Trapped burned gas fraction control for opposed-piston engines with uniflow scavenging
View Patent ↗A trapped burned gas fraction is controlled in a two-stroke cycle opposed-piston engine with uniflow scavenging by adjusting an external EGR setpoint in real time. The adjusted setpoint is used to control EGR flow in the engine's air handling system.
1. A uniflow-scavenged opposed-piston engine equipped with an air handling system, comprising:
at least one cylinder with a bore, longitudinally-spaced exhaust and intake ports, and a pair of pistons disposed in opposition in the bore and operative to open and close the exhaust and intake ports during operation of the engine;
a charge air channel to provide charge air to at least one intake port;
an exhaust channel to receive exhaust gas from at least one exhaust port;
an exhaust gas recirculation (EGR) loop having a loop input coupled to the exhaust channel and a loop output coupled to the charge air channel; and,
a control mechanization operable to:
determine a value of a trapped air handling parameter based on elements of combustion trapped in the cylinder by the last port of the cylinder to close during a cycle of engine operation and adjust the value of the trapped air handling parameter in response to a rate of EGR flow in the EGR loop; and
adjust the rate of EGR flow in the EGR loop based on the adjusted value of the trapped air handling parameter.
2. The opposed-piston engine of claim 1 , in which the control mechanization is operable to adjust the rate of EGR flow by operating a valve in the EGR loop to increase or decrease exhaust gas flow through the EGR loop.
3. The opposed-piston engine of claim 2 , in which the trapped air handling parameter is trapped burned gas fraction and the control mechanization is operable to:
determine a desired trapped burned gas fraction value for a current engine operating state;
determine a % EGR ratio defined by
%
E
G
R
=
W
egr
W
air
+
W
egr
in which W egr is a mass flow rate of EGR gas in the EGR loop and W air is a mass flow rate of air into the charge air channel;
determine an error value based upon a difference between a desired % EGR ratio and a measured % EGR ratio; and,
adjust EGR flow by operating the valve in the EGR loop in response to the error value.
4. The opposed-piston engine of claim 2 , in which the control mechanization is operable to correct the value of the trapped air handling parameter based upon a trapped temperature parameter.
5. The opposed-piston engine of claim 4 , in which the trapped air handling parameter is trapped burned gas fraction and the control mechanization is operable to:
determine a desired trapped burned gas fraction value for a current engine operating state;
determine a % EGR ratio defined by
%
E
G
R
=
W
egr
W
air
+
W
egr
in which W egr is a mass flow rate of EGR gas in the EGR loop and W air is a mass flow rate of air into the charge air channel;
determine an error value based upon a difference between a desired % EGR ratio and a measured % EGR ratio; and,
adjust EGR flow by operating the valve in the EGR loop in response to the error value.
6. A method of operating an opposed-piston engine, comprising:
generating exhaust gas in at least one ported cylinder of the engine;
transporting exhaust gas from an exhaust port of the ported cylinder through an exhaust channel;
recirculating a portion of the exhaust gas from the exhaust channel;
pressurizing fresh air;
mixing recirculated exhaust gas with the pressurized fresh air to form charge air;
pressurizing the charge air;
providing the charge air to an intake port of the ported cylinder;
determining a value of a trapped air handling parameter based on elements of combustion trapped in the cylinder by the last port of the cylinder to close during a cycle of engine operation;
adjusting the value of the trapped air handling parameter in response to a rate of EGR flow in the EGR loop; and
adjusting the rate of EGR flow in the EGR loop based on the adjusted value of the trapped air handling parameter.
7. The method of claim 6 , in which adjusting the rate of EGR flow includes operating a valve in the EGR loop to increase or decrease exhaust gas flow through the EGR loop.
8. The method of claim 7 , in which the trapped air handling parameter is trapped burned gas fraction and determining a value includes:
determining a desired trapped burned gas fraction value for a current engine operating state;
determining a % EGR ratio defined by
%
E
G
R
=
W
egr
W
air
+
W
egr
in which W egr is a mass flow rate of EGR gas in the EGR loop and W air is a mass flow rate of air into the charge air channel;
determining an error value based upon a difference between a desired % EGR ratio and a measured % EGR ratio; and,
adjusting EGR flow by operating the valve in the EGR loop in response to the error value.
9. The method of claim 7 , in which the control mechanization is operable to correct the value of the trapped air handling parameter based upon a trapped temperature parameter.
10. The method of claim 9 , in which the trapped air handling parameter is trapped burned gas fraction and determining a value includes:
determining a desired trapped burned gas fraction value for a current engine operating state;
determining a % EGR ratio defined by
%
E
G
R
=
W
egr
W
air
+
W
egr
in which W egr is a mass flow rate of EGR gas in the EGR loop and W air is a mass flow rate of air into the charge air channel;
determining an error value based upon a difference between a desired % EGR ratio and a measured % EGR ratio; and,
adjusting EGR flow by operating the valve in the EGR loop in response to the error value.
11. A uniflow-scavenged opposed-piston engine, comprising:
at least one cylinder with a bore, longitudinally-spaced exhaust and intake ports, and a pair of pistons disposed in opposition in the bore and operative to open and close the exhaust and intake ports during operation of the engine;
a charge air channel to provide charge air to at least one intake port;
an exhaust channel to receive exhaust gas from at least one exhaust port;
an exhaust gas recirculation (EGR) loop having a loop input coupled to the exhaust channel and a loop output coupled to the charge air channel; and,
a control mechanization operable to:
determine a trapped burned gas fraction value for a current engine operating state based on elements of combustion trapped in the cylinder by the last port of the cylinder to close during a cycle of engine operation;
determine a desired setpoint for EGR flow in the EGR loop for the trapped burned gas fraction value;
compare the desired setpoint with an actual rate of EGR flow in the EGR loop; and,
adjust the position of an EGR valve in the EGR loop based on the comparison.
12. The opposed-piston engine of claim 11 , in which the control mechanization is operable to adjust the actual rate of EGR flow in the EGR loop by adjusting the position of the EGR valve.
13. The opposed-piston engine of claim 12 , in which the control mechanization is operable to correct the trapped burned gas fraction based upon a trapped temperature value.
14. The opposed-piston engine of claim 13 , in which the control mechanization is operable to:
determine a % EGR ratio defined by
%
E
G
R
=
W
egr
W
air
+
W
egr
in which W egr is a mass flow rate of EGR gas in the EGR loop and W air is a mass flow rate of air into the charge air channel;
determine an error value based upon a difference between a desired % EGR ratio and a measured % EGR ratio;
correct the desired % EGR ratio based on the error; and,
adjust EGR flow by adjusting the position of the EGR valve based on the corrected desired % EGR.
15. The opposed-piston engine of claim 12 , in which the control mechanization is operable to:
determine a % EGR ratio defined by
%
E
G
R
=
W
egr
W
air
+
W
egr
in which W egr is a mass flow rate of EGR gas in the EGR loop and W air is a mass flow rate of air into the charge air channel;
determine an error value based upon a difference between a desired % EGR ratio and a measured % EGR ratio;
correct the desired % EGR ratio based on the error; and,
adjust EGR flow by adjusting the position of the EGR valve based on the corrected desired % EGR.