Patent Publication Number: US-2015066259-A1

Title: Engine Oil Maintenance Monitor For A Hybrid Electric Vehicle

Description:
TECHNICAL FIELD 
     One or more embodiments relate to an engine oil maintenance monitor for monitoring the quantity and quality of oil within an engine of a hybrid electric vehicle. 
     BACKGROUND 
     A hybrid electric vehicle (HEV) includes an internal combustion engine and one or more electric machines, wherein the energy source for the engine is fuel and the energy source for the electric machine may be electrical energy from a battery and/or electrical energy that is converted from the engine. In a HEV, the engine is the main source of energy for vehicle propulsion and the battery provides supplemental energy. A PHEV is like a HEV, but the PHEV has a larger capacity battery that is rechargeable from the external electric grid. In a PHEV, the battery is the main source of energy for vehicle propulsion during an electric vehicle (EV) mode, until the battery depletes to a low energy level, at which time the PHEV operates like a HEV for vehicle propulsion. The PHEV may operate for long periods of time using only battery power, for example, when the PHEV is used for shorter commutes, trips, and the like. The battery is recharged between these trips using a charging station and does not reach a state of charge where engine power is required to propel the vehicle. 
     The engine is started or stopped each time the powertrain transitions between a HEV mode and an EV mode. The engine, as in the case of conventional powertrain systems, requires a lubrication oil pump, which typically is driven by the engine as lubricating oil is circulated from an engine oil sump through moving components within the engine block. The oil is then drained back to the oil sump. In a HEV of the type described above, frequent engine stops and starts will reduce fuel consumption, but before each start there is a low oil pressure in the lubrication system. In a PHEV, infrequent restarts of the engine may result in much of the engine oil draining into the oil sump. Restarting the engine when there is insufficient engine oil can increase engine wear due to thin oil films on surfaces between relatively movable elements of the engine, which potentially affects engine life. Therefore, many HEVs and PHEVs include a secondary engine oil pump that is electrically powered (electric oil pump) to supplement the engine driven oil pump (mechanical oil pump) during the EV mode. However, such a dual oil pump system is redundant, and adds cost and weight to a HEV. 
     Additionally, for a PHEV, the engine oil quality may degrade or become stale during these periods of time when the vehicle is operating using primarily battery power. In some instances, this may lead to oil degradation such as water formation in the oil, and the like. 
     In HEVs and PHEVs the vehicle braking may include friction braking, regenerative braking and engine braking. The term engine braking usually refers to the braking effect caused by the closed-throttle partial-vacuum in petrol (gasoline) engines when the accelerator pedal is released. 
     Diesel engines do not have engine braking in the above sense. Unlike petrol engines, diesel engines vary fuel flow to control power rather than throttling air intake and maintaining a constant fuel ratio as petrol engines do. As they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects. However, some alternative mechanisms which diesel engines use that replace or simulate real engine braking include: a compression release brake, or jake brake. A jake brake is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, resulting in adiabatic expansion of the compressed air, so the large amount of energy stored in that compressed air is not returned to the crankshaft, but is released into the atmosphere. This type of brake is banned or restricted in many locations where people live because it creates loud objectionable sound. 
     SUMMARY 
     In one or more embodiments, a hybrid vehicle is provided with an engine having a crankshaft and an electric machine coupled to the crankshaft. The hybrid vehicle also includes a pump and a controller. The pump is driven by rotation of the crankshaft and coupled to the engine by a fluid circuit. The controller is configured to control the electric machine in response to a wheel torque request to drive the crankshaft with the engine off to provide lubricant to the engine. 
     In another embodiment, a method is provided for providing lubrication fluid to an engine in a hybrid vehicle. The fuel provided to an engine is disabled. An electric machine is controlled to drive a crankshaft of the engine and a pump coupled to the crankshaft in response to a wheel torque request exceeding an available regenerative braking torque, wherein the pump is coupled to the engine for providing lubrication fluid. 
     In yet another embodiment a vehicle system is provide with a pump and a controller. The pump is coupled to a crankshaft of an engine to provide lubricant thereto in response to rotation of the crankshaft. The controller is configured to control an electric machine to drive the crankshaft with the engine off after a predetermined time from a prior lubrication event. 
