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Technical Paper

Performance Assessment of Extended Stroke Spark Ignition Engine

2018-04-03
2018-01-0893
The performance of an extended stroke spark ignition engine has been assessed by cycle simulation. The base engine is a modern turbo-charged 4-stroke passenger car spark-ignition engine with 10:1 compression ratio. A complex crank mechanism is used so that the intake stroke remains the same while the expansion-to-intake stroke ratio (SR) is varied by changing the crank geometry. The study is limited to the thermodynamic aspect of the extended stroke; the changes in friction, combustion characteristic, and other factors are not included. When the combustion is not knock limited, an efficiency gain of more than 10 percent is obtained for SR = 1.5. At low load, however, there is an efficiency lost due to over-expansion. At the same NIMEP, the extended stroke renders the engine more resistant to knock. At SR of 1.8, the engine is free from knock up to 14 bar NIMEP at 2000 rpm. Under knocking condition, the required spark retard to prevent knocking is less with the extended stroke.
Technical Paper

Effects of Ethanol Evaporative Cooling on Particulate Number Emissions in GDI Engines

2018-04-03
2018-01-0360
The spark ignition engine particulate number (PN) emissions have been correlated to a particulate matter index (PMI) in the literature. The PMI value addresses the fuel effect on PN emission through the individual fuel species reactivity and vapor pressure. The latter quantity is used to account for the propensity of the non-volatile fuel components to survive to the later part of the combustion event as wall liquid films, which serve as sources for particulate emission. The PMI, however, does not encompass the suppression of vaporization by the evaporative cooling of fuel components, such as ethanol, that have high latent heat of vaporization. This paper assesses this evaporative cooling effect on PN emissions by measurements in a GDI engine operating with a base gasoline which does not contain oxygenate, with a blend of the gasoline and ethanol, and with a blend of the gasoline, ethanol, and a hydrocarbon additive so that the blend has the same PMI as the original gasoline.
Journal Article

Assessment of Gasoline Direct Injection Engine Cold Start Particulate Emission Sources

2017-03-28
2017-01-0795
The gasoline direct injection (GDI) engine particulate emission sources are assessed under cold start conditions: the fast idle and speed/load combinations representative of the 1st acceleration in the US FTP. The focus is on the accumulation mode particle number (PN) emission. The sources are non-fuel, combustion of the premixed charge, and liquid fuel film. The non-fuel emissions are measured by operating the engine with premixed methane/air or hydrogen/air. Then the PN level is substantially lower than what is obtained with normal GDI operation; thus non-fuel contribution to PN is small. When operating with stoichiometric premixed gasoline/air, the PN level is comparable to the non-fuel level; thus premixed-stoichiometric mixture combustion does not significantly generate particulates. For fuel rich premixed gasoline/air, PN increases dramatically when lambda is less than 0.7 to 0.8.
Journal Article

Analysis of NOx Emissions during Crank-Start and Cold Fast-Idle in a GDI Engine

2017-03-28
2017-01-0796
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.
Technical Paper

The Anatomy of Knock

2016-04-05
2016-01-0704
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
Journal Article

Reduction of Cold-Start Emissions through Valve Timing in a GDI Engine

2016-04-05
2016-01-0827
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.
Journal Article

Cycle-by-Cycle Analysis of Cold Crank-Start in a GDI Engine

2016-04-05
2016-01-0824
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.
Journal Article

Effect of Operation Strategy on First Cycle CO, HC, and PM/PN Emissions in a GDI Engine

2015-04-14
2015-01-0887
The impact of the operating strategy on emissions from the first combustion cycle during cranking was studied quantitatively in a production gasoline direct injection engine. A single injection early in the compression cycle after IVC gives the best tradeoff between HC, particulate mass (PM) and number (PN) emissions and net indicated effective pressure (NIMEP). Retarding the spark timing, it does not materially affect the HC emissions, but lowers the PM/PN emissions substantially. Increasing the injection pressure (at constant fuel mass) increases the NIMEP but also the PM/PN emissions.
Journal Article

On the Nature of Particulate Emissions from DISI Engines at Cold-Fast-Idle

2014-04-01
2014-01-1368
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.
Journal Article

EGR Effects on Boosted SI Engine Operation and Knock Integral Correlation

2012-04-16
2012-01-0707
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.
Technical Paper

Fuel Effects on HCCI Operation in a Spark Assisted Direct Injection Gasoline Engine

2011-08-30
2011-01-1763
The fuel effects on HCCI operation in a spark assisted direct injection gasoline engine are assessed. The low load limit has been extended with a pilot fuel injection during the negative valve overlap (NVO) period. The fuel matrix consists of hydrocarbon fuels and various ethanol blends and a butanol blend, plus fuels with added ignition improvers. The hydrocarbon fuels and the butanol blend do not significantly alter the high or the low limits of operation. The HCCI operation appears to be controlled more by the thermal environment than by the fuel properties. For E85, the engine behavior depends on the extent that the heat release from the pilot injected fuel in the NVO period compensates for the evaporative cooling of the fuel.
Journal Article

On the High Load Limit of Boosted Gasoline HCCI Engine Operating in NVO Mode

2010-04-12
2010-01-0162
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.
Journal Article

The Effects of Charge Motion and Laminar Flame Speed on Late Robust Combustion in a Spark-Ignition Engine

