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Two-stroke engines have to face the problems of insufficient charge for short intake time and the loss of intake air caused by long valve overlap. In order to promote the power of a two-stroke poppet valve diesel engine, measures are taken to help optimize intake port structure. In this work, the scavenging and combustion processes of three common types of intake ports including horizontal intake port (HIP), combined swirl intake port (CSIP) and reversed tumble intake port (RTIP) were studied and their characteristics are summarized based on three-dimensional simulation. Results show that the RTIP has better performance in scavenging process for larger intake air trapped in the cylinder. Its scavenging efficiency reaches 84.7%, which is 1.7% higher than the HIP and the trapping ratio of the RTIP reaches 72.3% due to less short-circuiting loss, 11.2% higher than the HIP.

In Spark Ignition (SI)-Controlled Auto Ignition (CAI) hybrid combustion, the in-cylinder temperature and total mass of dilution charge are usually increased compared to the traditional SI engine in order to achieve and control the auto-ignition combustion, which would in turn lead to the variations of the diluted flame propagation combustion. In this study, the optical measurements were performed to understand the flame characteristics at highly diluted conditions. The results showed that the decrease of the flame propagation speed of rich mixture was less than that of lean mixture at highly diluted conditions. However, the inhomogeneous distribution of residual gas led to asymmetric development of flame propagation. The high temperature, strong dilution and rich mixture created local auto-ignition sites which were located in front of the main flame and gradually merged with the main flame.

Electrically assisted turbochargers are a promising technology for improving boost response of turbocharged engines. These systems include a turbocharger shaft mounted electric motor/generator. In the assist mode, electrical energy is applied to the turbocharger shaft via the motor function, while in the regenerative mode energy can be extracted from the shaft via the generator function, hence these systems are also referred to as regenerative electrically assisted turbochargers (REAT). REAT allows simultaneous improvement of boost response and fuel economy of boosted engines. This is achieved by optimally scheduling the electrical assist and regeneration actions. REAT also allows the exhaust turbine to operate within a narrow range of optimal vane positions relative to the unassisted variable geometry turbocharger (VGT). The ability to operate within a narrow range of VGT vane positions allows an opportunity for a more optimal turbine design for a REAT system.

Controlled Auto-Ignition (CAI) combustion, also known as Homogeneous Charge Compression Ignition (HCCI), has the potential to simultaneously reduce the fuel consumption and nitrogen oxides emissions of gasoline engines. However, narrow operating region in loads and speeds is one of the challenges for the commercial application of CAI combustion to gasoline engines. Therefore, the extension of loads and speeds is an important prerequisite for the commercial application of CAI combustion. The effect of intake charge boosting, charge stratification and spark-assisted ignition on the operating range in CAI mode was reviewed. Stratified flame ignited (SFI) hybrid combustion is one form to achieve CAI combustion under the conditions of highly diluted mixture caused by the flame in the stratified mixture with the help of spark plug.

Controlled Auto-Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), can improve the fuel economy of gasoline engines and simultaneously achieve ultra-low NOx emissions. However, the difficulty in combustion phasing control and violent combustion at high loads limit the commercial application of CAI combustion. To overcome these problems, stratified mixture, which is rich around the central spark plug and lean around the cylinder wall, is formed through port fuel injection and direct injection of gasoline. In this condition, rich mixture is consumed by flame propagation after spark ignition, while the unburned lean mixture auto-ignites due to the increased in-cylinder temperature during flame propagation, i.e., stratified flame ignited (SFI) hybrid combustion.

In decade years, Spark Ignition-Controlled Auto Ignition (SI-CAI) hybrid combustion, also called Spark Assisted Compression Ignition (SACI) has shown its high-efficiency and low emissions advantages. However, high dilution causes the problem of unstable initial ignition and flame propagation, which leads to high cyclic variation of heat release and IMEP. The instability of SI-CAI hybrid combustion limits its dilution degree and its ability to improve the thermal efficiency. In order to solve instability problems and expand the dilution boundary of hybrid combustion, micro flame ignition (MFI) strategy is applied in gasoline hybrid combustion engines. Small amount of Dimethyl Ether (DME) chosen as the ignition fuel is injected into cylinder to form micro flame kernel, which can stabilize the ignition combustion process.

Poppet-valve two-stroke gasoline engines can increase the specific power of their four-stroke counterparts with the same displacement and hence decrease fuel consumption. However, knock may occur at high loads. Therefore, the combustion with stratified lean mixture was proposed to decrease knock tendency and improve combustion stability in a poppet-valve two-stroke direct injection gasoline engine. The effect of intake pressure and split injection on fuel distribution, combustion and knock intensity in lean mixture conditions at high loads was simulated with a three-dimensional computational fluid dynamic software. Simulation results show that with the increase of intake pressure, the average fuel-air equivalent ratio in the cylinder decreases when the second injection ratio was fixed at 70% at a given amount of fuel in a cycle.

