Patent Publication Number: US-2015068490-A1

Title: Multi-fuel engine with variable valve timing

Description:
TECHNICAL FIELD 
     The disclosure relates to reciprocating internal combustion engines, and in particular relates to compression ignition engines providing concurrent combustion of fuels having differing reactivity indices. 
     BACKGROUND 
     Compression ignition engines, particularly diesel cycle engines, provide power for many trucks and increasing numbers of automobiles throughout the world. The efficiency of diesel engines compares favorably with widely used spark ignition engines, however diesel engines are subject to legal restrictions relating to emissions of nitrous oxide compounds and emissions. The restrictions effect efficiency of operation of the engines. 
     The four stroke compression ignition engine which has been widely used in motor vehicles works by drawing air into an engine cylinder during a down stroke of the cylinder&#39;s piston, compressing the air on a subsequent up/compression stroke of the piston and injecting fuel into the cylinder as or just after the piston reaches the top of its compression stroke. The compression of the air results in increasing air temperature which in turn leads to ignition and combustion of the fuel in the cylinder as the fuel is injected at or near peak cylinder pressure. The combustion of the fuel with oxygen from the air increases pressure in the cylinder and powers the ensuing down or power stroke of the piston. The combustion by-product or exhaust is purged/scavenged from the cylinder during the following up stroke and the cycle repeats. The cylinder is provided with at least one intake valve which opens for the draw stroke and has otherwise been closed and at least one exhaust valve which opens for the exhaust stroke and has otherwise been closed. The coordination of intake and exhaust valve opening have been controlled by a cam shaft and have generally been fixed relative to the stroke position of the piston. 
     Basic diesel engine operation has been the subject of the application of electronic controls and through modifications of engines. Engine modifications have included: exhaust gas recirculation (EGR); a cooler in the EGR line; hydraulically controlled valve lifters, particularly intake valve lifters, which has allowed varying the timing of valve opening and closing (called variable valve actuation (VVA)) relative to piston position; solenoid control over hydraulic fuel injectors; and turbocharging. Electronic control over these elements in turn permits: selection of the time duration and pressure of fuel injection; varying the number of pulses of fuel injection which occur; varying the timing of injection relative to piston position; varying of the pressure boost to intake air; varying of the engine compression ratios by varying intake valve operation; and varying the temperature of the intake air. The ability to partially control these operational variables in turn increases control over the timing, progression and temperature at which combustion occurs. As a result engine operation can be varied dynamically in response to immediate vehicle conditions. 
     Concurrent combustion of dual fuels in a compression ignition engine has involved intake port injection of the lower reactivity fuel (e.g. gasoline) and direct in-cylinder injection of high-reactivity fuel (e.g. diesel). The more highly reactive fuel is injected near the top dead center (TDC) position of a piston in its compression stroke resulting in ignition of the more highly reactive fuel followed by combustion of the lower reactivity fuel. In effect the more highly reactive fuel replaces the spark source to ignite the charge, with the benefit that an injected 1 mg quantity of diesel can provide about 40× the energy of a spark promoting faster initiation of combustion. The near TDC piston position for injection of the higher reactivity fuel provides combustion stability yet reduces the effects of the dual fuels on the emission output and the efficiency of the combustion process. Pressure rise rates have been limited by de-rating the engine (lowering the power output) or reducing the compression ratio (depressing efficiency). The flexibility afforded in variable valve actuation and in operation of the fuel injection system allow an integrated control strategy to extend the benefits of fuel reactivity as obtained by high and low reactivity index fuels across an engine load range. 
     SUMMARY 
     An engine system comprises a cylinder having an intake port, an intake valve and an exhaust port, with the cylinder providing for compression of a charge received through the intake valve for combustion in the cylinder. An air induction sub-system coupled to the intake valve supplies air to the engine. A first fuel injector is connected to inject a fuel into the air induction sub-system or the intake port for each charge. An exhaust gas recirculation line connects exhaust gas produced by combustion of a charge and purged through the exhaust port to the air induction sub-system. A recirculated exhaust gas cooler and a valve for controlling recirculation of exhaust gas through the exhaust gas recirculation line and recirculated exhaust gas cooler allows control over dilution and temperature of a charge introduced to a cylinder to suppress auto-ignition of the charge in the cylinder. A second fuel injector injects fuel directly into the cylinder during the compression stroke of the piston for auto ignition. A variable valve actuator for opening and closing the intake valve extends control over charge pressure and temperature to extend suppression of auto-ignition of the charge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general schematic diagram of a multi-fuel motor vehicle engine system. 
