Abstract:
A hybrid electric powertrain includes a direct-injection, two-stroke, port scavenged engine in hybrid combination with an electric motor. The engine is configured to use a fuel with wide flammability limits, such as hydrogen, for extremely lean combustion to significantly reduce emissions. The two-stroke engine eliminates the engine displacement problems associated with extremely lean combustion, and the use of a hybrid powertrain allows the engine to be operated efficiently with reduced throttling compared to the prior art. A continuously variable transmission, or a transmission with closely-stepped ratios, is preferably employed.

Description:
TECHNICAL FIELD 
     This invention relates to vehicle powertrains having a two-stroke engine configured for combustion with at least 66% excess air and an electric motor in hybrid combination with the two-stroke engine. 
     BACKGROUND OF THE INVENTION 
     Hydrogen is a fuel that can burn reliably in a piston engine with a large amount of excess air. Burning hydrogen, or another fuel with wide flammability limits, with sufficient excess air produces only very small amounts of unburned fuel and very small amounts of oxides of nitrogen as emissions. Slightly lean combustion tends to be ideal for eliminating unburned fuel, and combustion in the presence of 66% or more excess air tends to eliminate oxides of nitrogen when hydrogen is the fuel. 
     However, prior art vehicle engines and powertrains cannot make very effective and efficient use of hydrogen fuel to take full advantage of extremely lean combustion. A hydrogen-fueled engine must process about twice as much air to effectively eliminate regulated emissions; excess air requires engine displacement be proportionately larger for the same amount of fuel burned and the power produced. Thus, for example, an engine operating with 100% excess air would require twice as much displacement as an engine operating with no excess air to achieve the same power output for the same amount of fuel. Increased size tends to make the engine less efficient, since its friction is relatively greater compared to the power produced. Since hydrogen is relatively expensive and difficult to store, it must be used efficiently. 
     Two-stroke engines with port scavenging are very simple. They lack separate intake and exhaust strokes and therefore do not keep intake and exhaust gases as well separated as four-stroke engines. The mixing of fresh and burned gases in the scavenging process normally makes the control and reduction treatment of emissions from the two-stroke engine relatively difficult. Without direct injection, escaping intake charge carries fuel to the exhaust, and the exhaust is always lean and cannot be cleaned by conventional catalytic converters. Burned gases remaining in the cylinder also reduce the maximum power that can be produced. As the engine is throttled to low torque and power, more burned gases remain, which can cause poor combustion and additional emissions. 
     SUMMARY OF THE INVENTION 
     A vehicle powertrain is provided that effectively and efficiently uses hydrogen to take advantage of lean combustion. The powertrain of the invention includes a torque-producing two-stroke engine in hybrid combination with an electric motor and a vehicle transmission. The two-stroke engine includes a cylinder, a piston in the cylinder reciprocally translatable between a top dead center position and a bottom dead center position for a compression stroke and a power stroke, an inlet port for admitting air into the cylinder, and a fuel injector configured to directly inject fuel into the cylinder for combustion. The engine is configured such that the fuel injector injects a quantity of fuel into the cylinder that results in at least 66% excess air in the cylinder during the combustion. 
     A direct injection port scavenged two-stroke engine configured for very lean combustion tends to eliminate the problems found in prior art lean-burn powertrains and prior art two-stroke engines. Residual gases that cannot be removed by the two-stroke engine are both a contributor to and substitute for excess air that a four-stroke engine must purposefully include to run very lean. A port scavenged two-stroke cylinder might typically contain one-third burned gases (“retained gases”) and two-thirds fresh gases in the midst of the compression stroke. In an engine operating with a large amount of excess air, almost half of the retained gases would actually be air, and all would be useful for limiting oxides of nitrogen. The two-stroke engine would then have an almost two-to-one advantage in power produced per unit of engine displacement over a four-stroke engine, and a port scavenged engine has no conventional valves, so its cost, size, and friction are much better. 
     The invention also improves upon the prior art by eliminating problems associated with throttling a two-stroke engine. The electric motor provides a substantial amount of peak power for vehicle acceleration, so the two-stroke engine can be configured to operate within a reduced range of power output. The transmission is preferably a continuously variable transmission (CVT) or a closely-stepped ratio transmission. The electric motor may provide some or most of the power for quick response to accelerator pedal “tip-in,” and the CVT or closely-stepped-ratio transmission can be overshifted so the engine can be run with little or no throttle margin for acceleration. 
