Internal Combustion Engine – Bartosz Ciechanowski

The invention of the internal combustion engine in the 19th century has revolutionized transportation over land, water, and air. Despite their omnipresence in modern day, the operation of an engine may be cryptic. Over the course of this article I’d like to explain the functionality of all the basic engine parts shown in the demonstration below. You can drag it around to see it from other angles: It’s hard to talk about a mechanical device without visualizing its motion, so many demonstrations in this blog post are animated. By default all animations are enabled, but if you find them distracting, or if you want to save power, you can globally pause them.disabled, but if you’d prefer to have things moving as you read you can globally unpause them. An engine like this may seem complicated, but we will build it up from first principles. In fact, we’ll start with a significantly simpler way of generating a rotational motion. Crank Let’s look at a simple crank. It consists of a handle, a crank arm, and a shaft. When a force is applied to the handle the shaft rotates which we can observe by looking at the attached disk: The force applied at a distance from the shaft generates torque. The harder we push on the handle, the bigger the torque on the shaft. This cranking mechanism is precisely what converts linear force into torque in a manual coffee grinder or a bicycle. It’s one thing to power something using our own muscles, but the entire point of building an engine is to avoid manual labor and have the device exert the effort instead. To do that we need to find a reliable source of a strong force that is easy to direct. Thankfully, such device was invented hundreds of years ago – a cannon does exactly what we need. In the demonstration below you can observe how a cannon ball is fired from a cannon. The diagonal lines indicate a cross section view – it lets us see what’s going on inside an otherwise obscured region: As the gun powder is set on fire it quickly produces a huge amount of gases, which push the cannon ball down the barrel. Since the ball snugly fits inside it can only go in one direction. While reliable and easy to direct, a cannon ball won’t be very effective at pushing the crank: We’ve only been able to do a partial turn of the shaft and the cannon ball is long gone.

However, with a few modifications we can harness the pushing power of the explosion in a significantly better way. Firstly, we’ll replace a cannonball with a piston that has a cylindrical shape and a hole drilled in it. We’ll then use a pin to attach to it a rod that can swing freely on a crankshaft: As the name implies, the crankshaft consists of both the rotating shaft and the crank on which a force is applied. By putting this assembly inside a simplified cannon shell, a cylinder, we’ve managed to solve the problem of the escaping cannon ball, as the piston is limited in its downward movement and will return up as the crankshaft keeps turning: Notice that the piston has now a minimum and a maximum position it can reach within the cylinder. A single movement over that length in either up or down direction is called a stroke. If we now trigger the explosion, the combustion gases will push the piston down, which turns the crankshaft: It’s still not a very exciting machine as it only does useful turning work once. To make it more practical we need to keep repeating the cycle of explosions – we have to add in new fuel, trigger a combustion process, and remove the exhaust gases, over and over again. Solid fuels like black powder are not very practical for an automated machine. It’s much easier to deal with fuels in fluid forms – their intake can be controlled by various valves. We’ll modify the cylinder we’ve built so far by adding new openings at the top of the combustion chamber: It may be hard to see how the various openings are laid out, so let’s take a look at the cross section view: Through the first large curved opening we’ll provide a mixture of gasoline and air and through the second one we’ll remove the exhaust gases. Those two openings will be guarded by the intake valve and the exhaust valve. Finally, to light the mixture, we’ll use an electric spark generated by an exposed ends of a wire. Let’s see how all the pieces fit together: We’re now ready to use this machine to do useful work. At first we’ll open the intake valve while the piston is moving down letting the air with fuel come in which I’ve symbolized using the yellow color. This is the intake stroke: Once the piston reaches its lowest position the intake valve closes, and the piston starts to move back which compresses the mixture of air and fuel which increases the thermal efficiency of the combustion.

This is the compression stroke: Voltage runs through the open ends of the wire, generating a spark which ignites the air-fuel mixture. The expanding gases created by the combustion push the piston down, creating torque on the crankshaft. This is the power stroke: Note that the flame propagation inside a cylinder is quite complex, and what you see here is a simplified visualization. The cylinder is now filled with the exhaust gases which we can vent out through another hole by opening the exhaust valve. This is the exhaust stroke: We’re now back to where we started and the cycle is complete. Let’s look at those four steps together: Since the piston moves down twice and up twice, it does a total of four strokes and the engine we’ve built is known as a four-stroke engine. Notice that it takes two revolutions of the crankshaft for the piston to do one full cycle of the work as it goes through the four phases: intake, compression, power, and exhaust. While functional, the engine we’ve built is more of a toy example that doesn’t show a lot of the engineering ingenuity behind many components of real internal combustion engines. Let’s build on the principles we’ve devised so far by constructing a more realistic machine – an engine that one could find in a car. Engine Block Let’s start with the biggest and heaviest part of an engine – the engine block. It forms the main body and mounting structure for other parts: Notice that this block contains four large cylindrical openings that define the four cylinders. Recall that a piston exerts a pushing force on the crankshaft only during the power stroke, so only for about a quarter of time. This uneven action creates a lot of vibration. While it’s often acceptable for smaller engines e.g. in a lawn mower, a typical car engine has more than one cylinder to ensure a more even delivery of power. I’ll discuss these concepts in more depth near the end of the article. Since the four cylinders are inline, the engine we’ll build is known as an inline four cylinder engine. Other engines may use different arrangements of cylinders, usually in a flat or V-shape configuration. The sides of the block are reinforced by various ribs to improve the rigidity of the structure – the body has to withhold the power of the explosions inside the cylinders. You may also have realized that the top part of the block is perfectly flat – we’ll soon attach another component there.