         FIG. 2  is a schematic diagram of a cylinder in the engine system of  FIG. 1 . 
         FIG. 3  is a data flow diagram for control over the engine system of  FIG. 1 . 
         FIG. 4  is a timing diagram for one cycle of a four stroke reciprocating compression ignition engine with fixed valve timing. 
         FIG. 5  is a timing diagram for one cycle of a four stroke reciprocating compression ignition engine with variable valve timing. 
         FIG. 6  illustrates engine torque over engine rpm. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. Furthermore, example sizes/models/values/ranges may be given with respect to specific embodiments but are not to be considered generally limiting. 
     Referring to  FIG. 1 , a reciprocating engine system  10  supports compression ignition while concurrently burning blends of different fuels each potentially having a different reactivity. Blending is dynamic, and is based on both exogenous inputs and current values for engine operating variables. In a configuration two fuels are burned and may be characterized as “low reactivity” and “high reactivity” fuels. A low pressure fuel injection system  15  draws a low reactivity fuel such as gasoline or natural gas from source  22  and delivers the fuel to a manifold injector  26   a  or to a port injector  26   b  for each cylinder. Natural gas is an example of a fuel for which a manifold injector  26   a  is used while gasoline is an example of a fuel for which a port injector  26   b  is used. In most applications only one of type of injector location, intake manifold  76  or port  16 , is used. The high pressure fuel injection system  17  draws high reactivity fuel from fuel source  36  to supply in-cylinder injectors  46  for each cylinder of engine  14 . Diesel grade fuels would be a high reactivity fuel in comparison to natural gas or gasoline. 
     Air management for engine system  10  is provided through an air induction sub-system  52 , an exhaust sub-system  66  and an exhaust gas recirculation (EGR) line  50 . The air induction sub-system draws air from the environment and boosts its pressure to support combustion in engine  14 . The exhaust sub-system  66  receives combustion by-products from engine  14  and may provide turbines  68 ,  70  to extract mechanical energy from the combustion by-products before releasing exhaust gas back to the environment after treatment (not shown). The EGR line  50  recirculates a controlled quantity of exhaust gas from the exhaust sub-system  66  back to the air induction sub-system  52  and more particularly to an intake manifold  76 . 
     A turbocharger sub-system  80  provides for extraction of energy from the exhaust sub-system  66  to boost air pressure in the induction sub-system  52  and to retain sufficient backpressure in the exhaust manifold  40  to force recirculated exhaust gas into an intake manifold  76 . The turbocharger sub-system  80  comprises a high pressure turbine  68  and a low pressure turbine  70  positioned in series downstream from the exhaust manifold  40  in the exhaust sub-system  66 . High pressure turbine  68  is mechanically coupled to drive a high pressure compressor/supercharger  56  and low pressure turbine  70  is mechanically coupled to drive a low pressure compressor/supercharger  54 . 
     The low and high pressure compressors  54 ,  56  are located connected in series with an intercooler  58  in the air induction sub-system  52 . Compressed air from low pressure compressor  54  is directed to high pressure compressor  56  through an intercooler  58  which extracts heat from the intake air and to reduce air temperature. Compressed air from high pressure compressor  56  flows through a high pressure stage cooler  60  into the intake manifold  76  where it is mixed with recirculated exhaust gas. Mechanically or electrically powered superchargers may be substituted for one or both compressors  54 ,  56  with a loss of energy recapture from the exhaust. Substitution of superchargers allows for simplification of the exhaust sub-system  66  and allows direct control over pressure in the exhaust manifold  40 . 
     Exhaust sub-system  66  carries exhaust from an exhaust port  18  (which includes an exhaust valve) through the high pressure turbine  68  and the low pressure turbine  70 . A control valve  72  connects exhaust from the low pressure turbine to the environment. Control valve  72  is used during engine  14  start to force maximum exhaust gas recirculation. The EGR line  50  is connected to the exhaust sub-system  66  upstream from the high pressure turbine  68  and discharges into the intake manifold  76  upstream from the intake port  16  and downstream from the high pressure stage cooler  60 . EGR line  50  includes an EGR control valve  62  which controls the portion of exhaust gas from exhaust manifold  40  which is recirculated after engine  14  has warmed to normal operating levels and an EGR cooler  64  for reducing the temperature of the recirculated exhaust gas. 
