Patent Application
20050252474
This invention relates to compression ignition engines. More particularly, the invention is concerned with cold starting of such engines.
Compression ignition engines are particularly susceptible to cold-start issues such as slow start times, excessive white smoke exhaust due to misfiring cycles, oil starvation, and poor idle stability. Cold starting means low temperature intake air that is coming inside the cylinder, low temperature walls, and low temperature piston heads. All of these make fuel evaporation difficult which in turn frustrates combustion. Cold starting also means compromised battery voltage which reduces its electrical current capability. The viscosity of oil increases dramatically with decreases in temperature, which results in increased frictional resistance during cold engine starts. The increased frictional drag is especially important when starting compression ignition engines because of the high minimum cranking speed required for starting. Cold temperatures therefore can result in undesirable engine emissions and wasted fuel, slow or no start conditions, battery depletion due to multiple start attempts and displeasing start idle feel. These issues are acute enough that a common practice is to continuously idle compression ignition engines in cold weather, resulting in wasted fuel, increased maintenance problem, and otherwise unnecessary emissions.
Many varied attempts at addressing the cold start issue have been proposed including: optimizing swirl patterns; optimizing fuel injection characteristics; optimizing valve timing events; varying cold start compression ratios; adding start-aid devices, including glow plugs, grid heaters, flame starters, and water heaters; adding passive thermal management to maintain engine/oil temperature above ambient; adding supplemental electrical storage devices such as supercapacitors which are substantially temperature independent; optimizing crankcase lubricants and lubrication systems; etc.
What is needed is a system and method for reliably starting a compression ignition engine during cold conditions which minimizes additional hardware including mechanical and electrical apparatus. Additionally, it is desirable to improve the idle start feel to the operator and a starting system meeting this objective is also needed.
The present invention provides a method for starting a compression ignition engine. The compression ignition engine is operatively coupled to an electric machine which is effective to spin up the engine during cranking. The starting sequence includes cranking the engine with the electric machine up to a first speed that is below the natural resonant speed of the coupled engine and electric machine combination. First speed cranking is maintained for a first duration and thereafter the engine is cranked up to a second speed that is above the natural resonant speed of the engine and motor combination. The first speed cranking terminates when the engine demonstrates relative stability at the first speed. Similarly, the second speed cranking terminates when the engine demonstrates relative stability at the second speed. Subsequent to the second speed cranking, the engine is cranked up to a third speed that is slightly below the engine idle speed. The third speed cranking terminates when the engine demonstrates relative stability at the third speed, whereafter engine cranking is terminated and normal engine control takes over. Relative stability at the various crank speeds may be determined for example by the engine speed being maintained by engine combustion torque above a predetermined offset from the crank speed for a predetermined time. The amount of the predetermined time may be substantially instantaneous with a high enough offset.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.
With reference first to
The exemplary powertrain of
The present invention requires that at least one electric motor be operatively coupled to the engine such that the engine can be spun up from a zero speed condition thereby. The motor may couple directly to the engine output shaft or may couple thereto via any variety of gearsets (including reduction gearing) or selectively engageable means such as a starting clutch, range clutch or ring and pinion gear arrangement such as a meshingly engaged starter pinion gear and engine flywheel.
With reference now to
A low transmission fluid temperature at step 101 results in execution of steps, beginning with step 103, uniquely executed during cold starts. At step 103, the engine cranking speed (CRANK SPEED) implemented by the motor control is set to a first reference speed Ref1 which is preferably substantially below any natural resonant frequency of the coupled engine and motor combination effective to avoid exciting undesirable resonant conditions. Additionally, this first reference speed is preferably higher than conventionally realized cold start cranking speeds of substantially 75 to 150 RPM. A cranking speed that is higher than about 150 RPM and preferably about 200 RPM will provide significantly more combustion favorable in cylinder temperatures conditions than conventionally realized cold start cranking speeds. Engine cranking at this controlled CRANK SPEED is a first stage of a stratified engine starting so labeled in
At step 105, engine speed, Ne, is compared to a first threshold comprising the first reference speed, Ref1, plus an additional offset, RPM1, e.g., 30 RPM. If the engine speed exceeds this first threshold for a predetermined time, T1, then it is determined that relative combustion stability at the first reference speed has been adequately demonstrated, for example to indicate some minimum degree of engine torque contribution to engine speed from successful in cylinder combustion events above the first reference speed. Relative combustion stability as used herein is relative to the particular engine speed reference to which it is compared. The engine speed control assists only to prop up the engine speed when it tends to sag below the reference speed, Ref1. It does not provide torque to the engine to establish speed above the reference speed. Any speed excursions above the reference speed, Ref1, is substantially due to combustion torque. An alternative condition which will indicate some minimum degree of engine torque contribution to engine speed from successful in cylinder combustion events above the first reference speed is also demonstrated by the engine speed, Ne, exceeding a second threshold. The second threshold comprises the first reference speed, Ref1, plus an additional offset, RPM2 which is larger than the first offset RPM1, e.g., 150 RPM. The time duration required for the second threshold to be exceeded is minimal and substantially instantaneous as provided by a single control loop.
