Abstract:
A method of boot strap starting a diesel having a plurality of cylinders comprises the steps of initiating cranking the diesel and responsive to cranking forcing exhaust valves for all cylinders open during compression strokes. Thereafter, responsive to the engine rotational speed signal exceeding a first threshold, an exhaust valve for one cylinder is allowed to open and close in synchronous with movement of a piston in the cylinder. Further responsive to engine rotational speed exceeding a second threshold higher than the first threshold, cranking is discontinued and the remaining exhaust valves are allowed to open and close in synchronous with movements of pistons in their respective cylinders.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates vehicle engine starting systems. 
     2. Description of the Problem 
     Designers of internal combustion piston engines have appreciated since their inception that such engines can function as pumps. Engine compression brakes exploit this aspect of engines to advantage. A compression brake operates by opening the cylinders&#39; exhaust valves at or just before top dead center (TDC) of the pistons&#39; cycles. Thus the energy used to compress the air in the cylinders is lost through the exhaust valves and no energy is returned to the crankshaft through the pistons during the expansion portions of the piston strokes. On motor vehicles this allows a substantial portion of an engine&#39;s rated power to be applied to braking. 
     Compression brake systems are implemented by installing controls for exhaust valves on many diesel engines. In one type of engine compression brake, the Jacobs engine brake, the exhaust valves are normally actuated with a standard camshaft. Normal actuation can be interrupted during braking by using energy from the injector push rod to open the exhaust valve for a cylinder at TDC of the piston. A detailed explanation of the principal of engine compression brakes, an in particular Jacobs engine brakes, may be found at pages 736-744 of Electric and Electronic Systems for Automobiles and Trucks by Robert N. Brady, Reston Publishing Company, Inc., Reston, Va. (1983). 
     Exhaust valve control has also been used on kick start motorcycles. Compression rebound occurring during attempts to start motorcycle engines could be dangerous to the riders. Compression rebound was caused when energy stored compressing an air mass in a cylinder was returned to the engine crankshaft during a down stroke of the piston. While less energy is returned than was put into the system compressing the air, the force can still be substantial. For that reason, prior to the widespread use automatic starters on motorcycles, some motorcycles came equipped with a manually activated valve switch, which allowed operators to roll the engine to top dead center (TDC) before attempts were made to kick start the vehicles&#39; engines and thereby avoid compression rebound. 
     Another area where the pump aspects of engines has undesirable results is in starting diesel engines. Diesels, which typically do not have ignition sources inside the cylinders, rely on compression heating to bring the fuel air mixture to its flash point. During cold weather, the engine block can serve as a substantial heat sink, meaning that compression must be reasonably fast to assure that the mixture reaches the ignition temperature before the temperature drops due to heat loss to the engine and to the environment. Typically, diesels must be cranked to 100 rpm before combustion can occur. Diesels have substantially higher compression ratios than do gasoline engines and require more energy input to compress the air in the cylinders than in gasoline engines. All of these aspects of diesels make engine cranking high on transient energy consumption. Starter motor power consumption can reach 20,000 watts at −20 degrees Fahrenheit with engine manufacturers&#39; recommended oil weights. Such power demands impose high loads on starter systems, starter motors and batteries, necessitating the use of large battery plants and large, heavy duty starter motors. Incomplete and failed combustion during cranking contributes to high levels of emissions releases during start up, particularly when the engine is cold. 
     The assignee of the present invention has implemented compression brakes on its diesels in a different manner than in the classic Jacobs engine brake. In the assignee&#39;s diesels, a hydraulic pump supplies engine oil at high pressure to the injectors and to exhaust valve override actuators as soon as the engine begins turning over. The exhaust valve override actuators are electronically controllable, allowing the exhaust valves to be opened at any point in the piston stroke for four stroke diesel engines. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to reduce power consumption during cranking and starting of internal combustion engines. 
     Another object of the invention is to reduce emissions during engine starts. 
     Still another object of the invention is to reduce power demands on starter motors and starter motor circuitry, allowing commensurate reductions in motor size and output. 
     Yet another object of the invention is to reduce the size of the battery plant. 
     One aspect of the invention is to provide a method of boot strap starting an internal combustion engine having a plurality of cylinders. Upon initial cranking of the engine the exhaust valves for each cylinder are opened during piston compression strokes. Upon engine rotational speed reaching a first minimum threshold, the exhaust valve for one cylinder is allowed to open and close normally in synchronous with movement of a piston in the cylinder. As combustion commences in the one cylinder operating normally, the remaining exhaust valves are allowed to open and close in synchronous with movements of pistons in their respective cylinders. Also responsive to combustion in the first cylinder, cranking of the engine by the starter motor is discontinued. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective partial cutaway view of a diesel engine equipped truck tractor with which the present is advantageously employed. 
