Controlled Auto-ignition Combustion Research Articles

Prospects of Controlled Auto-Ignition Based Thermal Propulsion Units for Modern Gasoline Vehicles

Gasoline engines employing the spatially distributed auto-ignition combustion mode, known as controlled auto-ignition (CAI), are a prospective technology for significantly improving engine efficiency and reducing emissions. This review paper provides an overview of developments in various gasoline CAI technologies and discusses their attendant strengths and weaknesses. Hybrid propulsion systems powered by high-efficiency gasoline CAI engines can provide a low-carbon pathway for mobility sector decarbonisation. Therefore, this paper focuses on the challenges and opportunities of CAI implementation, especially for electrified powertrains. Different control actuators that can extend the CAI operating range are discussed, and opportunities for synergistic operation between thermal and electric components of hybridised powertrains are identified. Such synergies can remove impediments in the way of CAI system adoption and can, thus, support CAI adoption and maximise efficiency gains from its implementation. The prospects of supporting CAI combustion for different powertrain electrification levels, hybrid architectures, engine size, and energy management systems are discussed. Load levelling offered by electrified powertrains through CAI-favouring energy management strategies has the potential to substantially relax the operating point requirements for CAI-based thermal propulsion units and to remove the need for expensive actuators. The highly flexible spark-assisted partially premixed compression ignition hybrid mode (SACI-PPCI) emerges as a promising CAI strategy for conventional powertrains, and the moderately flexible spark-assisted compression ignition (SACI) configuration can be a cost-effective thermal propulsion mode for electrified powertrains.

Strategies to achieve controlled auto-ignition (CAI) combustion: A review

Conventional gasoline engines suffer from low performance and NOx emissions. Controlled auto-ignition (CAI), sometimes referred to as homogeneous charge compression ignition (HCCI), is a promising concept to solve such problems. CAI has the potential to improve spark ignition (SI) engine fuel economy while at the same time solving the trade-off of NOx-soot emissions found in compression ignition (CI) engines. The CAI engine can reach a fuel economy comparable to that of a conventional diesel engine with ultra-low NOx and negligible soot emissions. However, controlling auto-ignition remains the biggest difficulty that hinders the implementation of CAI as a commercial engine. Research towards a cleaner and more efficient engine is driven by the progressively stringent emission regulation imposed worldwide. Therefore, the CAI was developed to meet the emissions target while maintaining engine performance. CAI works on the principle of lean mixture and auto-ignition. To obtain CAI combustion, the temperatures in the cylinder must be sufficient to initiate auto-ignition. Without the use of a spark plug or injector, the CAI suffers from a direct control mechanism to start the combustion. The most practical approach to controlling the initiation of auto-ignition in CAI is diluting the intake charge by either trapping the residual gas or recirculating the exhaust gas. Both approaches enable the engine to achieve CAI combustion without requiring significant modifications to control the onset of CAI combustion phase.

Modelling Study of Cycle-To-Cycle Variations (CCV) in Spark Ignition (SI)-Controlled Auto-Ignition (CAI) Hybrid Combustion Engine by Using Reynolds-Averaged Navier–Stokes (RANS) and Large Eddy Simulation (LES)

