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In this work we studied the effects of piston bowl design on combustion in a small-bore direct-injection diesel engine. Two bowl designs were compared: a conventional, omega-shaped bowl and a stepped-lip piston bowl. Experiments were carried out in the Sandia single-cylinder optical engine facility, with a medium-load, mild-boosted operating condition featuring a pilot+main injection strategy. CFD simulations were carried out with the FRESCO platform featuring full-geometric body-fitted mesh modeling of the engine and were validated against measured in-cylinder performance as well as soot natural luminosity images. Differences in combustion development were studied using the simulation results, and sensitivities to in-cylinder flow field (swirl ratio) and injection rate parameters were also analyzed.

Sector mesh modeling is the dominant computational approach for combustion system design optimization. The aim of this work is to quantify the errors descending from the sector mesh approach through three geometric modeling approaches to an optical diesel engine. A full engine geometry mesh is created, including valves and intake and exhaust ports and runners, and a full-cycle flow simulation is performed until fired TDC. Next, an axisymmetric sector cylinder mesh is initialized with homogeneous bulk in-cylinder initial conditions initialized from the full-cycle simulation. Finally, a 360-degree azimuthal mesh of the cylinder is initialized with flow and thermodynamics fields at IVC mapped from the full engine geometry using a conservative interpolation approach. A study of the in-cylinder flow features until TDC showed that the geometric features on the cylinder head (valve tilt and protrusion into the combustion chamber, valve recesses) have a large impact on flow complexity.

Diesel piston bowl geometry can affect turbulent mixing and therefore it impacts heat-release rates, thermal efficiency, and soot emissions. The focus of this work is on the effects of bowl geometry and injection timing on turbulent flow structure. This computational study compares engine behavior with two pistons representing competing approaches to combustion chamber design: a conventional, re-entrant piston bowl and a stepped-lip piston bowl. Three-dimensional computational fluid dynamics (CFD) simulations are performed for a part-load, conventional diesel combustion operating point with a pilot-main injection strategy under non-combusting conditions. Two injection timings are simulated based on experimental findings: an injection timing for which the stepped-lip piston enables significant efficiency and emissions benefits, and an injection timing with diminished benefits compared to the conventional, re-entrant piston.

In light-duty direct-injection (DI) diesel engines, combustion chamber geometry influences the complex interactions between swirl and squish flows, spray-wall interactions, as well as late-cycle mixing. Because of these interactions, piston bowl geometry significantly affects fuel efficiency and emissions behavior. However, due to lack of reliable in-cylinder measurements, the mechanisms responsible for piston-induced changes in engine behavior are not well understood. Non-intrusive, in situ optical measurement techniques are necessary to provide a deeper understanding of the piston geometry effect on in-cylinder processes and to assist in the development of predictive engine simulation models. This study compares two substantially different piston bowls with geometries representative of existing technology: a conventional re-entrant bowl and a stepped-lip bowl. Both pistons are tested in a single-cylinder optical diesel engine under identical boundary conditions.

In light- and medium-duty diesel engines, piston bowl shape influences thermal efficiency, either due to changes in wall heat loss or to changes in the heat release rate. The relative contributions of these two factors are not clearly described in the literature. In this work, two production piston bowls are adapted for use in a single cylinder research engine: a conventional, re-entrant piston, and a stepped-lip piston. An injection timing sweep is performed at constant load with each piston, and heat release analyses provide information about thermal efficiency, wall heat loss, and the degree of constant volume combustion. Zero-dimensional thermodynamic simulations provide further insight and support for the experimental results. The effect of bowl geometry on wall heat loss depends on injection timing, but changes in wall heat loss cannot explain changes in efficiency.

The design of modern diesel-powered vehicles involves optimization and balancing of trade-offs for fuel efficiency, emissions, and noise. To meet increasingly stringent emission regulations, diesel powertrains employ aftertreatment devices to control nitrogen oxides, hydrocarbons, carbon monoxide, and particulate matter emissions and use active exhaust warm-up strategies to ensure those devices are active as quickly as possible. A typical strategy for exhaust warm-up is to operate with retarded combustion phasing, limited by combustion stability and HC emissions. The amount of exhaust enthalpy available for catalyst light-off is limited by the extent to which combustion phasing can be retarded. Diesel cetane number (CN), a measure of fuel ignition quality, has an influence on combustion stability at retarded combustion phasing. Diesel fuel in the United States tends to have a lower CN (both minimum required and average in market) than other countries.

Heat loss through wall boundaries play a dominant role in the overall performance and efficiency of internal combustion engines. Typical engine simulations use constant temperature wall boundary conditions [1, 2, 3]. These boundary conditions cannot be estimated accurately from experiments due to the complexities involved with engine combustion. As a result, they introduce a large uncertainty in engine simulations and serve as a tuning parameter. Modeling the process of heat transfer through the solid walls in an unsteady engine computational fluid dynamics (CFD) simulation can lead to the development of higher fidelity engine models. These models can be used to study the impact of heat loss on engine efficiency and explore new design methodologies that can reduce heat losses. In this work, a single cylinder diesel engine is modeled along with the solid piston coupled to the fluid domain.

