Four-stroke
From Quad Hub Wiki
Today internal combustion engines in cars, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle as opposed to a two-stroke cycle. The four strokes refer to intake, compression, combustion (power) and exhaust strokes that occur during two crankshaft rotations per working cycle of the Gasoline engine and Diesel engine.
Contents |
Diagram
- Intake (induction) stroke
- Compression stroke
- Power stroke
- Exhaust stroke
In this example animation, the right blue side is the intake and the left yellow side is the exhaust. The cylinder wall is a thin sleeve surrounded by cooling water.
The cycle begins at top dead center (TDC), when the piston is furthest away from the axis of the crankshaft. On the intake or induction stroke of the piston, the piston descends from the top of the cylinder, reducing the pressure inside the cylinder. A mixture of fuel and air is forced (by atmospheric or greater pressure) into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s), and the compression stroke compresses the fuel–air mixture.
The air–fuel mixture is then ignited near the end of the compression stroke, usually by a spark plug. The resulting pressure of burning gases pushes the piston through the power stroke. In the exhaust stroke, the piston pushes the products of combustion from the cylinder through an exhaust valve or valves.
Design and Engineering Principles
Supercharging
One way to increase engine power is to force more air and fuel into the cylinder so that more power can be produced from each power stroke. This was originally done using a type of air compression device known as a supercharger which is powered by the engine crankshaft.
Supercharging increases the power output limits of four-stroke engine output but the supercharger is always running. Continuous compression of the intake air requires some mechanical energy to accomplish, so the supercharger has a cost of reduced fuel efficiency when the engine is operating at low power levels or when the engine is simply unloaded and idling.
Turbocharging
Turbocharging was designed as a part-time method of compressing more fuel and air into the cylinder head. It consists of a two piece high-speed turbine assembly with one side compressing the intake air and the other side powered by the exhaust gas outflow.
When idling and at low to moderate speeds, the turbocharger does not do much of anything and the engine operates in a naturally aspirated manner. When much more power output is required, the engine speed is increased until the exhaust gases are sufficient to spin up the turbocharger and start compressing much more air than normal into the intake manifold.
Turbocharging allows for more efficient engine operation at low to moderate speeds, but there is a design limitation known as turbo lag. The increased engine power is not immediately available, due to the need to sharply increase engine RPM and spin up the turbo, before the turbo starts to do any useful air compression.
