Search Results

The NOx emissions during the crank-start and cold fast-idle phases of a GDI engine are analyzed in detail. The NOx emissions of the first 3 firing cycles are studied under a wide set of parameters including the mass of fuel injected, start of injection, and ignition timing. The results show a strong dependence of the NOx emissions with injection timing; they are significantly reduced as the mixture is stratified. The impact of different valve timings on crank-start NOx emissions was analyzed. Late intake and early exhaust timings show similar potential for NOx reduction; 26-30% lower than the baseline. The combined strategy, resulting in a large symmetric negative valve overlap, shows the greatest reduction; 59% lower than the baseline. The cold fast-idle NOx emissions were studied under different equivalence ratios, injection strategies, combustion phasing, and valve timings. Slightly lean air-fuel mixtures result in a significant reduction of NOx.

The first 3 cycles in the cold crank-start process at 20°C are studied in a GDI engine. The focus is on the dependence of the HC and PM/PN emissions of each cycle on the injection strategy and combustion phasing of the current and previous cycles. The PM/PN emissions per cycle decrease by more than an order of magnitude as the crank-start progresses from the 1st to the 3rd cycle, while the HC emissions stay relatively constant. The wall heat transfer, as controlled by the combustion phasing, during the previous cycles has a more significant influence on the mixture formation process for the current cycle than the amount of residual fuel. The results show that the rise in HC emissions caused by the injection spray interacting with the intake valves and piston crown is reduced as the cranking process progresses. Combustion phasing retard significantly reduces the PM emission. The HC emissions, however, are relatively not sensitive to combustion phasing in the range of interest.

This work examines the effect of valve timing during cold crank-start and cold fast-idle (1200 rpm, 2 bar NIMEP) on the emissions of hydrocarbons (HC) and particulate mass and number (PM/PN). Four different cam-phaser configurations are studied in detail: 1. Baseline stock valve timing. 2. Late intake opening/closing. 3. Early exhaust opening/closing. 4. Late intake phasing combined with early exhaust phasing. Delaying the intake valve opening improves the mixture formation process and results in more than 25% reduction of the HC and of the PM/PN emissions during cold crank-start. Early exhaust valve phasing results in a deterioration of the HC and PM/PN emissions performance during cold crank-start. Nevertheless, early exhaust valve phasing slightly improves the HC emissions and substantially reduces the particulate emissions at cold fast-idle.

The combustion process after auto-ignition is investigated. Depending on the non-uniformity of the end gas, auto-ignition could initiate a flame, produce pressure waves that excite the engine structure (acoustic knock), or result in detonation (normal or developing). For the “acoustic knock” mode, a knock intensity (KI) is defined as the pressure oscillation amplitude. The KI values over different cycles under a fixed operating condition are observed to have a log-normal distribution. When the operating condition is changed (over different values of λ, EGR, and spark timing), the mean (μ) of log (KI/GIMEP) decreases linearly with the correlation-based ignition delay calculated using the knock-point end gas condition of the mean cycle. The standard deviation σ of log(KI/GIMEP) is approximately a constant, at 0.63. The values of μ and σ thus allow a statistical description of knock from the deterministic calculation of the ignition delay using the mean cycle properties

An ignition delay correlation encompassing the effects of temperature, pressure, residual gas, EGR, and lambda (on both the rich and lean sides) has been developed. The procedure uses the individual knocking cycle data from a boosted direct injection SI engine (GM LNF) operating at 1250 to 2000 rpm, 8-14 bar GIMEP, EGR of 0 to 12.5%, and lambda of 0.8 to 1.3 with a certification fuel (Haltermann 437, with RON=96.6 and MON=88.5). An algorithm has been devised to identify the knock point on individual pressure traces so that the large data set (of some thirty three thousand cycles) could be processed automatically. For lean and for rich operations, the role of the excess fuel, air, and recycled gas (which has excess air in the lean case, and hydrogen and carbon monoxide in the rich case) may be treated effectively as diluents in the ignition delay expression.

