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Emissions regulations for engine and vehicle manufacturers are bound to become more limiting to prevent greenhouse gas emissions and mitigate the negative effects that potentiate global warming. To fulfill the energy demand necessary in the transportation sector for the short-to-medium term, a parallel optimization of the internal combustion engine, powertrain and fuels is necessary. The combination of novel combustion modes like the reactivity-controlled compression ignition (RCCI), that seeks the benefits of both compression ignition and spark ignition engines, with the so-called e-fuels, that reduce the carbon footprint from well-to-wheel, is worth exploring. This work investigates the potential of the RCCI concept using OMEx-gasoline to reduce the engine-out emissions beyond the current EURO VI legislation. To do so, eight representative operating conditions from several driving cycles for heavy-duty vehicles will be explored experimentally.

CO2 is a greenhouse gas that is believed to be one of the main contributors to global warming. Recent studies show that a combination of methanol as a renewable fuel and advanced combustion concepts could be a promising future solution to alleviate this problem. However, high unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions can be stated as the main drawback in low load operations when using methanol under advanced combustion concepts. This issue can be mitigated by modifying the stratification of the local equivalence ratio to achieve a favorable level. The stratifications evolved, and the regimes that can simultaneously produce low emissions, and high combustion efficiency can be identified by sweeping the injection timing from homogeneous charge compression ignition (HCCI) to partially premixed combustion (PPC). Understanding how the stratification of the local equivalence ratio for methanol evolves during the sweep is essential to gain these benefits.

In the last two decades engine research has been mainly focused on reducing pollutant emissions. This fact together with growing awareness about the impacts of climate change are leading to an increase in the importance of thermal efficiency over other criteria in the design of internal combustion engines (ICE). In this framework, the heat transfer to the combustion chamber walls can be considered as one of the main sources of indicated efficiency diminution. In particular, in modern direct-injection diesel engines, the radiation emission from soot particles can constitute a significant component of the efficiency losses. Thus, the main of objective of the current research was to evaluate the amount of energy lost to soot radiation relative to the input fuel chemical energy during the combustion event under several representative engine loads and speeds. Moreover, the current research characterized the impact of different engine operating conditions on radiation heat transfer.

With ever-demanding emission legislations in Compression Ignition (CI) engines, new premixed combustion strategies have been developed in recent years seeking both, emissions and performance improvements. Since it has been shown that in-cylinder air flow affects the combustion process, and hence the overall engine performance, the study of swirling structures and its interaction with fuel injection are of great interest. In this regard, possible Turbulent Kinetic Energy (TKE) distribution changes after fuel injection may be a key parameter for achieving performance improvements by reducing in-cylinder heat transfer. Consequently, this paper aims to gain an insight into spray-swirl interaction through the analysis of in-cylinder velocity fields measured by Particle Image Velocimetry (PIV) when PCCI conditions are proposed. Experiments are carried out in a single cylinder optical Diesel engine with bowl-in-piston geometry.

This work investigates the particulates size distribution of reactivity controlled compression ignition combustion, a dual-fuel concept which combines the port fuel injection of low-reactive/gasoline-like fuels with direct injection of highly reactive/diesel-like fuels. The particulates size distributions from 5-250 nm were measured using a scanning mobility particle sizer at six engine speeds, from 950 to 2200 rpm, and 25% engine load. The same procedure was followed for conventional diesel combustion. The study was performed in a single-cylinder engine derived from a stock medium-duty multi-cylinder diesel engine of 15.3:1 compression ratio. The combustion strategy proposed during the tests campaign was limited to accomplish both mechanical and emissions constraints. The results confirms that reactivity controlled compression ignition promotes ultra-low levels of nitrogen oxides and smoke emissions in the points tested.

Gas engines (fuelled with CNG, LNG or Biogas) for generation of power and heat are, to this date, taking up larger shares of the market with respect to diesel engines. In order to meet the limit imposed by the TA-Luft regulations on stationary engines, lean combustion represents a viable solution for achieving lower emissions as well as efficiency levels comparable with diesel engines. Leaner mixtures however affect the combustion stability as the flame propagation velocity and consequently heat release rate are slowed down. As a strategy to deliver higher ignition energy, an active pre-chamber may be used. This work focuses on assessing the performance of a pre-chamber combustion configuration in a stationary heavy-duty engine for power generation, operating at different loads, air-to-fuel ratios and spark timings.

