I have begun the first stage of the mechanics module of The STEMpunk Project, my year-long attempt to learn as much about computers, electronics, mechanics, and robotics as I can. Naturally, this means I have been thinking about internal combustion engines, intriguing devices that can be found in a variety of machines ranging from lawnmowers to speed boats. All of them rely on the exothermic chemical process known as combustion, which occurs internally, hence their name.
The present essay is a broad-strokes discussion of the internal combustion engines found in modern motor vehicles, both because this knowledge will likely prove the most useful to me after my current project is finished and because I have to draw a line somewhere.
The majority of vehicles on the road are propelled by a four-stroke spark-ignition internal combustion engine. Vehicles on the less powerful end of the scale might only sport 3- or 4-cylinder engines while those at the other extreme contain 8- or even 16-cylinder engines.
There are a few different ways to arrange these cylinders within the engine block. In an inline configuration the cylinders are in a row, as the name suggests. Obviously there is a limit to how many cylinders can be made to fit in a straight line under the hood, so many engines have their cylinders in a “V” shape (hence the term “V-8”). One arm of the V will contain half the cylinders and the other arm will contain the other half, making better use of the space available. Less common than this are engines which have their cylinders laying sideways and the pistons moving left-to-right instead of up-and-down.
Each cylinder houses a piston, which is a metal drum that compresses the fuel-air mixture as it enters the cylinder cavity (also called the cylinder bore) and pushes exhaust out at the end of a full cycle. Each piston is connected to the crankshaft via a connecting rod, and it is the crankshaft which keeps all the pistons moving in sync. To prevent oil from leaking into the cylinder bore and exhaust from leaking out, each piston is wrapped in a set of rings which seals it in.
Internal combustion engines do not run on gasoline alone, but rather a mixture of air and gasoline together. In older vehicles and in simple, modern machines, the mixing of air and fuel is accomplished with a carburetor, but these days a fuel-injection system is more common. Air is brought into the engine and distributed to each cylinder by a series of tubes called an intake (or inlet) manifold. These come in numerous shapes, but a simple visualization of the manifold for a V-8 engine resembles a cylindrical octopus laying on top of the engine with one leg going to every cylinder.
Four-stroke engines are so called because each piston turns fuel into motion by going through four distinct strokes: intake, compression, power, and exhaust. During intake an intake valve at the top of the cylinder bore opens and the fuel-air mixture is drawn into the cylinder cavity by the downward motion of the piston. The piston then screams upward during compression, crushing the mixture into 1/8th or 1/10th its original volume, depending on the engine’s compression ratio. Then a spark plug ignites the mixture, beginning the “power” phase. The subsequent explosion drives the piston downward, with the resulting force being distributed to the tires and causing them to rotate.
Now the cylinder bore is filled with noxious fumes, and an exhaust valve opens at the top of the cylinder bore to allow the upward motion of the piston to expel them.
The spectacular synchronization between intake cylinders, exhaust cylinders, and pistons is achieved in part by a camshaft. The camshaft is a metal rod with tear-drop shaped lobes attached to it, each one of which connects through a rocker arm to either an intake valve or an exhaust valve. Rocker arms have a long side and a short side, and as the camshaft spins one lobe presses against the short side of a rocker arm which causes the long side to descend and open a valve.
These valves are spring loaded, so when the camshaft rotates and the lobe disengages its rocker arm the valve shuts again. Taken together the camshaft, valves, and rocker arms are called the valvetrain, and are connected to the crankshaft by a timing belt which keeps their motion in tune.
The result can be viewed as an exquisitely timed dance of fire and steel: a piston expels exhaust through an exhaust valve opened by a rocker arm, and is thus ready to begin its cycle anew; the crankshaft rotates, and the piston is drawn downward by its connecting rod; the camshaft rotates, synchronized to the crankshaft by a timing belt, and one of its lobes touches a rocker arm which opens the intake valve for the piston; the fuel-air mixture enters the cylinder bore, sucked in by the piston’s descent; the crankshaft continues to spin, now pushing the piston upward and compressing the mixture into a tiny space; there comes a spark, and an explosion, which fires the piston downward with great violence; the vehicle moves; the camshaft, bound by the timing belt to the crankshaft, now uses a different rocker arm to open the exhaust valve; the crankshaft sends the piston skyward again, and the exhaust is expelled.
This process naturally generates enormous amounts of friction. The engine is able to withstand this because much of its surface area is coated with oil, which in addition to lubrication also serves to marginally cool the engine down. The engine’s oil reservoir is called a sump, and usually sits below the crankshaft. An oil pump draws sends the oil to an oil filter before it is distributed by oil channels to the crankshaft bearings, cylinder bore, valvetrain, and anywhere else metal is touching metal. After performing it’s job the oil returns to the sump to be sent through the cycle again. Like everything oil breaks down eventually, which is why it must be regularly changed to keep the engine running smoothly.
But the cooling effects of oil are very minor compared to the tremendous amounts of heat created by combustion, which means that engines require an additional dedicated cooling system. While it is possible to air cool an engine, most vehicles rely on liquid cooling.
As the coolant of choice, water sits in a plastic tank waiting to be pumped throughout the engine. Because vehicles are expected to operate year round in a variety of different conditions the coolant must be protected from extreme cold by antifreeze and from extreme heat by being pressurized enough to push its boiling point up into a safe range.
A water pump sends the coolant through a number of hoses which spread like blood vessels through the engine, absorbing heat. When the coolant has absorbed as much heat as it is designed to, it is sent to the radiator. Consisting of many thin, usually horizontal tubes, the radiator is designed to “spread” the coolant out so that it releases the heat it has absorbed into the air, effectively carrying it away from the engine. Proper heat dissipation requires there to be a constant stream of air running over the radiator’s tubing; this is simple when the vehicle is going fast, but small electric fans are required to maintain airflow when the vehicle is going slow or at a stop.
Modern engines make use of ingenious devices to maintain the appropriate coolant pressure and temperature. The radiator pressure cap is built so that when pressure exceeds a certain threshold a small amount of coolant is let out into a reserve tank where it waits until it can be reintroduced into the cooling system. The mechanical thermostat is calibrated to open only when the coolant reaches a certain temperature. If the coolant is still cool it is recirculated through the engine, but if it has gotten hot it is sent to the radiator to cool down.
How then does the engine receive the initial spark it requires to turn over? Usually a lead-acid battery and an induction coil are used together to begin ignition, after which point the engine is somewhat self-sustaining. As the engine runs it spins an alternator, which is just a small generator inside the car that feeds energy back to the battery.
That covers the basics! Obviously there is a vast amount of additional material that could be included here. If time allows I’d like to write a bit about different engine types and engine improvements that could be on the horizon, as well as possibly getting a bit into the history of these remarkable contraptions that have done so much to shrink distances.
As always, thank you for reading.