Power plants – Fluid motor means driven by waste heat or by exhaust energy... – With supercharging means for engine
Reexamination Certificate
1999-01-26
2001-01-30
Koczo, Michael (Department: 3748)
Power plants
Fluid motor means driven by waste heat or by exhaust energy...
With supercharging means for engine
Reexamination Certificate
active
06178749
ABSTRACT:
TECHNICAL FIELD
This invention relates to turbocharged compression ignition engines and, more particularly, to methods of reducing turbo lag in turbocharged diesel engines having an exhaust gas recirculation (EGR) system.
BACKGROUND OF THE INVENTION
High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGTs) allow the intake airflow to be optimized over a range of engine speeds. This may be accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
A particular problem with turbocharged diesel engines is poor acceleration, particularly from idle or low engine speeds. This “turbo-lag” is due to the time delay associated with filling the intake manifold with enough fresh air to support the amount of fuel required to satisfy the operator's torque demand. To meet this requirement, however, the delivered fuel often must be limited as a function of the available air in order to maintain the air-to-fuel ratio above the threshold at which visible smoke occurs. The rate at which the air supply can be increased is limited by the dynamics of the turbocharger and the transport delay between the turbocharger compressor and the intake manifold of the engine.
A traditional control strategy for diesel engines having an EGR system and a VGT is two single loop controllers. In other words, the two devices are controlled independently with the EGR valve controlling the mass of airflow into the intake manifold (MAF), and the VGT controlling the intake manifold pressure (MAP, or boost). The desired values for compressor mass airflow (MAF) and boost pressure (MAP) are stored as lookup table values referenced to engine speed and load or fueling rate. For each engine speed and fueling rate, the control algorithm retrieves the desired values for MAP and MAF and controls the EGR and VGT to achieve those values.
The two single loop controllers can be represented as follows:
EGR=K
EGR
(
MAF−MAF
d
)
VGT=K
VGT
(
MAP−MAP
d
)
wherein the subscript “d” denote the desired setpoints for the given variable. The controller K is usually a proportional-integral-derivative (PID) controller, with gains scheduled on the speed-load condition of the engine.
The desired setpoints for MAP and MAF are typically optimized for steady-state engine operation. Because these values were optimized for steady-state engine operation, however, they are poorly suited for generating feedback errors to drive the EGR and VGT during transient conditions. This straightforward engine control strategy often results in excessively large turbo lag and slow engine torque response.
To improve acceleration, some engine control systems use a transient detection feature to turn off the feedback control to the EGR and close the EGR valve when fuel limiting is active. This is done to provide as much fresh air as possible to the intake manifold so that the maximum amount of fuel can be injected without violating the air/fuel threshold at which visible smoke occurs. The independent feedback control for MAF is then reinitiated after the transient condition is over and the engine is essentially operating at steady-state. Such engine control strategies fail to account for the interaction between the VGT and EGR, however, because of their independent control of the two systems.
Thus, there exists a need to control the EGR and VGT and, hence, MAF and MAP, to deliver fuel to the engine at a rate which generates the torque demanded by the driver, yet maintains the air/fuel ratio above the threshold at which visible smoke occurs.
DISCLOSURE OF THE INVENTION
One object of the present invention is to provide a fast airflow response during transient engine operating conditions to reduce turbo lag.
Another object is to modify the setpoints for MAP and MAF during transient engine operations to reduce turbo lag.
A further object of the invention is to coordinate the control of the VGT and EGR to speed up the system response and, thereby, reduce turbo lag.
The invention is advantageous in that it improves engine system performance while preventing the occurrence of visible smoke or, viewed another way, reduces the smoke content of the engine exhaust gas while maintaining engine performance.
According to the present invention, the foregoing and other objects and advantages are obtained by a method of reducing turbo lag in a compression ignition engine having an EGR system and a VGT. The method comprises the steps of determining an intake manifold pressure and intake manifold mass airflow setpoint, MAP
d
and MAF
d
, as a function of the current engine speed and requested fueling rate (W
f,REQ
) and generating modified setpoints, MAF
c
and MAP
c
, as a function of MAF
d
and MAP
d
, respectively. These modified setpoints are then compared to the actual MAF and MAP values to generate control signals for the turbocharger and EGR valve. The controller then dynamically adjusts the position of the EGR valve and turbocharger vanes to drive the turbocharger and EGR valve to the modified setpoints, thereby maximizing the amount of fresh air admitted to the engine during transient operation.
Thus, the setpoints for MAP and MAF are dynamically modified by a shaping filter before being fed to the controller. Accordingly, during transient conditions such as tip-in, the modified setpoints cause the EGR valve to close, and the VGT to initially open, then close, as the EGR valve moves toward the desired position. This coordinates the EGR and VGT actions during transient operation, insuring that the maximum amount of fresh air is admitted to the engine during transient conditions. Furthermore, this control strategy is implemented without modifying the feedback control law.
According to another aspect of the present invention, control of the EGR and VGT is coordinated to aggressively regulate airflow to the desired setpoint value, particularly during low engine speed conditions. This control strategy further includes the steps of generating the turbocharger control signal as a function of MAP, MAP
d
, MAF and MAF
d
, and generating the EGR valve control signal as a function of MAP, MAP
d
, MAF and MAF
d
. The method thus increases the MAF response by using multivariable control of both the EGR and VGT to aggressively regulate airflow to the desired setpoint, particularly during low airflow engine operation.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.
REFERENCES:
patent: 5123246 (1992-06-01), Younessi et al.
patent: 5187935 (1993-12-01), Akiyama, et al.
patent: 5520161 (1996-05-01), Klopp
patent: 5794445 (1998-08-01), Dungner
patent: 6029451
Kolmanovsky Ilya V.
Moraal Paul Eduard
Van Nieuwstadt Michiel J.
Drouillard Jerome R.
Ford Motor Company
Koczo Michael
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