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In this work, the authors assess the potential of the 2-stroke concept applied to Range Extender engines, proposing 3 different configurations: 1) Supercharged, Compression Ignition; 2) Turbocharged, Compression Ignition; 3) Supercharged, Gasoline Direct Injection. All the engines feature a single power cylinder of 0.49l, external air feed by piston pump and an innovative induction system. The scavenging is of the Loop type, without poppet valves, and with a 4-stroke like lubrication system (no crankcase pump). Engine design has been supported by CFD simulations, both 1D (engine cycle analysis) and 3D (scavenging, injection and combustion calculations). All the numerical models used in the study are calibrated against experiments, carried out on engines as similar as possible to the proposed ones.

The paper describes a numerical study, supported by experiments, on light aircraft 2-Stroke Direct Injected Diesel engines, typically rated up to 110 kW (corresponding to about 150 imperial HP). The engines must be as light as possible and they are to be directly coupled to the propeller, without reduction drive. The ensuing main design constraints are: i) in-cylinder peak pressure as low as possible (typically, no more than 120 bar); ii) maximum rotational speed limited to 2600 rpm. As far as exhaust emissions are concerned, piston aircraft engines remain unregulated but lack of visible smoke is a customer requirement, so that a value of 1 is assumed as maximum Smoke number. For the reasons clarified in the paper, only three cylinder in line engines are investigated. Reference is made to two types of scavenging and combustion systems, designed by the authors with the assistance of state-of-the-art CFD tools and described in detail in a parallel paper.

Engine downsizing is gaining popularity in the high performance engine market sector, where a new generation of highly downsized engines with specific power outputs around or above 150 HP/litre is emerging. High-boost and downsizing, adopted to increase power density and reduce fuel consumption, have to face the increased risks of pre-ignition, knock or mega-knock. To counterbalance autoignition of fuel/air mixture, such engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter reduces performance and induces an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC.

Two-stroke diesel engines could be a promising solution for reducing carbon dioxide (CO2) emissions from light-duty vehicles. The main objective of this study was to assess the potential of two-stroke engines in achieving a substantial reduction in CO2 emissions compared to four-stroke diesel baselines. As part of this study 1-D models were developed for loop scavenged two-stroke and opposed piston two-stroke diesel engine concepts. Based on the engine models and an in-house vehicle model, projections were made for the CO2 emissions for a representative light-duty vehicle over the New European Driving Cycle and the Worldwide Harmonized Light Vehicles Test Procedure. The loop scavenged two-stroke engine had about 5-6% lower CO2 emissions over the two driving cycles compared to a state of the art four-stroke diesel engine, while the opposed piston diesel engine had about 13-15% potential benefit.

Hybridization is a mainstream technology for automobiles, and its application is rapidly expanding in other fields. Marine propulsion is one such field that could benefit from electrification of the powertrain. In particular, for boats to sail in enclosed waterways, such as harbors, channels, lagoons, a pure electric mode would be highly desirable. The main challenge to accomplish hybridization is the additional weight of the electric components, in particular the batteries. The goal of this project is to replace a conventional 4-stroke turbocharged Diesel engine with a hybrid powertrain, without any penalty in terms of weight, overall dimensions, fuel efficiency, and pollutant emissions. This can be achieved by developing a new generation of 2-Stroke Diesel engines, and coupling them to a state-of-the art electric system. For the thermal units, two alternative designs without active valve train are considered: opposed piston and loop scavenged engines.

In recent years, interest has been growing in the 2-Stroke Diesel cycle, coupled to high speed engines. One of the most promising applications is on light aircraft piston engines, typically designed to provide a top brake power of 100-200 HP with a relatively low weight. The main advantage yielded by the 2-Stroke cycle is the possibility to achieve high power density at low crankshaft speed, allowing the propeller to be directly coupled to the engine, without a reduction drive. Furthermore, Diesel combustion is a good match for supercharging and it is expected to provide a superior fuel efficiency, in comparison to S.I. engines. However, the coupling of 2-Stroke cycle and Diesel combustion on small bore, high speed engines is quite complex, requiring a suitable support from CFD simulation.

The paper aims at defining a methodology for the prediction and understanding of knock tendency in internal combustion engine piston crevices by means of CFD simulations. The motivation for the analysis comes from a real design requirement which appeared during the development of a new high performance SI unit: it is in fact widely known that, in high performance engines (especially the turbocharged ones), the high values of pressure and temperature inside the combustion chamber during the engine cycle may cause knocking phenomena. “Standard” knock can be easily recognized by direct observation of the in-cylinder measured pressure trace; it is then possible to undertake proper actions and implement design and control improvements to prevent it with relatively standard 3D-CFD analyses.