     As such the vehicle and vehicle system provide advantages over existing HEVs that include dual oil pumps, by eliminating the electric oil pump and thereby saving cost and weight. The vehicle system analyzes the quantity and quality of the oil within the engine block. Based on this analysis, the controller makes an oil maintenance mode request that includes either: restarting the engine if the quality of the oil is insufficient or impure, or backdriving the engine using the generator to drive the oil pump if the quantity of the oil within the engine is not sufficient. Such backdriving of the engine provides engine braking which brakes or decelerates the vehicle. The vehicle system selectively utilizes engine braking to minimize any impact on the regenerative braking capabilities of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating an overview of an oil maintenance monitor for a vehicle according to one or more embodiments; 
         FIG. 2  is a schematic of a hybrid electric vehicle capable of implementing various embodiments of the present disclosure; 
         FIG. 3  is a schematic illustrating power flow through the hybrid electric vehicle of  FIG. 2  according to various operating modes; 
         FIG. 4  is a schematic illustrating power flow through the hybrid electric vehicle of  FIG. 2  according to an oil maintenance mode, according to one embodiment; 
         FIG. 5  is a graph illustrating a relationship between a quantity of oil within the engine and the time elapsed since a lubrication event; 
         FIG. 6  is a flow chart illustrating an oil maintenance monitor for monitoring the quantity of oil within an engine according to one or more embodiments; and 
         FIG. 7  is a flow chart illustrating an oil maintenance monitor for monitoring the quality of oil within an engine according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     With reference to  FIG. 1 , a vehicle system for monitoring oil maintenance is illustrated in accordance with one or more embodiments and is generally referenced by numeral  10 . The vehicle system  10  includes a controller  12 , an oil pump  14  and an engine  16 . The oil pump  14  provides lubricating oil to the engine  16  and is driven by the rotation of a crankshaft. The crankshaft rotates during normal engine operation as the engine combusts fuel. The vehicle system  10  also includes a generator  18  that is included in a transmission of the HEV (shown in  FIG. 2 ). The generator  18  is also coupled to the crankshaft and is configured to spin or backdrive the engine  16  when the engine  16  is disabled, or not combusting fuel for driving the oil pump  14 . 
     The controller  12  receives a plurality of input signals that are indicative of a present quantity and quality of the oil within the engine  16 . For example, the controller  12  receives input signals: Lubrication Event, Wheel Torque Request, Engine Revs, and Fuel Consumed, that are indicative of: an elapsed time since the oil pump  14  circulated oil through the engine  16 ; a driver&#39;s request for vehicle deceleration that is interpreted as a torque about the wheels; the number of engine revolutions since the last oil change; and the quantity of fuel consumed since the last oil change, respectively. In one or more embodiments the controller  12  also receives input (Crankshaft Position) that is indicative of a current angular position of the crankshaft. 
     The controller  12  then analyzes the input signals using an oil maintenance algorithm, which is described in greater detail below. Based on this analysis, the controller  12  makes an oil maintenance mode request that includes either: restarting the engine  16 , or backdriving the engine  16  using the generator  18 . Generally, the controller  12  may maintain normal HEV operation if the quantity and quality of the engine oil is sufficient; the controller  12  may restart the engine  16  if the quality of the engine oil is not sufficient; and the controller  12  may backdrive the engine without fuel, if the quantity of the oil at a location within the engine is not sufficient. 