The spark ignition (SI)-controlled auto-ignition (CAI) hybrid combustion is characterized by early flame propagation combustion and subsequent auto-ignition combustion. The application of combined SI–CAI hybrid combustion can be used to effectively extend the operating range of CAI combustion and achieve smooth transitions between SI and CAI combustion modes. However, SI–CAI hybrid combustion can produce significant cycle-to-cycle variations (CCV). In order to better understand the sources of CCV and minimize its occurrence, the large eddy simulation (LES) and Reynolds-averaged Navier–Stokes (RANS) approaches were employed in this study to model and understand the cyclic phenomenon of SI–CAI hybrid combustion. Both the multi-cycle LES and RANS simulations were analyzed against the experimental measurements in a single cylinder engine at 1500 rpm and a 5.43 bar average indicated the mean effective pressure (IMEP). The detailed analysis of the in-cylinder pressure traces, IMEP, in-cylinder peak pressure (PP), peak pressure rise rate (PPRR) and the crank angles with fuel mass burned fraction at 10%, 50%, 90% and mode transition was performed. The results indicate that overall, the adopted LES simulations could effectively predict the cyclic variations in the hybrid combustion observed in the experiments, while the RANS simulations failed to reproduce the cyclic characteristics at the chosen engine operating conditions. Based on the LES results, the correlation and visualization studies indicate that the cyclic variations in the local velocity around the spark plug lead to the variations in the early flame propagation, which in turn produce temperature fluctuations among the cycles and result in greater variations in the subsequent auto-ignition combustion events.

Combustion and emissions of controlled auto-ignition engine under stratified mixture conditions

Controlled auto-ignition (CAI) is an advanced combustion technology offering high thermal efficiency and low NOX emissions. One promising approach to achieve CAI combustion is application of negative valve overlap resulting in internal exhaust gas re-circulation. However, combustion control is still an issue which should be resolved before wide application of CAI combustion. Introduction of some degree of fuel stratification via direct fuel injection is interesting approach to control combustion process.In this study a gasoline-fuelled single-cylinder research engine was used to investigate the effects of stratification on combustion and emissions of CAI engine. The stratification was achieved using split direct fuel injection technique, where the second fuel injection controlled the degree of stratification. The results shown that variability of the second injection timing and mass of fuel injected is a viable method to control combustion on-set as well as its duration. However, the trends in both combustion and emissions were found to be non-monotonic, revealing complex relationships between mixture formation and combustion process. It has been found that small degree of mixture stratification delays combustion on-set, whereas large stratification advances combustion. The obtained results confirmed superior combustion controllability via mixture stratification, however with some limitations resulting from reduced efficiency at high degree of stratification.

Potential of water direct injection in a CAI/HCCI gasoline engine to extend the operating range towards higher loads

CAI (Controlled AutoIgnition) systems, also named HCCI (Homogeneous Charge Compression Ignition), are a promising way to improve gasoline engines. This combustion mode is more efficient than the standard SI (Spark Ignition) combustion and, additionally, it has very low emissions, especially NOx emissions, which represent a source of problems nowadays. The main problem of this combustion mode is the constrained operating range, caused, on the one hand, by the difficulty to ignite the fuel since it has to be auto-ignited by the control of the mixture reactivity, and, on the other hand, by its high heat release rates, causing high pressure gradients and, in some circumstances, knocking combustion. In this paper, the possibility to use directly injected water into the combustion chamber as a reactivity suppressor in order to extend the constrained load range of CAI operation is evaluated. For this study, a four-stroke single-cylinder gasoline engine has been modified to allow CAI combustion by means of adapted valve trains enabling to keep hot residual gases inside the cylinder, which will provoke the fuel autoignition. Additionally, a water direct injection system has been installed in the engine to carry out this study. The results show that water injection is an efficient strategy to increase the maximum affordable load in CAI conditions, since the reactivity of the mixture can be suitably controlled, thus reducing the pressure gradients and the knocking tendency of the combustion process, also keeping good levels of combustion stability. Nevertheless, the engine has to be significantly boosted and the necessary intake pressure compared to a conventional SI operation mode in stoichiometric conditions is much higher.

