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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 2014 change in Formula One regulations, from naturally aspirated to highly-downsized and heavily-boosted hybridized power units, led to a relevant increase of the internal combustion engine brake specific power output in comparison with former V-8 units. The newly designed “down-sized” engines are characterized by a fuel flow limitation and a relevant increase in the thermal loads acting on the engine components, in particular on those facing the combustion chamber. Furthermore, efficiency becomes an equivalent paradigm as performance. In the power unit layout, the air path is defined by the compressor, the intercooler and the piping from the intake plenum to the cylinder. Intake duct length is defined from intake plenum to valve seat and it is a key parameter for engine performance. In order to find the optimum length different design criteria can be applied: the so called “tuning”, the “un-tuning” or the “anti-tuning” are all valid possibilities, showing pros and cons.

Engine knock is emerging as the main limiting factor for modern spark-ignition (SI) engines, facing increasing thermal loads and seeking demanding efficiency targets. To fulfill these requirements, the engine operating point must be moved as close as possible to the onset of abnormal combustion events. The turbulent regime characterizing in-cylinder flows and SI combustion leads to serious fluctuations between consecutive engine cycles. This forces the engine designer to further distance the target condition from its theoretical optimum, in order to prevent abnormal combustion to severely damage the engine components just because of few individual heavy-knocking cycles. A RANS-based model is presented in this study, which is able to predict not only the ensemble average knock occurrence but also a knock probability. This improves the knock tendency characterization, since the mean knock onset alone is a poorly meaningful indication in a stochastic event such as engine knock.

A new generation of highly downsized SI engines with specific power output around or above 150 HP/liter is emerging in the sport car market sector. Technologies such as high-boosting, direct injection and downsizing are adopted to increase power density and reduce fuel consumption. To counterbalance the increased risks of pre-ignition, knock or mega-knock, currently made turbocharged SI 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 induce an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A possible solution to increase knock resistance is investigated in the paper by means of 3D-CFD analyses: water/methanol emulsion is port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins.

Control of the timing and magnitude of heat release is one of the biggest challenges for premixed compression ignition, especially when attempting to operate at high load. Single-fuel strategies such as partially premixed combustion (PPC) use direct injection of gasoline to stratify equivalence ratio and retard heat release, thereby reducing pressure rise rate and enabling high load operation. However, retarding the heat release also reduces the maximum work extraction, effectively creating a tradeoff between efficiency and noise. Dual-fuel strategies such as reactivity controlled compression ignition (RCCI) use premixed gasoline and direct injection of diesel to stratify both equivalence ratio and fuel reactivity, which allows for greater control over the timing and duration of heat release. This enables combustion phasing closer to top dead center (TDC), which is thermodynamically favorable.

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.

The present experimental engine efficiency study explores the effects of intake pressure and temperature, and premixed and global equivalence ratios on gross thermal efficiency (GTE) using the reactivity controlled compression ignition (RCCI) combustion strategy. Experiments were conducted in a heavy-duty single-cylinder engine at constant net load (IMEPn) of 8.45 bar, 1300 rev/min engine speed, with 0% EGR, and a 50% mass fraction burned combustion phasing (CA50) of 0.5°CA ATDC. The engine was port fueled with E85 for the low reactivity fuel and direct injected with 3.5% 2-ethylhexyl nitrate (EHN) doped into 91 anti-knock index (AKI) gasoline for the high-reactivity fuel. The resulting reactivity of the enhanced fuel corresponds to an AKI of approximately 56 and a cetane number of approximately 28. The engine was operated with a wide range of intake pressures and temperatures, and the ratio of low- to high-reactivity fuel was adjusted to maintain a fixed speed-phasing-load condition.

Reactivity Controlled Compression Ignition (RCCI) is an engine combustion strategy that utilizes in-cylinder fuel blending to produce low NOx and PM emissions while maintaining high thermal efficiency. The current study investigates RCCI and conventional diesel combustion (CDC) operation in a light-duty multi-cylinder engine over transient operating conditions using a high-bandwidth, transient capable engine test cell. Transient RCCI and CDC combustion and emissions results are compared over an up-speed change from 1,000 to 2,000 rev/min. and a down-speed change from 2,000 to 1,000 rev/min. at a constant 2.0 bar BMEP load. The engine experiments consisted of in-cylinder fuel blending with port fuel-injection (PFI) of gasoline and early-cycle, direct-injection (DI) of ultra-low sulfur diesel (ULSD) for the RCCI tests and the same ULSD for the CDC tests.

