Piston compound internal combustion engine with expander deactivation

A piston compound internal combustion engine is disclosed with an expander piston deactivation feature. A piston internal combustion engine is compounded with a secondary expander piston, where the expander piston extracts energy from the exhaust gases being expelled from the primary power pistons. The secondary expander piston can be deactivated and immobilized, or its stroke can be reduced, under low load conditions in order to reduce parasitic losses and over-expansion. Two mechanizations are disclosed for the secondary expander piston's coupling with the power pistons and crankshaft. Control strategies for activation and deactivation of the secondary expander piston are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a compound internal combustion piston engine and, more particularly, to a compound internal combustion piston engine with a secondary expander piston for improved efficiency at medium and high loads, where the secondary expander piston can be deactivated and made stationary under low load conditions in order to reduce parasitic losses and over-expansion.

2. Discussion of the Related Art

Internal combustion engines are a proven, effective source of power for many applications, both stationary and mobile. Of the different types of internal combustion engines, the piston engine is by far the most common in automobiles and other land-based forms of transportation. While engine manufacturers have made great strides in improving the fuel efficiency of piston engines, further improvements must be made in order to conserve limited supplies of fossil fuels, reduce environmental pollution, and reduce operating costs for vehicle owners.

One technique for improving the efficiency of piston engines is to employ a secondary expander piston to extract additional energy from exhaust gases before the exhaust gases are expelled to the environment. Secondary expander pistons can be effective at improving efficiency under relatively high loads, where exhaust gases still have a considerable amount of energy. However, secondary expander pistons are not very effective, and in fact can be counter-productive, under low load conditions, where parasitic losses can outweigh the benefit of any additional extracted energy. Because automobile engines inherently operate under widely varying conditions, including a substantial amount of low-load operation, traditional secondary expander piston engine designs have not proven beneficial.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a piston compound internal combustion engine is disclosed with an expander piston deactivation feature. A piston internal combustion engine is compounded with a secondary expander piston, where the expander piston extracts energy from the exhaust gases being expelled from the primary power pistons. The secondary expander piston can be deactivated and immobilized, or its stroke can be reduced, under low load conditions in order to reduce parasitic losses and over-expansion. Two mechanizations are disclosed for the secondary expander piston's coupling with the power pistons and crankshaft. Control strategies for activation and deactivation of the secondary expander piston are also disclosed.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed to a piston compound internal combustion engine with expander deactivation is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.

Obtaining the maximum fuel efficiency from internal combustion engines has long been an objective of engine designers. One technique which has been employed in the past is to incorporate a secondary expander piston into an engine, where the expander piston extracts additional energy from the engine's exhaust gases.

FIG. 1is a top view illustration of a piston engine which is compounded with a secondary expander piston. The engine10includes two power pistons12, which are the pistons normally found in an internal combustion engine. The power pistons12, in their respective cylinders, receive a charge of fuel and air through an inlet port13, which is then compressed, ignited, and expanded. After the combustion gases are expanded on the power stroke, the gases are exhausted from the power pistons' cylinders. In the compound engine10, instead of exhausting the gases from the power pistons12through an exhaust system to the environment, the exhaust gases are routed through a transfer port15to a secondary expander piston14, which extracts additional energy from the exhaust gases on its power stroke, then exhausts the gases to the environment through an exhaust port17. Because the gases have already been expanded once by the power pistons12, gas pressures are lower on the expander piston14. Therefore, the expander piston14has a considerably larger bore than the power pistons12.

A ratio of two of the power pistons12to one of the expander pistons14is ideal in a 4-stroke-per-cycle engine. This is because the two power pistons12, which are mechanically in phase (both at Top Dead Center (TDC) at the same time, etc.), are 360 degrees out of phase relative to their combustion cycles (one of the power pistons12is beginning an intake stroke when the other is beginning a power stroke, etc.). Therefore, each time the expander piston14reaches TDC, one of the power pistons12has reached Bottom Dead Center (BDC) on its power stroke and is ready to discharge its gases to the expander piston14through its respective transfer port15. Thus, the expander piston14operates in a 2-stroke mode, with a power stroke and an exhaust stroke on each crankshaft revolution.