Effects of Mixture Stratification on Combustion and Emissions of Boosted Controlled Auto-Ignition Engines

The stratification of in-cylinder mixtures appears to be an effective method for managing the combustion process in controlled auto-ignition (CAI) engines. Stratification can be achieved and controlled using various injection strategies such as split fuel injection and the introduction of a portion of fuel directly before the start of combustion. This study investigates the effect of injection timing and the amount of fuel injected for stratification on the combustion and emissions in CAI engine. The experimental research was performed on a single cylinder engine with direct gasoline injection. CAI combustion was achieved using negative valve overlap and exhaust gas trapping. The experiments were performed at constant engine fueling. Intake boost was applied to control the excess air ratio. The results show that the application of the late injection strategy has a significant effect on the heat release process. In general, the later the injection is and the more fuel is injected for stratification, the earlier the auto-ignition occurs. However, the experimental findings reveal that the effect of stratification on combustion duration is much more complex. Changes in combustion are reflected in NOX emissions. The attainable level of stratification is limited by the excessive emission of unburned hydrocarbons, CO and soot.

Analysis of the potential of a new automotive two-stroke gasoline engine able to operate in spark ignition and controlled autoignition combustion modes

The need to reduce the emissions coming from automobiles encourages the attempts to study different engine configurations and new combustion strategies. In this case, a two stroke engine able to operate in Controlled AutoIgnition (CAI) and Spark Ignition (SI) combustion modes is studied, with the purpose of getting lower NOx and CO2 emissions than with other currently employed solutions. The engine configuration retained for the research is a uniflow scavenging configuration with intake ports in the cylinder liner and exhaust valves in the cylinder head. These valves are controlled by a Variable Valve Timing (VVT) system. The scavenging is guaranteed by an external blower driven by the crankshaft. Finally, the fuel supply is performed by a direct injection (DI) air-assisted fuel injection system.Through this paper the adjusting parameters to control the engine operation, as well as their influence on the CAI and SI combustion modes have been studied, providing the most relevant information and knowledge for controlling and optimizing the engine performance. Once these controlling parameters were studied, an EGR system was introduced in order to analyze the effect of this other parameter over the combustion process, as well as to determine the potential benefits of introducing such a system in this type of engines.

Numerical investigation on low calorific syngas combustion in the opposed-piston engine

The aim of this study was to investigate a possibility of using gaseous fuels of a low calorific value as a fuel for internal combustion engines. Such fuels can come from organic matter decomposition (biogas), oil production (flare gas) or gasification of materials containing carbon (syngas). The utilization of syngas in the barrel type Opposed-Piston (OP) engine arrangement is of particular interest for the authors. A robust design, high mechanical efficiency and relatively easy incorporation of Variable Compression Ratio (VCR) makes the OP engine an ideal candidate for running on a low calorific fuel of various compostion. Furthermore, the possibility of online compression ratio adjustment allows for engine the operation in Controlled Auto-Ignition (CAI) mode for high efficiency and low emission. In order to investigate engine operation on low calorific gaseous fuel authors performed 3D CFD numerical simulations of scavenging and combustion processes in the 2-stroke barrel type Opposed-Piston engine with use of the AVL Fire solver. Firstly, engine operation on natural gas with ignition from diesel pilot was analysed as a reference. Then, combustion of syngas in two different modes was investigated – with ignition from diesel pilot and with Controlled Auto-Ignition. Final engine operating points were specified and corresponding emissions were calculated and compared. Results suggest that engine operation on syngas might be limited due to misfire of diesel pilot or excessive heat releas which might lead to knock. A solution proposed by authors for syngas is CAI combustion which can be controlled with application of VCR and with adjustment of air excess ratio. Based on preformed simulations it was shown that low calorific syngas can be used as a fuel for power generation in the Opposed-Piston engine which is currently under development at Warsaw University of Technology.

