The invention relates to an internal combustion engine provided with a system for controllably opening and closing exhaust and intake valves to the cylinders.
Within the area of vehicle technology, it is becoming increasingly common to use turbocharged engines, that is to say engines in which supercharging is effected by means of a compressor which is driven by an exhaust-driven turbine. At high power, greater charging is used than in an ordinary engine. In order, in this connection, to avoid knocking and excessively high combustion pressures, ignition is set relatively late. This in turn results in the expansion possibility for the combusted gases being small, at the same time as the temperature reduction during expansion does not have time to become as great. The exhaust-gas temperature is therefore high. The problems are aggravated by the fact that the proportion of residual gases in the cylinder is high, with attendant heating of the new charge and risk of premature ignition of the same. In order that, to the greatest possible extent, the exhaust-gas temperature is manageable, it is usual, at high power output, to use a rich mixture for the engine, with surplus fuel serving as a coolant in the cylinder.
At full throttle, a poor volumetric efficiency is obtained as a result of a negative pressure difference between inlets and outlets during the period when inlet valves and outlet valves are open simultaneously. At low engine speed, this negative pressure difference is caused by a disruptive pressure pulse in the exhaust branch pipe from the next cylinder igniting. At high engine speed, throttling of the engine at the exhaust-gas turbine also contributes to creating a negative pressure difference.
It has been shown that only the exhaust-gas pulse which is delivered to the exhaust-gas turbine in the bottom dead-centre position of the piston can be used effectively for driving the exhaust-gas turbine, while the remaining exhaust gases have a pressure which is far too low to make an additional contribution.
For the purpose of improving the operation of the exhaust-gas turbine, it is previously known from GB 2 185 286 to divide the exhaust-gas flow so that only the high-pressure pulse goes to the exhaust-gas turbine, while the low-pressure pulse bypasses the exhaust-gas turbine. In this way, disruptive pressure pulses are eliminated and the negative low-pressure cycle is converted into a positive low-pressure cycle. This is achieved by virtue of the fact that there are at least two exhaust valves in every cylinder, which open differently and feed different exhaust manifolds.
The result is better ventilation of the cylinder, by means of which the proportion of residual gases is reduced. The combustion is better and ignition can be set earlier as knocking only appears at a higher pressure than previously.
As the load increases, pressure limitations are required because of knocking, as a result of which the charging pressure must be limited at higher loads. This has a negative effect on the performance of the engine.
It is an object of the invention to provide an improved internal combustion engine. Still another object is to achieve better performance of the internal combustion engine at high load.
The object of the invention is achieved by producing an internal combustion engine as indicated in the introduction with the features which are indicated in Patent Claim 1.
An internal combustion engine in accordance with the invention makes it possible to bring about effective charging as the engine speed increases by carrying out a greater part of the compression outside the cylinder and by cooling compressed air before it enters the cylinder. This makes possible a good degree of filling and also a lower temperature in the cylinder than if all the compression took place in it. The result is that the temperature of the ready-compressed gas can be lowered on the order of 50-100xc2x0 C., with the attendant favorable effect on the knocking tolerance and the exhaust-gas temperature.
The invention can be realized in an embodiment in which the intake valve closes either early, while the piston is on the way down, or late, while the piston is on the way back up. In the former case, the air taken in has time to expand and cool in the cylinder before compression begins. In the latter case, some of the air taken in is discharged again while the piston is on its way up before compression.
The combination of the divided exhaust-gas period and the method selected for charging the cylinder (Miller principle) consequently makes possible an improvement in performance at higher engine speed and high power but presupposes good variable valve control. If, for example, the compression ratio in the cylinder is reduced from 10.0:1 to 7.5:1 by closing the inlet valve roughly 60xc2x0 later and at the same time increasing the absolute charging pressure by 50% (so as to maintain the pressure at the end of the compression stroke) and keeping the geometrical compression ratio unchanged, this has the same effect on the temperature at the end of the compression stroke as lowering the temperature in the inlet pipe by roughly 34xc2x0 C. The thermal load in the engine and the formation of NOx are also reduced by turbocharging according to the Miller principle. Moreover, the fuel consumption can be reduced by roughly 5% as a result of the combusted gas imparting less heat to the walls of the combustion chamber and as a result of the piston performing a smaller proportion of the total compression work. In other words, during the first part of the intake stroke, the pressure inside the cylinder is higher during charging according to the Miller principle, which increases the efficiency.
According to the invention, it is advantageous if the arrangement is such that the intake valve, since it closes earlier as the engine speed increases, opens earlier also. The same can apply correspondingly for the exhaust-gas valves. This facilitates effective gas exchange.
Further features and advantages of the solution according to the invention emerge from the description and the other patent claims.
The invention will be described in greater detail below with reference to exemplary embodiments shown in the drawings.