19-10-2012, 04:12 PM
NUMERICAL INVESTIGATION OF TURBOLAG REDUCTION IN HD CNG
ENGINES BY MEANS OF EXHAUST VALVE VARIABLE ACTUATION
AND SPARK TIMING CONTROL
NUMERICAL INVESTIGATION.pdf (Size: 2.55 MB / Downloads: 27)
ABSTRACT
Turbocharging port-injected Natural Gas (NG) engines allows them to recover gaseous-fuel related power gap
with respect to gasoline engines. However, turbolag reduction is necessary to achieve high performance during engine
transient operations and to improve vehicle fun-to-drive characteristics. Significant support for the study of turbocharged
Compressed Natural Gas (CNG) engines and guidelines for the turbo-matching process can be provided by 1-D numerical
simulation tools. However, 1-D models are predictive only when a careful tuning procedure is set-up and carried out on the
basis of the experimental data. In this paper, a 1-D model of a Heavy-Duty (HD) turbocharged CNG engine was set up in the
GT-POWER (Gamma Technologies Inc., Westmont, IL, US) environment to simulate transient operations and to evaluate the
turbolag. An extensive experimental activity was carried out to provide experimental data for model tuning. The model
buildup and tuning processes are described in detail with specific reference to the turbocharger model, whose correct
calibration is a key factor in accounting for the effects of turbine flow pulsations. The second part of the paper focuses on the
evaluation of different strategies for turbolag reduction, namely, exhaust valve variable actuation and spark timing control.
Such strategies were aimed at increasing the engine exhaust-gas power transferred to the turbine, thus reducing the time
required to accelerate the turbocharger group. The effects of these strategies were examined for tip-in maneuvers at a fixed
engine speed. Depending on the engine speed and the applied turbolag reduction strategy, turbolag reductions from 70% to
10% were achieved.
INTRODUCTION
NG-fuelled engines have recently emerged as a promising
solution for the transportation sector in industrialized
countries, thanks to the intrinsic environmental features of
NG and to the favorable geopolitical distribution of
reservoirs (d’Ambrosio et al., 2006). The application of
NG engines is most advantageous for public urban transportation.
Any limitations to the vehicle’s operating range,
due to the storage of fuel in a gaseous state, can be overcome
by scheduling refueling stops at stations that are
directly operated by the transportation providers. The
gaseous state of the fuel also reduces the engine power
output (Kato et al., 1999; Zhang et al., 1998). However,
that gap can be recovered by turbocharging (d’Ambrosio et
al., 2006), as in the new-generation high-performance NG
buses which exploit the high knock resistance of methane.
In contrast, the turbolag phenomenon is one of the major
concerns regarding these engines due to driver perception
of the vehicle’s performance. Turbolag introduces a delay
in the torque response under severe tip-in maneuvers. The
delay is due to the time required to increase the pressure in
the intake manifold, which is influenced by the acceleration
time of the turbocharger shaft.
Pipe and Flowsplit Submodels
GT-POWER solves the inviscid form of the conservation
laws of mass, momentum and energy. With reference to
pipes, these equations are discretized using a 1-D approach
and a finite volume technique. Pressure losses due to
friction are computed automatically by the code, taking the
Reynolds number and the surface roughness of the walls
into account. The modeled global heat exchange coefficient
was proportional to friction using the Colburn analogy.
In some cases, it may be necessary to tune friction and heat
transfer coefficients on the basis of experimental data regarding
gas pressure and temperatures at relevant points.
Flow-splits were specifically designed (Gamma Technologies,
2006) to account for the conservation of momentum
in three dimensions, even though the code is otherwise
one-dimensional. It is important to correctly specify the
flow-split parameters (expansion diameter, characteristic length
and orientation) to correctly reproduce wave phenomena
and friction without using friction multipliers that are too
far from unity.
RESULTS AND DISCUSSION
The developed GT-POWER engine model was applied to
the simulation of tip-in maneuvers at a constant N so as to
evaluate the effects of E-EVO-VVA, ComR and Combined
techniques on turbolag reduction.
E-EVO-VVA and ComR techniques were investigated
for a tip-in maneuver at N/Nmax = 0.55 (Figures 15, 16, 17).
Figures 15 and 16 show the values of Torque Rising Time
(tr) versus Average Torque ( ) for E-EVO-VVA strategies
with Prelift (Figure 15) and Full Lift (Figure 16) profiles.
The effects of different EVO advances (A = 65, 75, 85 CA
deg), switch-boost levels (0.60, 0.65, 0.70, 0.80) and prelift
values (P/Lmax = 0.2 in Figure 15(a); P/Lmax = 0.3 in Figure
15(b)) were examined. Figure 17 reports tr vs. for ComR
strategies featuring different ST retards (5, 10, 15 CA deg)
and switch-boost levels (0.60, 0.65, 0.70, 0.80). Values of tr
and are expressed as percentages with respect to the
corresponding values obtained in the baseline condition
(standard exhaust valve lift profile, no EVO advance and
no combustion retard).
CONCLUSION
In this paper, different strategies for turbolag reduction, Early-
Exhaust Valve Opening-Variable Valve Actuation (E-EVOVVA),
combustion retard (ComR) and Combined techniques
were assessed by numerical simulation. Such strategies
were aimed at increasing the engine exhaust-gas power
transferred to the turbine, thus reducing the time required to
accelerate the turbocharger group. The effects of these
strategies were examined for tip-in maneuvers at fixed
engine speeds. To this end, the 1-D model of an HD turbocharged
CNG engine was set up and calibrated in the GTPOWER
environment for the simulation of transient operations.