     A four cycle combustion process of drawing a charge of air, recirculated exhaust gas and low reactivity fuel, compressing the charge, combustion and purging of exhaust gas is implemented in engine  14  using low reactivity fuel and air introduced through intake port  16  during the intake cycle. High reactivity fuel may be introduced by in-cylinder injector  46  during the piston compression stroke. High reactivity fuel is introduced one or more times near completion of the compression stroke (top dead center or “TDC”) for auto-ignition. The energy released from auto-ignition ignites the low reactivity fuel and remaining high reactivity fuel. 
     An engine control module (ECM)  12  manages the combustion process in response to engine  14  operating conditions and at least one exogenous variable. Inputs to ECM  12  can come from several sources. An intake manifold  76  pressure sensor  28  reports manifold air pressure (MAP) to ECM  12 . An intake manifold  76  air temperature sensor  30  reports manifold air temperature (MAT). An oxygen sensor  24  in the exhaust manifold  40  reports exhaust oxygen levels (O2). Combustion feedback control via in-cylinder combustion phase sensor  48  or physical modeling of the combustion event is used. A combustion phase sensor  48 , if present, communicates with the interior of cylinder  14  and provides for detection of ignition and combustion in the cylinder. Presently combustion phase sensor  48  can be a pressure sensor, a knock sensor, or an ion sensor. Data generated by combustion phase sensor  48  is transmitted to ECM  12  as a combustion feedback signal (CBFK). A tachometer (not shown) generates an engine speed signal (N). A torque demand (TQ) signal may be considered as being generated externally. The engine speed signal (N) may and the torque demand signal (TQ) will usually be received by ECM  12  over a network from relatively remote sources. TQ is usually related to an accelerator position signal mediated by a vehicle body computer (not shown). If available an induction air mass sensor may provide a signal as well. 
     A number of elements of engine system  10  are controlled by ECM  12  to manage engine  14  operation. A variable valve actuator (VVA)  20  controls opening and closing of an intake valve in the intake port  16 . VVA  20  allows for varying the timing/phase of opening of the intake valve of intake port  16  relative to piston position. For example, the intake valve may be kept closed for part of the intake stroke which in effect reduces the intake displacement of a cylinder for a given cycle. Variable valve actuation may be extended to the exhaust valve (not shown). Control over either valve can be used to temporarily reduce engine  14  compression. 
     ECM  12  applies control signals to the low and high pressure fuel injection systems allowing it to set operating pressures. ECM  12  controls the timing, number and duration of injection pulses of fuel by injectors  26   a ,  26   b  and, most importantly, injector  46  for each engine  14  cylinder and for each combustion stroke in a cylinder. 
     Fuel injection is provided both in the induction sub-system  52 , usually the intake manifold  76  or intake port  16  using injectors  26   a    26   b , and directly into the cylinder  14  using injector  46 . Metering is provided of both fuel types through port and in-cylinder injection by ECM  12  control signals applied to the low and high pressure fuel injection systems  15 ,  17  and injectors  26   a ,  26   b  and  46 . In-cylinder injection pressure levels greater than 300 bar up to systems capable of 3000 bar are provided. 
     ECM  12  also controls the position of EGR control valve  62  in order to control the proportion of exhaust gas recirculated to the intake manifold  76 . This changes the boost provided intake air by compressors  54  and  56 . EGR line  50  is capable of recirculating 30% to 60% of exhaust gas to the intake manifold with the percentage being set by control signals applied to control valve  62  by ECM  12 . EGR cooler  64  cools the recirculated exhaust gas to a temperature near, but above, the condensation temperature of water. 
     The multi-turbocharger configuration of turbocharger sub-system  80  (low pressure and high pressure compressors  54 ,  56 ) provides delivery of enough oxygen to maintain combustion at lean to stoichiometic levels. 