There is growing global interest in using renewable alcohols to reduce the greenhouse gases and the reliance on conventional fossil fuels. Recent studies show that methanol combined with partially premixed combustion provide clear performance and emission benefits compared to conventional diesel diffusion combustion. Nonetheless, high unburned hydrocarbon (HC) and carbon monoxide (CO) emissions can be stated as the main PPC drawback in light load condition when using high octane fuel such as Methanol with single injection strategy. Thus, the present experimental study has been carried out to investigate the influence of multiple injection strategies on the performance and emissions with methanol fuel in partially premixed combustion. Specifically, the main objective is to reduce HC, CO and simultaneously increase the gross indicated efficiency compared to single injection strategy.

Concepts of premixed diesel Low Temperature Combustion (LTC) have been shown to be advantageous in greatly reducing engine-out nitrogen oxide (NOx) and particulate matter (PM) emissions, even below the minimum detection limit of standard opacity-based PM mass instruments. Previous research has revealed that significant changes to the PM size and number emissions still occur for changes to the LTC engine operating conditions. This work investigates the influence of reductions in intake oxygen concentration on PM (mass, size, and number), NOx, hydrocarbon (HC), and carbon monoxide (CO) emissions from select LTC engine operating conditions. Exhaust particle size distributions were measured for multiple engine operating conditions of premixed diesel LTC within a range of five intake oxygen concentrations from 9% to 13% (by volume) at three intake pressures from 1.325 to 1.6 bar.

The relationship between ignition, lift-off length and soot formation was investigated for a collection of fuels in an optically-accessible modified 2-stroke engine under a set of typical quasi-steady state Diesel DI conditions. Five fuels including biodiesel blends and Fischer-Tropsch fuels have been selected for their potential to substitute conventional diesel with no major modifications on the engine hardware, and were previously characterized under ambient pressure following ASTM standards. Fuels were injected into a large volume through a single-hole nozzle at three levels of injection pressure, by sweeping ambient temperatures at constant density, and ambient densities at constant temperature. The 8 ms single-shot injections were long enough to reach the stabilization of a free diffusion flame. The OH-chemiluminescence was imaged and lift-off length was measured via image post-processing.

Most of the new Diesel combustion concepts are mainly based on reducing local combustion temperatures and enhancing the fuel/air mixing with the aim of simultaneously reducing soot and NOx emissions. In this framework, Premixed Charge Compression Ignition (PCCI) has revealed as one of the best options to combine both low emissions and good combustion controllability. During last years, PCCI strategy has been widely explored using high EGR levels and different early or late injection timings to extend the ignition delay. Recently, the use of lower cetane fuels is under investigation. Despite the great quantity of research work performed, there are still some aspects related to PCCI combustion that are not completely well known. In this paper an experimental and numerical study is carried out focused on understanding the mixing and auto-ignition processes in PCCI combustion conditions using Diesel and Gasoline fuels.

Visualization of single-hole nozzles into quiescent ambient has been used extensively in the literature to characterize spray mixing and combustion. However in-cylinder flow may have some meaningful impact on the spray evolution. In the present work, visualization of direct diesel injection spray under both non-reacting and reacting operating conditions was conducted in an optically accessible two-stroke engine equipped with a single-hole injector. Two different high-speed imaging techniques, Schlieren and UV-Light Absorption, were applied here to quantify vapor penetration for non-reacting spray. Meanwhile, Mie-scattering was used to measure the liquid length. As for reacting conditions, Schlieren and OH* chemiluminescence were simultaneously applied to obtain the spray tip penetration and flame lift-off length under the same TDC density and temperature. Additionally, PIV was used to characterize in-cylinder flow motion.

Dual-mode dual-fuel combustion is a promising combustion concept to achieve the required emissions and CO2 reductions imposed by the next standards. Nonetheless, the fuel formulation requirements are stricter than for the single-fuel combustion concepts as the combustion concept relies on the reactivity of two different fuels. This work investigates the effect of the low reactivity fuel sensitivity (S=RON-MON) and the octane number at different operating conditions representative of the different combustion regimes found during the dual-mode dual-fuel operation. For this purpose, experimental tests were performed using a PRF 95 with three different sensitivities (S0, S5 and S10) at operating conditions of 25% load/950 rpm, 50%/1800 rpm and 100%/2200 rpm. Moreover, air sweeps varying ±10% around a reference air mass were performed at 25%/1800 rpm and 50%/1800 rpm. Conventional diesel fuel was used as high reactivity fuel in all the cases.

A comprehensive study is carried out in order to better understand combustion behavior in a direct injection Diesel engine working under multiple injection strategies, in particular when using post-injections. The aim of the study is to provide criteria to more easily define optimized injection strategies. During the study two main phenomena have been observed and characterized: an acceleration of the final stage of combustion and an apparent disconnection between the combustions of the two pulses (“split flame”). Thanks to the combustion acceleration phenomenon, if the post-injection is placed near enough the main injection, the end of combustion can take place even earlier compared to the case with a single main injection. In such conditions NOx emissions increase (most likely due to a higher temperature level during the last stage of combustion), but soot and specific fuel consumption decrease (due to a faster last phase of combustion).