     Referring to  FIG. 2 , the vehicle system  10  is depicted within a plug-in hybrid electric vehicle (PHEV)  20 . The vehicle  20  is propelled by two electric machines with assistance from the internal combustion engine  16  and connectable to an external power grid (not shown). The first electric machine is an AC electric motor/generator according to one or more embodiments, and depicted as the “motor”  22  in  FIG. 2 . The second electric machine is also an AC motor/generator and is depicted as the “generator”  18  in  FIG. 2 . Both the motor  22  and the generator  18  are configured to function as motors and convert electrical power into mechanical power (drive torque) to drive a pair of wheels  24  for vehicle propulsion. Both the motor  22  and the generator  18  are also configured to function as generators for converting mechanical power from the driven wheels  24  and/or the engine  16  into electrical power through regenerative braking 
     The vehicle  20  includes a transmission  26  having a power-split configuration, according to one or more embodiments. The transmission  26  includes the motor  22  and the generator  18 . The transmission  26  also includes a planetary gear set  27  which includes a sun gear (“SUN”), a planet carrier (“PC”) and a ring gear (“RING”). The ring gear is an outer gear member that circumscribes the sun. A plurality of planet gears are rotatably mounted to the planet carrier such that each planet gear meshes or engages both the ring and the sun. In the illustrated embodiment, the generator  18  is connected to the sun by a generator output shaft  28  and the engine  16  is connected to the planet carrier by a crankshaft  30 . The planetary gear set  27  combines the generator power and the engine power and provides a combined output power at the ring gear. The generator  18  and the planetary gear set  27  collectively function as an electronic continuously variable transmission (e-CVT), without any fixed or “step” ratios. 
     The transmission  26  includes countershaft gears  32  for combining the output power of both the planetary gear set  27  and the motor  22 . The countershaft gears  32  include a first gear, a second gear and a third gear, that mesh with a planetary output gear that is connected to the ring gear, a motor output gear that is connected to an output shaft of the motor  22 , and a transmission output gear that is connected to a driveshaft  34 , respectively. The driveshaft  34  is an output shaft of the transmission  26  and is connected to the pair of driven wheels  24  through a differential. 
     The vehicle  20  includes an energy storage device, such as a battery  36  for storing electrical energy. The battery  36  is a high voltage battery that is capable of outputting electrical power to operate the motor  22  and the generator  18 . The battery  36  also receives electrical power from the motor  22  and the generator  18  when they are operating as generators. The battery  36  is a battery pack made up of several battery modules (not shown), where each battery module contains a plurality of battery cells (not shown). Other embodiments of the vehicle  20  contemplate different types of energy storage devices, such as capacitors and fuel cells (not shown) that may supplement or be used as alternatives to the battery  36 . A high voltage bus electrically connects the battery  36  to the motor  22  and the generator  18 . 
     The vehicle  20  also includes a battery energy control module (BECM)  38  for controlling the battery  36 . The BECM  38  receives input that is indicative of vehicle conditions and battery conditions, such as battery temperature, voltage and current. The BECM  38  calculates and estimates battery parameters, such as battery state of charge (SOC) and the battery power capability and provides output that is indicative of such parameters to other vehicle systems and controllers. 
     The vehicle  20  also includes a variable voltage converter (“VVC”)  40  and an inverter  42  according to one or more embodiments. The VVC  40  and the inverter  42  are electrically connected between the battery  36  and the motor  22 ; and between the battery  36  and the generator  18 . The VVC  40  “boosts” or increases the voltage potential of the electrical power provided by the battery  36 . The VVC  40  may also “buck” or decrease the voltage potential of the electrical power provided to the battery  36 , according to one or more embodiments. The inverter  42  inverts the DC power supplied by the battery  36  (through the VVC  40 ) to AC power for operating the motor  22  and generator  18 . The inverter  42  also rectifies AC power provided by the motor  22  and the generator  18  to DC for charging the battery  36 . Other embodiments of the vehicle  20  contemplate multiple inverters (not shown) and/or no VVC. 
     The vehicle  20  includes a transmission control module (TCM)  44  for controlling the motor  22 , the generator  18 , the VVC  40  and the inverter  42 . The TCM  44  is configured to monitor, among other things, the position, speed, and power consumption of the motor  22  and the generator  18 . The TCM  44  also monitors electrical parameters (e.g., voltage and current) at various locations within the VVC  40  and the inverter  42 . The TCM  44  provides output signals corresponding to this information to other vehicle systems. 
     The controller  12  is a vehicle system controller (VSC) that communicates with other vehicle systems and controllers for coordinating their function. Although it is shown as a single controller, the VSC  12  may include multiple controllers that may be used to control multiple vehicle systems according to an overall vehicle control logic, or software. 
