![]() 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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