Engine Terminology: Donít Get Lost in Translation

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The best way to avoid a poor car purchase or unnecessary repairs is to know what you’re talking about. To help you understand your engine better, here are terms you need to know:

Engine Operation in a Nutshell

A precise mixture of fuel and air are introduced into an enclosed cylinder. From the bottom of the cylinder, a piston pushes up, compressing the mixture. When the piston has completely compressed the mixture, a spark plug fires inside the cylinder, igniting the fuel and causing an explosion (internal combustion). This explosion forces the piston back down through the cylinder. The crankshaft, connected by a rod to the piston, turns and delivers power to the drivetrain, which in turn rotates the vehicle’s wheels.

Engine Size/Capacity

Engine size is typically measured in terms of the volume of its cylinders. Each cylinder is measured in cubic centimeters or cc. The volume measurement of the whole engine is a cumulative measurement of all the cylinders, so it’s often expressed in liters — (1,000 cc is one liter). For example, a four-cylinder engine with 500 cc cylinders is a two-liter engine. This total volume is often called displacement, describing the amount of air displaced as the pistons move through the cylinders for one rotation.

A higher volume means a longer distance for the piston to travel and a larger surface area for the internal combustion to act on. In the past, this generally meant more power. However, with the advent of technologies like turbochargers, sometimes smaller engines have the edge.


A turbocharger increases the power of an engine by adding pressure and increasing the amount of air in the piston prior to combustion. More air in the air-fuel mixture allows the fuel to burn more thoroughly, causing a more intense explosion. The turbocharger achieves this by capturing exhaust from the engine and using it to spin a turbine, which operates a compressor wheel. The compressor wheel takes in ambient air, pressurizes it, and feeds it into the engine.

Fuel Injection

In most modern engines, fuel is delivered through an injection system. Fuel injectors atomize (spray) fuel into the engine’s cylinders. There are two types of injection systems: port injection and direct injection. Port injection sprays fuel into an intake port where it waits to be sucked into the cylinder. Direct injection sprays the fuel right into the cylinder, using a computer for precision timing.

Both systems have pros and cons: Direct injection significantly increases fuel economy, but adds complexity to the engine because it requires high pressure to spray directly into the cylinder. Additionally, the required parts need to be very high quality, increasing the engine’s cost. Port injection is the opposite side of the coin: it’s less expensive and less complex, but it isn’t as efficient.


RPM stands for Rotations Per Minute, or the number of times the crankshaft rotates in a minute, measured in the thousands. You can see how your RPM changes based on your speed on the tachometer gage of your dash. If the needle points at four, that means 4,000 RPM. If a four-cylinder, two-liter engine runs at 2,500 RPM, each cylinder is burning 500 cc of fuel-air mixture every minute. That’s a lot of explosions!


You’ve probably heard the term horsepower (HP) and know it refers to the power of the engine. Why the term horsepower? Way back when engines were new machinery, engineers and salesmen needed a way to easily express the working potential of the new devices. At the time, draft horses did most of the work that required pulling things like heavy farm equipment and carriages, so comparing the engine to the number of horses it could replace was a natural choice. In engines, HP measures the maximum rate of acceleration and top speed of the car.

Automotive horsepower can also be measured in brake horsepower (BHP). This term refers measuring the HP of an engine under lab-controlled conditions, without the loss of power caused by parts like the generator, gearbox, and water pump. It’s a more theoretical measurement of an engine’s power.

Congratulations! Now you’re familiar with the ins and outs of engines and can enter your automotive purchasing and repair negotiations with confidence.

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