EGR for a two-stroke cycle engine without a supercharger

Abstract

A two-stroke cycle, turbo-driven, opposed-piston engine with one or more ported cylinders and uniflow scavenging has no supercharger. The engine includes a high pressure EGR loop and a pump in the EGR loop to boost the pressure of the recirculated exhaust products.

Claims

1. A uniflow-scavenged, two-stroke cycle, opposed-piston engine comprising: at least one cylinder with piston-controlled exhaust and intake ports, the exhaust and intake ports being longitudinally spaced so as to be disposed near respective ends of the at least one cylinder; two crankshafts, in which each exhaust piston couples to a first crankshaft and each intake piston couples to a second crankshaft; an exhaust channel coupled to at least one exhaust port of the engine; a charge air channel coupled to at least one intake port of the engine; a power-assisted turbocharger with: a compressor output coupled to the charge air channel; a turbine coupled to the exhaust channel for being rotated by exhaust gas passing through the turbine; and a turbine output coupled to an exhaust pipe; a high pressure exhaust gas recirculation (EGR) loop having a loop input coupled to the exhaust channel upstream of the turbine and a loop output coupled to the charge aft channel downstream of the compressor and simultaneously connected to an inlet of at least one charge aft cooler; an electrically-driven pump in the high pressure EGR loop to pump exhaust gas through the high pressure EGR loop into the charge air channel; an electrically-controlled variable valve in the high pressure EGR loop between the loop input and the pump; and a control unit connected to provide control signals for the power-assisted turbocharger, the pump, and the valve, wherein the engine has no supercharger.

2. The uniflow-scavenged, two-stroke cycle, opposed-piston engine of claim 1, in which the EGR loop further includes an EGR cooler in series with the pump.

3. A method of operating the uniflow-scavenged, two-stroke cycle, opposed-piston engine, the opposed-piston engine including: at least one cylinder with piston-controlled exhaust and intake ports, the exhaust and intake ports being longitudinally spaced so as to be disposed near respective ends of the at least one cylinder; two crankshafts, in which each exhaust piston couples to a first crankshaft and each intake piston couples to a second crankshaft; an exhaust channel coupled to at least one exhaust port of the engine; a charge air channel coupled to at least one intake port of the engine; a power-assisted turbocharger with: a compressor output coupled to the charge air channel; a turbine coupled to the exhaust channel for being rotated by exhaust gas passing through the turbine; and a turbine output coupled to an exhaust pipe; a high pressure exhaust gas recirculation (EGR) loop having a loop input coupled to the exhaust channel upstream of the turbine and a loop output coupled to the charge air channel downstream of the compressor and simultaneously connected to an inlet of at least one charge air cooler; an electrically-driven pump in the high pressure EGR loop to pump exhaust gas through the high pressure EGR loop into the charge air channel; an electrically-controlled variable valve in the high pressure EGR loop between the loop input and the pump; a control unit connected to provide control signals for the power-assisted turbocharger, the pump, and the valve; in which the engine has no supercharger, the method comprising: pressurizing charge air via the compressor of the power-assisted turbocharger; cooling the charge air being pressurized in at least one cooler; delivering the charge air being pressurized and cooled to an intake port of each of the one or more cylinders; and pumping engine exhaust gas in the high pressure exhaust gas recirculation (EGR) loop to an inlet of the at least one air charge cooler by controlling the electrically-driven pump in the high pressure EGR loop to reduce Nox emissions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a conceptual schematic diagram of a two-stroke cycle engine of the opposed-piston type in which aspects of an air management system with EGR are illustrated.

(2) FIG. 2 is a conceptual schematic drawing illustrating a construction for EGR in a turbocharged two-stroke cycle opposed-piston engine without a supercharger.

DETAILED DESCRIPTION

(3) The EGR construction described in this specification is presented in an explanatory context that includes a uniflow-scavenging, two-stroke cycle engine of a type having at least one ported cylinder in which a pair of pistons is disposed with their end surfaces in opposition. A ported cylinder includes one or more of intake and exhaust ports formed or machined in a sidewall thereof. This explanatory context is intended to provide a basis for understanding a specific EGR construction embodiment by way of an illustrative example.