The engine10could operate on diesel fuel (compression ignition), or it could operate on gasoline or a variety of other fuels (spark ignition). The engine10could include only the two power pistons12and the one expander piston14, or the engine10could be scaled up to four or eight of the power pistons12, with one expander piston14for every two power pistons12. In automotive applications, the engine10could directly power the vehicle via a transmission and driveline, or the engine10could serve as an auxiliary power unit to provide electrical energy via a generator. The engine10could also be used in a wide variety of non-automotive applications, including primary or backup electrical generation, pumping, etc.

Although secondary expander piston engine designs have been known for some time, the concept has not proven viable for most engine applications, largely because the parasitic losses associated with the secondary expander piston14outweigh the additional energy extracted under low load conditions. Specifically, in situations where there is little energy remaining in the exhaust gases after the primary expansion by the power pistons12, the energy extracted from a secondary expansion of the exhaust gases is not enough to overcome the friction of the expander piston14in its cylinder. Because engines in automobiles—and most other applications—frequently operate at low load, little or no overall fuel efficiency improvement has been realized by secondary expander piston engines. However, if the expander piston14could be deactivated and made stationary at low loads, the parasitic losses associated with the expander piston14would be eliminated, and the engine's overall fuel efficiency would be significantly increased.

FIG. 2is a side view illustration of a first mechanization for coupling the secondary expander piston14to the engine's power pistons12and crankshaft, while allowing deactivation or reduced stroke of the expander piston14. The power pistons12(one shown) are coupled to a crankshaft16via a connecting rod18, in an arrangement typical of any piston engine. The crankshaft16is then coupled to a stroke adjustment link20via a connecting link22. The stroke adjustment link20includes a slot24which allows the position of the stroke adjustment link20to be adjusted relative to a pivot pin26. The pivot pin26is a “ground” point—that is, it is attached to the block of the engine10. A connecting rod28is connected at one end to the expander piston14, and at the other end to the stroke adjustment link20at a pivot point30.

By adjusting the position of the stroke adjustment link20relative to the pivot pin26, the stroke of the expander piston14can be increased or decreased. As shown inFIG. 2, with the pivot pin26approximately centered along the length of the stroke adjustment link20, the expander piston14will have approximately the same stroke as the power piston12. However, if the stroke adjustment link20is positioned such that the pivot pin26is at the far (right) end of the slot24, then the expander piston14will have a very short stroke. In practice, a design can be realized which allows the pivot point30to be positioned along the axis of the pivot pin26, thus resulting in no motion of the expander piston14. Under low load engine conditions, it may be desirable to completely deactivate and immobilize the expander piston14. However, as will be discussed below, under certain conditions it may be desirable to reduce the stroke of the expander piston14, but not completely immobilize it.

FIG. 3is a side view illustration of a second mechanization for coupling the secondary expander piston14to the engine's power pistons12and crankshaft16, while allowing deactivation of the expander piston14. In this embodiment, the secondary expander piston14is coupled to a secondary crankshaft32via a connecting rod34. The rotation of the secondary crankshaft32is coupled to the rotation of the crankshaft16via a clutch36. The clutch36must be a dog clutch or other such design that provides a positive mechanical engagement between the secondary crankshaft32and the crankshaft16—such that the rotational speeds of the two shafts are the same, and the required relative position is maintained. In this embodiment, the expander piston14can easily be deactivated and immobilized by disengaging the clutch36. A reduced stroke mode of operation is not inherently enabled in this embodiment, although a reduced stroke feature could be added to the secondary crankshaft32.

In both of the embodiments discussed above, which may collectively be referred to as de-stroking mechanisms, a controller38monitors engine conditions and establishes the desired stroke, or activation/deactivation, of the expander piston14. The controller38then actuates the link20or the clutch36to control the actual stroke of the expander piston14based on the desired stroke.

The controller38is a device typical of any electronic control unit (ECU) in an automobile, including at least a microprocessor and a memory module. The microprocessor is configured with a particularly programmed algorithm based on the logic described herein, using data from sensors—such as exhaust gas temperature sensors, an engine torque sensor, a throttle position sensor, etc.—as input.