Previous work has demonstrated the capabilities of gasoline compression ignition to achieve engine loads as high as 19.5 bar BMEP with a production multi-cylinder diesel engine using gasoline with an anti-knock index (AKI) of 87. In the current study, the low load limit of the engine was investigated using the same engine hardware configurations and 87 AKI fuel that was used to achieve 19.5 bar BMEP. Single injection, “minimum fueling” style injection timing and injection pressure sweeps (where fuel injection quantity was reduced at each engine operating condition until the coefficient of variance of indicated mean effective pressure rose to 3%) found that the 87 AKI test fuel could run under stable combustion conditions down to a load of 1.5 bar BMEP at an injection timing of −30 degrees after top dead center (°aTDC) with reduced injection pressure, but still without the use of intake air heating or uncooled EGR.

In order to improve fuel conversion efficiency, currently made spark-ignited engines are characterized by the adoption of gasoline direct injection, supercharging and/or turbocharging, complex variable valve actuation strategies. The resulting increase in power/size ratios is responsible for substantially higher average thermal loads on the engine components, which in turn result in increased risks of both thermo-mechanical failures and abnormal combustion events such as surface ignition or knock. The paper presents a comprehensive numerical methodology for the accurate estimation of knock tendency of SI engines, based on the integration of different modeling frameworks and tools. Full-cycle in-cylinder analyses are used to estimate the point-wise heat flux acting on the engine components facing the combustion chamber.

The paper reports the application of Proper Orthogonal Decomposition (POD) to LES calculations for the analysis of combustion and knock tendency in a highly downsized turbocharged GDI engine that is currently under production. In order to qualitatively match the cyclic variability of the combustion process, Large-Eddy Simulation (LES) of the closed-valve portion of the cycle is used with cycle-dependent initial conditions from a previous multi-cycle analysis [1, 2, 3]. Detailed chemical modelling of fuel's auto-ignition quality is considered through an ad-hoc implemented look-up table approach, as a trade-off between the need for a reasonable representation of the chemistry and that of limiting the computational cost of the LES simulations. Experimental tests were conducted operating the engine at knock-limited spark advance (KLSA) and the proposed knock model was previously validated for such engine setup [3].

Due to concerns regarding pollutant and CO2 emissions, advanced combustion modes that can simultaneously reduce exhaust emissions and improve thermal efficiency have been widely investigated. The main characteristic of the new combustion strategies, such as HCCI and LTC, is that the formation of a homogenous mixture or a controllable stratified mixture is required prior to ignition. The major issue with these approaches is the lack of a direct method for the control of ignition timing and combustion rate, which can be only indirectly controlled using high EGR rates and/or lean mixtures. Homogeneous Charge Progressive Combustion (HCPC) is based on the split-cycle principle. Intake and compression phases are performed in a reciprocating external compressor, which drives the air into the combustor cylinder during the combustion process, through a transfer duct. A transfer valve is positioned between the compressor cylinder and the transfer duct.

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 paper reports the application of a look-up table approach within a LES combustion modelling framework for the prediction of knock limit in a highly downsized turbocharged DISI engine. During experimental investigations at the engine test bed, high cycle-to-cycle variability was detected even for relatively stable peak power / full load operations of the engine, where knock onset severely limited the overall engine performance. In order to overcome the excessive computational cost of a direct chemical solution within a LES framework, the use of look-up tables for auto-ignition modelling perfectly fits with the strict mesh requirements of a LES simulation, with an acceptable approximation of the actual chemical kinetics. The model here presented is a totally stand-alone tool for autoignition analysis integrated with look-up table reading from detailed chemical kinetic schemes for gasoline.

