Exhaust back pressure and exhaust systems are more complicated than you think

Exhaust back pressure and exhaust systems

are more complicated than you think

    Most people know that when the engine is working, it needs the assistance of the intake system and the exhaust system. Air enters the combustion chamber through the intake system and mixes with fuel to form a combustible mixture. After compression, it is ignited by the spark plug, pushing the piston to move, and then driving the crankshaft to rotate. The burned exhaust gas is then discharged out of the car through the exhaust system. This completes the four-stroke working cycle of intake, compression, power and exhaust that we often call.

    This doesn't sound complicated, especially during the exhaust phase. In the eyes of many people, it is nothing more than the exhaust valve opening, the high-temperature exhaust gas being squeezed out of the cylinder by the piston, and being discharged out of the car along the exhaust pipe. But in fact, there are many factors to consider when designing the exhaust system. It is not simply connecting the exhaust pipe and the muffler together, and the design of the exhaust system will also affect the power output performance of the engine.

    If you have some knowledge about exhaust modifications, I believe the term "exhaust back pressure" is not unfamiliar to you, but not everyone really understands it. So what exactly is exhaust back pressure and how is it generated?

   When the power stroke is near the end, the engine piston is at the bottom dead center. At this time, the exhaust stroke is entered and the exhaust valve opens. Since the pressure inside the cylinder is higher than the pressure outside the cylinder at this time, it can almost reach 6-7 times the outside atmospheric pressure. Therefore, the high-temperature exhaust gas after combustion is quickly discharged from the cylinder through the exhaust valve. At the same time, the piston moves past the bottom dead center and moves up to the top dead center, forcing the exhaust gas out of the cylinder. When the piston reaches near the top dead center, the exhaust process ends.

     At the moment when the exhaust valve is opened, due to the relatively large air pressure difference between the inside and outside, a pressure wave will be formed and pushed out of the cylinder at the speed of sound. The combustion exhaust gas in the cylinder will also be brought out along with the pressure wave. When the pressure wave formed encounters changes in the diameter of the exhaust pipe, it will be partially reflected and return to the exhaust valve along the intake manifold. This is what we call exhaust back pressure.

    To simply understand, exhaust back pressure can also be regarded as exhaust resistance, which is generated by the bending, intersection, etc. of the exhaust pipe. The larger the exhaust back pressure is, the greater the exhaust resistance is and the harder it is for exhaust gas to be discharged; the smaller the exhaust back pressure is, the smaller the exhaust resistance is and the smoother the exhaust is.

     According to common sense, exhaust back pressure will hinder the smoothness of exhaust, thereby affecting the power output of the engine. But why can’t exhaust back pressure be avoided when designing the exhaust system?

     In fact, during the exhaust stroke, when the piston moves upward to about half way, the pressure difference between the inside and outside of the cylinder has basically disappeared. This means that the combustion exhaust gas in the cylinder will no longer be actively discharged from the exhaust valve to the outside of the cylinder. The problem arises at this time, we obviously do not want the combustion exhaust gas to stay in the cylinder, because this will reduce the efficiency of the next combustion cycle.

     At this time, if the reverse pressure wave generated by the exhaust back pressure can reach the exhaust valve at the right time, that is, within a specific speed range, just when the exhaust valve is closed, and the It is ideal to take away the remaining low-pressure combustion exhaust gas and form a vacuum zone again. Therefore, exhaust back pressure is beneficial under certain circumstances.

    Therefore, when designing the exhaust system, power system engineers will repeatedly simulate and calculate the length, diameter, bending method of the exhaust pipe, as well as the design of the exhaust pipe head section, etc. Comprehensively consider the relationship between exhaust speed and exhaust back pressure to maximize engine efficiency.

      Of course, when we perform exhaust modifications in the later stage, we cannot blindly reduce the exhaust back pressure and only consider the exhaust sound. Because although this can improve the engine's power performance at medium and high speeds, in most daily low-speed working conditions, the power performance will actually decrease.

About SNW

     SNW is mainly engaged in the Manufacturing and Wholesale of automobile exhaust system parts,The exhaust system parts mainly includes exhaust manifolds, headers, downpipes, cat-back systems,aluminum alloy intake pipes, T-pipe, Y pipe, X pipe, H pipe and other performance parts.