Learn How Car Engines Work Explained – Two-Stroke & Four-Stroke Engines, Air-Fuel Mixture, Valve System, Camshaft, Intake/Compression/Expansion/Exhaust Strokes, Combustion Chamber and Diesel Engines

 

Introduction:

Have you ever turned the key in your ignition, heard that satisfying roar, and wondered what's actually happening under the hood? I know I have. There's something magical about the way thousands of tiny explosions work together to propel you down the road. Today, we're going to take a friendly journey into the heart of your vehicle, the internal combustion engine. Whether you drive a zippy little compact or a rugged pickup truck, the basic principles are surprisingly similar. And trust me by the time we're done, you'll have a whole new appreciation for that mechanical marvel sitting under your hood.

In a passenger car or truck, the engine provides the rotating power to drive the wheels through the transmission and driving axles. It's the heart of the vehicle and like any heart, it needs to work just right to keep everything moving. All automotive engines both gasoline and diesel are classified as internal combustion engines because the combustion or burning takes place inside the engine. These systems require an air fuel mixture that arrives in the combustion chamber at the correct time and an engine constructed to withstand the temperatures and pressures created by the burning of thousands of fuel droplets.

The Combustion Chamber:

Let's start with the star of the show: the combustion chamber. This is the space between the top of the piston and the cylinder head. It's an enclosed area in which the fuel and air mixture is burned. The piston fits into a hollow metal tube called a cylinder, and it moves up and down in this cylinder. This up and down (reciprocating) motion must be converted to a rotary motion before it can drive the wheels of a vehicle. This change of motion is accomplished by connecting the piston to a crankshaft with a connecting rod. The upper end of the connecting rod moves with the piston as it moves up and down in the cylinder. The lower end of the connecting rod is attached to the crankshaft and moves in a circle. The end of the crankshaft is connected to the flywheel which transfers the engine's power through the drivetrain to the wheels.

Right Mix of Air, Fuel, and Heat:

In order to have complete combustion in an engine, the right amount of fuel must be mixed with the right amount of air. This mixture must be compressed in a sealed container then shocked by the right amount of heat (spark) at the right time. When these conditions exist, all the fuel that enters a cylinder is burned and converted to power, which is used to move the vehicle. Automotive engines have more than one cylinder. Each cylinder should receive the same amount of air, fuel and heat, if the engine is to run efficiently. It's like making sure every player on a team gets the same opportunities everyone needs to pull their weight!

The Valve System:

Although combustion must occur in a sealed cylinder, the cylinder must also have some means of allowing heat, fuel and air into it. There must also be a means to allow the burnt air fuel mixture out so a fresh mixture can enter and the engine can continue to run. To accommodate these requirements, engines are fitted with valves. There are at least two valves at the top of each cylinder. The air fuel mixture enters the combustion chamber through an intake valve and leaves after having been burned through an exhaust valve. The valves are accurately machined plugs that fit into machined openings. A valve is said to be seated or closed when it rests in its opening. When the valve is pushed off its seat, it opens.

The Camshaft:

A rotating camshaft, driven and timed to the crankshaft, opens and closes the intake and exhaust valves. Cams are raised sections of a shaft that have high spots called lobes. Cam lobes are oval shaped. The placement of the lobe on the shaft determines when the valve will open. The height and shape of the lobe determines how far the valve will open and how long it will remain open in relation to piston movement. As the camshaft rotates, the lobes rotate and push the valve open by pushing it away from its seat. Once the cam lobe rotates out of the way, the valve, forced by a spring, closes. The camshaft can be located either in the cylinder block or in the cylinder head. The camshaft is driven by the crankshaft through gears, or sprockets, and a cogged belt or timing chain. The camshaft turns at half the crankshaft speed and rotates one complete turn during each complete four stroke cycle.

The Four Stroke Cycle:

When the action of the valves and the spark plug is properly timed to the movement of the piston, the combustion cycle takes place in four strokes of the piston: the intake stroke, the compression stroke, the power stroke and the exhaust stroke. A stroke is the full travel of the piston either up or down in a cylinder's bore. The reciprocal movement of the piston during the four strokes is converted to a rotary motion by the crankshaft. It takes two full revolutions of the crankshaft to complete the four stroke cycle. One full revolution of the crankshaft is equal to 360 degrees of rotation; therefore, it takes 720 degrees to complete the four stroke cycle. During one piston stroke, the crankshaft rotates 180 degrees.

Why We Need a Flywheel?

