Search

Using high charge dilution low temperature combustion (LTC) strategies in a diesel engine offers low emissions of nitrogen oxides (NOx). These strategies are limited to part-load conditions and involve high levels of charge dilution, typically achieved through the use of recirculated exhaust gases (EGR). The slow response of the gas handling system, compared to load demand and fuelling, can lead to conditions where dilution levels are higher or lower than expected, impacting emissions and combustion stability. This article reports on the sensitivity of high-dilution LTC to variations in EGR rate and fuel injection timing. Impacts on engine efficiency, combustion stability and emissions are assessed in a single-cylinder engine and compared to in-cylinder flame temperatures measured using a borescope-based two-colour pyrometer. The work focuses on low-load conditions (300 kPa gross indicated mean effective pressure) and includes an EGR sweep from conventional diesel mode to high-dilution LTC, and sensitivity studies investigating the effects of variations in charge dilution and fuel injection timing at the high-dilution LTC condition. Key findings from the study include that the peak flame temperature decreased from ~2580 K in conventional diesel combustion with no EGR to 1800 K in LTC with low-NOx, low-soot operation and an EGR rate of 57%. Increasing the EGR to 64% reduced flame temperatures to 1400 K but increased total hydrocarbon (THC) and carbon monoxide (CO) emissions by 30–50% and increased fuel consumption by 5–7%. Charge dilution was found to have a stronger effect on the combustion process than the diesel injection timing under these LTC conditions. Advancing fuel injection timings at increasing dilution kept combustion instability below 2.5%. Peak in-cylinder temperatures were maintained in the 2000–2100 K range, while THC and CO emissions were controlled by delaying the onset of bulk quenching. Very early injection (earlier than 24 °CA before top-dead-centre) resulted in spray impingement on the piston crown, resulting in degraded efficiency and higher emissions. The results of this study demonstrate the potential of fuel injection timing modification to accommodate variations in charge dilution rates while maintaining low NOx and PM emissions in a diesel engine using low-temperature combustion strategies at part loads. [ABSTRACT FROM AUTHOR]

Cylinder deactivation, sometimes referred to as variable displacement engine technology, is a method being employed in state-of-the-art reciprocating engines to improve fuel economy. The approach involves disabling the valve actuation of one or more cylinders to deactivate them, thus forcing the engine to operate at a higher specific load across the remaining cylinders to produce the torque demanded. Operating at such a point with an increased throttle opening reduces the engine’s pumping losses and hence reduces fuel consumption. In this work, the spray morphology, combustion and emissions of a three-cylinder downsized gasoline turbocharged direct injection engine with variable displacement engine capability on one cylinder were studied. This investigation allowed the interaction between the fuel spray, engine performance and emissions immediately following the reactivation of the deactivated cylinder to be better understood. Three operation modes were examined which included running the engine at full displacement, at the reduced displacement and at full displacement with increased indicated mean effective pressure, matching that of the reduced displacement mode. The study showed that cylinder deactivation significantly reduced specific fuel consumption at the conditions tested in comparison to full displacement operation. It was also found that when running the engine at full displacement but with the reduced displacement–level indicated mean effective pressure, the specific fuel consumption was greater than for reduced displacement operation. In addition, it was observed that particulate number emissions increase transiently during the deactivation period due to the disturbances to the fuelling control caused by displacement transitions. Improved fuelling control, refinement of the engine calibration during reduced displacement operation or a gasoline particulate filter could be used to manage this particulate number level. [ABSTRACT FROM AUTHOR]

Liquefied natural gas is emerging as viable and potentially sustainable transportation fuel with intrinsic economic and environmental benefits. Liquefied natural gas possesses thermomechanical exergy amounting to ∼1 MJ kg-1 which is currently wasted on liquefied natural gas vehicles, while it could be used to produce useful work. The present investigation proposes an indirect means of obtaining useful work from liquefied natural gas through charge cooling and also demonstrates additional benefits in terms of NOx emissions and power density. A thermodynamic engine model was used to quantify the performance benefits of such a strategy for a homogeneous-charge, spark-ignited, stoichiometric natural gas engine. Four fuelling strategies were compared in terms of fuel consumption, mean effective pressure and NOx emissions. Compared to the conventional port-injected natural gas engine (where gaseous fuel is injected), it was found that directly injecting the liquid phase fuel into the cylinder near the start of the compression stroke resulted in approximately -8.9% brake specific fuel consumption, +18.5% brake mean effective pressure and -51% brake specific NOx depending on the operating point. Port-injection of the fuel in the liquid phase carried similar benefits, while direct injection of the fuel in the gaseous phase resulted in minor efficiency penalties (∼+1.3% brake specific fuel consumption). This work highlights the future potential of liquefied natural gas vehicles to achieve high specific power, high efficiency and ultra-low emissions (such as NOx) by tailoring the fuel system to fully exploit the cryogenic properties of the fuel. [ABSTRACT FROM AUTHOR]

This article reviews split-cycle internal combustion engine designs. The review includes historical work, assessment of prototypes and discussion of the most recent designs. There has been an abundance of split-cycle engine designs proposed since the first in 1872. Despite this, very few prototypes exist, and no split-cycle engines are reported to be in series production. The few split-cycle prototypes that have been developed have faced practical challenges contributing to limited performance. These challenges include air flow restrictions into the expansion cylinder, late combustion, thermal management issues, and mechanical challenges with the crossover valve actuation mechanism. The main promoted advantage of split-cycle engines is the increased thermal efficiency compared to conventional internal combustion engines. However, an efficiency improvement has not thus far been demonstrated in published test data. The thermodynamic studies reviewed suggest that split-cycle engines should be more efficient than conventional four-stroke engines. Reasons why increased thermal efficiency is not realised in practice could be due to practical compromises, or due to inherent architectural split-cycle engine design limitations. It was found that the number of split-cycle engine patents has increased significantly over recent years, suggesting an increased commercial interest in the concept since the possibility of increased efficiency becomes more desirable and might outweigh the drawbacks of practical challenges. [ABSTRACT FROM AUTHOR]