THERMAL MANAGEMENT OF AFTERTREATMENT DEVICES OF OPPOSED-PISTON ENGINES UNDER MOTORING CONDITIONS

A method of operating a two-stroke cycle, opposed-piston engine comprising a pumping device coupled to pump air to cylinders of the engine through a charge air cooler and an aftertreatment system of thermally-activated devices coupled to receive exhaust from the cylinders by which a thermal state of the exhaust sufficient to sustain thermal activation of one or more of the aftertreatment system devices may be maintained during a deceleration or motoring condition of operation by reducing the mass airflow to the engine.

TECHNICAL FIELD

The field is exhaust management strategies of two-stroke cycle, opposed-piston engines which maintain exhaust temperatures at levels suitable for effective operation of aftertreatment devices.

BACKGROUND

A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. One example of a two-stroke cycle engine is an opposed-piston engine in which two pistons are disposed in the bore of a cylinder for reciprocating movement in opposing directions along the central axis of the cylinder. Each piston moves between a bottom center (BC) location where it is nearest one end of the cylinder and a top center (TC) location within the cylinder where it is furthest from the one end. The cylinder has ports formed in the cylinder sidewall near respective BC piston locations. Each of the opposed pistons controls one of the ports, opening the port as it moves to its BC location, and closing the port as it moves from BC toward its TC location. One of the ports serves to admit charge air (sometimes called “scavenging air”) into the bore, the other provides passage for the products of combustion out of the bore; these are respectively termed “intake” and “exhaust” ports (in some descriptions, intake ports are referred to as “air” ports or “scavenge” ports). In a uniflow-scavenged opposed-piston engine, pressurized charge air enters a cylinder through its intake port as exhaust gas flows out of its exhaust port, thus gas flows through the cylinder in a single direction (“uniflow”)—from intake port to exhaust port.

The opposed-piston engine has an air handling system that manages the transport of charge air provided to, and exhaust gas produced by, the engine during operation of the engine. A representative air handling system construction includes a charge air subsystem and an exhaust subsystem. The charge air subsystem receives and compresses air and includes a charge air channel that delivers the compressed air to the intake port or ports of the engine. The charge air subsystem may comprise one or both of a turbine-driven compressor and a supercharger. The charge air channel typically includes at least one air cooler that is coupled to receive and cool the charge air (or a mixture of gasses including charge air) before delivery to the intake ports of the engine. The exhaust subsystem includes an exhaust channel that transports exhaust gas from engine exhaust for delivery to other exhaust subsystem components such as a turbine that drives the compressor, an exhaust gas recirculation (EGR) loop, and one or more aftertreatment devices.

As in conventional four-stroke engines the aftertreatment devices of opposed-piston engines cleanse exhaust gas of undesirable components as it is transported through the devices before being emitted into the atmosphere. The aftertreatment devices are constructed to convert components such as soot, NOx, and unburned hydrocarbons in the exhaust gas into harmless compounds by thermally-driven processes that may include one or more of catalyzation, decomposition, and filtration. The heat that causes the devices to operate is obtained from the exhaust gas itself, and the devices operate most effectively when exhaust gas temperatures are relatively high. One goal of an exhaust strategy for internal combustion engines equipped with aftertreatment devices is to maintain exhaust temperatures within a range of temperatures where the devices work most effectively. Ramesh, A. K., Gosala, D. B., Allen, C., Joshi, M., McCarthy Jr., J., Farrell, L., Koeberlein, E. D., and Shaver, G., “Cylinder Deactivation for Increased Engine Efficiency and Aftertreatment Thermal Management in Diesel Engines,” SAE Technical Paper 2018-01-384, 2018.

One engine operating condition which presents a challenge for such a strategy is motoring. In this regard, presume that the engine is installed in a vehicle, in which case motoring occurs during deceleration of the vehicle when the provision of fuel to the engine is interrupted and the engine continues to run in response to the vehicle's inertia. Motoring occurs in a dynamometer when the engine is run, without provision of fuel, by application of power to the engine's power train by a separate motor. The absence of combustion during motoring cools down exhaust flow which reduces the temperature of the aftertreatment devices. When deceleration is followed by acceleration and fueling, combustion occurs and mass exhaust flow is heated, thereby again heating exhaust flow. However, if the motoring period results in reduction of aftertreatment temperatures to suboptimal or non-operating levels, there may be an initial period of acceleration when undesirable emissions increase before the aftertreatment devices are once again heated to effective levels by the mass exhaust flow.