The piston moves by the pressure produced during combustion, but this moves the piston only about half a stroke or one quarter of a revolution of the crankshaft. This explains why a flywheel is needed. The flywheel stores some of the power produced by the engine. This power is used to keep the pistons in motion during the rest of the four stroke cycle. A heavy flywheel is only found on engines equipped with a manual transmission. Engines with automatic transmissions have a flexplate and a torque converter. The weight and motion of the fluid inside the torque converter serve as a flywheel.

1. Intake Stroke:

The first stroke of the cycle is the intake stroke. As the piston moves away from top dead center (TDC), the intake valve opens. The downward movement of the piston increases the volume of the cylinder above it reducing the pressure in the cylinder. This reduced pressure, commonly referred to as engine vacuum, causes the atmospheric pressure to push a mixture of air and fuel through the open intake valve. Some engines are equipped with a supercharger or turbocharger that pushes more air past the valve like giving the engine a little extra oomph! As the piston reaches the bottom of its stroke, the reduction in pressure stops, causing the intake of air fuel mixture to slow down. But it doesn't stop completely because of the weight and movement of the air fuel mixture. It continues to enter the cylinder until the intake valve closes. The intake valve closes after the piston has reached bottom dead center (BDC). This delayed closing of the valve increases the volumetric efficiency of the cylinder by packing as much air and fuel into it as possible.

2. Compression Stroke:

The compression stroke begins as the piston starts to move from BDC. The intake valve closes, trapping the air fuel mixture in the cylinder. The upward movement of the piston compresses the air fuel mixture, thus heating it up. At TDC, the piston and cylinder walls form a combustion chamber in which the fuel will be burned. The volume of the cylinder with the piston at BDC compared to the volume of the cylinder with the piston at TDC determines the compression ratio of the engine. This ratio is a big deal, it affects power, efficiency and even what kind of fuel your car needs.

3. Power Stroke:

The power stroke begins as the compressed fuel mixture is ignited. With the valves still closed, an electrical spark across the electrodes of a spark plug ignites the air fuel mixture. The burning fuel rapidly expands creating a very high pressure against the top of the piston. This drives the piston down toward BDC. The downward movement of the piston is transmitted through the connecting rod to the crankshaft. During this phase, a few crankshaft degrees before TDC, the air/fuel mixture is ignited by the spark which takes place between the spark plug electrodes. This is why we call them spark ignition or positive ignition engines. During combustion, a temperature peak of about 2,300°C is reached, which causes a remarkable heat release toward the cylinder walls. At the combustion end, a pressure peak typically 100 bar is reached which acts on the piston pushing it toward BDC. Expansion is the only active phase of the whole thermodynamic cycle. That's the stroke that actually does the work.

4. Exhaust Stroke:

The exhaust valve opens just before the piston reaches BDC on the power stroke. Pressure within the cylinder causes the exhaust gas to rush past the open valve and into the exhaust system. Movement of the piston from BDC pushes most of the remaining exhaust gas from the cylinder. As the piston nears TDC, the exhaust valve begins to close as the intake valve starts to open. The exhaust stroke completes the four stroke cycle. The opening of the intake valve begins the cycle again. This cycle occurs in each cylinder and is repeated over and over, as long as the engine is running. Before BDC, the exhaust valve opens to start the spontaneous removal of burnt gases from the cylinder since their pressure is still significantly higher than atmospheric pressure. During the subsequent upward piston stroke, the exhaust phase is completed. The exhaust valve closes after the TDC to take full advantage of the inertia of the exhaust gas flow. The pressure quickly drops to the atmospheric value.

Diesel Engines:

Now, let's talk about diesels. They work on the same basic four-stroke principle, but with some fascinating differences.

Spark Ignition vs. Compression Ignition:

Similarly to spark ignition engines, four stroke compression ignition engines make a complete working cycle every two crankshaft revolutions (720°). The main differences between direct injection turbocharged compression ignition engines today commonly used and spark ignition engines are significant.

Intake in Diesel Engines:

Through the intake duct, air is directly admitted to the intake valve. The valve opens at TDC or just slightly before it because the higher pressure due to turbocharging helps in allowing air in. Since diesel fuel is directly injected into the cylinder, the power is controlled by varying the quantity of fuel injected and no throttle valve is present. The pressure decrease in the intake duct is thus limited and substantially determined by friction in the duct and pressure drop in the air filter.