     The vehicle controllers, including the VSC  12 , the BECM  38  and the TCM  44  generally include any number of microprocessors, ASICs, ICs, memory (e.g., FLASH, ROM, RAM, EPROM and/or EEPROM) and software code to co-act with one another to perform a series of operations. The controllers also include predetermined data, or “look up tables” that are based on calculations and test data and stored within their memory. The VSC  12  communicates with other vehicle systems and controllers over one or more wired or wireless vehicle connections using common bus protocols (e.g., CAN and LIN). The VSC  12  receives input (PRND) that represents a current position of the transmission  26  (e.g., park, reverse, neutral or drive). The VSC  12  also receives input (APP) that represents an accelerator pedal position. The VSC  12  provides output that represents engine, motor and generator controls based on the input. 
     The vehicle  20  includes a brake system  48 . The braking system  48  includes a brake pedal, a booster, a master cylinder, as well as fluid lines (all not shown) for coupling to the driven wheels  24  to effect friction braking. The braking system  48  also includes position sensors, pressure sensors, or some combination thereof for providing information such as brake pedal position (BPP) that corresponds to a driver request for braking torque. 
     The vehicle  20  uses engine braking to brake or decelerate the vehicle  20  under certain operating conditions. Engine braking generally refers to the braking effect caused by the closed-throttle partial-vacuum in petrol (gasoline) engines when the accelerator pedal is released. An available engine braking torque corresponds to the size of the engine (e.g., the inertia of any moving components) and whether the engine is currently enabled or disabled. An engine is enabled or running when it is combusting fuel to generate output power. Even if an operator is not depressing the accelerator pedal, the engine is still enabled because the fuel delivery system and the ignition system are still operating, and the engine is idling. An engine is disabled when it is not combusting fuel. The vehicle  20  may still utilize engine braking when the engine  16  is disabled. The generator  18  is coupled to the crankshaft  30  by the planetary gear set  27 . Thus, the generator  18  may be controlled to spin or backdrive the engine  16  to effect engine braking, even when the engine  16  is disabled. Such engine braking complements other braking (friction braking and regenerative braking) that is available to the vehicle  20 . 
     The vehicle  20  also includes a brake system control module (BSCM)  50  that communicates with the VSC  12  and the TCM  44  to coordinate regenerative braking, engine braking and friction braking The BSCM  50  provides a wheel torque request input signal to the VSC  12  that corresponds to the brake pedal position (BPP). The VSC  12  then compares the wheel torque request to other vehicle information (e.g., vehicle mass, speed, acceleration, road gradient and battery conditions) to determine a total braking torque value that includes an available regenerative braking torque value, an engine braking torque value and a friction braking torque value. The VSC  12  provides a desired motor torque value and a desired generator torque value to the TCM  44  that corresponds to the regenerative braking torque value and the engine braking torque value, and a desired friction braking torque value to the BSCM  50 . In other embodiments, the BSCM  50  determines one or more of the braking torque values. 
     Generally, the vehicle  20  utilizes regenerative braking as the primary braking source, and supplements with friction braking when there is insufficient available regenerative braking torque to satisfy the wheel torque request. Regenerative braking recharges the main battery  36  and recovers much of the energy that would otherwise be lost as heat during friction braking Therefore regenerative braking improves the overall efficiency or fuel economy of the vehicle as compared to vehicles that are only configured for friction braking Engine braking may be used to supplement friction braking and regenerative braking under limited conditions, e.g., when the wheel torque request is greater than the available regenerative braking toque. 
     The vehicle  20  is configured as a PHEV according to one or more embodiments. The battery  36  periodically receives AC energy from an external power supply or grid, via a charge port  52 . The vehicle  20  also includes an on-board charger  54 , which receives the AC energy from the charge port  52 . The charger  54  is an AC/DC converter which converts the received AC energy into DC energy suitable for charging the battery  36 . In turn, the charger  54  supplies the DC energy to the battery  36  during recharging. Although illustrated and described in the context of a PHEV  20  with a power-split transmission  26 , it is understood that embodiments of the vehicle system  10  may be implemented in other types of HEVs having other types of transmissions in which the vehicle may be operated in EV mode for prolonged periods of time. 