     The hybrid powertrain also alleviates the effect of lean combustion and excess air on engine size; the motor contributes to power output and thus the engine size may be smaller in hybrid combination with the motor than without a motor. The low cost, mass, and size of the two-stroke engine tends to offset the cost, mass and size of hybrid and CVT or close ratio transmission components. The smoother operation of the two-stroke engine also helps to improve comfort in relatively high torque, low speed overshifted operation compared to the prior art. 
     The above features and advantages, and other features and advantages, of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic side view of a vehicle powertrain having a two-stroke engine configured for extremely lean combustion, an electric motor, and a transmission; 
     FIG. 2 is a schematic front view of the piston, connecting rod, and scotch yoke of the engine of FIG. 1; 
     FIG. 3 is a schematic side view of an alternative vehicle powertrain configuration; and 
     FIG. 4 is a truth table depicting step ratios for the transmission of the powertrain of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a hybrid vehicle powertrain  5  is schematically depicted. The powertrain  5  includes two-stroke engine  6 , an electric motor  7 , and a transmission  9 . The two-stroke engine  6  includes a crankshaft  10  operatively connected to the transmission  9  and the electric motor  7  so that the engine and the motor are in parallel hybrid combination. 
     The engine  6  includes a block  11  and a crankcase  12 . The block  11  defines a cylinder  14  in which a piston  16  resides. The piston  16  is mounted to connecting rod  18  and crankshaft  10  for reciprocating motion in the cylinder  14  between bottom dead center (shown in solid line at  16 ) and top dead center (shown in phantom line at  16 ′). The block  11  and the piston  16  cooperate to form a combustion chamber  20  on one side of the piston  16 , and an air intake pressure chamber  22  on the other side of the piston. The air intake pressure chamber  22  is separated from crankcase chamber  24  by wall  26 . The connecting rod  18  extends into the crankcase chamber  24  through a hole  28  in the wall  26 . The connecting rod  18  and hole  28  are sufficiently configured and shaped so that there is substantially no fluid communication between the crankcase chamber  24  and the air intake pressure chamber  22 . 
     The connecting rod  18  is rigidly connected to a scotch yoke  32 . Referring to FIG. 2, wherein like reference numbers refer to like components from FIG. 1, the scotch yoke  32  defines an elongated slot  34 . Member  36  is rotably connected to the crankshaft  10  and is slidingly contained within the slot  34  for translation therein. The scotch yoke  32  operatively interconnects the piston  16  and the crankshaft  10  in a manner that results in linear motion of the connecting rod  18  during reciprocation of the piston  16  in the cylinder  14 . 
     Referring again to FIG. 1, operably connected to the block  11  is intake manifold  38  and exhaust manifold  40 . The combustion chamber  20  communicates with the exhaust manifold  40  through exhaust port  42  in the block  11 . Intake manifold  38  communicates with the pressure chamber  22  through port  44 . An intake port  46 , also referred to as “inlet port, provides fluid communication between the combustion chamber  20  and the air intake pressure chamber  22 . Cylinder  14  is provided with a spark plug  48  and a fuel injector  50 . The fuel injector is configured and positioned to directly inject fuel  52  from fuel tank  54  into the combustion chamber  20 . The fuel  52  is preferably hydrogen or another fuel with wide flammability limits, such as dimethyl ether. 
     During an upstroke, piston  16  moves from bottom dead center in cylinder  14  toward top dead center. During the upward movement of the piston  16 , air intake port  46  and exhaust port  42  are closed off from the combustion chamber  20 , with air being inducted into pressure chamber  22  by the partial vacuum created by the piston  16 . Air  56  in combustion chamber  20  is mixed with fuel  52  from injector  50  and compressed until the spark plug  48  ignites the compressed mixture near the top of the stroke. As combustion  58  is initiated, the piston  16  begins its downstroke, decreasing the volume of pressure chamber  22  and the inducted air within. The air within the pressure chamber  22  is prevented from escaping through the intake manifold  38  by closure of a reed valve mechanism (not shown). Toward the end of the downstroke, piston  16  uncovers exhaust port  42  to release the combusted fuel and air (exhaust gas  60 ), followed by an uncovering of the intake port  46 , enabling the air  56  compressed within the pressure chamber  22  to flow through the intake port  46  into the combustion chamber  20 . The cycle begins anew when piston  16  reaches the bottom of its travel in cylinder  14 . 