The occurrence of knock is the most limiting hindrance for modern Spark-Ignition (SI) engines. In order to understand its origin and move the operating condition as close as possible to onset of this potentially harmful phenomenon, a joint experimental and numerical investigation is the most recommended approach. A preliminary experimental activity was carried out at IM-CNR on a 0.4 liter GDI unit, equipped with a flat transparent piston. The analysis of flame front morphology allowed to correlate high levels of flame front wrinkling and negative curvature to knock prone operating conditions, such as increased spark timings or high levels of exhaust back-pressure. In this study a detailed CFD analysis is carried out for the same engine and operating point as the experiments. The aim of this activity is to deeper investigate the reasons behind the main outcomes of the experimental campaign.

In spark-ignited direct-injected engines, the formation of fuel pools on the piston is one of the major promoters of unburnt hydrocarbons and soot: in order to comply with the increasingly stringent emission regulations (EU6 and forthcoming), it is therefore necessary to limit fuel deposit formation. The combined use of advanced experimental techniques and detailed 3D-CFD simulations can help to understand the mechanisms driving fuel pool formation. In the paper, a combined experimental and numerical characterization of pool formation in a GDI engine is carried out to investigate and understand the complex interplay of all the mentioned factors. In particular, a low-load low-rpm engine operation is investigated for different ignition phasing, and the impact of both fuel formulation and instantaneous piston temperature variations in the CFD analyses are evaluated.

The paper aims at providing information about the influence of the combustion chamber shape on the combustion process evolution in a high speed direct injection (HSDI) small unit displacement engine for off-highway applications. Small HSDI Diesel engines require a deep optimisation process in order to maximize specific power output, while limiting pollutant emissions without additional expensive pollutant aftertreatment equipments. Making reference to a current production engine, the purpose of this paper is to investigate the influence of combustion chamber design on both engine performances and combustion efficiency. The actual piston omega-shape is progressively distorted in order to assess the influence of some of the main bowl-features on both mean-flow evolution, mixture formation and pollutants.

Enhanced calibration strategies and innovative engine combustion technologies are required to meet the new limits on exhaust gas emissions enforced in the field of marine propulsion and on-board energy production. The goal of the paper is to optimize the control parameters of a 4.2 dm3 unit displacement marine DI Diesel engine, in order to enhance the efficiency of the combustion system and reduce engine out emissions. The investigation is carried out by means of experimental tests and CFD simulations. For a better control of the testing conditions, the experimental activity is performed on a single cylinder prototype, while the engine test bench is specifically designed to simulate different levels of boosting. The numerical investigations are carried out using a set of different CFD tools: GT-Power for the engine cycle analysis, STAR-CD for the study of the in-cylinder flow, and a customized version of the KIVA-3V code for combustion.

The paper presents a numerical investigation, aimed to explore the potential of 2-stroke Diesel engines, able to meet Euro VI requirements, for application to medium size commercial vehicles (power rate: 80 kW at 2600 rpm, max. torque 420 Nm from 1200 to 1400 rpm). The study is based on experimental performance of a highly developed 4-stroke engine. Two different designs are considered: Loop and Uniflow scavenging, the latter obtained through an opposed piston configuration. In both cases, no poppet valves are used, and the lubrication is provided by a 4-stroke-like oil sump. The study started with the development of a 4-stroke EURO VI engine, on the basis of a previous EURO IV version. A prototype of the new engine (named 430) was built and tested.

2-stage supercharging applied to HSDI Diesel engines appears a promising solution for enhancing rated power, low end torque, transient response and hence the launch characteristics of a vehicle. However, many open points still remain, in particular about the impact on emissions control and fuel economy at partial load conditions, generally requiring both high airflow and high EGR rates. The paper analyzes and compares two types of 2-stage supercharging systems: a) two turbochargers of different size; b) one turbocharger coupled to a positive displacement compressor. The goal of the paper is to assess pro and cons of the most feasible configurations for a typical automobile Diesel engine, complying with Euro V regulations and beyond. The base engine is the 2.8L, 4 cylinder in-line unit produced by VM Motori (Cento, Italy), equipped by a standard variable geometry turbocharger.

The paper compares two different design concepts for a range extender engine rated at 30 kW at 4500 rpm. The first project is a conventional 4-Stroke SI engine, 2-cylinder, 2-valve, equipped with port fuel injection. The second is a new type of 2-Stroke loop scavenged SI engine, featuring a direct gasoline injection and a patented rotary valve for enhancing the induction and scavenging processes. Both power units have been virtually designed with the help of CFD simulation. Moreover, for the 2-Stroke engine, a prototype has been also built and tested at the dynamometer bench, allowing the authors to make a reliable theoretical comparison with the well assessed 4-Stroke unit.