Compound disturbance rejection control of spark ignition–controlled-autoignition hybrid combustion for gasoline engines

Spark ignition–controlled-autoignition hybrid combustion is a promising concept because of its capability to achieve a smooth transition between spark ignition combustion and controlled-autoignition combustion, but it suffers from transient control owing to the high sensitivity to the operating conditions. In this paper, a control solution based on the principle of disturbance rejection is proposed for spark ignition–controlled-autoignition hybrid combustion. The complexity, the non-linearity and the cross-coupling inside are removed by idealizing the combustion process into three independent integrators, for the combustion timing channel, the indicated mean effective pressure channel and the λ (excessive air coefficient) channel respectively. All the other dynamics that deviate from the integrators (internal and external) are ‘lumped’ together as the total disturbance for each channel. With the total disturbance estimated in real time via the extended-state observer and eliminated by the disturbance rejection law, the enforced plant, i.e. the integrator, is controlled by a simple proportional controller. To enhance the response further, a non-linear model-inversion-based feedforward controller is added. In order to attenuate the slow time-varying disturbances, four correction factors for the model parameters are embedded in the model for online estimation. Validations by both simulations and experiments confirm the superiority of the proposed solution in terms of a fast transient response and a high robustness. By using the bandwidth-parameterization-based extended-state observer tuning method and a Kalman-filter-based extended-state observer, the controller is easy to tune, making it a promising candidate for applications of spark ignition–controlled-autoignition hybrid combustion.

Numerical investigation of CAI Combustion in the Opposed- Piston Engine with Direct and Indirect Water Injection

This paper is focused on the CAI combustion control in a turbocharged 2-stroke Opposed-Piston (OP) engine. The barrel type OP engine arrangement is of particular interest for the authors because of its robust design, high mechanical efficiency and relatively easy incorporation of a Variable Compression Ratio (VCR). The other advantage of such design is that combustion chamber is formed between two moving pistons - there is no additional cylinder head to be cooled which directly results in an increased thermal efficiency. Furthermore, engine operation in a Controlled Auto-Ignition (CAI) mode at high compression ratios (CR) raises a possibility of reaching even higher efficiencies and very low emissions. In order to control CAI combustion such measures as VCR and water injection were considered for indirect ignition timing control. Numerical simulations of the scavenging and combustion processes were performed with the 3D CFD multipurpose AVL Fire solver. Numerous cases were calculated with different engine compression ratios and different amounts of directly and indirectly injected water. The influence of the VCR and water injection on the ignition timing and engine performance was determined and their application in the real engine was discussed.

Performance and emission characteristics of an IC engine under SI, SI-CAI and CAI combustion modes

Controlled auto ignition (CAI) or, also known homogenous compressed ignition, is an advanced combustion technology allowing a lower NOx emission and fuel consumption that can relieve the energy demand based on petroleum. The objective of this study is to determine the effects of spark ignition (SI), spark assist controlled auto ignition (SI-CAI) and pure CAI combustion modes on the performance and emissions in a modified IC engine. For this purpose, the emissions, performance, and heat release analyses for these combustion modes were obtained at two different engine speeds (1500–2000 rpm) and various excess air ratio (EAR) in the range of 1.0–1.3 with 0.1 increments for both wide open throttle (WOT) and 50% opening throttle positions. The results showed that CAI combustion mode produced lower NOx emissions, although it resulted in higher CO and HC emission than those under SI and SI-CAI combustion modes. In addition, brake mean effective pressures (BMEP) were obtained as higher values under SI-CAI combustion mode. Therefore, SI-CAI combustion mode provided a wider operational range than that of the CAI combustion mode. However, the SI-CAI combustion modes resulted in high NOx emissions, but relatively lower values than those under SI combustion mode.

Effects of ethanol on combustion and emissions of a gasoline engine operating with different combustion modes