The engine and its exhaust flow behaviors are investigated in a turbo-charged gasoline direct injection engine under simulated cold-fast-idle condition. The metrics of interest are the exhaust sensible and chemical enthalpy flows, and the exhaust temperature, all of which affect catalyst light off time. The exhaust sensible enthalpy flow is mainly a function of combustion phasing; the exhaust chemical enthalpy flow is mainly a function of equivalence ratio. High sensible and chemical enthalpy flow with acceptable engine stability could be obtained with retarded combustion and enrichment. When split injection is employed with one early and one later and smaller fuel pulse, combustion retards with early secondary injection in the compression stroke but advances with late secondary injection. Comparing gasoline to E85, the latter produces a lower exhaust temperature because of charge cooling effect and because of a faster combustion.

Particulate emissions from a production gasoline direct injection spark ignition engine were studied under a typical cold-fast-idle condition (1200 rpm, 2 bar NIMEP). The particle number (PN) density in the 22 to 365 nm range was measured as a function of the injection timing with single pulse injection and with split injection. Very low PN emissions were observed when injection took place in the mid intake stroke because of the fast fuel evaporation and mixing processes which were facilitated by the high turbulent kinetic energy created by the intake charge motion. Under these conditions, substantial liquid fuel film formation on the combustion chamber surfaces was avoided. PN emissions increased when injection took place in the compression stroke, and increased substantially when the fuel spray hit the piston.

This work examines the effects of valve timing and back pressure on the engine out enthalpy flow which is critical to the light off of the catalyst. The engine behavior is observed under fast-idle condition using a turbocharged production direct injection spark ignition engine with variable cam phasing that could shift both the intake and exhaust valve timing by 50 deg. crank angle. The back pressure is adjusted by throttling the exhaust. The engine operates at a constant net indicated mean effective pressure of 2 bar. The valve timing effect is largely governed by the residual gas trapped. With increasing valve overlap, the exhaust enthalpy flow increases because of the increase in exhaust temperature due to a slower combustion, and of the increase in air and fuel flow to compensate for the lower efficiency due to the slower combustion. When the back pressure is increased, the engine through flow has to increase to compensate for the larger pumping loss.

The loss mechanisms associated with engine downsizing, boosting and compression ratio change are assessed. Of interest are the extents of friction loss, pumping loss, and crevice loss. The latter does not scale proportionally with engine size. These losses are deconstructed via a cycle simulation model which encompasses a friction model and a crevice loss model for engine displacement of 300 to 500 cc per cylinder. Boost pressure is adjusted to yield constant torque. The compression ratio is varied from 8 to 20. Under part load, moderate speed condition (1600 rpm; 13.4 Nm/cylinder brake torque), the pumping work reduces significantly with downsizing while the work loss associated with the crevice volume increases. At full load (1600 rpm; 43.6 Nm/cylinder brake torque), the pumping work is less significant. The crevice loss (normalized to the fuel energy) is essentially the same as in the part load case. The sensitivities of the respective loss terms to downsizing are reported.

The effects of cooled exhaust gas recirculation (EGR) on a boosted direct-injection (DI) spark ignition (SI) engine operating at stoichiometric equivalence ratio, gross indicated mean effective pressure of 14-18 bar, and speed of 1500-2500 rpm, are studied under constant fuel condition at each operating point. In the presence of EGR, burn durations are longer and combustion is more retard. At the same combustion phasing, the indicated specific fuel consumption improves because of a decrease in heat loss and an increase in the specific heat ratio. The knock limited spark advance increases substantially with EGR. This increase is due partly to a slower combustion which is equivalent to a spark retard, as manifested by a retarded value of the 50% burn point (CA50), and due partly to a slower ignition chemistry of the diluted charge, as manifested by the knock limited spark advance to beyond the value offered by the retarded CA50.

