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Optimally Controlled Flexible Fuel Vehicle
Robert Bosch LLC
, Gasoline Systems
38000 Hills Tech Dr., Farmington Hills, MI 48331, United States
Li Jiang
Hakan Yilmaz
Li.Jiang@us.bosch.com
Hakan.Yilmaz@us.bosch.com
+1 (248) 876-1446
+1 (248) 876-2353
Ricardo Inc.
, Powertrain Development
40000 Ricardo Dr., Van Buren Township, MI 48111, United States
Mark Christie
Mark.Christie@ricardo.com
+1 (734) 394-3874
University of Michigan
, Lay Auto Lab
1231 Beal Avenue, Ann Arbor, MI 48109, United States
Kyung-ho Ahn
Anna Stefanopoulou
akyungho@umich.edu
annastef@umich.edu
+1 (734) 936-0424
+1 (734) 615-8461
1
Optimally Controlled Flexible Fuel Vehicle
Introduction
Although fossil fuels (gasoline and diesel) remain the dominant energy source for in-
ternal combustion engines, there have been significant efforts into development of a
powertrain system that can be powered with alternative fuels. Ethanol, known for its
potentially neutral CO
2
cycle, has been recognized as a promising renewable fuel that
can serve as a substitute for conventional gasoline. In fact, current fuel standards in
United States (e.g. ASTM D4814) have already allowed up to 10% ethanol for regular
gasoline and the use of E85 (85% ethanol and 15% gasoline in volume).
Although today's flex-fuel vehicles are capable of running on gasoline-ethanol
fuels, their powertrain and engine management systems are not designed to fully ex-
ploit the potential benefits from such fuel flexibility. Instead, the main goal of the
current control calibration for flex-fuel vehicles is to improve the cold start perform-
ance [1]. Problems associated with cold start are caused by the lower vapor pressure
and the lower combustion heating values of ethanol that leads to requirements for a
higher fuel injection quantity. Apart from the cold start problems, the lower combus-
tion heating value of ethanol fuels results in higher fuel consumptions (miles/gallon).
However, as listed in Table 1, ethanol fuels also possess some advantageous proper-
ties. For instance, the higher octane number and latent heat of vaporization of ethanol
fuels could lead to higher knock resistance and stronger charge cooling effects. With
properly designed engine management system that can exploit these advantageous
properties, the use of ethanol fuels in combination with the current development of
turbocharged downsizing, direct injection, and variable valve timing can improve the
vehicle performance and mitigate the fuel consumption penalties associated with high
ethanol content fuels [2, 3].
Table 1: Properties of gasoline and ethanol
Property
Unit
Gasoline
Ethanol
Research Octane Number (RON)
-
92
111
kg/m
3
Density
747
789
Heat of combustion
MJ/kg
42.4
26.8
Stoichiometric air-to-fuel ratio
-
14.6
9.0
Latent heat of vaporization
kJ/kg
420
845
Boiling point
°
C
20-300
78.5
Dielectric constant
-
2.0
24.3
2
Optimally Controlled Flexible Fuel Vehicle
The primary objective of this study is to develop an optimized Flex-Fuel Vehi-
cle (FFV), targeting substantial fuel economy improvement with minimum drive-
ability and fuel consumption penalties using a direct injection turbocharged spark ig-
nition engine. Without significant hardware modifications from a gasoline-optimized
production engine, the proposed approach employs adaptive engine control strategies
combined with a novel ethanol content estimation scheme in order to optimize engine
performance for any fuel blend ranging from E0 up to E85.
In order to capture the combustion characteristics of ethanol fuels and their im-
pacts on the optimum control parameter settings, data from the target engine running
on fuel blends E0, E24, E55 and E85 were collected during dynamometer testing and
analyzed following the design-of-experiment (DoE) method. Then, engine simulation
models were developed using these experimental data to explore potential engine per-
formance benefits of different engine configurations, thus allowing engine optimiza-
tion studies. The testing was carried out on a 2.0L four-cylinder direct injection tur-
bocharged spark ignition engine with its specifications listed in Table 2. Intake and
exhaust valve timings could be varied independently to provide valve overlap periods
from negative values, when EVC occurs before IVO, to positive values up to 104CA
°
.