Compression in Diesel Engines:

During the piston travel from BDC to TDC, air is compressed in the combustion chamber reaching pressure and temperature values much higher than those typical of spark ignition engines due to the higher compression ratio. Temperatures as high as 750–800°C are reached at the end of compression close to the TDC, in present day engines.

Expansion in Diesel Engines:

The ignition mechanism is completely different. Contrary to what occurs in the spark ignition engines, the air/fuel mixture ignition takes place spontaneously due to the high temperature and pressure. This is why they're called compression ignition engines. The injector sprays the fuel into the combustion chamber just a few crankshaft degrees before TDC. The spontaneous ignition of the initial part of the mixture spray and, consequently, of the bulk of the injected fuel generates a sudden rise of the pressure inside the cylinder, pushing the piston down toward BDC. The sequence of the injection and spontaneous ignition immediately before and after the TDC (−5°, −1°, TDC, +1°). The pressure peak typically 150 bar takes place during combustion, and the temperature reaches values higher than those typical of spark ignition engines. The air/fuel charge is obviously not homogeneous, since the injector sprays only fuel and only air enters through the intake valve.

Exhaust in Diesel Engines:

Similarly to spark ignition engines, at the end of an expansion downward stroke, the exhaust valve opens before the BDC to promote spontaneous gas removal, and the phase continues as the piston moves toward the TDC. The temperature of the exhaust gases is always lower in compression ignition engines than in spark ignition engines.

Diesel Combustion Chambers:

Diesel combustion chambers are different from gasoline combustion chambers because diesel fuel burns differently. Three types of combustion chambers are used in diesel engines:

1. Open combustion chamber: 

Located directly inside the piston. Diesel fuel is injected directly into the center of the chamber. The shape of the chamber and the quench area produce turbulence.

2. Precombustion chamber: 

A smaller, second chamber connected to the main combustion chamber. On the power stroke, fuel is injected into the small chamber. Combustion is started there and then spreads to the main chamber. This design allows for lower fuel injection pressures and simpler injection systems.

3. Turbulence combustion chamber: 

Creates an increase in air velocity or turbulence in the combustion chamber. The fuel is injected into the turbulent air and burns more completely.

Fuel injection is used on all diesel engines. Older diesel engines had a distributor-type injection pump driven and regulated by the engine. The pump supplied fuel to injectors that sprayed the fuel into the engine's combustion chamber. Newer diesel engines are equipped with common rail systems. These are direct injection (DI) systems. The injectors nozzles are placed inside the combustion chamber. The piston top has a depression where initial combustion takes place. The injector must be able to withstand the temperature and pressure inside the cylinder and must be able to deliver a fine spray of fuel into those conditions.

These systems have a high pressure 14,500 psi fuel rail connected to individual solenoid type injectors. That's intense pressure. The injectors are controlled by a computer that attempts to match injector operation to the operating conditions of the engine. Newer diesel fuel injectors rely on stacked piezoelectric crystals rather than solenoids. Piezo crystals quickly expand when electrical current is applied to them. The crystals allow the injectors to respond very quickly to the needs of the engine. With this new style injector, diesel engines are quieter, more fuel efficient, cleaner and have more power.

Two Stroke Engines:

Diesel engines are also available in two stroke cycle models. Most diesels generally use the four stroke cycle while some larger diesels operate with the two stroke cycle. Two stroke diesels must use forced induction from either a turbocharger or a supercharger. These engines are ideal for some applications because they provide high torque for their displacement.

Two Stroke Gasoline Engines:

In the past, several imported vehicles have used two stroke engines. As the name implies, this engine requires only two strokes of the piston to complete all four operations: intake, compression, power and exhaust. Here's how it works:
1. Movement of the piston from BDC to TDC completes both intake and compression.
2. When the piston nears TDC, the compressed air fuel mixture is ignited causing an expansion of the gases. During this time, the intake and exhaust ports are closed.
3. Expanding gases in the cylinder force the piston down, rotating the crankshaft.
4. With the piston at BDC, the intake and exhaust ports are both open allowing exhaust gases to leave the cylinder and air fuel mixture to enter.

Although the two stroke cycle engine is simple in design and lightweight because it lacks a valve train, it has not been widely used in automobiles. Why? It tends to be less fuel efficient and releases more pollutants into the atmosphere than four stroke engines. Also, oil is often in the exhaust stream because these engines require constant oil delivery to the cylinders to keep the piston lubricated. Some of these engines require a certain amount of oil to be mixed with the fuel.