     The vehicle  20  includes an engine control module (ECM)  56  for controlling the engine  16 . The VSC  12  provides output (desired engine torque) to the ECM  56  that is based on a number of input signals including APP, and corresponds to a driver&#39;s request for vehicle propulsion. The desired engine torque may correspond to a request to start or stop the engine  16 . For example, the ECM  56  may stop the engine  16  in response to a desired engine torque of zero Nm. The engine  16  also includes a plurality of sensors for monitoring the present condition of the engine  16 , which are collectively represented by numeral  57  in  FIG. 2 . The sensors  57  monitor the temperature of the engine, the pressure of the engine oil, the speed or revolutions per minute (RPMs) of the engine, and the current angular position of the crankshaft  30 . The ECM  56  provides output that corresponds to the information monitored by the sensors to other vehicle controllers, such as the VSC  12 . 
     The oil pump  14  is driven by the crankshaft  30  and circulates oil from an oil sump or oil pan through the engine  16  for lubricating internal moving components (e.g., shafts, pistons, etc.). The oil pump  14  includes an input shaft  58  and an oil pump gear  60  that is fixed to the shaft  58 , according to the illustrated embodiment. The oil pump gear  60  engages an engine output gear  62  that is fixed to the crankshaft  30 . In other embodiments, the oil pump  14  includes an oil pump pulley that is coupled to a corresponding engine output pulley by a belt (not shown). The oil pump  14  is also coupled to the engine  16  by a fluid circuit (not shown) for providing the oil. Thus, as the crankshaft  30  rotates, it drives the oil pump  14 , which in turn circulates oil through the block of the engine  16  to lubricate internal moving components. The crankshaft  30  may be driven by engine power (e.g., internal combustion). 
     The oil pump  14  may also be driven by the generator  18  when the engine  16  is disabled (i.e. no fuel or spark is provided to the engine  16 ). The generator  18  is coupled to the crankshaft  30  by the planetary gear set  27 . The generator is configured to spin or backdrive the engine  16  when the engine is off, which in turn drives the oil pump  14 . 
     The engine  16  is started or stopped each time the transmission  26  transitions between a HEV mode and an EV mode. In a PHEV, such as the vehicle  20 , infrequent restarts of the engine  16  may result in much of the engine oil draining into the oil sump. Restarting the engine  16  when there is insufficient engine oil could increase engine wear due to thin oil films on surfaces between relatively movable elements of the engine, which could potentially affect engine life. 
     Many prior art HEVs and PHEVs include a secondary engine oil pump that is electrically powered (electric oil pump) to supplement the engine driven oil pump (mechanical oil pump) during the EV mode. However, such a dual oil pump system is redundant, and adds cost and weight to a HEV. 
     The vehicle system  10  is configured to provide oil to the engine  16  without a secondary electric oil pump. The vehicle system  10  monitors the quantity of oil within the engine  16 . If the vehicle system  10  determines that there is insufficient oil within the engine  16 , then the vehicle system  10  controls the generator  18  to drive the oil pump  14  and thereby provide the oil to the engine  16 . Thus the vehicle system  10  provides cost and weight savings over prior art HEVs having dual oil pump systems. 
       FIG. 3  illustrates the flow of power through the transmission  26  during various operating modes. The engine  16  receives fuel and provides engine power (τ e ,ω e ) to the planetary gear set  27 . The generator  18  provides power (τ g ,ω g ) to the planetary gear set  27  when acting as a motor, and receives power (τ g ,ω g ) when acting as a generator. The ring (r) of the planetary gear set  27  is connected to the countershaft gears  32  for providing power (τ r ,ω r ). The motor  22  provides power (τ m ,ω m ) to the countershaft gears  32  when acting as a motor, and receives power (τ m ,ω m ) when acting as a generator. The battery  36  provides electrical power to the generator  18  and the motor  22  when they are acting as motors, and receives electrical power from the generator  18  and the motor  22  when they are acting as generators. The countershaft gears  32  provide output power (Pout=τ s ,ω s ) to the driven wheels  24  which is based on the power provided by one or more of the engine  16 , the motor  22  and the generator  18 . 