(4) With reference to FIG. 2, an opposed-piston engine having a construction similar to that of the engine seen in FIG. 1 is equipped with an EGR loop that channels exhaust gas from the exhaust subsystem into the charge air subsystem, but without the aid of a supercharger in the charge air subsystem. Preferably, the EGR loop construction is a high pressure configuration. In this regard, a high pressure EGR loop circulates exhaust gas obtained from the exhaust channel 124 through a loop input upstream (prior to the input) of the turbine 121 to a mixing point downstream (following the outlet) of the compressor 122. In this EGR loop the EGR valve 138 is operated to shunt a portion of the exhaust gas from the exhaust manifold 125 through the conduit 131 to be mixed with charge air output by the compressor 122 into the conduit 126. If no exhaust/air mixing is required the EGR valve 138 is fully shut and charge air with no exhaust gas is delivered to the cylinders. As the EGR valve 138 is increasingly opened, an increasing amount of exhaust gas is mixed into the charge air. This loop subjects the exhaust gas to the cooling effects of the cooler 127. A dedicated EGR cooler 129 can be incorporated into the conduit 131 in series with the valve 138.

(5) EGR Loop Construction Including a Pump:

(6) The high-pressure EGR loop construction seen in FIG. 2 includes an EGR pump 200 in series with the EGR valve 138. The outlet of the valve 138 is connected to the input of the EGR pump 200 whose purpose is to raise the pressure of recirculated exhaust gas from the level in the exhaust manifold 125 to the level in the intake manifold 130. The pressure is applied by the pump 200 from a point in the conduit 131, as opposed to the application of pressure in the charge air subsystem by a supercharger. This pressure creates a pressure differential between the intake and exhaust manifolds that pumps a portion of exhaust gas from the exhaust manifold 125 to the conduit 126 where it is mixed with the charge air and recirculated therewith into the intake manifold 130. Preferably, the pump 200 is an electrically-controlled, variable-speed pump, but other pump types (hydraulically-controlled, for example) are possible.

(7) Power-Assisted Turbocharger:

(8) It is useful that the turbocharger 120 be assisted in order to ensure a continuous positive pressure differential across the manifolds 125, 130 while the engine 49 is operating. In this regard, the turbocharger 120 includes a power-assist system 210, which can comprise, for example an electric motor/generator unit, that boosts turbocharger operation during start and low load conditions so as to add energy to the charge air flow when unassisted turbocharger operation is inadequate for it. Alternative turbo power-assist devices include hydraulic or pneumatic mechanisms. A turbocharger with a power-assist system is referred to as a power-assisted turbocharger.

(9) Control Mechanization:

(10) An EGR control process for an EGR system that utilizes the construction illustrated in FIG. 2 is executed by an electronic control unit (ECU) 149 in response to specified engine operating conditions by automatically operating the valve 138, the pump 200, and the power assist system 210. Of course, operation of valves, throttles, and other associated elements that may be used for EGR and air management control can include any one or more of electrical, pneumatic, mechanical, and hydraulic actuating operations. For fast, precise automatic operation, it is preferred that valves, including the EGR valve 138, be high-speed, high-resolution, computer-controlled devices with a continuously-variable settings.

(11) Preferably an EGR control process automatically operates the EGR system described and illustrated herein based upon one or more parameters relating to recirculated exhaust gas and to a mixture of recirculated exhaust gas and charge air. Parameter values are determined by a combination of one or more of sensors, calculations, and table lookup so as to manage the values of individual parameters and one or more ratios of EGR and mixture parameters in one or more cylinders. The sensors involved in determining parameter values can include those shown in FIG. 2 located between the intake throttle valve 141 and the exhaust valve 140 on the exhaust pipe 128, such as one or more sensors for: air mass flow, ambient temperature, humidity, CAC out temperature, CAC out pressure, intake manifold pressure, intake manifold temperature, engine AP, turbo outlet pressure, turbo outlet temp, and engine out NOx. The sensors can also be located on the cylinder block, for example sensors for real pressure and coolant temperature in or around the cylinder block.

(12) An EGR construction for a two-stroke cycle engine without a supercharger has been described with reference to an opposed-piston engine having two crankshafts; however, it should be understood that various aspects of this EGR system can be applied to opposed-piston engines with one or more crankshafts. Moreover, various aspects of this EGR construction can be applied to opposed-piston engines with ported cylinders disposed in opposition, and/or on either side of one or more crankshafts. Accordingly, the protection afforded to this construction is limited only by the following claims.