     Electronic control module (ECM), or controller,  62  is typically a conventional digital computer used by those skilled in the art of engine control, and includes the standard elements of a central processing unit, random access memory, read only memory, analog-to-digital converter, input/output circuitry, and clock circuitry. The controller  62  is suited to receive information on various engine parameters from sensors connected to the engine. Upon receipt of such information, the controller  62  performs required computations and provides output signals which are transmitted to various operating systems which affect the operation of the engine  6 . 
     More specifically, the sensors include a mass air flow meter  66  connected to the intake manifold  38 , and a proportional oxygen sensor  70  connected to the exhaust manifold  40 . The mass air flow meter  66  is configured to measure, and transmit a signal  74  indicative of, the air flow rate through the intake manifold  38 . The oxygen sensor  70  is configured to measure, and transmit a signal  78  indicative of, the amount of oxygen in the exhaust manifold  40 . 
     The controller  62  is configured receive and process signals  74 ,  78 , and transmit control signals  82  to which the fuel injector  50  is responsive thereby to control the amount of fuel  52  injected by the fuel injector  50  and maintain a predetermined air/fuel ratio. The controller  62  is configured to operate the engine  6  such that at least 66% excess air is present in the combustion chamber  20  for combustion with the hydrogen fuel  52 , i.e., Lambda equals 1.66. 
     In the context of the present invention, the percentage of excess air is the percentage of air that is in excess of the amount required for stoichiometric combustion of fuel. Thus, if the cylinder contains no more air than the amount necessary for stoichiometric combustion of the fuel in the cylinder, then the cylinder contains zero percent excess air. If the cylinder contains twice the amount of air necessary for stoichiometric combustion of the fuel in the cylinder, then the cylinder contains 100% excess air. Preferably, the controller causes the engine to run with at least 100% excess air, i.e., a Lambda value of 2 or greater. Lambda is equal to the air/fuel ratio divided by the stoichiometric air/fuel ratio. In the context of the present invention, air” in the cylinder during combustion includes fresh air admitted by an inlet port, as well as exhaust gases that were not exhausted through the exhaust port after a preceding combustion event (residual gases). 
     Electric motor  7  is operably connected to an energy storage device such as battery  86  that selectively transmits energy  90  to the motor  7  so that the motor contributes to power output of the transmission  9 . The transmission  9  is a continuously variable transmission. More specifically, the transmission  9  depicted in FIG. 1 is an electronically variable transmission (EVT). Accordingly, transmission  9  includes a second electric motor  94 , an input shaft  102  that is connected to the crankshaft  10 , an output shaft  104 , and differential gearing  98  operatively connected to the motors  7 ,  94 , the input shaft  102  and the output shaft  104 . The controller  62  is operatively connected to the battery  86  and motors  7 ,  94  to control the speed of the motors and thereby vary the speed ratio between the input shaft  102  and the output shaft  104 . An exemplary EVT is described in U.S. Pat. No. 6,527,658, issued Mar. 4, 2003 to Holmes et al, which is hereby incorporated by reference in its entirety. 
     Referring to FIG. 3, wherein like reference numbers refer to like components from FIGS. 1 and 2, an alternative embodiment is schematically depicted. Engine  6 ′ does not include a pressure chamber on one side of piston  16 . Rather, an air compressor  108  in intake manifold  38 ′ provides sufficient pressure to air entering cylinder  14  for scavenging. 
     Transmission  9 ′ is configured to provide a plurality of discrete, successive speed ratios between input shaft  102 ′ and output shaft  104 ′. Those skilled in the art will recognize and understand various transmission configurations that result in a plurality of discrete, successive speed ratios. Referring to FIG. 4, a table depicts the ratio steps of the transmission  9 ′. The transmission has seven forward speed ratios; the average of all forward ratio steps is equal to or less than 1.34:1. In the embodiment depicted, the average of all the ratio steps is 1.33:1. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.