     FIG. 2 is a schematic view of a vehicle electrical control system. 
     FIG. 3 is a flow chart illustrating the method of the invention. 
     FIG. 4 is a schematic illustration of a full set of stroke cycles in a cylinder of an engine to which a preferred embodiment of the invention as been applied. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a perspective view of a vehicle  11  and of an electrical control system  10  installed on the vehicle. Vehicle electrical system  10  comprises a network which may, in one embodiment, comprise a twisted pair (either shielded or unshielded) cable operating as a serial data bus  18 . One node of bus  18  is an electrical system controller (ESC)  30 , which is a major component of a vehicle electronic control system. ESC  30  manages a number of vocational controllers. Collectively, bus  18 , ESC  30  and the vocational controllers attached thereto, form a controller area network (CAN). 
     Active vehicle components, such as engine  24 , are typically controlled by one of a group of autonomous, programmable, vocational controllers, which include an instrument and switch bank  12 , a gauge cluster  14 , an engine controller  20 , a transmission controller  16 , and an antilock brake system (ABS) controller  22 . The autonomous controllers are all connected for data communication to ESC  30  and to one another over a serial data bus  18 . The autonomous controllers include local data processing and programming and are typically supplied by the manufacturer of the controlled component. For each autonomous controller there is a defined set of variables used for communications between the autonomous controller and other data processing components on the network or attached to the network. Although the autonomous controllers handle many functions locally and are functionally defined without reference to ESC  30 , they report data to ESC  30  and can receive operational requests from ESC  30 . Bus  18  is preferably a twisted pair cable constructed in accordance with SAE standard J1939. 
     FIG. 2 is a schematic illustration of an electrical control system  13  for a vehicle that may be used to implement control over individual engine cylinders  32  for easing starting of engine  24  in the preferred embodiment of the invention. Engine  24  is a multiple cylinder diesel engine comprising a plurality of cylinders  32 . Cylinders  32  exhaust gas containing byproducts of the combustion process through exhaust valves  34 . For the sake of simplicity only one exhaust valve is shown but it will be understood that each of the cylinders  32  has its own exhaust valve  34 . It will also be realized by those skilled in the art that cylinders may have more than one exhaust valve and reference to a valve for a cylinder in the singular is not intended to exclude multiple vaive per cylinder arrangements. 
     In normal operation exhaust valve  34  position is controlled by a camshaft  36 . Camshaft  36  rotates in synchronous with crank shaft  52 , which in turn is coupled to pistons (shown in FIG.  4 ). Camshaft  36  coordinates intake and exhaust valve positions for each cylinder  32  with piston movement in the cylinder and the stage of the intake, compression, combustion and exhaust that the cylinder is in for a four stroke engine through an hydraulic actuation system. The engine controller  20  can assume control over the exhaust valves  34  through exhaust valve override actuators  38  to open the valves at any point in the pistons&#39; strokes. Control over the exhaust valve override actuators is provided by using high pressure engine oil from an hydraulic oil pump  39 . High pressure engine oil becomes available as soon as engine  24  begins turning. During braking, fuel flow to the cylinders is cut off and exhaust valves are opened just before piston TDC in the compression strokes, in effect converting the engine into a compression pump. The result is that vehicle forward momentum, coupled through the vehicle transmission to the engine  24  crankshaft  52 , is used to compress air. 
     The mechanism for exhaust valve control found with an engine compression brake system on a vehicle provides a convenient tool for implementing the boot strap engine starting method of the present invention. In the preferred embodiment, the invention operates by utilizing the valve control features of the assignee&#39;s engine compression brake to override valve position control by the camshaft. In engines equipped either with the assignee&#39;s engine compression brake, or in proposed engines where no mechanical camshaft is present and valve control is electronic, the invention may be implemented as software routines for exhaust valve position control. Such engines must provide an alternative indicator of piston position for the cam shaft, such as the engine crankshaft. 
