Most people think of a car as something they fill with petrol and drive from point A to point B. But have you ever wondered, "How does it actually do that?" What causes it to move? Unless you currently drive an electric car on a daily basis, the wonder of how comes down to the internal-combustion engine—that noisy thing beneath the hood. But how precisely does an engine work?
Internal-combustion engines are heat engines in the sense that they transform energy from the heat of burning petrol into mechanical work, or torque. To move the car, torque is delivered to the wheels.
Pistons in engines move up and down inside metal tubes called cylinders. Consider riding a bicycle: To turn the pedals, your legs go up and down. Pistons are connected to a crankshaft by rods (similar to your shins), and they move up and down to spin the engine's crankshaft, just as your legs spin the bike's—which in turn powers the bike's or car's drive wheels. The engine of a vehicle normally has between two and twelve cylinders, each with a piston that moves up and down.
Where Engine Power Comes From Thousands of tiny controlled explosions occur each minute, powering the pistons up and down, formed by combining fuel with oxygen and igniting the mixture. The combustion, or power, stroke occurs each time the fuel ignites. This miniexplosion's heat and expanding gases push the piston down in the cylinder.
Almost all of today's internal-combustion engines (to keep things simple, we'll concentrate on petrol engines here) are four-stroke. There are three more strokes in addition to the combustion stroke, which pulls the piston down from the top of the cylinder. These are intake, compression, and exhaust.
To burn fuel, engines require air (specifically, oxygen). During the intake stroke, valves open, allowing the piston to descend downward like a syringe, taking in ambient air through the engine's intake system. When the piston hits the bottom of its stroke, the intake valves close, effectively sealing the cylinder for the compression stroke, which is the inverse of the intake stroke. The piston's upward movement compresses the intake charge.
The Different Engine Types
there are exceptions and minor variances between the internal-combustion engines on the market. For example, Atkinson-cycle engines alter the valve timing to produce a more efficient but less powerful engine. Turbocharging and supercharging, which are grouped together as forced-induction alternatives, push additional air into the engine, increasing available oxygen and hence the quantity of fuel that can be burned—resulting in greater power when you want it and better efficiency when you don't. All of this is accomplished by diesel engines without the use of spark plugs. But, as long as the engine is internal-combustion, the fundamentals of how it operates stay the same. And now you know who they are.
Four strokes
Petrol is delivered straight into the cylinders towards the peak of the compression stroke in today's most modern engines. (In some engines, the air and fuel are premixed during the intake stroke.) Spark plugs ignite the air and fuel combination immediately before the piston hits the top of its movement, known as top dead centre.
During the combustion stroke, the piston is pushed in the opposite direction (down) by the expansion of hot, burning gases. This is the stroke that causes your car's wheels to turn, exactly like when you push down on your bike's pedals.
Exhaust valves open as the combustion stroke approaches bottom dead centre, allowing the combustion gases to be forced out of the engine (similar to a syringe discharging air) when the piston rises again. When the exhaust is ejected (it travels through the car's exhaust system before exiting the back of the vehicle), the exhaust valves close at top dead centre, and the cycle begins again.
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