Search Results

In order to extend the CAI operation range in 4-stroke mode and maximize the benefit of low fuel consumption and emissions in CAI mode, 2-stroke CAI combustion is revived operating in a GDI engine with poppet valves, where the conventional crankcase scavenging is replaced by boosted scavenging. The CAI combustion is achieved through the inherence of the 2-Stroke operation, which is retaining residual gas. A set of flexible hydraulic valve train was installed on the engine to vary the residual gas fraction under the boosting condition. The effects of spark timing, intake pressure and short-circuiting on 2-stroke CAI combustion and its emissions are investigated and discussed in this paper. Results show the engine could be controlled to achieve CAI operation over a wide range of engine speed and load in the 2-stroke mode because of the flexibility of the electro-hydraulic valvetrain system.

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. Currently, CAI combustion is constrained at part load operation conditions because of misfire at low load and knocking combustion at high load, and the lack of effective means to control the combustion process. Extending its operating range including high load boundary towards full load and low load boundary towards idle in order to allow the CAI engine to meet the demand of whole vehicle driving cycles, has become one of the key issues facing the industrialisation of CAI/HCCI technology. Furthermore, this combustion mode should be compatible with different fuels, and can switch back to conventional spark ignition operation when necessary. In this paper, the CAI operation is demonstrated on a 2-stroke gasoline direct injection (GDI) engine equipped with a poppet valve train.

One-dimensional engine simulation programmes are often used in the engine design and optimization studies. One of the key requirements of such a simulation programme is its ability to predict the heat release process during combustion. Such simulation software has built in it the heat release models for spark ignited premixed flame and compression ignited diesel combustion. The recent emergence of Controlled Auto Ignition (CAI) combustion, also known as Homogeneous Charge Compression Ignition (HCCI), has generated the need for a third type of heat release models for this new combustion process. In this paper, a heat release correlation for CAI combustion has been derived from extensive in-cylinder pressure data obtained from a Ricardo E6 single cylinder research engine and a multi-cylinder Port Fuel Injection (PFI) gasoline engine running with CAI combustion. The experimental data covered a wide range of air/fuel ratios, speed and percentage of residual gas.

To achieve homogeneous charge compression ignition (HCCI) combustion in the range of low speeds and loads, special camshafts with low intake/exhaust cam lift and short intake/exhaust cam duration were designed. The camshafts were mounted in a Ricardo Hydra four-stroke single cylinder port fuel injection gasoline engine. HCCI combustion was achieved by controlling the amount of trapped residuals from previous cycle through negative valve overlap. The results show that indicated mean effective pressure (IMEP) depends on valve timings, engine speeds and lambda. Early exhaust valve closing (EVC) timings result in high residual fractions in the cylinder and low air mass sucked into the cylinder. As a result, combustion duration increases, IMEP and peak pressure decrease. However, pumping losses decrease. High engine speed has the similar effect on HCCI combustion characteristics as early EVC timings do. But inlet valve opening timings have slight effect on IMEP compared to EVC timings.

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.

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.

An accurate prediction of residual burned gas within the combustion chamber is important to quantify for development of modern engines, especially so for those with internally recycled burned gases and HCCI operations. A wall-guided GDI engine has been fitted with an in-cylinder sampling probe attached to a fast response NDIR analyser to measure in-situ the cycle-by-cycle trapped residual gas. The results have been compared with a model which predicts the trapped residual gas fraction based on heat release rate calculated from the cylinder pressure data and other factors. The inlet and exhaust valve timings were varied to produce a range of Residual Gas Fraction (RGF) conditions and the results were compared between the actual measured CO2 values and those predicted by the model, which shows that the RGF value derived from the exhaust gas temperature and pressure measurement at EVC is consistently overestimated by 5% over those based on the CO2 concentrations.

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.

In recent years, in order to develop more efficient and cleaner gasoline engines, a number of new engine operating strategies have been proposed and many have been studied on different engines but there is a lack of comparison between various operating strategies and alternative fuels at different SI modes. In this research, a single cylinder direct injection gasoline engine equipped with an electro-hydraulic valve train system has been commissioned and used to study and compare different engine operation modes. In this work, the fuel consumption, gaseous and particulate emissions of gasoline and its mixture with ethanol (E15 and E85) were measured and analysed when the engine was operated at the same load but with different load control methods by an intake throttle, reduced intake valve duration, and positive overlap.

The two-stroke operation is one of the most effective approaches to significantly increase the torque and power of a 4-stroke engine without the necessary requirement of intensifying the engine. Scavenging process is one of the key factors determining the performance of the two-stroke engine. In this work, a structure of top entry intake ports with poppet valves was employed on a 2-stroke single cylinder diesel engine with the conventional horizontal intake ports replaced. By this way, the reversed tumble flows in the cylinder were formed during the intake process to improve the scavenging performance of 2-stroke operation. In the meanwhile, the effects of valve timings and intake port structures on scavenging processes were estimated respectively through the1D and 3D simulation of the gas exchange process.

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.

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.

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.

Two-stroke cycle is one of the most effective methods to increase the torque and power output of a four-stroke engine due to the doubled firing frequency compared to four-stroke cycle at the same engine speed. As the two-stroke cycle lacks separate intake and exhaust strokes, the positive pressure difference between intake and exhaust ports is required to drive fresh charge into the cylinder, and is affected by intake port structures due to the different amounts of short-circuited fresh charge during scavenging process. To evaluate the effects of intake port structures on the high-load performance of a boosted poppet-valved two-stroke diesel engine, one-dimensional gas dynamic model and three-dimensional computational fluid dynamics model were established and used to predict the high-load performance of the boosted two-stroke diesel engine with top-entry intake ports, inclined side-entry intake ports, and side-entry intake ports, respectively.

A strategy to actualize the dual-mode (SI mode and HCCI mode) operation of gasoline engine was investigated. The 4VVAS (4 variable valve actuating system), capable of independently controlling the intake and exhaust valve lifts and timings, was incorporated into a specially designed cylinder head for a single cylinder research engine and a 4VVAS-HCCI gasoline engine test bench was established. The experimental research was carried out to study the dynamic control strategies for transitions between HCCI and SI modes on the HCCI operating boundaries. Results show that equipped with the 4VVAS cylinder head, the engine can be operated in HCCI or SI mode to meet the demands of different operating conditions. 4VVAS enables the rapid and effective control over the in-cylinder residual gas, and therefore dynamic transitions between HCCI and SI can be stably achieved. It is easier to achieve transition from HCCI to SI than reversely due to the influence of thermo-inertia.

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.

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.

With the application of valve timing strategy to inlet and exhaust valves, Homogeneous Charge Compression Ignition (HCCI) combustion was achieved by varying the amount of trapped residuals through negative valve overlap on a Ricardo Hydra four-stroke port fuel injection engine fueled with ethanol. The effect of ethanol on HCCI combustion and emission characteristics at different air-fuel ratios, speeds and valve timings was investigated. The results indicate that HCCI ethanol combustion can be achieved through changing inlet and exhaust valve timings. HCCI ethanol combustion range can be expanded to high speeds and lean burn mixture. Meanwhile, the factors influencing ignition timing and combustion duration are valve timing, lambda and speeds. Moreover, NOx emissions are extremely low under HCCI combustion. The emissions-speed and emissions-lambda relationships are obtained and analyzed.

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.