International Journal of Automotive Technology, Vol. 10, No. 2, pp. 141−149 (2009)
DOI 10.1007/s12239−009−0017−1
Copyright © 2009 KSAE
1229−9138/2009/045−02
STRATEGIES FOR IMPROVING THE MODE TRANSITION IN A
SEQUENTIAL PARALLEL TURBOCHARGED AUTOMOTIVE DIESEL
ENGINE
J. GALINDO, H. CLIMENT, C. GUARDIOLA and J. DOMÉNECH
*
CMT Motores Térmicos, Universidad Politécnica de Valencia, Camino de Vera s/n, Valencia 46022, Spain
(Received 1 August 2007; Revised 2 September 2008)
ABSTRACT−Parallel sequential turbocharging systems are able to operate in different modes, which are defined according
to the turbochargers that simultaneously boost the engine, and are controlled by means of specific valves. In order to cover the
full engine operating range, a smooth transition between turbocharging operating modes must be ensured. However, important
disturbances affect both boost and exhaust pressure when shifting the operation mode, thus causing non-negligible torque
oscillations. This paper presents different methods for smoothing such undesirable effects during mode transition. Strategies
covering optimal synchronization of the control valves, control of the valves’ position, and correction of the injected fuel
during the transition are analysed. A fully instrumented passenger car engine is used for illustrating the different torque
smoothing methods, and experimental results for transitions during both steady operation and engine accelerations are shown.
KEY WORDS : Sequential turbocharging, Diesel engine, Transition, Boost control
NOMEMCLATURE
The problem of the turbo-lag is of great importance
when an acceleration is demanded and exhaust gas energy
availability is low, which occurs at low engine speed and
load. In this case, the driver’s torque demand produces a
step increase in the fuel supplied; however, the required air
to ensure a correct combustion process is not supplied by
the compressor as fast as it should be. This situation results
in an increase in the fuel-to-air ratio, and hence, characteristic ‘black smoke’ appears. In order to limit smoke
emissions, current diesel engines limit the injected fuel
during the accelerations on the basis of minimum air-tofuel ratio criteria, which also negatively affects engine performance during load transients.
Current demands on diesel engine performance require
the solving of problems related to the turbo-lag. Downsizing is a general trend that leads to engines that have the
same power output with lower displacement. The downsized engine has lower emissions and consumption but the
low end torque is worsened. As a result, some new technologies have been developed, such as the variable geometry
turbines (VGT) (Watson and Janota 1982; Arnold et al.,
2002), which are able to adapt the effective area of the
stator. Another method to improve the performance in
transient conditions is to employ a smaller turbocharger
since: a) the turbine is better designed for working with low
exhaust gases mass flow, and b) it accelerates faster because
of its lower inertia. Both methods allow faster transient
engine response. However, small size turbochargers limit
the maximum engine speed and load. In order to overcome
this problem, sequential operation of turbochargers arrang-
1T
: engine operation mode with a single turbocharger
2T
: engine operation mode with two turbochargers
C1
: compressor of TC1
C2
: compressor of TC2
TC1 : turbocharger number 1
TC2 : turbocharger number 2
T1
: turbine of TC1
T2
: turbine of TC2
VC2 : control valve of C2
VGT : variable geometry turbine
Vrecirc: valve of the recirculation circuit
VT2 : control valve of T2
WG : waste-gate
1. INTRODUCTION
Turbocharging is a widely used method that improves
internal combustion engine performance. However, turbocharged engines cause a delay during load transients (Watson
and Janota, 1982).
The causes of the delay are mechanical, thermo- and
fluid dynamic. The first involves the inertia of the turbocharger, the second includes the processes of mass and
energy transfer between the exhaust valve and the turbine,
and between the compressor and the cylinders (Benajes et
al., 2002; Payri et al., 2002).
*Corresponding author. e-mail: carguaga@mot.upv.es
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