Vacuum management system for engine with variable valve lift

A vacuum management system for an engine with variable valve lift includes a vacuum control valve at the entrance to the intake manifold to increase vacuum within the manifold as needed and preferably only when it can be done without impairing fuel economy or engine performance. Vacuum may then be used for any of various vacuum-assisted devices and functions, for example, boosting a vehicle braking system. The numerical relationships among important operating parameters are determined in a laboratory, and a programmable engine control module (ECM} is provided with algorithms and tables of such values by which the ECM is able to vary valve lift and vacuum control valve position to provide optimum flow across the intake valves and optimum manifold vacuum under all engine operating conditions.

TECHNICAL FIELD

The present invention relates to internal combustion engines; more particularly, to such engines wherein devices for variably controlling the lift of intake valves are the primary throttling means of the engine; and most particularly, to a system for providing and managing manifold vacuum in such an engine to optimize fuel economy and enable vacuum-assisted devices such as a brake booster.

BACKGROUND OF THE INVENTION

Fuel-injected internal combustion engines are well known, especially for automotive applications. Torque output of such an engine is typically controlled by moderating airflow into the engine via a throttle device. The throttle, usually a butterfly valve disposed at the entrance to the engine intake manifold, may be directly actuated by a driver's foot pedal or may be electronically governed through a digital or analog controller. Under typical driving conditions, the engine is substantially throttled. Because the engine is a positive displacement pump, a vacuum is created in the intake manifold downstream of the throttle valve.

Recently, some engines are known to be provided with means for varying the lift of one or more engine cylinder intake valves to improve fuel economy (also known as variable valve actuation, VVA, and referred to herein as variable valve lift, VVL). Typically, the lift of a plurality of valves in a multiple-cylinder engine is reduced or modulated during operating periods of low engine load to reduce fuel consumption, the amount of lift being directed by an engine control module (ECM) responsive to various performance inputs, operator pedal position, and programmed algorithms.

In some such engines, it is known to control engine torque by directly utilizing the variable valve lift means to controllably throttle the flow of air into each of the individual cylinders, thereby obviating the need for any conventional throttle valve at the inlet to the intake manifold.

A first unfavorable consequence of eliminating a manifold throttle valve is that the air pressure within the manifold is substantially the same as atmospheric pressure outside the engine; i.e., there is no useful level of manifold vacuum. However, a variety of standard engine and other automotive subsystems have evolved over many years which utilize vacuum as the source of actuation. The engine intake manifold has previously been a “free” source of vacuum for operating such devices and functions, which may include brake boosting, evaporative canister purging, exhaust gas recirculation, and HVAC systems among others. Providing an auxiliary vacuum pump for auxiliary automotive devices adds cost to a vehicle, consumes valuable onboard space, and parasitically decreases fuel economy. Engine functions, such as improving fuel preparation for cold starting, inducing exhaust gas recirculation into the intake manifold, and reducing cylinder-to-cylinder air volume differences at light engine loads, require manifold vacuum and cannot be accomplished by addition of an auxiliary vacuum pump.

A second unfavorable consequence of eliminating a manifold throttle valve is that fuel economy typically is sub-optimal when there is no manifold vacuum.

It is a principal object of the present invention to provide a substantially non-parasitic system for creating and managing vacuum for operating vacuum-assisted devices and functions in a vehicle powered by a VVL-equipped engine wherein primary throttling has heretofore been provided exclusively by variable valve lifting.

It is a further object of the invention to provide such a system whereby fuel economy is improved.

It is a still further object of the invention to provide a failsafe means for operating a VVL-equipped and throttled engine in the event that the VVL control fails and the valves assume a full-lift mode.