Mechanical load and thermal load are the two main barriers limiting the engine power output of heavy duty (HD) diesel engines. Usually, the peak cylinder pressure could be reduced by retarding combustion phasing while introducing the drawback of higher thermal load and exhaust temperature. In this paper, Miller cycle with late intake valve closing was investigated at high speed high load condition (77 kW/L) on a single cylinder HD diesel engine. The results showed the simultaneous reduction of mechanical and thermal loads. In the meanwhile, higher boosting pressure was required to compensate the Miller loss of the intake charge during intake and compression process. The combustion temperature, cylinder pressure, exhaust temperature and NOx emission were reduced significantly with Miller cycle at the operating condition. Furthermore, the combustion process, smoke number and fuel consumption were analysed.

Controlled Auto-Ignition (CAI), also known as Homogeneous charge compression ignition (HCCI), has been the subject of extensive research because of their ability to providing simultaneous reduction in fuel consumption and NOx emissions in a gasoline engine. However, due to its limited operation range, combustion mode switching between CAI and spark ignition (SI) combustion is essential to cover the overall operational range of a gasoline engine for passenger car applications. Previous research has shown that the SI-CAI hybrid combustion has the potential to control the ignition timing and heat release process during both steady state and transient operations. However, it was found that the SI-CAI hybrid combustion process is often characterized with large cycle-to-cycle variations, due to the flame instability at high dilution conditions.

With the introduction of CO2 emissions legislation in Europe and many countries, there has been extensive research on developing high efficiency gasoline engines by means of the downsizing technology. Under this approach the engine operation is shifted towards higher load regions where pumping and friction losses have a reduced effect, so improved efficiency is achieved with smaller displacement engines. However, to ensure the same full load performance of larger engines the charge density needs to be increased, which raises concerns about abnormal combustion and excessive in-cylinder pressure. In order to overcome these drawbacks a four-valve direct injection gasoline engine was modified to operate in the two-stroke cycle. Hence, the same torque achieved in an equivalent four-stroke engine could be obtained with one half of the mean effective pressure.

The spark ignition-controlled auto-ignition (SI-CAI) hybrid combustion is promising in achieving smooth transition between SI and CAI combustion but, it is limited by the combustion cyclic-variation at late combustion phasing to avoid too high pressure rise rate (PRR). In this paper, to stabilize the combustion and reduce PRR, the in-cylinder fuel-stratification strategy is investigated in a gasoline engine, equipped with port fuel injection combined with single pulse direct injection (PFI-DI). Experimental results confirm the benefits of employing PFI-DI in comparison with PFI and single-pulse DI strategy. The influence of DI timing (Start of injection, SOI) on the combustion process is found to be quite complicated, in terms of combustion phasing, combustion stability, PRR and thermal efficiency. It makes the optimal-SOI calibration time-intensive, since complex trade-off between PRR and thermal efficiency is needed.

Controlled Auto Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), is one of the most promising combustion technologies to reduce the fuel consumption and NOx emissions. In order to take advantage of the inherent ability to retain a large and varied amount of residual at part-load condition and its potential to achieve extreme engine downsizing of a poppet valve engine running in the 2-stroke cycle, a single cylinder 4-valves camless direct injection gasoline engine has been developed and employed to investigate the CAI combustion process in the 2-stroke cycle mode. The CAI combustion is initiated by trapped residual gases from the adjustable scavenging process enabled by the variable intake and exhaust valve timings. In addition, the boosted intake air is used to provide the in-cylinder air/fuel mixture for maximum combustion efficiency.

Controlled Auto Ignition (CAI), also known as Homogeneous Charge Compression Ignition (HCCI), is one of the most promising combustion technologies to reduce the fuel consumption and NOx emissions. Most research on CAI/HCCI combustion operations have been carried out in 4-stroke gasoline engines, despite it was originally employed to improve the part-load combustion and emission in the two-stroke gasoline engine. However, conventional ported two-stroke engines suffer from durability and high emissions. In order to take advantage of the high power density of the two-stroke cycle operation and avoid the difficulties of the ported engine, systematic research and development works have been carried out on the two-stroke cycle operation in a 4-valves gasoline engine. CAI combustion was achieved over a large range of operating conditions when the relative air/fuel ratio (lambda) was kept at one as measured by an exhaust lambda sensor.

