Car engine displacement is a term relating to the combustion process of the vehicle, which generates the power to get the wheels moving. You might not have heard of the phrase before, but you are likely to know plenty about it already. It is not something that can be altered easily, or requires repairs, but you will need to know about it for an auto mechanic career.
Luckily, it is a pretty straightforward concept, so let’s look at the things you need to know about engine displacement.
How to Calculate Car Engine Displacement
Engine cubic inches (or displacement) refers to the total volume of all the cylinders in an engine combined, typically measured in cubic inches (in³) or liters (L). It is a measure of the total capacity of the engine’s combustion chambers.
To determine the displacement of an engine, you will need a couple of key measurements and a quick geometry refresher (or you can skip the math and simply plug numbers into one of many).
Each cylinder of an engine is, as the name implies, cylindrical. As you might recall from high school, the formula to determine the volume of a cylinder is pi (rounded to 3.14) multiplied the radius squared times height.
In engine-speak, the height of the cylinder is known as the stroke. That is the distance the piston travels up or down the cylinder. The radius is equal to half of the cylinder’s bore, which describes the diameter of the hole drilled, or bored, into the engine block to accept a piston. These dimensions are the same for each cylinder in an engine. To get the total displacement then, multiply the volume of one cylinder by the number of cylinders the engine has.
Back in the 1960s and ’70s, U.S. automakers talked about engine size in cubic inches, often advertising it directly on the vehicle in the form of badges or even as part of the model’s name as with the Ford Mustang Boss 302 and 429. Today’s preferred automotive displacement units are expressed as liters or cubic centimeters.
For example, a four-cylinder might have a bore of 8.25 cm and a stroke of 9.28 cm. Plugging those numbers into our volume equation gives us a displacement of 1,984 cubic centimeters, which rounds up to 2.0 liters.
The Effect of Car Engine Displacement on Vehicle Performance
The bigger the engine displacement volume, the more air that can be pushed into the cylinders. This boosts the combustion process and allows the engine to generate more power. This does, of course, have an obvious downside. Fuel efficiency is likely to drop when displacement increases, so drivers will have more frequent trips to the gas station. Low displacement engines are more likely to be found in family cars as a result, where power is less necessary.
Engine Volume Always Stays the Same
The total volume of the engine’s cylinders always remains constant, despite all the piston movement taking place. It is one of the most fascinating parts of engine design, because no two pistons are ever at the same stage of the combustion process. One could be about to start its cycle, while the others are mid-stroke or completing their cycle. This synchronization keeps the engine volume the same throughout any trip in a healthy vehicle.
Car Engine Displacement Vs Horsepower
The relationship between car engine displacement and horsepower is not direct, but there is a general correlation:
- Displacement and Torque: In general, larger engine displacement tends to produce more torque, especially at lower engine speeds. This is because a larger displacement allows for more air and fuel to be drawn into the cylinders, resulting in greater combustion forces.
- Torque and Horsepower: Torque is a measure of the engine’s twisting force, while horsepower is a measure of the engine’s ability to do work over time. While torque is what you feel when accelerating, horsepower reflects how quickly the engine can produce that torque. The formula for calculating horsepower is: Horsepower = (Torque × Engine Speed) / 5252.
- Effect on Horsepower: Generally, larger engine displacement can contribute to higher horsepower, especially if other factors like compression ratio, cylinder head design, intake/exhaust systems, and engine management are optimized for performance. However, it is important to note that horsepower is influenced by many factors beyond just displacement.
- Efficiency and Technology: Modern engine design and technology have allowed smaller engines to produce more power efficiently through methods such as turbocharging, supercharging, direct injection, variable valve timing, and advanced engine management systems. This means that a smaller-displacement engine may produce as much or even more horsepower than a larger engine while being more fuel-efficient.
Finally
In summary, while car engine displacement can provide a general indication of a car’s potential power output, horsepower is influenced by numerous factors beyond just displacement. It is essential to consider the engine’s overall design, technology, and tuning to accurately assess its horsepower output.
Car engine displacement does matter to the engine, but it probably matters very little to you. When shopping for a vehicle, you should focus on a vehicle’s output — horsepower and torque — more than cylinder count and engine size.