     Referring to  FIG. 2 , a representative cylinder  21  from engine  14  is shown. Cylinder  21  encloses a reciprocating piston  23  which is used to vary the volume of a combustion chamber  25 . An intake valve  27  and an exhaust valve  29  may be used to connect the combustion chamber  25  to the intake manifold  76  and to the exhaust manifold  40 . Valve  72  in the exhaust manifold  40  represents restrictive features of the exhaust manifold used to generate backpressure which urges some portion of exhaust gas to recirculate to the intake manifold  76  through EGR line  50 . A manifold fuel injector  26   a  provides for injection of low reactivity fuel into the intake manifold  76  and an in-cylinder injector  46  provides for injection of high reactivity fuel directly into combustion chamber  25 . 
     Concurrent combustion of multiple fuels in reciprocating engine  14  is done using a premixed charge air and recirculated exhaust gas with a low reactivity fuel (typically inserted into the port  16  for gasoline like fuels or into the intake manifold  76  for natural gas) and direct injection high-reactivity fuel (typically Diesel) into the combustion cylinder  14 . One or more high-reactivity injection events (multiple shots) may be used. The timing of the injection of high-reactivity fuel will range from early in the compression stroke (yielding nearly premixed conditions) to closer to TDC. The combination of dual fuel injection, in-cylinder combustion charge mixture and compression ratio control through variable valve actuation and exhaust gas level, and combustion sensing feedback for combustion adjustments in a cycle-to-cycle basis allows flexibility in adjusting boundary conditions so that the mixture has a reactivity which improves phasing for improved fuel efficiency. 
     Multiple combustion modes are achieved through specific fuel injection strategies and variable valve actuation control coupled with combustion feedback to expand the robust operating range of premixed combustion. Improved efficiency (approximately 5-10% over current  2010  benchmarks) and the possible elimination NOx after-treatment due to lower operating temperature may be achieved by use of these combustion modes. 
     In general the injection of a low reactivity fuel and a high-reactivity fuel is distributed between a lower pressure system for port or manifold injection of the low reactivity fuel and high pressure in-cylinder injection for the high reactivity fuel. Recirculated exhaust gas is mixed with induction air and low reactivity fuel charge and the mixture is delivered by port injector  26  to an engine cylinder. Recirculated exhaust gas suppresses auto-ignition of the fuel/gas/air mixture drawn into cylinder  14  before injection of fuel by the high pressure in-cylinder injector  46  at or near TDC of piston  23 . The recirculated exhaust gas is cooled and mixed with the air from the induction sub-system  52 . The induction air is cooled by high pressure stage cooler  60  and intercooler  58 . Manifold temperature of the charge and the degree of dilution of the charge with exhaust gas are targeted to control auto-ignition. The turbo-charger system  80  is a high boost system encompassing multi-stage compressors  54 ,  56  to provide the sufficient air to run the system lean and above stoichiometric (for efficiency) and provide sufficient pressure to enhance the reactivity of the mixture in the presence of high exhaust gas recirculation rates. The VVA system  20  coupled to the intake port  16  provides further in-cylinder cooling by controlling the compression ratio and for control over charge air-to-fuel ratio and oxygen concentration. One or more high-reactivity injection events (multiple shots) may be used. The timing of the high-reactivity fuel will range from early in the compression stroke (yielding nearly premixed conditions) to closer to TDC. Combustion feedback data is provided either through combustion phase sensing or trough modeling of the combustion phenomenon. 
     There are several issues to be addressed when mixing two dissimilar fuels with different reactivity indices (such as gasoline&#39;s less reactive and Diesel&#39;s high reactive quality). The issues are: (1) How to meter and schedule the introduction of fuels in a full engine operating map; (2) Accurately establishing combustion phasing; and (3) Limiting pressure rise rates or knocking. Referring to  FIG. 3  a data flow diagram  31  for setting one of two combustion modes in engine system  14 . Combustion Mode 1 (CM1) uses the injection timing of the high reactivity fuel to control the combustion phasing. In the event of multiple injections with the injections closer to (or the injection closest to) TDC control(s) the phasing. Combustion Mode 2 (CM2) uses early injection timings (one or multiple shots), and the combustion phasing is controlled by the mass ratio between the low and high reactivity fuels. The more amount of high-reactivity fuel use will move the combustion phasing earlier. CM 2 is highly premixed. 