The goal of exhaust temperature maintenance may be attained by a management process that reduces the flow of air through the engine, which may result in higher exhaust temperatures. Ramesh, et al., op. cit. An opposed-piston engine operated in a two-stroke cycle mode has no intake stroke with which to pump air through the engine. Instead the air handling system comprises one or more devices for this purpose. In many cases the preferred pumping device is a supercharger. Manifestly, when the engine is motored in a moving vehicle (or in a dynamometer) by cessation of fueling, operation of the pumping device continues to push air through the engine, often through a charge air cooler. The continued flow of air unheated by combustion may cool aftertreatment devices and make them ineffective—causing emission of undesirable exhaust components for a period of time after resumption of combustion while the devices are brought to their effective operating temperatures.

SUMMARY

Certain embodiments of the invention include a method of operating a fuel-injected, opposed-piston engine comprising a pumping device coupled to pump air to cylinders of the engine and an aftertreatment system of thermally-activated devices coupled to receive exhaust from the cylinders by which a thermal state of the mass exhaust flow sufficient to sustain thermal activation of one or more of the aftertreatment system devices may be maintained during a deceleration or motoring condition of operation by reducing the mass airflow to the engine.

In some embodiments, a thermal state of the mass exhaust flow sufficient to sustain thermal activation of one or more of the aftertreatment system devices may be maintained during deceleration of the engine by recirculating a portion of the mass airflow to an inlet of the pumping device.

In some other embodiments, a thermal state of the mass exhaust flow sufficient to sustain thermal activation of one or more of the aftertreatment system devices may be maintained during motoring of the engine by recirculating a portion of the mass airflow to an inlet of the pumping device.

DETAILED DESCRIPTION

With reference toFIG. 1, a two-stroke cycle internal combustion engine is embodied in an opposed-piston engine49having at least one ported cylinder50. For example, the engine may have one ported cylinder, two ported cylinders, three ported cylinders, or four or more ported cylinders. Each ported cylinder50has a bore52and exhaust and intake ports54and56formed or machined in the vicinity of respective ends of a cylinder wall. Each of the ports54and56includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid bridge. In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions shown inFIG. 1. Pistons60and62are slideably disposed in the bore52of each cylinder with their end surfaces61and63opposing one another. Movements of the pistons60control the operations of the exhaust ports54. Movements of the pistons62control the operations of the intake ports56. Thus, the ports54and56are referred to as “piston controlled ports”. Pistons60controlling the exhaust ports (“exhaust pistons”) are coupled to a crankshaft71. Pistons62controlling the intake ports of the engine (“intake ports”) are coupled to a crankshaft72.

As pistons60and62approach respective TC locations, a combustion chamber is defined in the bore52between the end surfaces61and63. Fuel is injected directly into the combustion chamber through at least one fuel injector nozzle100positioned in an opening through the sidewall of a cylinder50. The fuel mixes with charge air admitted through the intake port54. As the mixture is compressed between the end surfaces it reaches a temperature that causes the fuel to ignite; in some instances, ignition may be assisted, as by spark or glow plugs. Combustion follows.

With further reference toFIG. 1, the engine49includes an air handling system51that manages the transport of charge air provided to, and exhaust gas produced by, the engine49. A representative air handling system construction includes a charge air subsystem and an exhaust subsystem. In the air handling system51, the charge air subsystem includes a charge air source that receives intake air and processes it into charge air, a charge air channel coupled to the charge air source through which charge air is transported to the at least one intake port of the engine, and at least one air cooler in the charge air channel that is coupled to receive and cool the charge air before delivery to the intake port or ports of the engine. Such a cooler can comprise an air-to-liquid and/or an air-to-air device, or another cooling device. Hereinafter, such a cooler is denoted as a “charge air cooler”. The charge air subsystem also includes a supercharger that pumps charge air in the charge air channel to intake ports of the engine. The exhaust subsystem includes an exhaust channel that transports exhaust products from exhaust ports of the engine to an exhaust outlet.

