Experimental and Numerical Investigations of Close-Coupled Pilot Injections to Reduce Combustion Noise in a Small-Bore Diesel Engine

For further improvement in brake thermal efficiency of diesel engines, not only increase in indicated work but decrease in energy losses, for example, cooling loss and mechanical loss, are essential. Nevertheless, these demands are generally in trade-off with limited ability of combustion control by conventional diesel fuel injection equipment even though it can precisely control injection timing, duration, pressure, and frequency. To overcome the trade-off, the potential of heat release rate control with more degrees of freedom was first investigated by means of a zero-dimensional thermodynamic engine model. The result indicated that the optimum brake thermal efficiency is not achieved with constant-volume combustion (Otto) cycle, not with constant-pressure combustion (Diesel) cycle, but with Sabathe (limited-pressure or Seiliger) cycle. To experimentally confirm the numerical analysis result by a heavy-duty single-cylinder diesel engine, a new diffusion-combustion-based concept with the combination of multiple fuel injectors and soup plate type piston cavity has been developed. One injector was mounted vertically at the cylinder center like in a conventional direct-injection diesel engine, and two additional injectors were slant-mounted at the piston cavity circumference. The side injectors sprayed fuel downstream the swirl to prevent both spray interference and spray impingement on the cavity wall, while improving air utilization near the center of the cavity. The desired heat release rate profile was well achieved by independent control of injection timing and duration (fuel injection pressure was kept in constant) for each fuel injector. Results showed reduced friction loss, heat loss, and nitrogen oxides (NO x) emissions, while maintaining indicated thermal efficiency by suppressing the peak cylinder pressure, cylinder-averaged temperature, and spray flame impingement to the cavity wall. Additionally, a simultaneous reduction in smoke and NO x emissions was achieved, without any deterioration in carbon monoxide (CO) and total hydrocarbon emissions, even compared with conventional diesel combustion.

Section: Combustion Observation By a Bottom-view Transparent Enginementioning

confidence: 99%

A new concept of actively controlled rate of diesel combustion for improving brake thermal efficiency of diesel engines: Part 1—verification of the concept

Uchida¹,

Okamoto

Watanabe³

2017

“…The ignition delay and physical properties of conventional diesel fuel can be matched with this blend as reported by previous studies. 12,13 Combustion imaging setup A simultaneous high-speed flame natural luminosity imaging and OH* chemiluminescence imaging technique is developed to provide temporally and spatially resolved information about the ignition and the early combustion process. OH* chemiluminescence imaging was chosen for determining the high-temperature ignition because excited state species, such as OH*, are formed during the stoichiometric combustion of hydrocarbon fuels 14 and soot formation generally occurs right after the onset of high-temperature reactions.…”

Section: Experimental Apparatusmentioning

confidence: 99%

The influence of pilot injection on high-temperature ignition processes and early flame structure in a high-speed direct injection diesel engine

Park

Busch

2017

Self Cite

Simultaneous high-speed natural luminosity and OH* chemiluminescence imaging is used to characterize hightemperature ignition processes in conventional diesel combustion with a pilot-main injection strategy in a single-cylinder, light-duty optical diesel engine. High-speed imaging provides temporally and spatially resolved information in terms of high-temperature ignition processes and flame structure during the combustion. Using these imaging measurements, the high-temperature inflammation and the diffusion flame development processes are analyzed. The chemiluminescence signal shows a hot, reactive mixture, which gradually decreases after the peak release of the pilot combustion and lasts long after the apparent heat release has ended. Therefore, when the reactive pilot mixture exists near the main injection jets, the high-temperature ignition of the main injection is apparently initiated through interactions with the reactive pilot mixture. High-temperature autoignition, another process by which ignition of the main injection occurs, is observed in main injection plumes where the chemiluminescence signal of the reactive pilot mixture becomes very weak or is absent at the start of main injection. As the reaction of the main injection continues, the non-premixed main injection jet structure is developed and the high-temperature reacting region expands throughout the jet.

“…Shibata and colleagues 17,18 developed combustion noise simulation further and applied it to two-stage HTHR NCS combustion in 2016 and discussed the NCS combustion mechanism. Furthermore, Busch et al 19,20 reported the combustion noise reduction by short pilot-main injections and much research has been conducted with combustion noise spectrum analysis. The fuel penetration rate data provide evidence that rate shaping of the initial phase of the main injection is occurring in the engine and that this rate shaping is largely consistent with the injection rate data; however, the results demonstrate that these changes are not directly responsible for the observed trends in combustion noise.…”

Section: Introductionmentioning

confidence: 99%

Optimization of multiple heat releases in pre-mixed diesel engine combustion for high thermal efficiency and low combustion noise by a genetic-based algorithm method

Shibata

Ogawa

Amanuma

et al. 2018

The reduction of diesel combustion noise by multiple fuel injections maintaining high indicated thermal efficiency is an object of the research reported in this article. There are two aspects of multiple fuel injection effects on combustion noise reduction. One is the reduction of the maximum rate of pressure rise in each combustion, and the other is the noise reduction effects by the noise canceling spike combustion. The engine employed in the simulations and experiments is a supercharged, single-cylinder direct-injection diesel engine, with a high pressure common rail fuel injection system. Simulations to calculate the combustion noise and indicated thermal efficiency from the approximated heat release by Wiebe functions were developed. In two-stage high temperature heat release combustion, the combustion noise can be reduced; however, the combustion noise in amplification frequencies must be reduced to achieve further combustion noise reduction, and an additional heat release was added ahead of the two-stage high temperature heat release combustion in Test 1. The simulations of the resulting three-stage high temperature heat release combustion were conducted by changing the heating value of the first heat release. In Test 2 where the optimum heat release shape for low combustion noise and high indicated thermal efficiency was investigated and the role of each of the heat releases in the three-stage high temperature heat release combustion was discussed. In Test 3, a genetic-based algorithm method was introduced to avoid the time-consuming loss and great care in preparing the calculations in Test 2, and the optimum heat release shape and frequency characteristics for combustion noise by the genetic-based algorithm method were speedily calculated. The heat release occurs after the top dead center, and the indicated thermal efficiency and overall combustion noise were 50.5% and 86.4 dBA, respectively. Furthermore, the optimum number of fuel injections and heat release shape of multiple fuel injections to achieve lower combustion noise while maintaining the higher indicated thermal efficiency were calculated in Test 4. The results suggest that the constant pressure combustion after the top dead center by multiple fuel injections is the better way to lower combustion noise; however, the excess fuel injected leads to a lower indicated thermal efficiency because the degree of constant volume becomes deteriorates.