     Boot strap starting of engine  24  begins when the engine controller  20  receives indication over the system bus  18  from electrical system controller (ESC)  30  that a start button  56  has been depressed and when gauge controller  14  indicates that the ignition position  58  is at ON. At this point, crank shaft  52  and camshaft  36  should be motionless. Cam angle position sensor  42 , which provides a cam angle position signal to engine controller  20 , will indicate no changes in position of the cam. A tachometer routine  46  derives an engine speed signal in rpm from the cam position signal. The cam angle position sensor is also used by a piston position determination routine  44  to determine the positions for all pistons. Throttle input  54  to the engine controller  20  is disabled. 
     Engine controller  20  through exhaust valve override actuator controller  40  commands the opening of all exhaust valves  34  by directing the opening of exhaust valve override actuators  38  for compression strokes of pistons. Engine controller  20  further causes starter motor  50  to begin turning crank shaft  52 , which makes pressurized engine oil available. As engine oil under pressure becomes available, the exhaust valve override actuators  38  disable the hydraulic valve actuation by camshaft  36  for the compression stroke for each cylinder in turn. As engine speed reaches the minimum speed for compression combustion to begin, engine controller  20  causes exhaust valve override actuator  38  for one cylinder  32  to cease operation, allowing the exhaust valve  34  for the cylinder to operate normally. Fuel flow to the normally operating cylinder  32  is initiated through injector control  48  as timed by piston position determination  44 . Typically the first minimum or threshold engine speed is about 100 rpm. This figure is a good cold start figure for ambient temperatures in the range of −20 degrees F. for an engine having the manufacturer&#39;s recommended oil for operation at such temperatures. It is possible to make the first threshold figure a function of engine oil weight, ambient temperature and engine temperature to optimize operation of the invention. 
     As soon as combustion begins to occur in the first cylinder  32 , the exhaust valves for the remaining cylinders  32  are allowed to operate normally. Combustion is determined by engine speed reaching a second minimum threshold, for example about 375 rpm. Combustion may also be indicated by a sudden increase in engine rpm. At this point the starter motor  50  ceases cranking and cranking is taken over by the first cylinder  32 . 
     The method of the invention is represented by a flow chart in FIG.  3 . Upon start all exhaust valves are commanded to open at step  60  for compression strokes. Next, at step  62  the starter motor is engaged. Once engine speed exceeds 100 rpm, the process advances beyond decision block  64  to start normal operation of one cylinder at step  66 . When combustion begins in one cylinder, engine speed will increase which is detected at step  68 . With one cylinder firing the starter motor is dropped at step  70  and all cylinders are returned to normal operation at step  72 . If more sophisticated electronics are available it is possible that cylinders can be brought into operation in stages. 
     FIG. 4 illustrates movement of piston  74 , intake valve  78  and exhaust valve  34  in implementing operation of the invention for a four stroke diesel engine  24 . The strokes are labeled in sequence by the letters, A, B, C, D, E and F. Letter A is associated with a forced exhaust stroke during boot strap starting. Piston  74  moves upwardly in cylinder  32  in response to a crank shaft  52  turning under the influence of starter motor  50 . Intake valve  78  is closed and exhaust valve  34  is open. The contents of cylinder  32  are ambient air and are exhausted. At letter B piston  74  has passed TDC and moves downwardly. Exhaust valve  34  has closed while intake valve  78  has opened. Ambient air with no fuel is drawn through intake valve  78 . At letter C piston  74  has passed bottom dead center and the exhaust cycle of letter A is repeated. Letter D repeats letter B, and these cycles continue until engine rpm exceeds the first minimum threshold where upon a conventional compression stroke E and combustion stroke F occur. 
     The invention reduces power consumption during cranking and starting of a diesel by allowing the engine to crank up to the minimum speed required for starting without imposing the load of compressing air in the cylinders on the starter motor. Once a minimum speed is achieved a start is attempted on only one cylinder, keeping the load imposed on the starter motor to a minimum. Once combustion is achieved in that cylinder, the firing cylinder carries the load of cranking the motor until the remaining cylinders are brought into ignition. Diesels are prone to exhausting unburned and partially burned hydrocarbons during start up, which are seen as black smoke and particulate emission. The invention reduces these start up emissions by reducing the number of cylinders in which ignition is being attempted at low rpms. Once engine speeds exceed about 350 rpms, particulate emission is substantially reduced. Diesels can obtain an engine speed of 350 rpms on one cylinder operation, but providing an electrical starter motor capable of such speeds would add substantial weight and expense to vehicles. The invention avoids any need to provide an oversize starter motor and in fact, allows a smaller motor to used than is the current practice. Because less power is required for starting, the number or size of batteries used for starting may also be reduced. 
     While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.