If you look at the cross section of the block you’ll notice that the areas around cylinders are empty: Those passages are there for the coolant to flow around the cylinders and take the heat of the combustion away. While I’m not going to dive into details of engine cooling, it’s worth noting that engines should run at a specific operating temperature and the coolant pump, thermostats, and radiators make sure that the engine isn’t running too cold or too hot. Crankshaft Let’s look at the first big part we’ll mount onto the engine – the crankshaft: Notice that the crankshaft has five main cylindrical parts that define its axis of rotation, they’re called the main journals. There are also four rod journals that are positioned off-axis. All the journals are connected via webs. Note that while the sections have different colors here, the entire crankshaft is made from a single piece of metal. You may wonder why the two inner rod journals are offset differently than the two outer ones, so let’s pop the piston assemblies on and see how they’ll move on the crankshaft: Since the rod journals are at different locations each of the four pistons can run at a different phase of the four stroke cycle. Notice that the distance between the center of the main journals and the rod journals defines how far up and down the piston goes in the cylinder. A real piston and its connecting rod have some mass so they end up creating a weight imbalance on a rotating crankshaft. To counteract that mass, the webs have elongated shape to form a counterweight that helps to even out the inertial forces on the shaft. One could assume the installation of the crankshaft in the engine block is as simple as putting it directly in a designated spot at the bottom: Unfortunately, that wouldn’t really work. During engine operation the pistons exert a lot of force on the crankshaft and the main journals would just rub against the housing creating a lot of friction that would wear the parts down. To fix that we need to firstly put in some bearings that will help to make the rotation of the crankshaft smooth: These strips of metal don’t look like much, but bearings are usually made from a softer material which causes them to wear first which prevents degradation of the crankshaft itself in case any contact occurs. Most of the time, however, the crankshaft doesn’t actually touch the bearings at all.

Notice the small hole in the bearing that matches the corresponding hole inside the engine block: Through that hole the engine pumps oil under pressure. The crankshaft’s diameter is slightly smaller than the bearings' inner diameter so oil fills the tiny gap between the two surfaces. Presence of oil is critical here as it creates conditions for hydrodynamic lubrication. Oil sticks to the bearings and the crankshaft, but since the crankshaft rotates it creates a variation in velocity of oil between the two surfaces. In the demonstration below the small arrows symbolize the local velocity of the liquid: The difference of diameters causes a wedge-like shape to develop which then creates an area of increased pressure that lifts the crankshaft journal away from the bearings. Note that the size of the gap in the demonstration is not to scale, but in real running engines the rotating crankshaft should float completely on a very thin surface of oil. You may have noticed that one half of the bearings also contains a small gutter which creates a small pool of oil under pressure. Moreover, the crankshaft has small holes in it: Those passages are actually connected inside and the oil from the pool in the bearing travels through the little passages in the crankshaft itself. This brilliant solution distributes the oil from the main journals to the rod journals, which are constantly changing their position inside the engine. The demonstration below shows one of the many typical arrangements of these passages and the presence of oil in and on the crankshaft: Let’s finally put the crankshaft in. We’ll clamp it down using five end caps that have their corresponding bearings put in and we’ll screw everything together: Those screws have to be tightened to a precise torque – it has to be high enough so that end caps are able to keep the crankshaft in place despite the force of explosions pushing down on it through the piston rods, but the torque on the screws can’t be too high to avoid any deformation of the circular shape of the final opening in which the crankshaft lies. Pistons The crankshaft itself is there to receive the force from the pistons, so let’s look at one up close: Firstly, notice all the empty spaces inside the piston. They’re there to reduce the weight – a piston should be as light as possible to minimize the inertial forces created by its reciprocating motion. In this piston the top part known as the piston crown has a dish-like cavity in it. Other pistons may be flat or have more complicated shapes.