Car engines, more specifically the four-cycle internal combustion engine, have been around for decades. New technology has made some differences in the way engines operate to increase efficiency and reduce emissions, but the underlying mechanisms are still the same.

A functional engine requires fuel, spark and compression. If one of those three is missing or not working correctly, the engine will not run. A four-cycle engine like the kind used in automobiles has (as the name implies) four cycles for each piston. The cycle starts with the piston drawing back and sucking in fresh air and fuel. In the second cycle, the piston moves forward and compresses the mixture, and a spark ignites it when the piston is near the top of the stroke. This causes an explosion that forces the piston down, creating power as the third part of the cycle. After the power stroke, the piston then moves back up and forces out the exhaust gases for the fourth cycle. When the piston starts moving down again, it draws in fresh air and fuel, starting the cycle again.

Early car engines used a carburetor. This device used the air flow created from the piston to turn gasoline into a mist to mix with the air. Modern vehicles now use pressurized fuel with fuel injectors. A fuel injector is an electric valve and spray nozzle controlled by the vehicle's computer. Fuel injectors have the advantage of more precise control of how much fuel and when the fuel is delivered. Original fuel injection systems used a throttle body in which a single fuel injector sprayed the fuel mist into the air intake manifold. The fuel was then sucked into all the pistons. Most vehicles today have a fuel injector for each piston near the intake valve of that piston, allowing more precise control.

Engine Output

Source: ProjectManhattan / CC BY-SA 3.0 On the output side of a modern vehicle, oxygen sensors measure the products of combustion to determine how much oxygen is left in the mixture. The amount of oxygen in the exhaust correlates to how completely the fuel has burned. Based on the readings from the oxygen sensor, the computer will adjust the amount of fuel injected to optimize efficiency and power. In older cars, this feedback loop and control mechanism did not exist.

The spark in an engine is created in the spark plug, a ceramic/metal part located at the top of the piston. Without the spark, the fuel wouldn’t ignite, and the spark must happen at a precise time in the cycle. Typically, a spark fires just before the piston reaches the top, known as top dead center (TDC). This timing allows the gas to ignite and reach pressure just as the momentum of the piston has carried it past TDC so it maximizes combustion. To achieve this timing, the exact position of the engine is measured. Based on the sensor data, the computer sends an electrical pulse to trigger a spark at just the right moment. Modern engines have a coil for each piston, which is triggered by the car computer. Older vehicles use a single coil to create the high voltage spark and then send the power to each piston by wires and a device called a distributor.

Each piston in an engine is connected to a crank shaft, which turns the up and down movement into rotational movement. A crank shaft works in the same way that pushing down on pedals of a bicycle is turned into rotation. Each cylinder also needs valves — two per cylinder. One valve is for the intake where fresh air and fuel come in, and the other valve is where the exhaust gases exit. To achieve the four cycles, the valves must be in exact time with the piston and open and close differently for each part of the cycle. When fresh air is pulled in, the intake valve is open and the exhaust valve closed. For compression and the power stroke, both valves are closed and then for the exhaust part, the exhaust valve must open with the intake valve closed. To achieve this valve timing, one or several cam shafts are used. These shafts turn with the engine by way of the timing chain or belt. The cam shaft has oblong sections that push as it spins to open the valve.

Engine Input

On the intake side of things is the throttle body to control an engine's speed. It has a valve in it called a butterfly valve that opens when a driver steps on the gas. This valve allows more air into the intake as well as the throttle position sensor telling the engine to spray more gas from the fuel injectors. The output side is a little simpler — a manifold combines the exhaust gases from each cylinder and sends them to the catalytic converter, which helps clean the exhaust. The muffler then reduces the noise of the engine.

One other important part of an engine is that it must be kept cool and lubricated. Engines are designed with passageways to allow liquid coolant to flow around the outside of the cylinders. A pump is connected to the engine to keep coolant flowing through the engine and to the radiator to remove the excess heat. Engines also require lubrication, achieved with engine oil, which circulates around the crank shaft, cam shaft and on the back side of the pistons. Engine oil is also pumped through the engine and filtered to keep the engine running smooth.

It takes a lot of moving parts, and many engines are quite complicated in design, but in the end what counts is that an engine runs and gets a driver where they need to go.