12/22/2023 0 Comments Airflow performance injectionDoubling the charge density of an engine more than doubles its power output, provided an optimal air/fuel ratio is maintained. Supercharging and turbocharging are very potent ways to increase an engine’s power output. The combustion of an exhaust turbine coupled to a compressor on the intake side of the engine is called a turbocharger. However, if the compressor is driven by a turbine placed in the exhaust system of the engine, that combination of turbine and compressor is called a turbocharger (see below). If that compressor is driven by a mechanical linkage to the engine, such as a belt drive or gear drive, the compressor is simply called a supercharger. If we use some sort of compressor to increase the pressure above atmospheric pressure, that’s called “supercharging” the engine. And of course, the exhaust must exit against that same atmospheric pressure. On a non-supercharged engine, the only pressure available to force air into the engine is normal atmospheric pressure. For a non-supercharged engine, that’s simply atmospheric pressure, or about 14.7 pounds per square inch, measured at sea level (see below). But most of all, the biggest factor is the pressure of the air available to flow into the engine on its intake stroke. Even when the throttle is wide open, restrictions in either the intake or exhaust passages effectively become throttles to the engine’s airflow capability.Įven the temperature of the incoming air will affect its density (see “Cool Air Equals Power” elsewhere on this site). The amount of power an engine can make is directly related to the free flow of incoming air and outgoing exhaust through all of the engine’s passages. To make maximum power that ratio would fall to approximately 12.5:l.īesides throttle position, many things affect airflow, such as restrictions in either the intake or exhaust paths (which can become de facto throttles in themselves), or the design of the camshaft to control valve openings and closings (see below). For normal cruising operation, a gasoline engine operates at around a 14.7:l air-to-fuel ratio, so it would need roughly 14.7 pounds of air to mix with every pound of fuel. To say it another way, assuming you mix the correct amount of fuel with the air, how much power an engine can make is dependent on airflow. The more oxygen (air) that flows into the engine, the more fuel that can be burned, and the more power the engine can make. Let’s think of charge density as the amount of oxygen available to support the combustion of the fuel. This is often called the “charge density”. As the throttle opens, more air mass can flow in to fill the engine, and the density will increase. Thus, the air will be of very low density. When the throttle is closed, very little air mass flows into the engine, so that small amount has to expand to fill our 300 cubic inches. And while it is true that more air mass flows into the engine when the throttle opens, the engine’s size, or displacement, never changes, so the only actual difference is the density of the air that fills that displacement. The engine takes in more air when the throttle is open. It does this whether the throttle is open or closed. Two revolutions of the crankshaft, four distinct cycles – it’s the basic Otto-cycle piston engine.īy its very design, this means our 300-cubic-inch engine takes in 300 cubic inches of air every two revolutions of the crankshaft. The following upward movement of the piston is the exhaust stroke. On the next revolution of the crankshaft, the power stroke occurs as the air/fuel mixture burns pushing the piston down. These first two strokes occur during one revolution of the crankshaft (see Fig.1). Most of you know how an engine works, but as a simple review, a four-cycle engine has an intake stroke to draw the air/fuel mixture into the cylinder as the piston moves down the cylinder bore, followed by a compression stroke during the following upward movement of the piston. Suppose you have a 300-cubic-inch gasoline four-cycle engine. Each cycle is often referred to as a “stroke” of the piston. These four cycles occur over two revolutions of the crankshaft. The four cycles of an Otto-cycle engine: intake, compression, power, and exhaust cycles. It begins with airflow, but it doesn’t end there.
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