ASVAB Test Blog
Automotive Information | How Engines Work!
Automotive Information Subtest
Automobiles may appear as simple machines, but they are in fact rather complex. Here in this guide, we discuss one of the most asked questions on the automotive and shop information subtest – namely, how engines work.
Remember – the automotive and shop information subtest does not form part of your AFQT score. Therefore, you do not need to score high on this test to be eligible to enlist in the US military.
However, the automotive and shop exam is important for many MOS – or military occupational specialty. Many line scores depend on success at this subtest, and if your MOS score includes the AS subtest, you must spend extra time preparing for this part of the exam.
Luckily, many candidates hoping to join an engineering/automotive-related role in the military tend to have hands-on experience with how engines work. And moreover, many of the ASVAB test questions asked on this topic tend to be basic and straightforward. An elementary understanding of the topic is often enough to get you over the line.
Structure and Function of Automotive Engines
The function of an engine is to produce energy. That energy is then channeled through the engine to support systems – such as:
- Electrical systems
- Cooling systems
- Ignition systems
- Drive systems
- Brake systems
We will delve deep into these systems another time. For now, though, we will focus on how engines work – how that energy is produced by the engine to support the many complex systems that come together to form a functioning automobile.
Two elements are needed to produce energy within engines:
- Gas – the fuel mixture
In engines, the gas-air mixture is burned – which causes it to expand. The pressure caused by this combustion produces energy, and it is this energy that fuels the systems needed to help the automobile function – from accelerating the vehicle to functioning the drive system, to cooling the engine.
Automobiles move according to a fixed cycle of four phases:
- Intake – the gas-air mixture enters the engine through an intake valve. This valve opens in response to the connecting rod pulling the piston down.
- Compression – with the gas-air mixture now inside, the valves shut. Now, the connecting rod pushes the piston back up – which compresses the gas-air mixture.
- Power – a device known as a spark plug ignites the gas-air mixture. Now, the piston is forced back down – pushing down the connecting rod, turning the crankshaft which, in turn, turns the flywheel. This is the mechanism that keeps the engine running.
- Exhaust – exhaust valves are also present in engines to remove any ignited gas. This is controlled by the push rod, which accurately times when valves should open and close to intake and remove gases from the engine.
This is the fundamental process by which engines produce energy. Go through this process with the aid of the diagram above to provide a step-by-step guide. Many ASVAB test practice questions ask about the structure and function of how vehicles work.
These four processes – intake, compression, power, and exhaust – are referred to as four-strokes of one cycle of an engine. You may hear engines referred to as ‘four-stroke, one cycle’, and the processes explained above is what this refers to.
Once the exhaust stroke is completed, the process begins anew. You may have noticed on your automobile’s tachymeter figures such as ‘3,600 rpm’ – which means that the engine is performing 3,600 of these cycles every minute.
- rpm refers to revolutions per minute.
Fuel Injection and the Throttle
For this 4-stroke, one cycle process to even begin, the fuel-air mixture must enter the engine. There are two major ways in which this can occur, with:
- Fuel injectors: as their name suggests, fuel injectors inject fuel directly into the engine system. Typically, this requires a pump. Most automobiles these days use fuel injectors, which replaced the older mechanism, the carburetor.
EFI is an acronym for Electronic Fuel Injection – and EFI computers are installed into vehicles to calculate the amount of fuel in the vehicle, as well as the optimum amount of fuel that the vehicle needs to operate.
- Carburetors: carburetors mix air and gas mechanically. Air moves through the carburetor, creating a vacuum. This, in turn, attracts more fuel into the mixture. As we have learned, carburetors are common in older vehicles, typically those manufactured before 1995.
Throttles are essential devices that are used to control how engines work – systems used by vehicles to regulate power. More specifically, throttles are used to regulate the amount of air that enters the engine. Depending on the vehicle, throttles may be electronically connected to the EFI computer or connected to the carburetor. The accelerator is the gas pedal which, when pressed, opens the throttle to allow more fuel to be transported to either the fuel injector or the carburetor.
When the gas-air mixture is ignited too early, this can cause what is known as engine knock.
Octane ratings are used to inform the buyer of the degree to which gasoline can be compressed before it ignites spontaneously. In other words, the higher the octane rating, the greater degree of compression the fuel can withstand before detonating.
Typically, there are three gradings of octane rating – measured in AKI (anti-knock index):
- Grade 87
- Grade 89-90
- Grade 91-94
If the engine does not knock at the recommended gasoline octane rating, then there is no need to switch to a higher grade. Furthermore, higher grade gasoline costs more, too – so there is no need to unnecessarily add to costs. Higher octane gas should only be used when the engine knocks at the recommended level. There is no other advantage to using higher grade gasoline – as it does not improve performance, speed, or mileage.
Engine knock should be avoided. Over time, it can cause damage to the engine.
That about completes our study guide of how engines work. We hope you found this ASVAB study guide helpful. Check back to ASVAB Test Practice soon for more exclusive content to help you master the exam and make it through to boot camp!
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