enabled as well. Table 4 shows
the simulation results for this case.
A constant improvement in fuel economy is observed
again. Thus far, the hybrid environment has not changed the
engine torque speed operating areas in any of the engine
operating cases. The hybrid has only prevented idle fuel
consumption and eliminated engine launch torque. Yet to
affect the overall cycle engine efficiency, the hybrid environment
can shift the engine torque areas. The engine speed is
still linked to the wheel speed. The significant improvement in
emissions is due to the fact that the high engine loads during
accelerations are reduced by the additional torque provided by
the electrical launch support.
Basic hybrid control
In this basic hybrid control strategy the motor is used as
a motor-generator. Since the engine is mainly operated in 5th
gear, the control strategy will use the motor as a generator
when the engine torque is low to increase the engine efficiency.
In this case, when the engine torque output is less than
100 Nm in 5th gear, the motor loads the engine to 100 Nm,
thus operating the engine at higher load and increasing the
engine efficiency. The extra energy is used to propel the
vehicle through launches and accelerations until the energy is
depleted. Then, the engine takes over as the sole power
source. Fig. 8 shows the engine torque–speed information in
the hybrid operation. The 100 Nm torque line is clearly
defined.
Conclusions
The empirical data for the supercharged 2.3 L hydrogen
engine demonstrates efficiencies as high as 37%. At constant
torque conditions, the engine efficiency increases by about
1–2% when the air/fuel ratio is increased from l ¼2 to l ¼ 3.
The NOx emissions are reduced to single digit ppm levels at an
air fuel ratio of 3 and increase exponentially as air fuel ratios
of 2.25 are reached. Running the engine lean has great emissions
and efficiency advantages but reduces the maximum
torque output by up to 30%. Thus as the engine torque demand
increases from low loads to high loads the air/fuel ratio is
varied from 3 to 2 in the variable l strategy. This strategy
increases the overall engine efficiency in the torque–speed
map and improves the NOx emissions compared to a constant
l ¼ 2 strategy while maintaining the torque envelop.