Effects Of Variable Valve Timing Research Articles

Effects of variable valve timing and duration on catalyst heating using optically accessible MPI engine

The effects of intake and exhaust valve timing and duration were investigated using an optically accessible multi-point injection engine under catalytic heating conditions. The target engine features a quartz cylinder and an extended piston with a quartz piston crown. To understand the effects of valve timing and duration on the in-cylinder flow, particle image velocimetry (PIV) was applied to visualize the in-cylinder flow. The acquired images were used to quantitatively evaluate the in-cylinder flow characteristics, and the effects of these flow characteristics on the flame propagation were analyzed through flame visualization experiments. The results indicate that when the intake valve timing was advanced by 20°, the average flow velocity was 59.5% higher, the tumble ratio was 72.2% greater, and the flame propagation velocity was also the fastest. The longest duration of 218° was advantageous for the intake valve duration, and the formation of a tumble flow pattern was important for fuel and air mixing. Finally, in the exhaust valve timing and duration experiment, the exhaust valve closing timing affected the amount of residual gas inside the cylinder. Therefore, the ratio of diffusion flames increased when the exhaust valve timing was advanced or retarded compared to the reference exhaust valve timing.

Influence of variable enhanced LIVC miller cycle coupled with high compression ratio on the performance and combustion of a supercharged spark ignition engine

A novel variable enhanced late intake valve closing (LIVC) Miller cycle with asynchronous valve opening was proposed. Relevant tests were carried out in a supercharged spark ignition (SI) engine with high compression ratio, which mainly included the effects of variable valve timing on performance and combustion characteristics, as well as the comparison with the baseline engine. The purpose of this study was to explore the effect of different intake and exhaust opening timing on the performance and combustion characteristics of the enhanced LIVC Miller cycle with asynchronous valve opening, and further explore the energy saving potential of this Miller cycle. The results showed that the application of variable enhanced Miller cycle effectively improved the anti-knock ability and pumping loss of the engine with high compression ratio. On the one hand, the optimization of pumping loss substantially improved the fuel economy under small load conditions. On the other hand, better combustion performance and air-fuel ratio could be achieved at medium and high loads. Meanwhile, the exhaust valve should be opened as earlier as possible to achieve the best fuel economy. Correspondingly, under optimal valve timing, compared with the baseline engine, the asynchronous intake valve opening variable enhanced Miller cycle engine with high compression ratio had a fuel economy optimization of more than 3.5 % under all loads, and the maximum fuel economy optimization was 12.74 % under the condition of 11 bar.

Experimental study of the effect of variable valve timing on hydrogen-enriched ammonia engine

To reduce carbon emissions and increase the efficiency of internal combustion engines (ICEs), it is necessary to use renewable alternative fuels and to use Miller cycle technology. Ammonia is an energy storage medium as well as a hydrogen storage medium. In this experiment, a Miller cycle ICE using ammonia-hydrogen fuel was investigated at 1500 rpm and 60% throttle opening. The excess air factor of 1 and the ammonia volume share of 70% were maintained during the experiment. The objective of the experiment was to vary the intake valve timing and investigate its effect on engine performance. The results show that when the intake valve timing is advanced and the valve overlap angle is kept larger, the combustion stability deteriorates, the flame development period (CA0-10) is prolonged, the cycle variation increases, COVpmax will exceed 25%, and the brake thermal efficiency (BTE) quickly dropped from 32.8% to 30%. When the intake valve timing is retarded by 25°CA, IMEP and BMEP can be increased by 8% and 16% compared with the minimum values. Generally speaking, the Miller cycle hydrogen-enriched ammonia engine is not suitable for the early opening strategy of the intake valve.

Effects of variable valve timing on the air flow parameters in an electromechanical valve mechanism – A cfd study

In this study, volumetric efficiency, air flow rates, and cylinder pressure values were experimentally determined for a single-cylinder spark-ignition engine having conventional camshafts. Cold flow analysis was carried out using the computational fluid dynamics (CFD) method with importing combustion chamber and flow volumes of the suction and exhaust ports of the engine. After validating analyzes results with experimental data for different revolutions (RPMs), analyses were repeated using electromechanical valve mechanism (EMV) cam profiles. In the case of using EMV, advancing the intake valve opening time and advancing and retarding exhaust valve closing were simulated with CFD software. Simulations where different cam profiles were used for each revolution, volumetric efficiency, in-cylinder pressure, swirl and tumble ratio, in-cylinder total mass, and air flow rates in the cylinder were investigated numerically. As a result of the obtained data, it was seen that the use of EMV at low RPMs caused a decrease in the volumetric efficiency according to the conventional camshaft valve lifting amount and timing and it caused an increase in the volumetric efficiency at high RPMs. According to the results, it was understood that the effect of intake valve closing time is important on volumetric efficiency. Additionally, this study shows that the swirl ratio is significantly affected by EMV system.

Investigating the Effects of Variable Valve Timing on Spark Ignition Engine Performance

Investigating the Effects of Variable Valve Timing on Spark Ignition Engine Performance

An Investigation on the Effect of Variable Valve Timing on Piston Engine for Lightweight Aircraft

As the Lycoming engine had failed its attempt on using variable valve timing for aircraft piston engine back in 1940s, the idea of the technology was abandoned as the turbines were then introduced in the aviation for better performance and greater power. Since piston engines produce smaller power efficiently in the low speed than turbine engines, they are presently still practically used in most of lightweight aircraft. With the use of a variable valve timing mechanism, it may help to increase the amount of air inlet and to provide more power output with lesser fuel consumption. With the use of this new valve system, improvements in the performance of automobile engines have been recorded. The indicated improvements, however, are limited to automobile engines running with high revolutions only. Engine simulation program was run in this investigation as an attempt to predict engine performances that are appropriate for lightweight aircraft.