2010-04-12
2010-01-0350
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.
Technical Paper

A New Approach to Ethanol Utilization: High Efficiency and Low NOx in an Engine Operating on Simulated Reformed Ethanol

2008-10-06
2008-01-2415
The use of hydrogen as a fuel supplement for lean-burn engines at higher compression ratios has been studied extensively in recent years, with good promise of performance and efficiency gains. With the advances in reformer technology, the use of a gaseous fuel stock, comprising of substantially higher fractions of hydrogen and other flammable reformate species, could provide additional improvements. This paper presents the performance and emission characteristics of a gas mixture of equal volumes of hydrogen, CO, and methane. It has recently been reported that this gas mixture can be produced by reforming of ethanol at comparatively low temperature, around 300C. Experiments were performed on a 1.8-liter passenger-car Nissan engine modified for single-cylinder operation. Special pistons were made so that compression ratios ranging from CR= 9.5 to 17 could be used. The lean limit was extended beyond twice stoichiometric (up to lambda=2.2).
Journal Article

Primary Reference Fuel Behavior in a HCCI Engine near the Low-Load Limit

2008-06-23
2008-01-1667
In a previous study, a wide range of gasolines with RON∼90 were tested in a single cylinder engine operated in HCCI mode using negative valve overlap, and all were found to have very similar behavior near the low-load limit. Here we broaden the range of gasolines to include PRF90 and PRF60. At high engine speed, both PRF60 and PRF90 behave similarly to all the other gasolines tested. However, at 1000 RPM, PRF90 is very different from all the other gasolines: it ignites very late, and the engine cannot be operated at low load. Simulations using a popular fuel chemistry model cannot distinguish PRF60 and PRF90 under these conditions. However, a new fuel chemistry model correctly shows the onset of fuel sensitivity at low engine speed. Sensitivity analyses indicate the low-load limit at low engine speed strongly depend on both the chemistry parameters and on the heat-transfer parameters.
Technical Paper

Assessing the Windage Tray Blockage Effect on Aeration in the Oil Sump

2007-10-29
2007-01-4109
The windage tray effect on aeration in the engine sump was assessed by replacing much of the windage tray materials with wire meshes of various blockages. The mesh was to prevent direct impact of the oil drops spinning off the crank shaft onto the sump oil, and simultaneously, to provide sufficient drainage so that there was no significant build up of windage tray oil film that would interact with these droplets. Aeration at the oil pump inlet was measured by X-ray absorption in a production V-6 SI engine motoring at 2000 to 6000 rpm. Within experimental uncertainty, these windage tray changes had no effect on aeration. Thus activities in the sump such as the interaction of the oil drops spun from the crank shaft with the sump oil or with the windage tray, and the agitation of the sump oil by the crank case gas, were not major contributors to aeration at the pump inlet.
Technical Paper

Effects of Variations in Market Gasoline Properties on HCCI Load Limits

2007-07-23
2007-01-1859
The impact of market-fuel variations on the HCCI operating range was measured in a 2.3L four-cylinder engine, modified for single-cylinder operation. HCCI combustion was achieved through the use of residual trapping. Variable cam phasing was used to maximize the load range at each speed. Test fuels were blended to cover the range of variation in select commercial fuel properties. Within experimental measurement error, there was no change in the low-load limit among the test fuels. At the high-load limit, some small fuel effects on the operating range were observed; however, the observed trends were not consistent across all the speeds studied.
Technical Paper

A Novel Strategy for Fast Catalyst Light-Off without the Use of an Air Pump

2007-01-23
2007-01-0044
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.
Technical Paper

Contribution of Liquid Fuel to Hydrocarbon Emissions in Spark Ignition Engines

2001-09-24
2001-01-3587
The purpose of this work was to develop an understanding of how liquid fuel transported into the cylinder of a port-fuel-injected gasoline-fueled SI engine contributes to hydrocarbon (HC) emissions. To simulate the liquid fuel flow from the valve seat region into the cylinder, a specially designed fuel probe was developed and used to inject controlled amounts of liquid fuel onto the port wall close to the valve seat. By operating the engine on pre-vaporized Indolene, and injecting a small amount of liquid fuel close to the valve seat while the intake valve was open, we examined the effects of liquid fuel entering the cylinder at different circumferential locations around the valve seat. Similar experiments were also carried out with closed valve injection of liquid fuel at the valve seat to assess the effects of residual blowback, and of evaporation from the intake valve and port surfaces.
Technical Paper

Throttle Movement Rate Effects on Transient Fuel Compensation in a Port-Fuel-Injected SI Engine

2000-06-19
2000-01-1937
Throttle ramp rate effects on the in-cylinder fuel/air (F/A) excursion was studied in a production engine. The fuel delivered to the cylinder per cycle was measured in-cylinder by a Fast Response Flame Ionization detector. Intake pressure was ramped from 0.4 to 0.9 bar. Under slow ramp rates (∼1 s ramp time), the Engine Electronic Control (EEC) unit provided the correct compensation for delivering a stoichiometric mixture to the cylinder throughout the transient. At fast ramp rates (a fraction of a second ramps), a lean spike followed by a rich one were observed. Based on the actual fuel injected in each cycle during the transient, a x-τ model using a single set of x and τ values reproduced the cycle-to-cycle in-cylinder F/A response for all the throttle ramp rates.
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