SUMMARY OF THE INVENTION

Briefly described, a vacuum creation and management system for an engine with variable valve lift includes a vacuum control valve at the entrance to the intake manifold connected to a programmable engine control module (ECM) to increase vacuum within the manifold as needed. Vacuum may then be used for any of various vacuum-assisted devices and functions, for example, boosting a vehicle braking system. Numerical values for important operating parameters are determined in a laboratory, and the ECM is provided with algorithms and tables of such values according to which the ECM varies valve lift and throttle valve position to provide optimum manifold vacuum under all engine operating conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, a fuel-injected engine12with variable valve lift (VVL) means14for actuation of intake valve15includes a programmable engine control module20(ECM). (It should be understood that engine12is a multiple-cylinder engine and that valve15is individually representative of a plurality of engine valves in a plurality of engine cylinders.) Intake manifold18is connected for air flow19to engine head22via runner24which supports a conventional fuel injector25. Head22supports intake valve15and exhaust valve23. The ECM is electrically connected to VVL lift means14via first lead30for varying the lift of intake valve15. Engine throttling and consequent torque control is provided by varying the lift of the intake valves via ECM20in response to engine load request from an electronic pedal module27connected via second lead28and responsive to positional input of accelerator pedal29from operator31. ECM20may be further connected to other engine and vehicle inputs and may be provided with algorithms for determining the instantaneous performance of engine12. Typically, during operation of engine12as shown inFIG. 1there is substantially no vacuum in manifold18.

Referring toFIG. 2, a vacuum system10in accordance with the invention includes an engine12and components substantially as shown in FIG.1. In addition, a controllable vacuum control valve16is disposed in the entrance17to intake manifold18and is connected to ECM20by third lead26for sensing the rotary position thereof and for actuating valve16to move to a different rotary position in response to an algorithm in the ECM. Manifold18is further provided with a pressure sensor33connected to ECM20via fourth lead35for sensing pressure (vacuum) therein.

Referring toFIG. 3, in Venn diagram36, regional boundary38encompasses automotive and engine functions benefiting from an engine manifold vacuum system; regional boundary40encompasses those automotive functions which may be performed by an auxiliary vehicle vacuum pump in the absence of a manifold vacuum control system such as a system in accordance with the invention. Power brake vacuum assist42and HVAC control44are readily though expensively accommodated by an auxiliary vacuum pump, and power brakes may also be accommodated electrically without vacuum assist. Vacuum purge46of a fuel tank emissions canister might also be accommodated at a cost of more vehicle expense and complexity.

The functions within region38but outside of region40are engine functions requiring manifold vacuum and cannot be accommodated by either engine12inFIG. 1or an auxiliary vacuum pump. Functions48,50, and52may each be optimized when the engine intake valves are operated at a slightly higher lift permitted by the presence of a slight vacuum in the manifold.

Function48refers to improving atomization of fuel when an engine is cold, which improves fuel efficiency and reduces tailpipe emissions.

Function50refers to improving the uniformity of air and fuel flow to the cylinders. With no manifold vacuum, under low load conditions the valves may be nearly closed; small absolute differences in manufacture or wear of valves can cause large percentage differences in fueling and even torque pulses in an engine. Providing a manifold vacuum requires a higher valve lift for the same flow, thereby increasing the open area of the valve throat and reducing the percentage flow differences between valves.

Function52refers to improving fuel economy by causing a slight amount of internal recycling of engine exhaust back through the opening intake valve at the end of the exhaust stroke. It is well known in the art that dilution of fresh fuel/air mix with exhaust gas can improve thermal efficiency and reduce NOX formation; indeed, such is the basis of external exhaust gas recirculation (EGR).

Function54is the well-known external recirculation of a portion of the engine exhaust (EGR) into the intake manifold, as just recited, and requires a positive pressure differential between the exhaust and intake manifolds.

Function46, noted above, is the stripping of collected adsorbed fuel from a charcoal-filled canister in communication with a vehicle fuel tank. Fuel vapors are collected by the canister during refueling and are stripped into the engine subsequently, most conveniently in response to intake manifold vacuum.

Function56refers to prevention of a full-torque condition in engine12ofFIG. 1in the event that the VVL system fails and the intake valves commence operation at maximum lift. The presence of vacuum control valve16in accordance with the invention permits the ECM to instantaneously convert engine control to conventional electronic throttle control of valve16by operator31, thereby avoiding a runaway vehicle.