As perFIG. 1, the preferred charge air subsystem includes a supercharger110, which can be driven by an electrical motor, or by a gear, chain, or belt apparatus coupled to a crankshaft. The supercharger110can be a single-speed or multiple-speed device, or a fully variable-speed device. In some aspects, the air management system51includes a turbocharger120with a turbine121and a compressor122that rotate on a common shaft123. The turbine121is coupled to the exhaust subsystem and the compressor122is coupled to the charge air subsystem. The turbine121can be a fixed-geometry or a variable-geometry device. The turbocharger120extracts energy from exhaust gas that exits the exhaust ports54and flows into the exhaust channel124directly from the exhaust ports54, or from an exhaust manifold125. In this regard, the turbine121is rotated by exhaust gas passing through it. This rotates the compressor122, causing it to generate charge air by compressing intake air. The charge air output by the compressor122flows through a conduit126to a charge air cooler127, whence it is pumped by the supercharger110to the intake ports. Air compressed by the supercharger110is output from the supercharger through a charge air cooler129to an intake manifold130. The intake ports56receive charge air pumped by the supercharger110, through the intake manifold130. Preferably, but not necessarily, in multi-cylinder opposed-piston engines, the intake manifold130is constituted of an intake plenum that communicates with the intake ports56of all cylinders50.

The air handling system51may be equipped to control emissions of nitrous oxide (NOx) by recirculating exhaust gas through the one or more cylinders of the opposed-piston engine. The recirculated exhaust gas is mixed with charge air to lower peak combustion temperatures, which lowers NOx emissions. This process is referred to as exhaust gas recirculation (“EGR”). The opposed-piston engine49seen inFIG. 1may be equipped with an EGR loop that channels exhaust gas from the exhaust subsystem into the charge air subsystem. An example of a specific EGR loop construction (which is not intended to be limiting) is a high pressure configuration illustrated inFIG. 1. In this regard, a high pressure EGR loop circulates exhaust gas obtained from a source upstream of the turbine121to a mixing point downstream of the compressor122. In this EGR loop the conduit131and an EGR valve138shunt a portion of the exhaust gas from the exhaust manifold125to be mixed with charge air output by the compressor122into the conduit126. If no exhaust/air mixing is required the valve138is fully shut and charge air with no exhaust gas is delivered to the cylinders. As the valve138is increasingly opened, an increasing amount of exhaust gas is mixed into the charge air.

An air handling system200for a two-stroke cycle, opposed-piston engine201such as the engine illustrated byFIG. 1is shown in the schematic diagram ofFIG. 2. The air handling system200includes a supercharger210which receives input rotary power from a drive unit212. The supercharger210includes an inlet213and an outlet214. The air handling system200may also include a turbocharger220with a turbine221and a compressor222. The turbine221is coupled to an exhaust channel224and the compressor222is coupled to a charge air channel225. The turbine221is spun by exhaust gas expelled from the exhaust ports156of the engine201and transported through the exhaust channel224. This spins the compressor222, causing it to generate charge air by compressing inlet air that flows into the charge air channel. Compressed charge air output by the compressor222is transported through the charge air channel225to a charge air cooler227. In this configuration, the supercharger210constitutes a second stage of compression in the air handling system200(following the compressor222). In any case, the supercharger210compresses air in the charge air channel and provides compressed charge air (also called “boost”) to the intake ports154of the opposed-piston engine. In some instances, a charge air cooler229may be provided to cool the output of the supercharger210. Optionally, the air handling system may include an EGR loop230to transport exhaust products from the exhaust channel224to the charge air channel225via an EGR mixer226.

The exhaust channel224includes an exhaust aftertreatment system228downstream of the turbine221. Exhaust gas flowing from the outlet of the turbine221flows through devices of the aftertreatment system228and, from there, out of a tailpipe. The aftertreatment system228may be constituted of one or more aftertreatment devices. For example, the aftertreatment system228may include one or more devices to convert components such as soot, NOx, and unburned hydrocarbons in the exhaust gas into harmless compounds by thermally-driven processes that may include one or more of catalyzation, decomposition, and filtration. In this regard, the aftertreatment system may include a diesel oxidation catalyst (DOC) device, a diesel particulate filter (DPF) device, a selective catalytic reduction (SCR), and/or an ammonia slip catalyst (ASC) device. Such an aftertreatment system would be comparable to a typical exhaust after-treatment system on a commercial heavy duty diesel four-stroke engine. The heat that causes aftertreatment devices of the aftertreatment system to operate is obtained from the exhaust gas itself, and the devices operate most effectively when exhaust gas temperatures are relatively high.