Analysis of Optimum Valve Timing Based on Diesel Engine Comprehensive Performance

In order to investigate the effect of variable valve timing on engine power performance, fuel economy performance and emission performance quantitatively, proceed to optimize it, a high speed diese Specific Fuel Consumption, quantity of NOx and Soot, mass of fresh charge were calculated by 3-D CFD method l engine contains variable valve timing system (VVT) was researched. The effective power, at 6 speeds conditions which valve timing parameters were modulated. A method of calculating CRG was suggested. The results indicate that exhaust advance angle has a little impact on sweep and emission performance, but a big impact on ventilation losses. The optimum exhaust advance angle should be increased as engine speed rises. Increasing intake lag angle in low engine speed could increase coefficient of charge, thereby increase power performance and fuel economy performance. Comprehensive performance could be increased by decreasing intake delay angle under high speed.

Improvement of Diesel Engine Performance by Variable Valve Train System

The effects of variable valve timing and lift are studied in order to improve the thermal efficiency of a diesel engine, while maintaining low emission levels. At high load conditions, early closing of one of the intake valves or early intake valve opening realizes an enhancement of swirl intensity without increased pumping losses, and retarded intake valve closing reduces the effective compression ratio, both of which result in an increased exhaust gas recirculation ratio and an advanced fuel injection timing. Consequently low NO x formation and an improved thermal efficiency can be achieved simultaneously. At low load conditions, the injected fuel is dispersed in the cylinder by air swirl because of the small fuel quantity, and the increased effective compression ratio achieved by the early intake valve closing becomes effective at reducing hydrocarbon emissions. It is confirmed that the variable valve timing and lift system introduced in this research can flexibly change the engine parameters that govern engine combustion at various engine operating conditions. As a result, a 40 per cent reduction of engine-out NO x emissions and 4 per cent improvement of fuel consumption in the New European Driving Cycle (NEDC) are achieved. Furthermore, low-end torque could be increased by 40 per cent, utilizing exhaust pressure pulsation by matching of exhaust valve opening timing, and the overlap of intake and exhaust valve opening around top dead centre in the intake stroke. To enhance these benefits a new piston chamber with deep valve pockets is developed and its effect is investigated.

A modelling study into the effects of variable valve timing on the gas exchange process and performance of a 4-valve DI homogeneous charge compression ignition (HCCI) engine

Homogeneous charge compression ignition (HCCI) combustion mode is a relatively new combustion technology that can be achieved by using specially designed cams with reduced lift and duration. The auto-ignition in HCCI engine can be facilitated by adjusting the timing of the exhaust-valve-closing and, to some extent, the timing of the intake-valve-opening so as to capture a proportion of the hot exhaust gases in the engine cylinder during the gas exchange process. The effects of variable valve timing strategy on the gas exchange process and performance of a 4-valve direct injection HCCI engine were computationally investigated using a 1D fluid-dynamic engine cycle simulation code. A non-typical intake valve strategy was examined; whereby the intake valves were assumed to be independently actuated with the same valve-lift profile but at different timings. Using such an intake valves strategy, the obtained results showed that the operating range of the exhaust-valve-timing within which the HCCI combustion can be facilitated and maintained becomes much wider than that of the typical intake-valve-timing case. Also it was found that the engine parameters such as load and volumetric efficiency are significantly modified with the use of the non-typical intake-valve-timing. Additionally, the results demonstrated the potential of the non-typical intake-valve strategy in achieving and maintaining the HCCI combustion at much lower loads within a wide range of valve timings. Minimizing the pumping work penalty, and consequently improving the fuel economy, was shown as an advantage of using the non-typical intake-valve-timing with the timing of the early intake valve coupled with a symmetric degree of exhaust-valve-closing timing.

An investigation and evaluation of variable-valve-timing and variable-valve-actuation strategies in a diesel homogeneous charge compression ignition engine using three-dimensional computational fluid dynamics

A three-dimensional computational fluid dynamics modelling was carried out to investigate effects of variable valve timing (VVT) and variable valve actuation (VVA) on gas exchange and fuel-air mixing processes in a diesel homogeneous charge compression ignition (HCCI) engine with early fuel injection. Four VVT or VVA strategies were conducted for this study: first, a negative valve overlap (NVO) strategy with fixed exhaust-valve-opening (EVO) and intake-valve-closing (IVC) timings but variable valve lifts, referred to as the NVO strategy; second, the NVO strategy with fixed valve profiles but variable EVO and IVC timings, referred to as the EVO strategy; third, the NVO strategy with fixed valve lifts and fixed EVO and IVC timings but variable exhaust-valve-closing (EVC) and intake-valve-opening timings, referred to as the EVC strategy; fourth, VVA with just variable valve lifts, referred to as the VMAX strategy. The results indicate that suitable NVO settings will enhance in-cylinder tumble and then increase turbulence intensity before compression end, although the increased NVO has a negative contribution to swirl ratio. It was found that reducing valve lifts alone is not an efficient way to retain the residual gas, but the function of reduced valve lifts will become significantly obvious by combining it with increasing NVO. For the effect of NVO on in-cylinder temperature, longer NVO not only will increase in-cylinder temperature because of the higher residual gas rate but also will improve the in-cylinder temperature homogeneity. On lowering the maximum valve lift or increasing the NVO, the unmixed region of the in-cylinder charge shrinks. The fuel-rich region expands because of the high intake velocity and enhanced turbulence intensity. This is beneficial to the forming of a global homogeneous charge. It has been noted from the current study that, as the droplet distribution may be influenced more by the in-cylinder air motion caused by NVO when the average droplet size is smaller, it is recommended that future studies explore the effects of VVT and VVA on diesel HCCI mixing and combustion with various advanced fuel injection strategies.