The introduction of fuel economy and CO2 emission legislations for passenger cars in many countries and regions has spurred the research and development of more efficient gasoline engines. The pumping loss at part-load operations is a major factor for the higher fuel consumption of spark-ignition gasoline engines than the diesel engines. Various approaches have been identified to reduce the pumping loss at part-load operations, leading to improved fuel economy, including early intake-valve closing, positive valve overlap and controlled auto-ignition combustion. On the other hand, in order to reduce the CO2 emissions from the fossil fuel, ethanol produced from renewable resources is becoming widely used in the gasoline engine. In this article, the performance, combustion and emissions were measured, analysed and compared between gasoline and its mixture with ethanol (E15 and E85) at a typical part-load condition when a direct-injection gasoline engine was operated with the controlled auto-ignition combustion by means of the negative valve overlap and spark-ignition combustion by means of the intake throttled, early intake-valve closing and positive valve overlap. An electro-hydraulic actuated camless system enabled the engine to be operated with controlled auto-ignition combustion and spark-ignition combustion of different valve timings and durations at the same load. The results showed that the controlled auto-ignition combustion reduces nitrogen oxide emissions by more than 90%. The positive valve overlap results in better mixture preparations and improved combustion efficiency and best fuel economy compared to all the other modes. The early intake-valve closing operation led to a moderate improvement in the fuel conversion efficiency over the throttled spark-ignition operation, but it was characterised by the slowest combustion and worst hydrocarbon and carbon monoxide emissions. Fewer and smaller particle numbers were detected in early intake-valve closing using E0 and E15 fuel blends. Using ethanol blends reduces the knocking combustion in controlled auto-ignition modes by about 50%. The use of E85 resulted in an increased number of particulate emissions in early intake-valve closing but increased indicated specific fuel consumption in all the modes. The particulate emission results showed that soot is the dominant particle in the exhaust.

INFLUENCE OF HOT BURNED GAS UTILIZATION ON THE EXHAUST EMISSION CHARACTERISTICS OF A CONTROLLED AUTO-IGNITION TWO-STROKE CYCLE ENGINE

A controlled auto-ignition (CAI) two-stroke cycle engine suggests an exceptional aspect and promising future for internal combustion engines (ICEs), such as a higher power-toweight ratio, higher combustion efficiency and lower exhaust gas emissions. Conventional two-stroke cycle engines emit higher exhaust gas emissions and offer lower fuel saving economy. Most of these drawbacks can be addressed if CAI combustion is associated with a two-stroke cycle engine. An experimental investigation is carried out based on a single-cylinder CAI two-stroke cycle engine using Internal and External Exhaust Gas Recirculation (In-EGR and Ex-EGR) and fuels with different octane numbers to investigate the exhaust emissions characteristics. The experimental results indicate a remarkable improvement in the engine's exhaust gas emissions. The concentration of uHC and CO emissions decreased with application of In/Ex-EGR. However, NOx emission increased with the use of In-EGR

Thermal and chemical effects of fuel direct injection on kinetically controlled combustion of alcohol and gasoline fuels

Direct fuel injection into hot residual gases is an effective means to promote and control the autoignition timings during controlled autoignition combustion of gasoline engines. In order to understand better the underlying physical and chemical processes involved, a systematic experimental study was carried out on a single-cylinder engine with optical access by means of thermodynamic analysis, high-speed chemiluminescence imaging, and in-cylinder sampling-based gas chromatography and mass spectroscopy measurements. The relative effects of enthalpy of evaporation and gas expansion from direct liquid fuel injection were quantified. Alcohol fuels (methanol and ethanol), gasoline, and primary reference fuels were investigated and clarified. Chemiluminescence imaging and heat release analysis demonstrated the presence of fuel reforming reactions during the recompression stroke. In-cylinder gas speciation measurement showed that the fuel pyrolysis was the dominant process. In addition, exothermic fuel reformation reactions of alcohol resulted in an increase in the gas temperature and produced many intermediate species during negative valve overlap period. As a result, the start of main heat release process of alcohol fuel occurred earlier in the subsequent compression stroke.

A Converted Two-Stroke Cycle Engine for Compression Ignition Combustion

A new kind of alternative combustion concept that has attracted attention intensively in recent years is called controlled auto-ignition (CAI) combustion. CAI combustion has been proposed and partially implemented with the aim of both improving the thermal efficiency of internal combustion engines, achieving cleaner exhaust emissions and lower cyclic variation. An experimental study is conducted through a CAI two-stroke cycle engine in order to investigate the influence of internal exhaust gas recirculation (In-EGR) and external exhaust gas recirculation (Ex-EGR) variation in relation to combustion cyclic variability and exhaust emissions characteristics. Results implied that cyclic variation of both combustion-related and pressure-related parameter is substantially improved. Furthermore remarkable decreased exhaust emissions, unburned hydrocarbon (uHC), carbon monoxide (CO) and nitric dioxide (NOX), was observed.