       FIG. 4  illustrates the power flow through the transmission  26  when the generator  18  is driving the engine  16  and the oil pump  14  (shown in  FIG. 2 ). The engine  16  is off and not receiving fuel. The generator  18  acts as a motor and provides power (τ g ,ω g ) to the planetary gear set  27 . The planet carrier (pc) of the planetary gear set  27  is connected to the engine  16  for providing power (τ pc ,ω pc ). The motor  22  provides power (τ m ,ω m ) to the countershaft gears  32  when acting as a motor, and receives power (τ m ,ω m ) when acting as a generator. The battery  36  provides electrical power to the generator  18  and the motor  22  when they are acting as motors, and receives electrical power from the generator  18  and the motor  22  when they are acting as generators. The countershaft gears  32  provide output power (Pout=τ s ,ω s ) to the driven wheels  24  which is based solely on the power provided by the motor  22 . The ring (r) of the planetary gear set  27  is connected to the countershaft gears  32  for receiving power (τ r ,ω r ) to provide a reaction torque while the generator  18  is driving the engine  16 . 
       FIG. 5  is a graph illustrating a relationship between a quantity of oil within the engine block and an elapsed time since a lubrication event. The elapsed time includes time when the vehicle is not operating, (e.g., parked or off). This relationship is represented by line  510 . The engine oil travels through many small passages or channels that are formed in the engine block and therefore it is difficult to measure the quantity of oil within the engine. However the time since a lubrication event may be used to estimate the quantity of oil within the engine block. A lubrication event occurs when the engine has operated at a predetermined speed for longer than a predetermined period of time to drive the oil pump  14  to sufficiently lubricate the engine  16 . For example, in one embodiment, a lubrication event occurs once the engine  16  has operated at idle speed for a short period of time (e.g., five to ten seconds). In one embodiment, the ECM  56  monitors for the occurrence of a lubrication event and resets a timer after each occurrence. Such a reset is represented by point  511  on line  510 . In other embodiments, the engine  16  includes a sensor (not shown) for measuring the fluid level within the oil sump. The vehicle system  10  then determines a quantity of oil within the engine block based on both the elapsed time since a lubrication event and the quantity of oil within the oil sump. 
     To differentiate between engine restart sensitivity with respect to an estimated amount of oil within the engine block, the fluid level range is partitioned into several regions that are defined by predetermined boundaries that are based on the time elapsed since a lubrication event. These boundaries are each represented by horizontal dashed lines that separate the oil quantity operating range into High, Medium and Low oil level regions. 
     In one embodiment, the engine has an oil capacity of 5.0 L, of which the engine block has a capacity of 1.0 L, and requires a minimum quantity of 0.4 L of oil within the block during engine restart without damaging any internal components. This minimum value corresponds to the amount of oil left in the engine block eighty hours after a lubrication event, and is represented by a Low threshold value  512 . Additionally, the engine  16  may be restarted without any potential damage when there is at least 0.8 L of oil in the block. This value corresponds to the amount of oil remaining in the engine block forty hours after a lubrication event, and is represented by a High threshold value  514 . Further, the engine  16  may be damaged if restarted numerous times when there is less than 0.6 L of oil in the block. This value corresponds to the amount of oil remaining in the engine block sixty hours after a lubrication event, and is represented by an intermediate threshold value  516 . As illustrated in  FIG. 5 , a region above the high threshold value  514  is designated as a “High” fluid region; a region between the high threshold value  514  and the intermediate threshold value  516  is designated as a “Medium” fluid region; and a region between the medium threshold value  516  and the low threshold value  512  is designated as a “Low” fluid region. The fluid levels provided in this example are merely exemplary and non-limiting, and the threshold values associated with each engine and application will differ. 
     With reference to  FIG. 6 , an oil maintenance algorithm or method for monitoring a quantity of oil within an engine is illustrated in accordance with one or more embodiments and generally referenced by numeral  610 . The method  610  is implemented using software code contained within the VSC  12  according to one or more embodiments. In other embodiments the software code is shared between multiple controllers (e.g., the VSC  12 , the ECM  56  and the TCM  44 ). While the flowchart is illustrated with a number of sequential operations or steps, one or more operations may be omitted and/or executed in another manner without deviating from the scope and contemplation of the present disclosure. 
