Engine
Not everyone who takes the ASVAB will specialize in mechanics, but it’s still important to study and understand the Auto Information section. At its core, this section is about systems, which are a collection of parts working together to form a whole.
This chapter and the next few cover the four main systems of a vehicle: Engine, Cooling, Chassis, and Electronic. We’ll start with the Engine system here, then move to Cooling, Chassis, and Electronic in later chapters.
Engine
There are many different types of engines, including electric motors and internal combustion engines. For the Auto section, we’ll primarily look at internal combustion engines, as they power most cars and trucks.
In an internal combustion engine, a mixture of fuel and air is ignited to create motion. Let’s break down our component parts to see how each piece contributes to this process.
Components
Engine block - This is the main structure of the engine. It contains the cylinders, pistons, crankshaft, and other necessary components.
Cylinder - A hollow cylindrical space inside the engine block where a piston moves up and down. Combustion occurs here. Different engines have different numbers of cylinders, each containing one piston.
Piston - A piston is a metal plunger inside the cylinder. Expanding gases push it downward during combustion, transferring the force to the connecting rod. Piston rings around the piston seal it against the cylinder wall, so combustion gases don’t leak down past it.
Connecting rod - The piece that connects the piston to the crankshaft.
Crankshaft - The crankshaft converts the up-and-down movement of the pistons into a continuous rotational motion.
Cylinder head - The cylinder head sits on top of the engine block, sealing the cylinders during combustion.
Spark plug - The spark plug creates the spark that ignites the air-fuel mixture inside the cylinder. It sits at the top of the cylinder. It screws into the cylinder head, and its reach, the length of its threaded end, must suit that head. The spark jumps the plug’s gap, the small space between its center electrode and its ground electrode. A plug’s heat range describes how easily its tip heats up: a “hot” plug heats up easily and is slow to shed heat, while a “cold” plug sheds heat readily and runs cooler.
Intake port - The intake port is a passage that allows the fuel-air mixture into the cylinder. The fuel system supplies that fuel: it stores fuel in the tank and pumps it to the engine.
Exhaust port - The exhaust port is a passage that allows spent gases from the combustion reaction to exit the cylinder. The exhaust system then carries these gases away from the engine and out through the tailpipe.
Valves - Metal components that open and close to control airflow. They let the fuel-air mixture into the cylinder and let exhaust gases out. When closed, they seal the cylinder during combustion.
Camshaft - A rotating shaft containing raised sections called cams (or cam lobes). Its rotation causes the intake and exhaust valves to open and close at precise intervals.
Cams - Cams are raised sections of the camshaft that press down on the valves to open them. When the cams rotate away, valve springs close the valves again.
Timing belt - Links the crank gear on the crankshaft to the cam gear on the camshaft, so the pistons and valves move in sync.
Cam gear - A gear that connects the camshaft to the timing belt and rotates the camshaft.
The four-stroke cycle
Now, let’s see how these parts work together. A gasoline vehicle uses a four-stroke combustion cycle, in which each stroke is the piston’s movement up or down.
First stroke: intake stroke
The camshaft rotates, allowing the intake valve to open, and the piston moves down. This pulls in a mixture of fuel and air through the intake port and into the cylinder. Once the mixture is inside the cylinder, all valves shut.
Second stroke: compression stroke
The piston moves to the top of the chamber, compressing the air-and-fuel mixture against the roof of the cylinder.
Third stroke: power stroke
The spark plug at the top of the cylinder ignites the compressed air-fuel mixture. The combination of compression and ignition creates a combustion reaction, in which the heated gases expand outwards. The force of this rapid combustion drives the piston back down. That energy transfers from the piston to the connecting rod, then to the crankshaft.
Fourth stroke: exhaust stroke
The camshaft rotates again, allowing the exhaust valve to open. The piston moves upwards once more to push the spent mixture out of the exhaust port.
Repeat
As the camshaft rotates, the piston moves downwards again, the intake valve opens, and the cycle repeats.
Arrangement of cylinders
Cylinders can be arranged multiple ways. Four of the most common layouts are in-line, V-type, boxer, and radial.
In-line
An in-line cylinder configuration is just as described: the cylinders sit in a line, and all the pistons are connected to a single crankshaft. This is a popular choice for automobile designs because it is simple, easy to maintain, and inexpensive due to its fewer moving parts.
V-type
If an engine has six or more cylinders, it will often be organized in a V-type configuration. In a V-type engine, two banks of cylinders angle up and away from each other, meeting at a single crankshaft at the bottom of the V.
This lets it fit more cylinders in the same space, so it is common in high-powered vehicles.
Boxer
Boxer engines are a type of flat engine. The cylinders lie in flat pairs, and the pistons move simultaneously towards and away from each other. When they move inwards together, it looks similar to boxers touching gloves before a fight.
Due to their flat design, boxer engines have low vibration and a low center of gravity. They also allow a lower hood line, making the vehicle smoother and more aerodynamic.
Radial
Radial engines have cylinders arranged in a circle around a shared crankshaft. They were commonly used in older aircraft, though many modern aircraft now use gas-turbine engines instead.
Diesel engines
Diesel engines and gasoline-powered engines operate a little differently. In a gasoline engine, a spark plug ignites the fuel in the combustion chamber. In a diesel engine, air is compressed in the combustion chamber to the point that it becomes extremely hot. Fuel is then injected into this hot air, allowing it to spontaneously ignite without a spark.
Diesel engines can also produce more torque at lower engine speeds (RPMs) than gasoline engines, which makes them ideal for towing and hauling heavy equipment. Diesel fuel also has a higher flash point and is therefore less flammable than gasoline, making it safer to store and handle.
Diesel engines can be either 2-stroke or 4-stroke.
A 2-stroke diesel engine completes a power cycle in just two piston movements instead of four, producing power with every crankshaft revolution. It’s also typically lighter and more compact than a 4-stroke engine. The trade-off is that a 2-stroke engine is typically less fuel-efficient and produces higher emissions than a 4-stroke engine.
A 4-stroke diesel engine completes a power cycle in 4 piston movements, producing power with every second crankshaft revolution. The motions are similar to a gasoline engine, but a diesel engine still does not need a spark to ignite the fuel. Because it undergoes a more complete combustion process, a 4-stroke engine is typically more fuel-efficient and produces fewer emissions than a 2-stroke engine. Most military and commercial diesel-engine vehicles use a 4-stroke design.
Electric and hybrid power
Besides gasoline and diesel engines, vehicles can also be powered by electric motors or hybrid systems.
Electric motors use electromagnetic fields to produce rotational motion, converting electrical energy into mechanical energy to propel the vehicle. Because they can convert energy without combustion, electric propulsion systems are extremely quiet and do not emit exhaust fumes. Electric engines can also produce instant torque and acceleration as soon as they’re turned on, without needing to go through a combustion cycle.
There are, however, downsides to electric engines. The lithium-ion batteries that power them need to be charged, which limits a vehicle’s usable range and increases downtime. These batteries are also a safety hazard if damaged or mishandled, as they can overheat, become a fire risk, or even explode.
Hybrid systems combine an internal combustion engine with an electric motor and battery. They can therefore switch between using one propulsion system or another, or use a combination of both. This combination gives these systems flexibility, extends their operating range, and improves fuel efficiency.