The high load limit of a boosted homogeneous-charge-compression-ignition (HCCI) engine operating on negative-valve-overlap (NVO) was assessed. When operating under stoichiometric condition with no external dilution, the load, as measured by the net indicated mean effective pressure (NIMEP), increased with increase in manifold absolute pressure (MAP), and with decrease in trapped amount of residual gas. The maximum pressure rise rate (MPRR), however, also increased correspondingly. When the MAP and the amount of residual gas were adjusted so that the engine operating point could be held at a constant MPRR value, the NIMEP increased with the simultaneous decrease in MAP and residual until the misfire limit was reached. Therefore if a MPRR ceiling is imposed, the high load limit of an HCCI engine is at the intersection of the constraining MPRR line and the misfire line.

The effects of charge motion and laminar flame speeds on combustion and exhaust temperature have been studied by using an air jet in the intake flow to produce an adjustable swirl or tumble motion, and by replacing the nitrogen in the intake air by argon or CO₂, thereby increasing or decreasing the laminar flame speed. The objective is to examine the "Late Robust Combustion" concept: whether there are opportunities for producing a high exhaust temperature using retarded combustion to facilitate catalyst warm-up, while at the same time, keeping an acceptable cycle-to-cycle torque variation as measured by the coefficient of variation (COV) of the net indicated mean effective pressure (NIMEP). The operating condition of interest is at the fast idle period of a cold start with engine speed at 1400 RPM and NIMEP at 2.6 bar. A fast burn could be produced by appropriate charge motion. The combustion phasing is primarily a function of the spark timing.

The effects of the directed port flow produced by a Charge-Motion-Control-Valve (CMCV) on mixture preparation in a Port-Fuel-Injection engine were assessed under conditions typical of fast idle in a cold start process. The port fuel was found to comprise two components: a “valve” puddle (at the vicinity of the valve) that built up quickly, and that was mainly responsible for the delivery of the fuel to the cylinder charge; a “port” puddle located significantly upstream. The latter was mainly created by the reverse back flow process and built up slowly. Although the fuel amounts in these two components were roughly the same, the latter did not significantly interact with the fuel transport to the cylinder charge. The CMCV only weakly affected the purging or filling time of the valve puddle, hence the dynamics of the fuel delivery process was not materially affected.

Knock in a HCCI engine was examined by comparing subjective evaluation, recorded sound radiation from the engine, and cylinder pressure. Because HCCI combustion involved simultaneous heat release in a spatially large region, substantial oscillations were often found in the pressure signal. The time development of the audible signal within a knock cycle was different from that of the pressure trace. Thus the audible signal was not the attenuated transmission of the cylinder pressure oscillation but the sound radiation from the engine structure vibration excited by the initial few cycles of pressure oscillation. A practical knock limited maximum load point for the specific 2.3 L I4 engine under test (and arguably for engines of similar size and geometry) was defined at when the maximum rate of cycle-averaged pressure rise reached 5 MPa/ms.

The impact of intake air temperature and humidity on gasoline HCCI engine operation was assessed. The 2.3 L I4 production engine modified for single cylinder operation was controlled by using variable cam phasing on both the intake and exhaust valve in the negative-valve-overlap mode. Exhaust cam phasing was mainly used to control load, and intake cam phasing was mainly used to control combustion phasing. At stoichiometric condition, higher intake air temperature advanced combustion phasing and promoted knock, resulting in a 19% reduction of the Net Indicated Mean Effective Pressure (NIMEP) at the high load limit at 1500 rpm when intake temperature was changed from −10 to 100° C. Higher ambient humidity delayed combustion phasing. For stoichiometric operation, this delay allowed a small extension (a few tenths of a bar in NIMEP) in the high load limit when the moisture concentration was changed from 3 to 30 g/m3 (corresponding to 10-100% relative humidity at 28° C).