Control of the gas transport configuration of the air handling system is implemented by a mechanization that includes a programmable ECU (engine control unit)240, air handling processes executed on the ECU, air handling valves and associated actuators, the supercharger210, and engine sensors. The ECU240is programmed to execute fuel handling algorithms and air handling algorithms under various engine operating conditions. Such algorithms are embodied in control modules that are part of an engine systems control program executed by the ECU240while the engine is operating. For a common rail direct injection system with which the engine may be equipped, the ECU240can control injection of fuel into the engine's cylinders by issuing rail pressure commands and injector commands.

Air handling system control is exercised by settings of variable valves. In this regard, for example, a supercharger bypass valve231(also referred to as a “recirculation” or “shunt” valve) bleeds charge air produced by the supercharger210through a bypass channel232so as to modulate charge air pressure, and dampen surges, at the intake ports154. An EGR valve233adjusts the amount of exhaust gas that is transported through the EGR loop230to the charge air channel225for control of emissions. A wastegate valve235shunts exhaust gas around the turbine221in order to control the amount of exhaust gas flowing through the turbine221, thereby modifying the turbine work and the exhaust gas temperature downstream of the turbine. A backpressure valve237regulates exhaust pressure at the turbine outlet in order to increase the gas pressure inside the engine and warm the engine quickly during start-up. For fast, precise automatic operation, it is preferred that these and other valves in the air handling system be high-speed, computer-controlled devices, with continuously-variable settings. The ECU240is in control communication with actuators (not shown) that operate the valves in response to ECU-issued valve-setting commands. In cases where the supercharger210is operated by a variable drive, the ECU240also controls gas transport by issuing drive commands to actuate the supercharger drive212. And, in those instances where the turbine221may be configured as a variable geometry device, the ECU240also controls the transport of gas by issuing VGT commands to set the aspect ratio of the turbine.

The ECU240monitors air handling system operating conditions by way of various air handling sensors. In this regard, such sensors may include, without limitation, accelerator position, engine speed, fuel rail pressure, mass airflow, mass EGR flow, intake manifold, exhaust manifold, supercharger inlet, supercharger outlet, coolant temperature, and so on. For purposes of this specification these and other sensors may comprise physical measurement instruments and/or virtual systems.

In most cases, to obtain the rotary power necessary to its operation, the supercharger210is directly coupled to the engine-usually via a crankshaft-driven drive apparatus. In these cases the speed of the supercharger is dependent on the speed of the engine. In some instances, the drive212may be equipped with a transmission that enables the supercharger to be driven, under command of the air handling control mechanization, at a continuously-, or incrementally-, variable speed, independently of a crankshaft. In some of these instances, the supercharger bypass valve231may be redundant. That is to say, the greater the variability in supercharger speed afforded by the variable-speed drive, the less likely a bypass valve would be needed to modulate boost pressure. However, there may be instances wherein a drive unit is constructed to provide a limited number of speeds (two speeds, for example) and flexibility in control of boost pressure may require the operations of the supercharger bypass valve231.

Under control of an air handling process executed by the ECU240, exhaust temperature management is provided by reducing the flow of air through the engine in order to maintain effective operation of thermally activated devices of the aftertreatment system of an opposed-piston engine. The desired result is obtained by taking advantage of the fact that such an opposed-piston engine has negligible internal pumping. Instead one or more external devices are provided for this purpose. In many cases the preferred pumping devices comprise the supercharger and the turbocharger. When the engine is decelerated in a moving vehicle (or motored in a dynamometer), only the supercharger continues to push (i.e., pump) air through the engine, often through one or more charge air coolers. With nothing more, one would expect the continued flow of air unheated by combustion to cool thermally-actuated aftertreatment devices and make them ineffective—causing emission of undesirable exhaust components during subsequent acceleration until the devices are brought to their effective operating temperatures.