Referring toFIG. 4, a vacuum management system58in accordance with the invention includes a brake booster vacuum assist60connected to manifold18via a vacuum tube62including a check valve64; a canister purge valve66; an EGR valve68; and a mass flow sensor70in entrance17to manifold18.

FT=throttle valve flow72, or flow past valve16

FP=purge valve flow74, or flow past valve66

FE=EGR valve flow76, or flow past valve68

FV=intake valve flow78, or flow past intake valves15

TA=temperature80of the atmosphere outside the engine

θ=position of vacuum control valve16

Thus:
FV=FT+FE+FP(Eq.1)
=f(PM, {acute over (ω)}, I, TM)  (Eq. 1a)
Flow across intake valve15is the sum of flows across vacuum control valve16, EGR valve68, and purge valve66, and is a function of manifold pressure, engine speed, intake valve lift, and manifold temperature.
FT=f((PM/PA), θ,TM)  (Eq. 2)
Flow across vacuum control valve16is a function of pressure drop across valve16, the position of valve16, and the manifold temperature.
FE=f((PM/PE),XE, TE)  (Eq. 3)
Flow across EGR valve68is a function of pressure drop across valve68, the position of valve68, and the temperature of the exhaust gas.
FP=f((PM/PA),XP, TM)  (Eq. 4)
Flow across purge valve66is a function of pressure drop across valve66, the position of valve66, and the manifold temperature.
PM—DESIRED=f(PB, FE—DESIRED, FP—DESIRED)  (Eq. 5)
The desired manifold pressure (vacuum) is a function of brake booster pressure88, the desired EGR flow76, and the desired purge flow74.
FT—DESIRED=FV—DESIRED−(FE+FP)  (Eq. 6)
The desired flow across valve16equals the desired flow across intake valve15minus the flows76,74across EGR valve68and purge valve66.
ΘDESIRED=f((PM—DESIRED/PA),FT—DESIRED)  (Eq. 7)
The desired angular position of valve16is a function of the ratio of the desired manifold pressure to atmospheric pressure and the desired flow across valve16.

For simplicity, ECM20is omitted fromFIG. 4; however, it should be understood that ECM20is in communication with manifold pressure sensor33and with similar appropriate means (not shown) for measuring and transmitting TA, TM, TE, PA, PB, PE, {acute over (ω)}, θ, XP, XE, and I to ECM20.

In a control method in accordance with the invention, all of the above relationships are measured on a test engine under simulated use conditions in an engine laboratory, and the relationships are numerically quantified and mapped, primarily for optimum fuel efficiency. From these data, algorithms are developed in known fashion and programmed into ECM20.

The primary objective of vacuum management system58is to provide optimum flow across intake valves15at an optimum manifold pressure, PM—DESIRED, at any given time, taking into account all of the above factors and relationships. The ECM algorithm considers all of the above parameters, decides on an engine condition based primarily on load (inputted by operator31), engine speed, and manifold temperature, and establishes a height of valve lift98and a desired flow across valves15, FV—DESIRED, for those conditions. The algorithm then establishes the desired air flow72across vacuum control valve16, FT—DESIRED, in accordance with Eqs. 6 and 1, at the desired manifold pressure, PM—DESIRED, and sets the position θ of valve16in accordance with the parametric maps provided from the laboratory determinations. As engine conditions change, for example, when purging of the fuel canister is complete and valve66is closed, ECM20automatically varies the lift of valves15and the position θ of vacuum control valve16to maintain the optimum flow and manifold pressure.

A vacuum management system in accordance with the invention, such as system58, provides insurance against an inadvertent full-torque event. Engines throttled solely by VVL means, like engine12inFIG. 1, are vulnerable to unexpected full-torque conditions if the VVL control system fails and the intake valves assume a full-lift mode. The presence of vacuum control valve16in accordance with the invention permits ECM20to be programmed to switch throttle control to valve16, responsive to pedal input from operator31in essentially a conventional engine operating mode, until the VVL control means can be repaired.