     At operation  612  the vehicle system  10  starts or initializes and receives input data and signals including: a current operating mode of the vehicle (Mode), an elapsed time since a lubrication event, a wheel torque request, available regenerative braking torque, and a current crankshaft position. 
     At operation  616  the vehicle system  10  evaluates the operating mode input to determine if the vehicle  20  is currently operating in an EV mode. If the vehicle  20  is operating in an EV mode, the vehicle system  10  proceeds to operation  618  to determine if the elapsed time since a lubrication event is greater than forty hours. In one embodiment, a lubrication event occurs once the engine has operated at idle speed for a short period of time (e.g., five to ten seconds). If the determination at operation  616  or  618  was negative, then this would indicate that there is a high level of oil within the engine block and the oil level is sufficient for restarting the engine, and therefore the vehicle system returns to operation  614 . If the elapsed time since the last lubrication event is greater than forty hours, then the vehicle system proceed to operation  620 . 
     At operation  620  the vehicle system  10  compares the wheel torque request to the available regenerative braking torque. As stated above, the available regenerative braking torque is based on vehicle speed and battery conditions, such as state of charge. If the wheel torque request is greater than the available regenerative torque, then the vehicle system  10  proceeds to operation  622 , and drives the crankshaft  30  using the generator  18  to effect engine braking and to drive the oil pump  14  for lubricating the engine  16 . As described above, regenerative braking is an important feature for conserving energy within the vehicle  20 . Therefore the vehicle system  10  limits any interruption of the regenerative braking. Here since the wheel torque request is greater than the available regenerative braking torque, the vehicle system  10  is not displacing any potentially conserved energy by engine braking. Rather, the vehicle system  10  is displacing friction braking, and thereby helps preserve the friction braking components. 
     During operation  622  the vehicle system  10  controls the generator  18  to backdrive the engine  16  at approximately idle speeds (e.g., between 500 and 1000 rpms) and limits the duration of such an operation. The vehicle system  10  opportunistically lubricates the engine  16  by driving the crankshaft  30  with the engine off  16 , when the high wheel torque request exceeds the available regenerative braking torque, in order to provide lubrication without affecting the regenerative braking efficiency of the vehicle. If the determination at operation  620  is negative, then this would indicate that the vehicle system  10  would limit regenerative braking if it were to use engine braking Therefore the vehicle system proceeds to operation  622  to evaluate additional conditions before performing such engine braking. 
     At operation  622 , the vehicle system  10  determines if the elapsed time since the last lubrication event is greater than sixty hours. If the determination at operation  622  is negative, then this would indicate that it has been somewhere between forty and sixty hours since the last lubrication event. The quantity of oil within the engine block is within the medium region and sufficient for a restart, and therefore the vehicle system  10  will delay engine braking The vehicle system  10  then returns to operation  614 . If the elapsed time since the last lubrication event is greater than sixty hours, then the vehicle system proceeds to operation  624 . 
     At operation  624  the vehicle system compares the wheel torque request to the available engine braking torque. If the wheel torque request is less than or equal to the available engine braking torque, then this would indicate that if engine braking were applied then the deceleration of the vehicle would be greater than that desired by the driver, and this would likely be perceptible to the driver. However, if the wheel torque request is greater than the available engine braking torque the vehicle system  10  proceeds to operation  622  and applies engine braking If the determination at operation  624  is negative, the vehicle system  10  proceeds to operation  626 . 
     At operation  626  the vehicle system  10  determines if the elapsed time since the last lubrication event is greater than eighty hours. After eighty hours, an engine restart may damage the engine. Therefore, if the elapsed time is greater than eighty hours the vehicle system  10  proceeds to operation  622  and applies engine braking Such engine braking will likely be perceptible to the driver because they have not requested such vehicle deceleration. Therefore, in one or more embodiments the vehicle system  10  communicates the low engine oil/engine braking status to the driver via a user interface such as a display or audio message (not shown). If the determination at operation  626  is negative, the vehicle system  10  returns to operation  614 . 
     In one or more embodiments, the method  610  also includes optional operations for monitoring the condition of engine components and for monitoring the quality of the oil within the engine  628 . The vehicle system  10  may proceed to optional operation  628  in response to a negative determination in operation  626 . 