Traditionally the NOx prediction models use to take into account exclusively the NOx formation process. Nowadays diesel engines use to operate with high EGR rates and high fuel/air equivalence ratios, in opposition with what it was standard in the past years. In such conditions a considerable amount of NOx generated in the previous cycle (coming from the recirculated exhaust gases) or in the previous combustion is re-entrained by the flame, where a highly reducing region exists (due to a lack of oxygen and the presence of hydrocarbons and a high temperature level). Face to this fact a question arises: what happens with these re-entrained NOx?

In this paper the effect of the engine load and the EGR (exhaust gas recirculation) rate on the combustion process and the pollutant emissions when using RME (rapeseed methyl-ester) is investigated. For this purpose a parametric study in a single-cylinder HSDI (high speed direct injection) engine in a wide range of operating conditions (thus trying to maximize the generality of the results) has been carried out. All the output parameters are compared with the corresponding ones for a reference diesel fuel at equivalent engine performances and operating conditions. To perform a rigorous comparison, a specific methodology has been designed based on the comparison at equivalent engine load and oxygen mass fraction in the intake manifold, so as to remove the effect of the fuel properties (derived from the different oxygen content, mainly) on the engine performances.

Despite the development in NOx aftertreatment for Diesel engines, EGR is a cost-effective solution to fulfill current and future emission regulations. There is a wide bibliography discussing the global effects of EGR on combustion and emissions. However, little has been published concerning the effects of the unsuitable EGR and air distribution among cylinders. Since current HSDI engines operate with EGR rates as high as 50% the effect of the unequal EGR distribution becomes important. In addition, cylinder-to-cylinder charge dispersion becomes a critical aspect on the control of low temperature combustion systems. In concordance with the aspects outlined before, the aim of this paper is to study the effects of the EGR cylinder to cylinder distribution on the engine performance and emissions. To cope with this objective, experiments have been conducted in a HSDI engine with two different EGR systems.

Post injections are a commonly used strategy to reduce soot and NOx emissions in DI diesel engines. This strategy has been widely explored and studied for several years, however, there are still some aspects of the behavior of the combustion process when it is used that are not completely well known. In this paper a numerical study is carried out in order to better understand the improvement on mixing/combustion phenomena using post injection compared with a single injection case. For this purpose, CFD simulations using commercial code Star CD were performed in realistic engine conditions: the combustion of a single injection case (Pilot + Main injection) was compared with that of post injection case (Pilot + Main + Post) evaluating different post injection timings and comparing the simulated results with previously developed experimental tests which reveal the mentioned improvement on combustion behavior.

The purpose of the study reported in this paper was to exploit the possibility of adjusting some injection parameters in a diesel engine fitted with a common-rail injection system with the final goal of reducing pollutant emissions. Starting from the original settings, several injection parameters like nozzle hole diameter, injection pressure and injection duration, were adjusted following three different injection strategies, trying to produce some specific fuel spray patterns (spray penetration and cone angle, air entrainment, etc). Additionally, boost pressure was modified, in order to control spray-air interaction, and EGR was introduced to achieve the required NOx reduction. The adjusted injection setting allowed to generate starting values in pollutants emissions very tolerant to EGR, in such a way that the achieved reduction of NOx was not frustrated by an excessive increase in PM emissions.

The main objective of the study described in this paper is to explore the potential of different post-injection patterns, with a plain common rail system, for reduction of soot emissions in HD diesel engines. Test have been carried out in a single-cylinder engine at several critical engine operation points from the European Steady state test Cycle (ESC). At these operation points, EGR was introduced to reduce NOx emissions to a given value, and then different post-injection patterns were produced. A parametric study was performed, considering the time between injections and the post-injected fuel mass as the main variables. In every case the total injected fuel mass was kept constant. Aside from the experimental data obtained in the engine tests, a diagnosis model was applied to calculate heat release laws and other parameters depicting the combustion process.

In the past few years’ various studies have shown how the application of a highly premixed dual fuel combustion for CI engines leads a strong reduction for both pollutant emissions and fuel consumption. In particular a drastic soot and NOx reduction were achieved. In spite of the most common strategy for dual fueling has been represented by using two different injection systems, various authors are considering the advantages of using a single injection system to directly inject blends in the chamber. In this scenario, a characterization of the behavior of such dual-fuel blend spray became necessary, both in terms of inert and reactive ambient conditions. In this work, a light extinction imaging (LEI) has been performed in order to obtain two-dimensional soot distribution information within a spray flame of different diesel/gasoline commercial fuel blends. All the measurements were conducted in an optically accessible two-stroke engine equipped with a single-hole injector.