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In order to stabilize the hybrid combustion process, the port fuel injection (PFI) combined with gasoline direct injection (GDI) strategy is proposed in this study to form the in-cylinder fuel stratification to enhance the early flame propagation process and control the auto-ignition combustion process. The effect of bowl piston shapes and fuel injection strategies on the fuel stratification characteristics is investigated in detail using three-dimensional computational fluid dynamics (3-D CFD) simulations. Three bowl piston shapes with different bowl diameters and depths were designed and analyzed as well as the original flat piston in a single cylinder PFI/GDI gasoline engine.

SCHC (SI-CAI hybrid combustion), also known as spark-assisted HCCI, has been proved to be an effective method to stabilize combustion and extend the operation range of high efficiency, low temperature combustion. The combustion is initiated by the spark discharge followed by a propagation of flame front until the auto-ignition of end-gas. Spark ignition and the spark timing can be used to control the combustion event. The goal of this research is to study the effect of flame propagation on the auto-ignition timing in SCHC by means of chemiluminescence imaging and heat release analysis based on an optical engine. With higher EGR (exhaust gas recirculation) rate, more fuel is consumed by the flame propagation and stronger correlation between the flame propagation and auto-ignition is observed.

The goal of this research was to study and quantify the effect of exhaust valve timing and residual gas dilution on in-cylinder flow patterns, flame propagation and heat release characteristics in a spark ignition engine. Experiments were carried out in a recently developed single cylinder optical engine. Particle image velocimetry (PIV) was applied to measuring and evaluating the in-cylinder flow field. Detailed analysis of flame images combined with heat release data was presented for several engine operating conditions, giving insight into the combustion process in terms of visible flame area and flame expansion speed. Results from PIV measurement indicates that the limited alteration of the in-cylinder bulk flow could be observed with the variation of exhaust valve timing. The in-cylinder fluctuating kinetic energies and their Coefficient of Variations (COVs) decrease with the advance of the exhaust valve timing.

Internal combustion engines are subjected to part-load operation more than in full load during a typical vehicle driving cycle. The problem with the Spark Ignition (SI) engine is its inherent low part-load efficiency. This problem arises due to the pumping loses that occur when the throttle closes or partially opens. One way of decreasing the pumping losses is to operate the engine lean or by adding residual gases. It is not possible to operate the engine unthrottled at very low loads due to misfire. However, the load can also be controlled by changing the intake valve closing timing - either early or late intake valve closing. Both strategies reduce the pumping loses and hence increase the efficiency. However the early intake valve closure (EIVC) can be used as mode transition from SI to CAI combustion.

The RCS (Rankine cycle system) used to recover the exhaust gas energy from internal combustion engines has been regarded as one of the most promising ways to achieve higher efficiency. However, it is a big challenge to keep the RCS still in good performance under variable driving cycle. This paper aims at revealing the reasons resulting in the low efficiency under driving cycle, comparing to that under steady-state condition. The dynamic operating process of the RCS under driving cycle is analyzed, and then the RCS applied on an 11.6L heavy duty diesel engine is modeled. Based on that, the dynamic performance of the RCS under an actual driving cycle is discussed. The results indicate that the average efficiency under a piece of Tianjin bus driving cycle is as low as 3.63%, which is less than half of that (7.77%) under the rated point (1300rpm and 50%load). The reasons leading to the low efficiency under driving cycle is interpreted from three aspects.

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In order to investigate the effect of the thermal boundary condition on the hybrid combustion, the experiments with different coolant temperatures are performed to adjust the chamber wall temperature in a gasoline engine. The experimental results indicate that increasing wall temperature would advance the combustion phasing, enlarge the peak heat release rate and shorten the combustion duration. While the capacity of the wall temperature effect on the hybrid combustion characteristics are more notable in the auto-ignition dominated hybrid combustion.

Direct injection gasoline engines have the potential for improved fuel economy through principally the engine down-sizing, stratified charge combustion, and Controlled Auto Ignition (CAI). However, due to the limited time available for complete fuel evaporation and the mixing of fuel and air mixture, locally fuel rich mixture or even liquid fuel can be present during the combustion process of a direct injection gasoline engine. This can result in significant increase in UHC, CO and Particulate Matter (PM) emissions from direct injection gasoline engines which are of major concerns because of the environmental and health implications. In order to investigate and develop a more efficient DI gasoline engine, a camless single cylinder DI gasoline engine has been developed. Fully flexible electro-hydraulically controlled valve train was used to achieve spark ignition (SI) and Controlled Autoignition (CAI) combustion in both 4-stroke and 2-stroke cycles.