FIG. 2shows how this undesirable effect may be reduced or eliminated in an opposed-piston engine operated in a two-stroke cycle mode. Inlet air flows form the compressor222into the supercharger210through the charge air cooler227. The supercharger210may be driven using a 2-speed drive in order to control the drive ratio between the engine crankshaft to which the drive is coupled and the input shaft of the supercharger210. The supercharger210may also act as a pump to draw recirculated exhaust through the EGR loop230. Charge air (with or without recirculated exhaust) flows through the supercharger210into the engine intake manifold130through the charge air cooler229. The supercharger bypass valve231enables the ECU240to accurately control the mass airflow going into the engine, and therethrough to the exhaust subsystem. All of the pumping work required to move the mass of air is provided by the turbocharger222and the supercharger210. Pistons do not contribute to pumping. During deceleration or motoring when the exhaust enthalpy is negligible, the supercharger210does virtually all of the pumping. As a result, during deceleration or motoring, it is possible to significantly reduce the cold air flow going into the exhaust subsystem through the engine by controlling the position of the supercharger bypass valve231. During deceleration or motoring this reduction can contribute to keeping the aftertreatment system238of the opposed-piston engine warm unlike four-stroke engines in which piston motion continues to pump cold air through the aftertreatment system238. AlthoughFIG. 2shows the aftertreatment system228located on the downstream side of the backpressure valve237, this is not the only location where this desirable effect may be produced. The benefits of reducing cold airflow during deceleration or motoring may be realized when the components of the aftertreatment system238are situated between the downstream side of the wastegate valve235and the downstream side of the backpressure valve.

FIG. 3illustrates the above-described process as a method of airflow control for thermal management of aftertreatment devices of an opposed-piston engine, which may be executed by the ECU240during two-stroke operation of an opposed-piston engine equipped with an air handling system such as that ofFIG. 2. The airflow control process300is initiated at302by operation of the engine either in a vehicle or a dynamometer. Operation of the engine may include injecting fuel into the engine while operating the engine in a two-stroke cycle mode, operating the supercharger to provide a mass airflow into the engine, and transporting a mass exhaust flow from the engine to the aftertreatment system.

When the engine is decelerated or motored at304, fueling of the engine is shut at306by one or more fuel stop commands (best seen inFIG. 2). At308, exhaust flow out of the engine into the exhaust subsystem is reduced by reducing mass airflow into the engine, which may be accomplished by reduction of airflow out of the supercharger210. In the preferred embodiment, this reduction is accomplished by opening the bypass valve231, which causes charge air to be circulated or shunted from the outlet214to the inlet213of the supercharger. As perFIG. 2, when the recirculated charge air is diverted into the bypass channel232downstream of the charge air cooler229and returned to the supercharger210via the inlet of the charge air cooler227, the combined cooling effect of the two coolers on the reduced air mass delivered to the aftertreatment exhaust subsystem is minimized. In other implementations, the bypass channel232can be modified to obtain the output of the supercharger from a point between the supercharger outlet214and the inlet of the charge air cooler229. Upon resumption of combustion by the engine at310, the control process returns to302.

Variations on the airflow control process ofFIG. 3may be implemented. For example, when the supercharger bypass valve231is opened during deceleration or motoring the backpressure valve237may be partially or fully closed. Further, when the supercharger bypass valve231is opened and the backpressure valve237is closed, the EGR valve233may be opened. Opening the bypass channel and the EGR loop may allow for lower restriction and even lower supercharger loads. If the drive212is equipped with a clutch, the drive may be de-clutched to reduce the supercharger rotation speed to zero, which may eliminate all pumping from the engine and essentially cut off exhaust flow. Alternatively, If the supercharger drive is variable, a lower ratio can be selected.

A surprising result of these strategies may be that the engine motoring torque or engine drag may be reduced enough to allow the vehicle to roll without power for a longer distance and thereby reduce fuel consumption. Further with the exhaust backpressure valve remaining closed, the engine drag can be modulated to provide a desired level of deceleration of the vehicle or to compensate for other vehicle loads to meet vehicle drivability requirements.

The desired exhaust management strategy can be triggered based on a variety of parameters including reduction of the fuel flow request to zero. This strategy can be enabled within a given vehicle speed range, within a given engine speed range, within a given coolant temperature range, within a given catalyst temperature range, within a given ambient temperature range, and/or within a given ambient pressure range.

In the case when there is a specific request for a different engine drag torque, a model of the supercharger power consumption based on supercharger inlet and outlet pressure and temperature may be used to adjust the bypass valve and/or EGR valve to achieve the requested drag torque.

In the foregoing specification, embodiments have been described with reference to numerous specific details that can vary from implementation to implementation. Certain adaptations and modifications of the described embodiments can be made. Other embodiments can be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

It is also intended that the sequence of steps, acts, or states shown in figures are only for illustrative purposes and are not intended to be limited to any particular sequence of steps. As such, those skilled in the art can appreciate that these steps, acts, or states can be performed in a different order while implementing the same method.