In both design embodiments, the proper geometric relationship between the power pistons12and the expander piston14is maintained. That is, when the power piston12is at TDC, the expander piston14is at BDC, and vice versa. This relationship is inherently maintained by the linkage of the first embodiment (FIG. 2), and maintained by way of the design of the clutch36in the second embodiment (FIG. 3).

InFIG. 3, it is even conceivable to allow the expander piston14and the secondary crankshaft32to operate independent of any mechanical coupling to the crankshaft16. For example, in an electrical power generation application, the secondary crankshaft32could drive a small secondary generator. The valving of the exhaust gases from the power pistons12to the expander piston14would inherently tend to drive the secondary crankshaft32at the same speed as, and at the correct phase relationship to, the crankshaft16.

A variety of control strategies can be envisioned which take advantage of the piston compound internal combustion engine with expander deactivation or stroke adjustment. As discussed above, it is known that expander deactivation is desirable at low load conditions. Other factors also come into consideration. For example, exhaust gas after-treatment devices, such as catalytic converters, are only effective when they reach a certain minimum temperature. In a real world automotive application, it would not be desirable to extract so much energy from the exhaust gases that the exhaust after-treatment system drops below its minimum effective temperature. This criterion can be incorporated into a control strategy. Also, in practice, it may be desirable to add a hysteresis effect to the control of the expander piston14, such that it is not repeatedly activated and deactivated at high frequency.

FIG. 4is a flowchart diagram40of a method for activating and deactivating the secondary expander piston14in order to optimize engine performance and efficiency. The controller38would be configured to follow the method steps of the flowchart diagram40. At start box42, the engine10is started. When the engine10is started, the expander piston14is deactivated and immobilized. At box44, exhaust system temperature is measured. At decision diamond46, the exhaust system temperature is compared to a first threshold temperature. If the exhaust system temperature is below the first threshold, which is the minimum effective temperature of the exhaust after-treatment devices, then the expander piston remains deactivated and immobilized, and the process loops back to again measure the exhaust system temperature at the box44after some time delay.

If the exhaust system temperature is above the first threshold temperature at the decision diamond46, then engine output torque is measured at box48. Engine output torque is considered to be a good indicator of whether engine load is high enough to warrant the engagement of the secondary expander piston14. It is certainly conceivable to use other measurements, individually or in combination, as an indication of engine load level. Such other measurements could include fuel flow rate, cylinder head temperature (for the power piston12), cylinder pressure (for the power piston12), etc. In any case, some reliable indication of engine load is needed, and is obtained at the box48, for control of the expander piston14.

At box50, exhaust system temperature is again measured. At box52, a control algorithm is used to determine the desired stroke of the expander piston14, and the process loops back to again measure engine output torque. The control algorithm can be adapted to handle variable stroke engine designs, where the stroke of the expander piston14may be normalized to vary from zero (immobilized) to one (full or maximum stroke possible for the engine mechanization). The algorithm can also be adapted to allow only full activation and deactivation of the expander piston14, but not variable stroke.

The control algorithm may advantageously use a strategy which considers both engine load (torque) and exhaust system temperature, while including a hysteresis effect to avoid rapid repeated activation and deactivation of the expander piston14. For example, if engine torque is below a first torque threshold or exhaust system temperature is below the first temperature threshold, the expander piston14would be deactivated. If engine torque is above a second torque threshold and exhaust system temperature is above a second temperature threshold, the expander piston14would be activated at full stroke. If the engine10supports variable stroke of the expander piston14, then the stroke can be adjusted between the values of zero and one as a function of the engine torque and the exhaust system temperature relative to their respective thresholds. If the engine10supports only full activation and deactivation of the expander piston14, only one temperature threshold and one torque threshold may be used, where the expander piston14is activated when both thresholds are exceeded. Hysteresis can be added, for example by requiring several consecutive measurement cycles at a certain condition before changing the stroke of the expander piston14.

By adding a deactivation feature or a variable stroke feature to a piston compound internal combustion engine as described above, the fuel efficiency improvement of a secondary expander piston can be realized when an engine is operating at medium or high load, but the parasitic losses of the expander piston can be eliminated when the engine is operating at low load. This selective expander piston de-stroking offers another approach to increasing fuel efficiency, which is so important to both automakers and consumers.