Where relative combustion stability is not adequately demonstrated at the first reference speed, step 107 next determines whether the engine cranking at the first reference speed, Ref1, within this first stage of cranking, has exceeded a predetermined duration, T4. The time T4 is designed to prevent over draining of the battery system to allow for subsequent start attempts and prevent deeply discharging the battery system. If the cranking has been occurring in the present stage in excess of the acceptable time period, T4, then the current engine starting attempt is aborted at step 123. However, if the acceptable time period, T4, has not been exceeded, a voltage test is performed at step 109 on the battery to determined whether the battery voltage, V_batt is less than an acceptable minimum battery voltage, V_min. If the battery system is deeply discharged, then the current engine starting attempt is aborted at step 123. Where neither the time in the current cranking stage nor the battery voltage condition warrants aborting the cranking attempt, the routine returns to step 101 to continue with the current cranking stage.
Where the relative combustion stability is adequately demonstrated at the first reference speed, step 111 establishes CRANK SPEED implemented by the motor control to a second reference speed Ref2 which is preferably substantially above any natural resonant frequency of the coupled engine and motor combination. The second reference speed used to establish the CRANK SPEED is labeled Ref2 in
At step 113, engine speed, Ne, is compared to a third threshold comprising the second reference speed, Ref2, plus an additional offset, RPM3, e.g., 50 RPM. If the engine speed exceeds this third threshold for a predetermined time, T2, then it is determined that relative combustion stability at the second reference speed has been adequately demonstrated, for example to indicate some minimum degree of engine torque contribution to engine speed from successful in cylinder combustion events above the second reference speed. Once again, the engine speed control assists only to prop up the engine speed when it tends to sag below the reference speed, Ref2. It does not provide torque to the engine to establish speed above the reference speed. Any speed excursions above the reference speed, Ref2, is substantially due to combustion torque. An alternative condition which will indicate some minimum degree of engine torque contribution to engine speed from successful in cylinder combustion events above the second reference speed is also demonstrated by the engine speed, Ne, exceeding a fourth threshold. The fourth threshold comprises the second reference speed, Ref2; plus an additional offset, RPM4 which is larger than the third offset RPM3, e.g., 100 RPM. The time duration required for the fourth threshold to be exceeded is minimal and substantially instantaneous as provided by a single control loop.
Where relative combustion stability is not adequately demonstrated at the second reference speed, step 115 next determines whether the engine cranking at the second reference speed, Ref2, within this second stage of cranking, has exceeded a predetermined duration, T5. The time T5 is designed to prevent over draining of the battery system to allow for subsequent start attempts and prevent deeply discharging the battery system. If the cranking has been occurring in the present stage in excess of the acceptable time period, T5, then the current engine starting attempt is aborted at step 123. However, if the acceptable time period, T5, has not been exceeded, a voltage test is performed at step 117 on the battery to determined whether the battery voltage, V_batt is less than an acceptable minimum battery voltage, V_min. If the battery system is deeply discharged, then the current engine starting attempt is aborted at step 123. Where neither the time in the current cranking stage nor the battery voltage condition warrants aborting the cranking attempt, the routine returns to step 101 to continue with the current cranking stage.
Where the relative combustion stability is adequately demonstrated at the second reference speed, step 119 establishes CRANK SPEED implemented by the motor control to a third reference speed Ref3 which is preferably slightly below the engine idle speed, typically about 800 RPM. The third reference speed used to establish the CRANK SPEED is labeled Ref3 in
At step 121, engine speed, Ne, is compared to a third threshold comprising the third reference speed, Ref3, plus an additional offset, RPM3, e.g. 50 RPM. If the engine speed exceeds this third threshold for a predetermined time, T3, then it is determined that relative combustion stability at the third reference speed has been adequately demonstrated, for example to indicate some minimum degree of engine torque contribution to engine speed from successful in cylinder combustion events above the third reference speed. Once again, the engine speed control assists only to prop up the engine speed when it tends to sag below the reference speed, Ref3. It does not provide torque to the engine to establish speed above the reference speed. Any speed excursions above the reference speed, Ref3, is substantially due to combustion torque.
Where relative combustion stability is not adequately demonstrated at the third reference speed, the routine returns to step 101 to continue with the current cranking stage. This third stage cranking also serves as the normally invoked warm cranking mode. As previously described, where it is determined at step 101 that the cold cranking routine of the previously described steps are not required, as indicated for example by warm transmission fluid, this third stage routine is performed and the first two stages are bypassed as unnecessary for successful engine starting at present conditions.
Where the relative combustion stability is adequately demonstrated at the third reference speed, step 121 exits the start routine and engine control is turned over to normal engine control routines, including engine speed control routines to maintain idle speed and engine torque control routines responsive to operator torque demands.
While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.
