نشریه تحقیقات موتور
پیاپی 47 (تابستان 1396)

In low ambient temperatures the engine will have poor output torque and power generation at the first cycles of its performance, long start time and huge noxious emissions because of insufficient temperature and quality of air-fuel mixture and failure of flame to burn the charge. It is noted that any attempt to increase the charge temperature leads to better combustion conditions. By applying thin ceramic coating over the piston top surface and increasing the temperature of engine inner walls, more efficient combustion in shorter time can be provided for the engine. In current research, a transient thermal study is developed for simulating the engine performance under cold starting conditions and predicting the effect of insulating layer thickness on the charge temperature before ignition timing. The results demonstrate that the ceramic coating can provide higher temperature for piston crown and effectively improve the temperature of charge during the first cranking cycles of engine cold start stage.

Implementing emission regulations in the transport sector is enforcing manufacturers to improve performance of Heavy Duty (HD) vehicles as they are accountable for 25% of CO2 emissions within EU. The currently reported research was involved with evaluating partial replacement of hydrogen with diesel fuel experimentally and intake air enrichment with oxygen in a heavy-duty diesel single cylinder engine. Fumigation of hydrogen was done into the intake system at two particular engine loads (6 and 12 bar IMEPn). Indicated efficiency was increased up to 4.6% at 6 bar IMEPn and 2.4% at 12 bar IMEPn, while reducing CO2 emissions by 58% and 32% at 6 and 12 bar IMEPn respectively. Applying diesel pre-injection was required in order to mitigate the high pressure rise rates known as combustion noise. Furthermore, intake air enrichment with oxygen resulted in faster combustion process. This could curb soot and minimised CO emissions to the detriment of NOx increase.

In the present work, the experimental study was performed to investigate the flame propagation and combustion characteristics of methane (CH4)-air mixture in a constant-volume combustion vessel (CV) under lean (f= 0.7) and diluted engine-like condition. In addition, propane (C3H8) in two volume fractions of 20% and 40% was added to methane as the impurity and EGR gas. Schlieren photography of propagating spherical flame using a high speed camera and edge finding of flame pictures via a Matlab code was used to find the rate of flame instantaneous radius. The unstretched flame propagation speed and Markstein lengths were obtained via a nonlinear methodology in different chamber pressures. Flame speed modeling was performed using an improved version of kinetics model. The result showed that adding propane to methane increases the laminar flame speed and propensity of instability. The cellular instabilities for the diluted methane/propane–air flames were interpreted and evaluated in the viewpoint of the hydrodynamic and diffusional-thermal instabilities.

In recent years diesel direct injection engines have been received more attention due to their high fuel efficiency and torque characteristics compared with gasoline engines. However, the cold start difficulties and cold diesel fuel behavior are still important issues which need improvement. In this study, a novel experimental facility has been developed using Stirling engine for fuel cooling and estimating the gel point and CFPP of diesel fuel and evaluating of the very cold diesel fuel behavior inside high-pressure common rail diesel fuel injection system. The results showed that the diesel fuel that is produced inside Iran has the capability to flow inside fuel injection system at temperature of -250C without using anti-freeze. The results also provided a good understanding of cold diesel fuel behavior and the test rig can be used for evaluating the fuel injection system components under ultra-cold conditions.

The homogeneous charge compression ignition (HCCI) is an advantageous combustion concept for internal combustion engines in terms of efficiency and pollutant emissions. This combustion strategy is one of the main types of low temperature combustion (LTC) concepts. It is a spontaneous multi-point combustion of a highly diluted premixed fuel-air mixture. However, its limited operating range suppressed its successful utilization. One applicable method to use this combustion concept in engines is to combine it with other combustion strategies while each combustion concept come into work during its optimum operating range. The idea of an HCCI combustion engine involving direct fuel injection into the combustion chamber is an applicable strategy aiming at this target. In the HCCI engines there is no direct control method for the time of auto-ignition. The octane number or fuel auto-ignitability characteristic is one of the most influential parameters on the combustion timing. Therefore, this study investigates the effect of the octane number on the direct injection (DI) HCCI combustion. The mathematical investigations are based on a novel two-stage combustion model. The combustion is predicted with the single-zone model from the intake valve closing to the end of injection. Thereupon, it is predicted by the multi-zone model. The results indicate that the octane number has profound effects on combustion phasing and delivered power. Therefore, cycle-by-cycle and cylinder-to-cylinder control of the combustion phasing by octane number adjustment is possible.

Reactive mufflers depend on reducing the exhaust noise through a volume depending on reflection of sound noises. Muffler analysis is always challenging task due to complex design, shape and size limitation for specific application. That is why the reduction of exhaust noise generated from engine is in today's world an important issue. Attaching a muffler in the exhaust pipe is the good option for reducing noise. But muffler requires specific design and construction considering various noise parameters produced by the engine. Since early development of mufflers, the main objective of design was attenuation of sound in regular mufflers. A typical sound generated by vibrational waves is usually based on the experience and design experience, which can be enhanced by optimizing the internal structure of the exhaust system. The use of baffles and perforations has a significant effect on reducing the transmission loss in a silencer exhaust system. In order to evaluate the response effect of a silencer, the effect of baffle and perforations, without baffles and perforations, was evaluated in COMSOL5.2 software. Which results in numerical results with close experimental results and the effect of baffle and perforations, have shown a good reduction in transmissibility, and their partial differences are due to the features of geometry design.