     At operation  626  the vehicle system  10  compares the current angular position of the crankshaft  30  to historical information regarding prior resting angular positions of the crankshaft  30 . If the current angular position of the crankshaft is “uneven” or a position in which the crankshaft  30  has rested for an unproportional length of time as compared to other positions, then the vehicle system  10  will proceed to operation  622  and backdrive the engine  16 . A crankshaft  30  may wear unevenly, if the crankshaft  30  rests in the same general angular position for a prolonged period of time. This is due to the crankshaft  30  rocking slightly back and forth during EV mode. Although, it may be difficult for the vehicle system  10  to precisely stop the engine crankshaft at a specific angular position, such an operation may help to avoid uneven wear. 
     The vehicle system  10  may proceed to optional operation  630  in response to a negative determination at operations  622 ,  626  or  628 . Generally, at operation  628  the vehicle system evaluates input signals that correlate to the quality of the oil within the engine. If the quality of the oil is low and there is sufficient oil within the engine block, then the vehicle system  10  may restart the engine  16  to improve the quality. 
     With reference to  FIG. 7 , an oil maintenance algorithm or method for monitoring the quality of oil within an engine is illustrated in accordance with one or more embodiments and generally referenced by numeral  710 . The method  610  is implemented using software code contained within the VSC  12  according to one or more embodiments. In other embodiments the software code is shared between multiple controllers (e.g., the VSC  12 , the ECM  56  and the TCM  44 ). While the flowchart is illustrated with a number of sequential steps, one or more steps may be omitted and/or executed in another manner without deviating from the scope and contemplation of the present disclosure. In one or more embodiments the method  710  is included within operation  628  of method  610 . 
     At operation  712  the vehicle system  10  starts or initializes and receives input data and signals including: a current operating mode of the vehicle (Mode), an elapsed time since a lubrication event, the number of engine revolutions since the last oil change, and the amount of fuel consumed since the last oil change. 
     At operation  716  the vehicle system  10  evaluates the operating mode input to determine if the vehicle  20  is currently operating in an EV mode. If the vehicle  20  is operating in an EV mode, the vehicle system  10  proceeds to operation  718  to determine if the elapsed time since a lubrication event is greater than forty hours. If the determination at operation  718  is positive, the vehicle system  10  proceeds to operation  720  to monitor the quantity of oil within the engine block. In one or more embodiments, operation  720  corresponds with the method  610  of  FIG. 6 . If the elapsed time since a lubrication event is less than forty hours, the vehicle system  10  proceeds to operation  722 . 
     At operation  722  the vehicle system  10  compares the number of engine revolutions since the last oil change to a predetermined revolution value. If the number exceeds the predetermined revolution value this would indicate that the oil is stale and may contain impurities (e.g., water and fuel). To remove such impurities, the vehicle system  10  proceeds to operation  724  and restarts the engine  16 . If the determination at operation  722  is negative, then the vehicle system proceeds to operation  726  and compares the amount of fuel consumed since the last oil change to a predetermined fuel consumption value. If the number exceeds the predetermined fuel consumption value, then this would indicate that the oil is stale and may contain impurities (e.g., water and fuel). To remove such impurities, the vehicle system  10  proceeds to operation  724  and restarts the engine  16 . 
     As such the vehicle system  10  provides advantages over existing HEVs that include dual oil pumps, by eliminating the electric oil pump and thereby saving cost and weight. The vehicle system  10  also provides advantages over other systems that may include additional sensors for monitoring the engine  16 . The vehicle system  10  monitors the quantity and quality of the oil within the engine block using existing sensors, and analyzes this information using an oil maintenance algorithm. Based on this analysis, the controller  12  makes an oil maintenance mode request that includes either: restarting the engine  16  if the quality of the oil is insufficient or impure, or backdriving the engine  16  using the generator  18  to drive the oil pump  14  if the quantity of the oil within the engine is not sufficient. Such backdriving of the engine provides engine braking which brakes or decelerates the vehicle  20 . The vehicle system  10  selectively utilizes engine braking to minimize any impact on the regenerative braking capabilities of the vehicle  20 . 
     While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art will recognize that one or more features or characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes may include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described herein that are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.