09-11-2012, 04:01 PM
Intake Air Dynamics on a Turbocharged SI-Engine with Wastegate
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Abstract
On turbocharged spark-ignited (SI) engines with wastegate the position of the
wastegate changes the exhaust manifold pressure. A secondary eect of this is
that the residual gas mass trapped inside the cylinder at exhaust valve closing
changes and causes the volumetric eciency to change. The volumetric
eciency is used to estimate air-mass-to-cylinder which is important for good
air/fuel ratio control.
Air-mass to-cylinder is not directly measurable so observers for air-mass
flow to the cylinder are therefore often proposed. For observers with one state
for intake manifold pressure and proportional feed-back from measured state,
there is a tradeo whether to estimate intake manifold pressure or air-mass-tocylinder.
A new nonlinear air-mass-to-cylinder observer is suggested with two
states: one for intake manifold pressure and one for the in-cylinder air-mass
oset compared to expected using the volumetric eciency.
The exhaust manifold pressure can change rapidly in an engine with wastegate.
A method to estimate the exhaust manifold pressure is presented for
diagnosis of wastegate and turbocharger on SI-engines. It does not use any
extra sensors in the exhaust system after the calibration. The exhaust manifold
pressure estimator is validated using a series of wastegate steps. The exhaust
pressure estimation is designed for steady-state conditions and the validation
shows that it works well and converges within 1 to 4 seconds.
Introduction
Today turbocharged spark-ignited (SI) engines are getting more popular as they
provide good fuel economy and high power output. On most of these engines
there is a device called wastegate (Watson and Janota, 1982; Heisler, 1997),
which is located in the turbocharger, on the exhaust side, with the purpose to
control the power to the turbine. When the wastegate is opened the power drops
and vice versa, and often this device is controlled by a pneumatic actuator which
is coupled to the boost pressure after the compressor. The valve setting of the
wastegate or the actuator is not normally measured. Few sensors are located on
the exhaust side of the engine; usually there are only oxygen sensors. On the
other hand, on most engines there are more sensors in the intake system. Here,
the information that is the result of a change in wastegate setting is studied
using the available sensors in the intake system. An experiment to open the
wastegate at constant speed and load is made to give some indications of what
kind of information that is present in the intake system when the wastegate is
moved. During the experiment the air-mass flow is governed by a controller
whose objective is to maintain constant air-mass flow. The result of the experiment
is shown in Figure 1.1, where the exhaust manifold pressure drops
when the wastegate is opened. What is interesting is that the intake manifold
pressure also drops when the exhaust pressure drops. Thus, information about
exhaust manifold conditions is present in the intake system.
Contributions and Publications
1. A study of how common speed-density methods handle air-to-cylinder
estimation during a wastegate step is made and a new observer for airmass-
to-cylinder is developed. This work was published at the SAE conference
in Detroit 2001 (Andersson and Eriksson, 2001a). The air-mass-tocylinder
estimation problem for turbocharged SI-engines for various wastegate
settings is illustrated. The contribution is an air-mass-to-cylinder
observer that estimates the in-cylinder air-mass-oset.
2. An exhaust manifold pressure estimator for a turbocharged SI engine with
wastegate is proposed. This application extracts information from the
intake system about exhaust manifold conditions and does not require
any additional sensors after calibration. It was published at the IFAC
workshop Advances in Automotive Control in Karlsruhe 2001, (Andersson
and Eriksson, 2001b). The contribution is a model based estimator for
exhaust manifold pressure with few parameters.
Air/Fuel Ratio Control
Air/fuel ratio control for spark ignited (SI) engines is a well studied topic over
the years. Air/fuel control is necessary since the combustion in SI-engines is
only possible for air/fuel ratios around stoichiometric. For slightly rich mixtures
at the high temperatures and pressures inside the cylinder carbon monoxide is
formed since there is not enough oxygen to fully oxidize the fuel to carbon
dioxide. Rich mixtures can be used to maximize torque at full load. For lean
mixtures, the eciency on the other hand peaks, depending on lower pumping
losses, lower heat transfer and higher ratio of specic heats for the mixture. This
is another reason for air/fuel control since it provides a possibility of better fuel
economy at part load by running the engine slightly lean. Good driveability is
another issue especially during transients since the eciency and torque development
strongly depends on the air/fuel path. With bad air/fuel ratio control
the engine torque fluctuates in a non-comfortable way.