Experimental investigation of the influence of internal and external EGR on the combustion characteristics of a controlled auto-ignition two-stroke cycle engine

A two-stroke cycle engine incorporated with a controlled auto-ignition combustion approach presents a high thermodynamic efficiency, ultra-low exhaust emissions and high power-to-weight ratio features for future demand of prime movers. The start of auto-ignition, control of the auto-ignition and its cyclic variability, are major concerns that should be addressed in the combustion timing control of controlled auto-ignition engines. Several studies have been performed to examine the effect of internal exhaust gas recirculation utilization on auto-ignited two-stroke cycle engines. However, far too little attention has been devoted to study on the influence of external exhaust gas recirculation on the cyclic variation and the combustion characteristics of controlled auto-ignition two-stroke cycle engines. The purpose of this study is to examine the influence of external exhaust gas recirculation in combination with internal exhaust gas recirculation on the combustion characteristics and the cyclic variability of a controlled auto-ignition two-stroke engine using fuel with different octane numbers. In a detailed experimental investigation, the combustion-related and pressure-related parameters of the engine are examined and statistically associated with the coefficient of variation and the standard deviation. The outcomes of the investigation indicates that the most influential controlled auto-ignition combustion phasing parameters can be managed appropriately via regulating the internal and external exhaust gas recirculation and fuel octane number. In general, start of auto-ignition and its cyclic variability are predominantly affected by external exhaust gas recirculation variation rather than internal exhaust gas recirculation. Furthermore, although the magnitude of low temperature heat release is substantially influenced by external exhaust gas recirculation variation, timing of low temperature heat release is more influenced by internal exhaust gas recirculation approach.

An experimental study on the influence of EGR rate and fuel octane number on the combustion characteristics of a CAI two-stroke cycle engine

Having the higher power-to-weight ratio feature of two-stroke cycle engines and higher thermodynamic efficiency of controlled auto-ignition (CAI) combustion proposes a promising concept for future internal combustion engines. The control of start of auto-ignition (CA10) and its cyclic variation are major issues that should be addressed. The research to date has tended to focus on the influence of internal EGR (In-EGR) utilization on the combustion characteristics of CAI two-stroke cycle engine rather than external EGR (Ex-EGR) utilization. This experimental study aims to investigate the influence of Ex-EGR incorporated with In-EGR utilization on the combustion characteristics and the cyclic variability of a single-cylinder CAI two-stroke cycle engine using different fuel octane number (ON). Engine's combustion-related parameters were examined statistically associated with Coefficient of Variation (COV) and Standard Deviation (STD). It is perceived that the significant CAI combustion parameters, including HRRmax, θHRRmax, LTHR, θLTHR, CA10, CA50, MFB and TCA10 are strongly correlated to variation of In/Ex-EGR and fuel ON and they suggest an effective means to control CAI combustion phasing and cyclic variability. Furthermore, CA10 and its cyclic variability exhibited that are more sensitive to altering Ex-EGR compared to altering In-EGR.

