Question

Research how an internal combustion engine operates. Describe four steps of a combustion cycle. What materials go in and out of the engine during each step? How many cylinders are involved in one cycle? What energy processes take place during each stroke? In which steps is work done?

Answer 1

Internal combustion engine operation

Combustion, also known as burning, is the basic chemical process of releasing energy from a fuel and air mixture. In an internal combustion engine (ICE), the ignition and combustion of the fuel occurs within the engine itself. The engine then partially converts the energy from the combustion to work. The engine consists of a fixed cylinder and a moving piston. The expanding combustion gases push the piston, which in turn rotates the crankshaft. Ultimately, through a system of gears in the powertrain, this motion drives the vehicle’s wheels.

There are two kinds of internal combustion engines currently in production: the spark ignition gasoline engine and the compression ignition diesel engine. Most of these are four-stroke cycle engines, meaning four piston strokes are needed to complete a cycle. The cycle includes four distinct processes: intake, compression, combustion and power stroke, and exhaust.

Spark ignition gasoline and compression ignition diesel engines differ in how they supply and ignite the fuel. In a spark ignition engine, the fuel is mixed with air and then inducted into the cylinder during the intake process. After the piston compresses the fuel-air mixture, the spark ignites it, causing combustion. The expansion of the combustion gases pushes the piston during the power stroke. In a diesel engine, only air is inducted into the engine and then compressed. Diesel engines then spray the fuel into the hot compressed air at a suitable, measured rate, causing it to ignite.

Four steps of a combustion cycle

A four-cycle engine works with 4 basic steps to a successful rotation of the crankshaft: the intake, compression, power and exhaust stroke. Each engine cylinder has four openings for the intake, exhaust, spark plug and fuel injection. The piston is driven by the engine's crankshaft whereas the intake and exhaust valves are driven by the camshaft. The crankshaft and camshaft are connected by a timing belt/chain to maintain synchronization between them. The various processes comprising the cycles of a four-stroke engine are explained below:

Intake Stroke: The intake stroke is where the intake valves are open and the air is drawn into the cylinder. The fuel injector sprays the fuel into the cylinder to achieve the perfect air-fuel ratio. The downward movement of the piston causes the air and fuel to be sucked into the cylinder.

Compression Stroke: The next is the compression cycle where both the intake and exhaust valves are closed. The upward movement of the piston causes the air-fuel mixture to be compressed upwards towards the spark plug. The compression makes the air-fuel combination volatile for easier ignition.

Combustion/Power Stroke: During the power/combustion stroke, both the intake and exhaust valves are still closed. The spark plug produces a spark to ignite the compressed air-fuel mixture. The resulting energy of the combustion forcefully pushes the piston downward.

Exhaust Stroke: The last cycle is the exhaust stroke, when the exhaust valves open and the exhaust gases are forced up by the returning piston.

Most cars have four, six, or eight cylinders. If the car has four cylinders, called the straight-four engine, all of its cylinders are designed to be in a straight line. This configuration is common to cars that have a 2.4-liter engine displacement. An engine of a car with six cylinders is called a V6 engine.