All tests on the engine were carried out under fully warm, steady-state operating con-
ditions that covered possible settings of intake valve open, exhaust valve close, spark
advance, injection angle, and fuel rail pressure over a wide operation range. Kistler
6125B pressure transducers were flush-mounted in each cylinder, from which cylinder
pressure measurements were collected using the RedLine Combustion Analysis Sys-
tem (CAS). Engine-out emissions and filter smoke were recorded using a Horiba
MEXA 7100 DEGR emission bench and an AVL415 smoke meter, respectively. In
addition, a Siemens ethanol sensor was installed to measure the fuel ethanol content.
Table 2: Specification of the test engine
Variable
Unit
Values/Description
Test engine configuration
In-line four cylinder
Bore
mm
86
Stroke
mm
86
Compression ratio
9.25:1
dm
3
Displacement
1998
DOHC – double overhead camshaft, 4-
valve, variable valve timing
Camshaft layout
Boosting system
Twin scroll turbocharger
Maximum power
kW
190 at 5800 RPM
Maximum torque
N·m
353 at 2000-5000 RPM
3
Optimally Controlled Flexible Fuel Vehicle
Engine Performance
The DoE study results in [4] has shown that a more sophisticated calibration scheme
that covers the intake/exhaust valve timing and the fuel rail pressure, in addition to the
fuel injection and spark timing, can improve the fuel economy of a flex-fuel engine by
2-3%. Experiments have also been conducted to investigate and compare the wide-
open-throttle performance of the tested engine when it was operated on fuels E0, E24,
E55, and E85 achieving the same torque curve. As illustrated in Figure 1, the stronger
charge cooling effects of ethanol fuels reduce the exhaust temperature and thus allow
stoichiometric combustion over a wider range of engine speeds, which mitigates the
fuel consumption penalties of ethanol fuels.
E0
E24
E55
E85
E0
E24
E55
E85
1000
1.1
900
1
800
0.9
700
0.8
600
0.7
500
0.6
0
1000
2000
3000
4000
5000
6000
7000
0
1000
2000
3000
4000
5000
6000
7000
Engine Speed [RPM]
Engine Speed [RPM]
Figure 1: Turbine inlet temperature and lambda measurements when the tested engine was oper-
ated on fuel blends E0, E24, E55 and E85 to achieve the same WOT torque curve
In order to exploit the potential benefits of ethanol’s higher knock resistance, a
simulation model was developed in WAVE to evaluate the performance of an engine
with an increased compression ratio and a higher tolerable maximum cylinder pres-
sure. In addition, the WAVE model is used to predict the engine knocking behavior
with different variable valvetrain timings and engine compression ratios. As shown in
the left plot of Figure 2, with a higher maximum cylinder pressure of 150bar, the en-
gine is predicted to be able to achieve a maximum torque of 500N·m over 2500-
4000RPM when running on E85. Despite the benefits exploited from fuels with high
ethanol content, the performance of an engine with a higher compression ratio will, in
the mean time, suffer from more severe knocking when running on gasoline. In this
study, late intake valve closing strategy, as a result of modified intake cam profile and
increase phaser authority, is employed to reduce the effective compression ratio to
mitigate problems associated with knocking. The simulations shown in the right plot
of Figure 2 compare the effects of two intake cam profiles, with an extended duration
4
Optimally Controlled Flexible Fuel Vehicle
of 20CA
°
and 40CA
°
, on the torque output of an engine with an increased compres-
sion ratio when it is run on E0.
Figure 2: Predicted output torque from an engine, running on E85, (a) Left: with higher maxi-
mum cylinder pressures (120 to 150bar in the arrow direction); (b) Right: with a higher compres-
sion ratio and two different intake cam profiles
Engine Optimization
In this section, several necessary hardware modifications of the engine component de-
sign and fuel system specification will be discussed. These modifications were made
to improve the engine performance with high ethanol content fuels without penalizing
its performance with gasoline.
Engine Design
According to engine optimization investigation conducted based on the 1-D simula-
tion model, the compression ratio and the maximum cylinder pressure of the opti-
mized flex-fuel engine are determined to be 11.3:1 and 140bar, respectively. In order
to achieve the desired compression ratio, the piston bowl design was modified, as il-
lustrated in Figure 3. Such design also takes into consideration of the change of valve
movements, resulted from the change of cam profile and phaser authority for the late
intake valve closing strategy, and the change of injection spray targeting for better
emissions. In addition, engine components with higher strength, such as the cylinder
block and cylinder head, are used to withstand a higher maximum cylinder pressure.
5
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