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.
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.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
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.
A ratio of two of the power pistons 12 to one of the expander pistons 14 is ideal in a 4-stroke-per-cycle engine. This is because the two power pistons 12, 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 pistons 12 is beginning an intake stroke when the other is beginning a power stroke, etc.). Therefore, each time the expander piston 14 reaches TDC, one of the power pistons 12 has reached Bottom Dead Center (BDC) on its power stroke and is ready to discharge its gases to the expander piston 14 through its respective transfer port 15. Thus, the expander piston 14 operates in a 2-stroke mode, with a power stroke and an exhaust stroke on each crankshaft revolution.
The engine 10 could operate on diesel fuel (compression ignition), or it could operate on gasoline or a variety of other fuels (spark ignition). The engine 10 could include only the two power pistons 12 and the one expander piston 14, or the engine 10 could be scaled up to four or eight of the power pistons 12, with one expander piston 14 for every two power pistons 12. In automotive applications, the engine 10 could directly power the vehicle via a transmission and driveline, or the engine 10 could serve as an auxiliary power unit to provide electrical energy via a generator. The engine 10 could 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 piston 14 outweigh 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 pistons 12, the energy extracted from a secondary expansion of the exhaust gases is not enough to overcome the friction of the expander piston 14 in 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 piston 14 could be deactivated and made stationary at low loads, the parasitic losses associated with the expander piston 14 would be eliminated, and the engine's overall fuel efficiency would be significantly increased.
By adjusting the position of the stroke adjustment link 20 relative to the pivot pin 26, the stroke of the expander piston 14 can be increased or decreased. As shown in
In both of the embodiments discussed above, which may collectively be referred to as de-stroking mechanisms, a controller 38 monitors engine conditions and establishes the desired stroke, or activation/deactivation, of the expander piston 14. The controller 38 then actuates the link 20 or the clutch 36 to control the actual stroke of the expander piston 14 based on the desired stroke.
The controller 38 is 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 pistons 12 and the expander piston 14 is maintained. That is, when the power piston 12 is at TDC, the expander piston 14 is at BDC, and vice versa. This relationship is inherently maintained by the linkage of the first embodiment (
In
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 piston 14, such that it is not repeatedly activated and deactivated at high frequency.
If the exhaust system temperature is above the first threshold temperature at the decision diamond 46, then engine output torque is measured at box 48. 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 piston 14. 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 piston 12), cylinder pressure (for the power piston 12), etc. In any case, some reliable indication of engine load is needed, and is obtained at the box 48, for control of the expander piston 14.
At box 50, exhaust system temperature is again measured. At box 52, a control algorithm is used to determine the desired stroke of the expander piston 14, 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 piston 14 may 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 piston 14, 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 piston 14. For example, if engine torque is below a first torque threshold or exhaust system temperature is below the first temperature threshold, the expander piston 14 would be deactivated. If engine torque is above a second torque threshold and exhaust system temperature is above a second temperature threshold, the expander piston 14 would be activated at full stroke. If the engine 10 supports variable stroke of the expander piston 14, 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 engine 10 supports only full activation and deactivation of the expander piston 14, only one temperature threshold and one torque threshold may be used, where the expander piston 14 is 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 piston 14.
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.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
This application claims the benefit of the priority date of U.S. Provisional Patent Application Ser. No. 61/721,958, titled PISTON COMPOUND INTERNAL COMBUSTION ENGINE WITH EXPANDER DEACTIVATION, filed Nov. 2, 2012.
Number | Date | Country | |
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61721958 | Nov 2012 | US |