Several factors affecting the combustion process of automotive engines
  • Automotive

First, the antiknock performance of gasoline and the antiknock performance of evaporative gasoline are one of the most important factors that affect the combustion process of gasoline engines. The ability to prevent detonation and combustion of fuel during combustion is generally evaluated by octane number. The vaporization of gasoline is usually characterized by distillate composition and vapor pressure. During the gasoline distillation test, the temperature and final temperature of 10%, 50% and 90% fractions evaporated were measured respectively. 10% the distillation temperature is related to the cold start performance of the gasoline engine. The lower the temperature is, the easier it is to start the engine. The 50% distillate temperature indicates that the vaporization of the middle fraction of the gasoline has an effect on the preheating, acceleration and working stability of the engine. The lower the temperature, the faster the engine preheating, the faster the engine is accelerated from low speed, and the mixture can be evenly distributed to the cylinders, and the engine can not stall at idle speed at idle speed. 90% distillate temperature and final distillation temperature can determine the content of heavy distillates that are not easily evaporated in gasoline. The lower the 90% distillate temperature, the less the fuel is high boiling point, and the less heavy fraction is not easily evaporated; this temperature is too high, because the evaporation will not complete the combustion, increase fuel consumption, and the unevaporated fuel will leak into the crankcase from the cylinder wall. Dilute the lubricating oil. The vaporization of gasoline is also limited by saturated vapor pressure. The vaporization of gasoline is too high, and the excessive steam pressure caused by evaporation at a certain temperature will make the fuel system the barrier of steam bubbles (steam resistance), reducing or interrupting the supply of fuel and affecting the normal work of the carburetor. Two. When ignition advance angle is too large, detonation is easily caused, but the conditions of slight detonation are also beneficial to normal combustion, so the ignition advance angle is generally adjusted to a little bit later at the start angle of detonation. The optimal ignition advance angle also varies with the size of the speed and load, as the speed increases, the load decreases, and the best ignition advance angle needs to be properly increased, so some gasoline engine ignition system has an automatic adjustment device with the ignition advance angle. Three. The mixture gas mixture becomes thinner and the flame propagation velocity decreases, but the fuel utilization is improved and the combustion tends to be complete. When the mixture ratio is a=1.05 to 1.15, the combustion is most complete. At this time, the flame propagation velocity is not too much, and the heat loss is reduced. If the mixture further dilute, the distance between the fuel molecules is too large, if the special measures are not adopted, the combustion condition will deteriorate. When the gasoline engine works with too thin mixture, the carburetor tempering will occur (this is due to the low burning rate), so that the combustion process is elongated to the end of the exhaust stroke and continues when the valve is opened, thus burning the combustible mixture in the intake system. When the mixture is too strong (mixture ratio is a <0. 85 to 0.95), the propagation velocity of flame front will be greatly reduced due to lack of oxygen. Because the distribution of mixed gas and the composition of mixed gas in each cylinder of a multi cylinder gasoline engine are not necessarily uniform, the air cylinder has the dilution effect on the new gas or the stratification of the residual gas and the new gas. It will slow the flame propagation in the operation of the gasoline engine. When the mixture is too large or an hour, the cylinder is short of fire or intermittent fire, which causes the cylinder to cause the cylinder. The stability and uniformity of work are reduced. Four. When the speed of the gasoline engine is increased, the speed of the inlet valve of the mixed air flow is increased and the velocity of the piston is also increased in the compression process, so the swirl and turbulence intensity in the cylinder are strengthened, and the mixture of fuel and air is improved and the heat exchange between the burned and unburned gases is accelerated, and the velocity of the flame propagation is roughly proportional. Big. This is the reason why modern gasoline engines can become high speed engines and can also develop at high speed. In general, with the increase of speed, the time used in the combustion process is reduced, but it is not necessarily proportional to the reduction of the positive ratio. The crankshaft angle is often increased and the afterburning increases. In order to ensure the normal combustion process, the ignition time should be advanced in advance. As the speed increases, the duration of the working cycle shortens, the possibility of self ignition of the fuel in the afterburning mixture is smaller by the two stage preparation, and the relative residual waste gas in the combustion chamber is also increased, so the possibility of deflagration is reduced and the conditions of normal combustion have a certain degree of deterioration. When the rotating speed is increased, the temperature of the combustion chamber wall is increased, which will help the combustion process. Five, load reduction and throttle opening decrease, this is the way of gasoline engine load adjustment. At this time, the fresh charge decreases and the residual gas coefficient increases, making the flame front formation longer. As the intake throttle reduces the pressure and temperature of the final compression, the combustion speed decreases, the maximum combustion pressure and the maximum combustion temperature drop, the combustion quality becomes worse, and the afterburning increases, and the loss of heat increases, so the vacuum regulator is required to increase the ignition advance angle automatically. These factors also reduce deflagration tendency, so that deflagration can reduce load in order to eliminate it. Six, the compression ratio can reduce the residual gas coefficient and increase the mixed pressure and temperature of the spark ignition. Thus it is beneficial to the formation of the flame forward, shorten the initial period, accelerate the combustion period of the main combustion period, and the maximum pressure close to the top stop. The high compression ratio reduces the volume of the combustion chamber, the shape tends to be flat, the face volume ratio is increased, the relative amount of mixing gas at the piston top extrusion face at the wall of the combustor is increased, which reduces the heat release of the main combustion period, and also increases the mixed gas amount of the post combustion, and the effective exothermic difference. Increasing the compression ratio will increase the emission of HC and NOx due to the increase of face ratio, the expansion of the maximum combustion temperature and the decrease of temperature at the later stage of exhaust.