The effects of dual-coil ignition and axial swirl on spark-assisted controlled autoignition

The work involved an experimental study of a dual-coil ignition system for improved combustion stability and control during four-stroke gasoline controlled autoignition combustion. The ignition system was based upon the ‘redundant-coil’ set-up previously used in motorsport applications, where two conventional induction coils are connected to a common spark plug via an individually assigned diode. Such systems have recently been used for improved tolerance to exhaust gas recirculation in automotive spark ignition engines, but their effects have apparently not been previously reported for spark-assisted controlled autoignition. The basis for the work was therefore to validate such operation at low loads, where spark assist has been most widely suggested as a means of stabilising controlled autoignition combustion. The experiments were undertaken using a thermodynamic single-cylinder research engine equipped with an electrohydraulic, fully variable valvetrain on the inlet and the exhaust. This valvetrain was primarily used to invoke negative valve overlap and later, additionally, to allow study of the asymmetric intake valve operation and the influence of the increased axial swirl ratio within the bulk gas flow. The key observation from the work was that a staged single strike per coil strategy was found to improve the combustion stability significantly when operating at the low-load controlled autoignition boundary, especially when combined with increased axial swirl to promote ignitability between successive strikes. Overall, it was concluded that the system provides a new method for ensuring robust controlled autoignition operation at the lowest loads, with additional ability to help to control the combustion noise.

Optimisation of boosting strategy for controlled auto-ignition combustion in a four-valve camless gasoline direct injection engine running in two-stroke cycle

Due to their ability to simultaneously reduce fuel consumption and NOx emissions, controlled autoignition or homogeneous charge compression ignition combustion processes have been extensively researched over the last decade and adopted on prototype gasoline engines. Despite the fact that they were initially achieved on conventional two-stroke ported gasoline engines, there have been much fewer studies carried out on controlled autoignition combustion in two-stroke engines because of the inherent problems associated with the conventional two-stroke engine with intake and exhaust ports. In the meantime, engine downsizing has been actively researched and developed as an effective means to improve the vehicle’s fuel economy by operating the engine at higher load and more efficient regions and by reducing the number of cylinders. But the aggressive engine downsizing of the current four-stroke gasoline engine is limited by the knocking combustion and high peak cylinder pressure. As an alternative approach to engine downsizing, the boosted two-stroke cycle in a poppet valve engine is proposed and researched. It has been shown that the controlled autoignition combustion in the two-stroke cycle could be readily achieved and it led to significant reduction in carbon monoxide and unburned hydrocarbon emissions than the spark ignition combustion. In this study, extensive engine experiments have been performed to determine the optimum boosting for minimum fuel consumption in a single-cylinder gasoline direct injection camless engine operating in the two-stroke cycle. In order to minimise the air short-circuiting rate, the intake and exhaust valve timings were adjusted. Lean boost was applied to the engine operation, which was found to extend the range of controlled autoignition combustion, result in higher combustion and thermal efficiencies and significantly lower carbon monoxide and hydrocarbon emissions. By means of the cycle-resolved in-cylinder measurements and heat release analysis, the improvement in combustion and thermal efficiencies was attributed to the improved in-cylinder mixture, optimised autoignition and combustion phases.

Controlled Auto-Ignition Combustion in a Two-Stroke Cycle Engine Using Hot Burned Gases

A new combustion concept, which is viewed increasingly as a probable solution to these issues is Controlled Auto-Ignition (CAI) Combustion. In such an engine, a homogeneous mixture of air, fuel and residual gases is compressed until auto-ignition occurs. Due to its significantly low temperature combustion, NOx will be dramatically reduced while the mixture will be under ultra-lean fuel-air condition, thus able to achieve high efficiency and low emission. In the case of two-stroke engine, problem of poor combustion efficiency and excessive white smoke emission can be addressed by the incorporation some features that will ultimately convert a typical two-stroke engine into an efficient CAI engine demonstrating the best of both features. Due to its inherent high internal residual gas rate in partial load operation, the two-stroke engine has been the first application to benefit from the unconventional CAI combustion process. This paper will concisely discuss the utilization of hot burned gas for induction thus imposing a CAI combustion feature onto two-stroke cycle engine. Among the features incorporated are the increasing in the level of Exhaust gas Recirculation and cycle-by-cycle uniformity of the air-fuel ratio (AFR) supplied to cylinder, which will be crucial in creating a suitable temperature within the engine’s combustion chamber.