This study uses Fourier analysis to investigate the relationship between the heat release event and the frequency composition of pressure oscillations in a variety of combustion modes. While kinetically-controlled combustion strategies such as HCCI and RCCI offer advantages over CDC in terms of efficiency and NOX emissions, their operational range is limited by audible knock and the possibility of engine damage stemming from high pressure rise rates and oscillations. Several criteria such as peak pressure rise rate, ringing intensity, and various knock indices have been developed to quantify these effects, but they fail to capture all of the dynamics required to form direct comparisons between different engines or combustion strategies. Experiments were performed with RCCI, HCCI, and CDC on a 2.44 L heavy-duty engine at 1300 RPM, generating a significant diversity of heat release profiles.

Advanced combustion systems that simultaneously address PM and NOx while retaining the high efficiency of modern diesel engines, are being developed around the globe. One of the most difficult problems in the area of advanced combustion technology development is the control of combustion initiation and retaining power density. During the past several years, significant progress has been accomplished in reducing emissions of NOx and PM through strategies such as LTC/HCCI/PCCI/PPCI and other advanced combustion processes; however control of ignition and improving power density has suffered to some degree - advanced combustion engines tend to be limited to the 10 bar BMEP range and under. Experimental investigations have been carried out on a light-duty DI multi-cylinder diesel automotive engine. The engine is operated in low temperature combustion (LTC) mode using 93 RON (Research Octane Number) and 74 RON fuel.

The paper presents the development of a novel approach to the solution of detailed chemistry in internal combustion engine simulations, which relies on the analytical computation of the ordinary differential equations (ODE) system Jacobian matrix in sparse form. Arbitrary reaction behaviors in either Arrhenius, third-body or fall-off formulations can be considered, and thermodynamic gas-phase mixture properties are evaluated according to the well-established 7-coefficient JANAF polynomial form. The current work presents a full validation of the new chemistry solver when coupled to the KIVA-4 code, through modeling of a single cylinder Caterpillar 3401 heavy-duty engine, running in two-stage combustion mode.

An investigation of high speed direct injection (DI) compression ignition (CI) engine combustion fueled with gasoline injected using a triple-pulse strategy in the low temperature combustion (LTC) regime is presented. This work aims to extend the operation ranges for a light-duty diesel engine, operating on gasoline, that have been identified in previous work via extended controllability of the injection process. The single-cylinder engine (SCE) was operated at full load (16 bar IMEP, 2500 rev/min) and computational simulations of the in-cylinder processes were performed using a multi-dimensional CFD code, KIVA-ERC-Chemkin, that features improved sub-models and the Chemkin library. The oxidation chemistry of the fuel was calculated using a reduced mechanism for primary reference fuel combustion chosen to match ignition characteristics of the gasoline fuel used for the SCE experiments.

Many recent studies have shown that the Reactivity Controlled Compression Ignition (RCCI) combustion strategy can achieve high efficiency with low emissions. However, it has also been revealed that RCCI combustion is difficult at high loads due to its premixed nature. To operate at moderate to high loads with gasoline/diesel dual fuel, high amounts of EGR or an ultra low compression ratio have shown to be required. Considering that both of these approaches inherently lower thermodynamic efficiency, in this study natural gas was utilized as a replacement for gasoline as the low-reactivity fuel. Due to the lower reactivity (i.e., higher octane number) of natural gas compared to gasoline, it was hypothesized to be a better fuel for RCCI combustion, in which a large reactivity gradient between the two fuels is beneficial in controlling the maximum pressure rise rate.

In automotive industry it has been a challenge to retain diesel-like thermal efficiency while maintaining low emissions. Numerous studies have shown significant progress in achieving low emissions through the introduction of common-rail injection systems, multiple injections and exhaust gas recirculation and by using a high octane number fuel, like gasoline, to achieve adequate premixing. On the other hand, low temperature combustion strategies, like HCCI and PCCI, have also shown promising results in terms of reducing both NOx and soot emissions simultaneously. With the increasing capacity of computers, multi-dimensional CFD engine modeling enables a reasonably good prediction of combustion characteristics and pollutant emissions, which is the motivation behind the present research. The current research effort presents an optimization study of light-duty compression ignition engine performance, while meeting the emission regulation targets.