This study examines the dynamics and control of an engine operated with late intake valve closure (LIVC) timings in a dual-fuel combustion mode. The engine features a fuel delivery system in which diesel is direct-injected and natural gas is port-injected. Despite the benefits of LIVC and dual-fuel strategy, combining these two techniques resulted in efficiency losses due to the variability of the combustion process across cylinders. The difference in power production across cylinders ranges from 9% at an IVC of 570°ATDC* to 38% at an IVC of 620 °ATDC and indicates an increasingly uneven fuel distribution as the intake valve remains open longer in the compression stroke. This paper describes an approach for controlling the amount of fuel injected into each cylinders’ port of an inline six- cylinder heavy-duty dual-fuel engine to minimize the variations in fuel distribution across cylinder.

Advanced internal combustion engines, although generally more efficient than conventional combustion engines, often encounter limitations in multi-cylinder applications due to variations in the combustion process. This study leverages experimental data from an inline 6-cylinder heavy-duty dual fuel engine equipped with a fully-flexible variable intake valve actuation system to study cylinder-to-cylinder variations in power production. The engine is operated with late intake valve closure timings in a dual-fuel combustion mode featuring a port-injection and a direct-injection fueling system in order to improve fuel efficiency and engine performance. Experimental results show increased cylinder-to-cylinder variation in IMEP as IVC timing moves from 570°ATDC to 610°ATDC, indicating an increasingly uneven fuel distribution between cylinders.

Dual-fuel combustion using port-injection of low reactivity fuel combined with direct injection of a higher reactivity fuel, otherwise known as Reactivity Controlled Compression Ignition (RCCI), has been shown as a method to achieve high efficiency combustion with moderate peak pressure rise rates, low engine-out soot and NOx emissions. A key requirement for extending to high-load operation is reduce the reactivity of the premixed charge prior to the diesel injection. One way to accomplish this is to use a very low reactivity fuel such as natural gas. In this work, experimental testing was conducted on a 13L multi-cylinder heavy-duty diesel engine modified to operate using RCCI combustion with port injection of natural gas and direct injection of diesel fuel. Natural gas/diesel RCCI engine operation is compared over the EPA Heavy-Duty 13 mode supplemental emissions test with and without EGR.

Dual-fuel combustion using port-injected gasoline with a direct diesel injection has been shown to achieve low-temperature combustion with moderate peak pressure rise rates, low engine-out soot and NOx emissions, and high indicated thermal efficiency. A key requirement for extending high-load operation is moderating the reactivity of the premixed charge prior to the diesel injection. Reducing compression ratio, in conjunction with a higher expansion ratio using alternative valve timings, decreases compressed charge reactivity while maintain a high expansion ratio for maximum work extraction. Experimental testing was conducted on a 13L multi-cylinder heavy-duty diesel engine modified to operate dual-fuel combustion with port gasoline injection to supplement the direct diesel injection. The engine employs intake variable valve actuation (VVA) for early (EIVC) or late (LIVC) intake valve closing to yield reduced effective compression ratio.

Stricter CO2 and emissions regulations are pushing spark ignition engines more and more towards downsizing, enabled through direct injection and turbocharging. The advantages which come with direct injection, such as increased charge density and an elevated knock resistance, are even more pronounced when using low carbon number alcohols instead of gasoline. This is mainly due to the higher heat of vaporization and the lower air-to-fuel ratio of light alcohols such as methanol, ethanol and butanol. These alcohols are also attractive alternatives to gasoline because they can be produced from renewable resources. Because they are liquid, they can be easily stored in a vehicle. In this respect, the performance and engine-out emissions (NOx, CO, HC and PM) of methanol, ethanol and butanol were examined on a 4 cylinder 2.4 DI production engine and are compared with those on neat gasoline.

The low temperature combustion concept is very attractive for reducing NOx and soot emissions in diesel engines. However, it has potential limitations due to higher combustion noise, CO and HC emissions. A multiple injection strategy is an effective way to reduce unburned emissions and noise in LTC. In this paper, the effect of multiple injection strategies was investigated to reduce combustion noise and unburned emissions in LTC conditions. A hybrid surrogate fuel model was developed and validated, and was used to improve LTC predictions. Triple injection strategies were considered to find the role of each pulse and then optimized. The split ratio of the 1st and 2nd pulses fuel was found to determine the ignition delay. Increasing mass of the 1st pulse reduced unburned emissions and an increase of the 3rd pulse fuel amount reduced noise. It is concluded that the pulse distribution can be used as a control factor for emissions and noise.