The internal combustion engine (ICE) for a series hybrid vehicle must be very compact, fuel efficient reliable and clean; furthermore it should possess excellent NVH features; finally, the cost should be as low as possible. An unconventional but not exotic solution, potentially ideal to fulfill all the above mentioned requirements, is represented by a 2-Stroke externally scavenged GDI engine, without poppet valves. BRC (Cherasco, Italy) and PRIMAVIS (Turin, Italy) are currently developing an engine of this type, incorporating a patented rotary valve for the control of the charge induced to cylinder. The development is supported by extensive CFD simulations, which are able to predict all the main engine performance characteristics. The paper analyzes, from a theoretical point of view, the installation of the engine on an electric vehicle, previously optimized for a small Diesel engine (Smart 0.8 l CDi).

The trend of powertrain electrification is quickly spreading from the automotive field into many other sectors. For ultra-light aircraft, needing a total installed propulsion power up to 150 kW, the combination of a specifically developed internal combustion engine (ICE) integrated with a state-of-the-art electric system (electric motor, inverter and battery) appears particularly promising. The dimensions and weight of ICE can be strongly reduced (downsizing), so that it can operate at higher efficiency at typical cruise conditions; a large power reserve is available for emergency maneuvers; in comparison to a full electric airplane, the hybrid powertrain makes possible to fly at zero emissions for a much longer time, or with a much heavier payload. On the other hand, the packaging of a hybrid powertrain into existing aircraft requires a specific design of the thermal engine, that must be light, compact, highly reliable and fuel efficient.

The detailed study of part-load conditions is essential to characterize engine-out emissions in key operating conditions. The relevance of part-load operations is further emphasized by the recent regulations such as the new WLTP standard. Combustion development at part-load operations depends on a complex interplay between moderate turbulence levels (low engine speed and tumble ratio), low in-cylinder pressure and temperature, and stoichiometric-to-lean mixture quality (to maximize fuel efficiency). From a modelling standpoint, the reduced turbulence intensity compared to full-load operations complicates the interaction between different sub-models (e.g., reconsideration of the flamelet hypothesis adopted by common combustion models). In this article, the authors focus on chemistry-based simulations for laminar flame speed of gasoline surrogates at conditions typical of part-load operations. The analysis is an extension of a previous study focused on full-load operations.

The paper reviews the theoretical and experimental development of the engine powering the 2011 Formula SAE single seater of the University of Modena and Reggio Emilia (UNIMORE). The general design criteria followed by the UNIMORE team are discussed and compared to those chosen by other competitors. In particular, the reasons supporting the selection of the engine type (single cylinder by Husqvarna) are explained in details. The adoption of a single cylinder, instead of the more powerful four-in-line, required a much bigger effort for getting an acceptable level of brake power. Therefore, the development was massively supported by CFD simulation (both 1D and 3D) and by experiments. It was found that the most important design areas for the single cylinder are: the intake system, including the restrictor (20 mm), the intake runner and the plenum, and the muffler.

One of the main challenges related to the use of Hydrogen in Internal Combustion Engines is the trade-off between NOx emissions and brake power output: on the one hand, a lean premixed charge (Lambda ≈2.5) is generally able to provide a regular and efficient combustion, yielding near-zero NOx emissions; on the other hand, the power density tends to be very poor, due to the huge amount of air required by the thermodynamic process. As a further penalization, the injection of a gaseous fuel during the intake process has a negative impact on volumetric efficiency. Supercharging can be a solution for addressing the problem, but at the cost of an increase of complexity, cost and overall dimensions. An alternative path is represented by the 2-stroke cycle, and, in particular, by the opposed piston (OP) design. Most of the existing OP engines are compression ignited, but Spark ignition and direct fuel injection can be implemented without relevant modifications to the layout of cylinders.

The paper presents a preliminary study on a virtual 2-stroke 3-cylinder 0.9 L DI SI supercharged engine running on Hydrogen (H2), able to meet both high performance targets and ultra-low emissions limits (NOx<20 ppm). Combustion is similar to a conventional 4-stroke H2 DI engine, while the design of the cylinder and the actuation law of both intake and exhaust valves are specifically optimized for the 2-stroke cycle. In comparison to a more conventional 2-stroke loop scavenged engine, with piston-controlled ports, the use of poppet valves enables a more flexible control of the gas exchange process and to maintain the same design of a 4-stroke engine for pistons, cylinders block, crankcase and lubrication system. On the other hand, it is more difficult to avoid the short-circuit of the fresh charge, while permeability of the valves becomes quite critical at high engine speed.