A novel engine management strategy for achieving fast catalyst light-off without the use of an exhaust air pump in a port-fuel-injected, spark ignition engine was developed. A conventional 4-cylinder engine was operated with three cylinders running rich and the fourth one as an air pump to supply air to the exhaust manifold. Under steady-state cold coolant conditions, this strategy achieved near total oxidation of CO and HC with sufficiently retarded spark timing, resulting in a 400% increase in feedgas enthalpy flow and a 90% reduction in feedgas HC emissions compared to conventional operation. The strategy was also evaluated for crank starts. Using the existing engine hardware, implementing the strategy resulted in a reduction in catalyst light-off time from 28.0 seconds under conventional operation to 9.1 seconds.

The fuel property effects on first cycle mixture preparation were assessed by measuring the in-cylinder fuel equivalence ratio (Φ) with a Fast Flame Ionization Detector (FFID) using four different fuels. The Engine Coolant Temperature (ECT) was varied between -6°C and 80°C. The Φ values increased with both ECT and amount of injected fuel mass. The delivery fraction (fraction of the injected fuel that went into the combustible charge), however, increased with ECT but decreased with increase in injected fuel. The minimum required injected mass to produce a combustible mixture increased sharply with decrease in ECT below 20°C. There was, however, no single fuel parameter that would correlate with the measurements over the entire temperature range. Instead, the minimum required injected mass correlated to different distillation points on the ASTM distillation curve; e.g. at ECT of -6°C, it correlated to T20; at 40°C, it correlated to T50.

A strategy to facilitate the mixture preparation process in PFI engines is to delay the Intake Valve Opening (IVO) by shifting the cam phasing so that the cylinder pressure is sub-atmospheric when the valve opens. The physics of the effect are discussed in terms of the pressure differential between the manifold and the cylinder, and the resulting flow and charge temperature history. The effect was evaluated by measuring the equivalence ratio of the trapped charge and the exhaust HC emissions in the first cycle of cranking in a 2.4L engine. When the IVO timing was changed from 18° BTDC to 21° ATDC, the in-cylinder fuel equivalence ratio increased by approximately 10%. This increase was attributed mainly to the enrichment of the charge by displacing the leaner mixture at the top of the cylinder in the period between BDC and IVC. The exhaust HC, however, increased by 40%. No conclusive explanation was established for this increase in HC emissions.

The catalyst oxygen storage capacities were measured over a test matrix consisted of fuels with 7, 33, 266 and 500 ppm sulfur, and of dynamometer-aged catalysts simulating 4K, 50K and 150K vehicle miles. A methodology was developed to relate the post-catalyst λ sensor response following an engine lean-step-transient to the total oxygen storage sites available. The time resolved NOx concentration profiles (which were most sensitive to the oxygen storage) in the catalyst were measured with the engine operating at λ modulations of various amplitudes and frequencies. The oxygen storage capacity deterioration comprised two multiplicative factors - an age factor which decreased quickly initially (by a factor of 2 from 4K to 50K miles), and then more modestly (by 30% from 50 to 150K miles), and a fuel sulfur factor which decreased by 10% for every 150 ppm increase in sulfur level.

Engine Hydrocarbon (HC) emissions behaviors in the shut down and re-start processes were examined in a production 4-cylinder 2.4 L engine. Depending on when the power to the ECU was cut off relative to the engine events, there could be two or three mis-fired cylinders (i.e. cylinders with fuel injected but no ignition). The total HC pumped out by the engine into the catalyst in the stopping process was ∼ 4 mg (approximately equaled to the amount of one injection at idle condition). Because the size of the catalyst was larger than the total exhaust volume in the stopping process, this HC was not observed at the catalyst exit. The catalyst temperature was also not affected. When the engine was purged after shut down (by cranking the engine with the injectors and ignition disconnected), the total exit HC was 33 mg. In a restart 90 minutes after shut down, the integrated amount of HC emissions due to residual fuel from the stopping process was 16 mg.