Diesel combustion and emissions is largely spray and mixing controlled. Spray and combustion models enable characterization over a range of conditions to understand optimum combustion strategies. The validity of models depends on the inputs, including the rate of injection profile of the injector. One method to measure the rate of injection is to measure the momentum, where the injected fuel spray is directed onto a force transducer which provides measurements of momentum flux. From this the mass flow rate is calculated. In this study, the impact of impingement distance, the distance from injector nozzle exit to the anvil connected to the force transducer, is characterized over a range of 2 - 12 mm. This characterization includes the impact of the distance on the momentum flux signal in both magnitude and shape. At longer impingement distances, it is hypothesized that a peak in momentum could occur due to increasing velocity of fuel injected as the pintle fully opens.

Recent developments in advanced combustion engines have demonstrated the potential increases in efficiency and reductions in emissions through low temperature combustion (LTC). These combustion modes often rely on high exhaust gas recirculation (EGR), early fuel injection systems, and in some cases a combination of fuels with different reactivities. Despite the advantages of LTC, such operations are highly sensitive to the in-cylinder pre-combustion conditions and face significant challenges in multi-cylinder operation due to cylinder-to-cylinder variations of the combustion process. The cause of cylinder-to-cylinder variations is strongly tied to non-uniform trapped mass. In particular, in-cylinder oxygen concentration plays a critical role in the combustion process of each cylinder and can be leveraged to predict combustion characteristics and to develop control algorithms that mitigate cylinder-to-cylinder variation.

The Leaner Lifted-Flame Combustion (LLFC) strategy offers a possible alternative to low temperature combustion or other globally lean, premixed operation strategies to reduce soot directly in the flame, while maintaining mixing-controlled combustion. Adjustments to fuel properties, especially fuel oxygenation, have been reported to have potentially beneficial effects for LLFC applications. Six fuels were selected or blended based on cetane number, oxygen content, molecular structure, and the presence of an aromatic hydrocarbon. The experiments compared different fuel blends made of n-hexadecane, n-dodecane, methyl decanoate, tri-propylene glycol monomethyl ether (TPGME), as well as m-xylene. Several optical diagnostics have been used simultaneously to monitor the ignition, combustion and soot formation/oxidation processes from spray flames in a constant-volume combustion vessel.

Dual-fuel combustion using liquid fuels with differing reactivity has been shown to achieve low-temperature combustion with moderate peak pressure rise rates, low soot and NOx emissions, and high indicated efficiency. Varying fractions of gasoline-type and diesel-type fuels enable operation across a range of low- and mid-load operating conditions. Expanding the operating range to cover the full operating range of a heavy-duty diesel engine, while maintaining the efficiency and emissions benefits, is a key objective. With dissimilar properties of the two utilized fuels lying at the heart of the dual-fuel concept, a tool for enabling this load range expansion is altering the properties of the two test fuels - this study focuses on altering the reactivity of the diesel fuel component. Tests were conducted on a 13L six-cylinder heavy-duty diesel engine modified to run dual-fuel combustion with port gasoline injection to supplement the direct diesel injection.

This study evaluates iso-butanol as a pathway to introduce higher levels of alternative fuels for recreational marine engine applications compared to ethanol. Butanol, a 4-carbon alcohol, has an energy density closer to gasoline than ethanol. Isobutanol at 16 vol% blend level in gasoline (iB16) exhibits energy content as well as oxygen content identical to E10. Tests with these two blends, as well as indolene as a reference fuel, were conducted on a Mercury 90 HP, 4-stroke outboard engine featuring computer controlled sequential multi-port Electronic Fuel Injection (EFI). The test matrix included full load curves as well as the 5-mode steady-state marine engine test cycle. Analysis of the full load tests suggests that equal full load performance is achieved across the engine speed band regardless of fuel at a 15-20°C increase in exhaust gas temperatures for the alcohol blends compared to indolene.

This paper evaluates the potential of adding higher alcohols to gasoline blendstock in an attempt to improve overall fuel performance. The alcohols considered include ethanol, normal- and iso-structures of propanol, butanol and pentanol as well as normal-hexanol (C2-C6). Fuel performance is quantified based on energy content, knock resistance as well as petroleum displacement and promising multi-component blends are systematically identified based on property prediction methods. These promising multi-component blends, as well as their respective reference fuels, are subsequently tested for efficiency and emissions performance utilizing a gasoline direct injection, spark ignition engine. The engine test results confirm that combustion and efficiency of tailored multi-component blends closely match those of the reference fuels. Regulated emissions stemming from combustion of these blends are equal or lower compared to the reference fuels across the tested engine speed and load regime.

Low temperature combustion through in-cylinder blending of fuels with different reactivity offers the potential to improve engine efficiency while yielding low engine-out NOx and soot emissions. A Navistar MaxxForce 13 heavy-duty compression ignition engine was modified to run with two separate fuel systems, aiming to utilize fuel reactivity to demonstrate a technical path towards high engine efficiency. The dual-fuel engine has a geometric compression ratio of 14 and uses sequential, multi-port-injection of a low reactivity fuel in combination with in-cylinder direct injection of diesel. Through control of in-cylinder charge reactivity and reactivity stratification, the engine combustion process can be tailored towards high efficiency and low engine-out emissions. Engine testing was conducted at 1200 rpm over a load sweep.