     The issue of metering and scheduling the introduction of fuels is addressed by using CM 1 at low loads, where the use of the high-reactivity is necessary to ignite the mixture, switching to CM2 at mid-loads, where the fully premixed characteristics of the charge yields efficiency and emissions and combustion phasing controlled with the reactivity ratio, and reverting to CM1 at high load, where excessive premixed fuel can lead to too high combustion pressure rise rates. 
     Providing accurate combustion phasing is controlled via injection timing of the high-reactivity fuel (CM1) or by the setting reactivity of the mixture through control of the relative ratio of low to high reactivity fuel (CM2). The ignition delay is modeled according to the reactivity index, temperature and pressure in the manifold (MAT and MAP) and in the cylinder at the time to intake valve closing, the dilution ratio is established by the rates of EGR applied and boost, applying combustion feedback by means of detecting the start of combustion (CBFK). Limiting the high pressure rise rates can be attained by reducing the premix amount of low reactivity fuel, however doing so results in a lowering of engine efficiency and compromises engine  14  emissions. To counteract this effect, or to maintain the high level of premix fuel at high loads, variable valve actuation timing (VVA) in the intake valve  16  (early or late valve closing timing of the intake valve  27  relative to the intake stroke of the piston  23 ) is used to reduce the effective compression ratio. This application of VVA preserves a cylinder&#39;s expansion ratio. 
       FIG. 3  illustrates the relation of signal inputs to control outputs. The inputs are engine speed (N)  39 , torque demand (TQ)  41 , (intake) manifold air pressure (MAP)  43 , (intake) manifold air pressure (MAP)  45 , exhaust oxygen level (O2)  47  and, optionally, a combustion phase feedback value (CBFK)  49 . A possible additional input  51 , such as mass flow, is reserved. If combustion phasing is determined through a model  35  it is estimated from the values for MAP, MAT and O2. Alternatively combustion phasing may be known directly from CBFK in which case “combustion estimation” is passed through block  37 . The combustion estimation value from either block  35  or  37  is passed to a fuel charge system control routine  33 . In addition fuel charge system control receives engine speed N and torque demand  41  which indicate the loads imposed on engine  14 . The loads are broadly characterized here as “light”, “medium” or “heavy.” 
     The fuel charge system control block  33  determines the energy output target to be met by the fuel delivered and the oxygen level needed to burn the fuel. These values are passed to a reactivity target block  61  and to the charge air system block  53 , respectively. The fuel charge system  33  also selects one of the two combustion modes. Depending upon the combustion mode selected and the load level the charge air system  53  and the reactivity target block  61  establish mixes of recirculated exhaust gas with fresh air to supply and relative quantities of low reactivity and high reactivity fuel to supply. Reactivity of the fuel mix is also dependent upon timing of in-cylinder injection of high reactivity fuel, which is provided for by a fuel module  63 . The charge air system  53  applies appropriate control signals relating to turbo boost  55 , recirculation of exhaust gas  57  and variable valve actuation timing and duration  59 . The fuel module can apply the appropriate fuel injectors for injection low reactivity fuel  65  and high reactivity fuel  67 . 
       FIGS. 4 and 5  provide a comparison through four cycles of an engine cylinder between operation of a cylinder having fixed valve timing ( FIG. 4 ) and operation of a cylinder with variable valve actuation on the intake valve ( FIG. 5 ). In the example operation is identical for light and medium loads, but for heavy loads the cylinder having VVA for the intake valve closes the intake valve early and uses a higher ratio of low reactivity fuel (gasoline). The combustion modes provide a functional approach to implement dual fuels in a traditional engine with fixed valve timing. The constraints of auto-ignition and excess pressure rise rates limits the application of the low reactivity fuel and directs the switch of combustion modes. This is illustrated in  FIG. 4 . The addition of VVA allows the extended use of the low-reactivity fuel and effectively extends the use of CM2 to higher loads, retaining the engine efficiency. 
     The introduction of two combustion modes to exploit the fuel reactivity properties provided by multi-fuels leads to very low engine our emissions (NOx and PM below the 2010 US regulations) while improving the engine efficiency. The effect of the application of the combustion modes can be extended by use of VVA, which effectively extends the operation of the premix characteristics of what is termed combustion mode 2 allowing retaining high ratios of the low-reactivity fuel as shown in  FIG. 6  graph (B) as compared to graph (A).