The Internal Combustion Engine, Explained

The all-you-need-to-know guide to that mechanical thing under the hood powering your car.


The modern combustion engine is a technological marvel, a mechanical miracle that requires little knowledge of its workings in order to use. Unless you’re a car geek, you probably don’t think all that much about your car’s engine.

Until something goes wrong under the hood, of course. When things go bad, the issues and causes can befuddle many drivers, for whom terms like “piston” and “crankcase” are obscure nomenclature, and “boxer” brings to mind Muhammed Ali, not Ferdinand Porsche.

So in order to provide a little clarity about what’s going on under the hood, we at Gear Patrol have pulled together a quick primer on how a combustion engine works and a rundown of the various types of combustion engines available in mainstream consumer automobiles.

Terms to Know

Carburetor: A device that mixes air and fuel in the proper ratio for combustion. The system is mechanical, not electronic like modern fuel injection or direct injection engines; as such, it’s less efficient.
Crankcase: Part of the engine block that houses the crankshaft. Usually made from a one or two pieces of aluminum or cast iron.
Crankshaft: The engine component connected to the pistons that provides rotational motion when combustion occurs.
Cylinder: The portion of the engine block that houses the piston and connecting rod, and the location where combustion occurs.
Direct Injection: A method by which gasoline is pressurized and injected into the cylinder’s combustion chamber. Unlike fuel injection, where gas is injected into the cylinder’s intake port.
Harmonic Balancer: Also known as a dampener, a circular device made of rubber and metal attached to the front of the crankshaft to absorb vibrations and reduce crankshaft wear. It reduces engine harmonics that occur when multiple cylinders move along the crankshaft.
Piston: A component housed within the cylinder walls and secured by piston rings. It moves up and down during the four-stroke combustion process, providing force when exploding fuel and air moves it.
Rev Matching: Technology in manual transmission cars that utilize sensors on the clutch pedal, gear shift, and transmission, sending signals to the electronic control unit that tell it to rev the engine automatically if revolutions per minute fall too low. Rev matching also occurs during the downshift, bringing rpms higher to match the lower gear. This reduces wear on the engine and smooths the shifting process.
Torsional Vibration: Vibration that occurs due to rotating shafts within a car.

The Combustion Engine

Once you get past the protective plastic engine cover found on most new cars, the vehicle’s heart is laid bare: an engine surrounded by a radiator, fluid reservoirs, airbox, and battery. Regardless of how complicated engines can be—thanks in part to features like direct injection, rev matching, etc.—most vehicles make use of what’s known as a four-stroke combustion cycle to convert fuel into kinetic energy. In a nutshell, your engine 1. draws air and fuel in, 2. compresses it, 3. ignites it, pushing the pistons down and generating the mechanical force that moves the car, and 4. expels the air to make room for the next round of the cycle.

Though the actual process is significantly more complicated, the four stages can basically be summed up as such:

Intake stroke: Air and fuel are drawn into the cylinder as the piston moves downward.
Compression Stroke: The air brought into the engine and the fuel are compressed when the cylinder moves into the upstroke position.
Combustion Stroke: A spark from the spark plug ignites the air/fuel mixture, creating pressure. The expanding mixture pushes the piston downward.
Exhaust Stroke: The resulting gas mixture created by the ignition and expansion is expelled from the cylinder as waste.

Engine output varies greatly, depending on the number of cylinders, the configuration of the engine, and technologies like turbocharging and supercharging. Horsepower isn’t just about adding cylinders or displacement; in fact, many of today’s high-performance four-cylinder engines can easily match or exceed the outputs of their six-cylinder brethren. These days, it’s also a game of technology; mate a smaller gasoline engine with an electric motor, and you have a recipe for added acceleration. (Case in point: the BMW i8, which combines a turbocharged 1.5-liter inline three-cylinder with an electric motor for a total of 357 horsepower and 420 lb-ft of torque.)

Engine Types

Modern combustion engines have come a long way since 1876, when German-born Nicolaus Otto built the first four-stroke internal combustion engine. Today, automotive engineers perform regular miracles by extracting maximum horsepower and efficiency from the design. And although hybrid and electric powertrains are on the rise, for now, combustion engines—inline/straight, V-type, and boxer/flat, using gasoline or diesel fuel‚ own the road.

Inline/Straight Engines

Examples of Inline/Straight Engines
Inline/Straight-Three: BMW i8
Inline/Straight-Four: Honda Civic Si
Inline/Straight-Six: BMW X3 / X4 M

In an “inline” or “straight” engine, the cylinders are arranged in a straight line. The overwhelming majority of four-cylinder cars on the road are “inline-four” engines, so the industry generally refers to them as “four cylinders.” Inline four-cylinder engines tend to be found in economy cars, since they are less expensive to build and easier to maintain—the cylinders line up along a single crankshaft that drives the pistons.

The inline/straight six-cylinder engine is inherently balanced, due to the fact that there are no secondary harmonics generated by pairs of pistons moving at odd angles or on a different axis from one another, resulting in much less vibration than straight four-cylinder engines. Currently, only BMW and Mercedes-Benz make inline/straight six-cylinder engines for their passenger cars — and they have a stellar reputation for smoothness and balance.

V-Type Engines

Examples of V-Type Engines
V-4: Porsche 919 Hybrid Le Mans
V-6: Toyota 4Runner
V-8: Dodge Challenger
V-10: Lamborghini Huracán
V-12: Ferrari 821 Superfast

“V-6” and “V-8” are so embedded into the American vocabulary, some people may not know engines come in any other format. V-type engines typically have two rows of cylinders set at a 90-degree angle to each other — hence the “V” formation — with each row bearing half the number of total cylinders. As a result, V-type engines are shorter and take up less room than straight ones, enabling carmakers to decrease the size of the engine compartment and increase crumple zones and passenger space. It’s also easier to set them lower in the vehicle, benefiting handling.

If you fancy yourself a motorsport fan, you have an appreciation for V-type engines, due to their frequent use in race cars. The rigid construction and robust materials used in V-type engines allow it to take on high stresses. This also allows for low torsional vibration forces, providing for smooth delivery during gearshifts and high rpms.

Boxer/Flat Engine

Examples of Boxer/Flat Engines
Flat-Four: Subaru WRX
Flat-Six: Porsche 911 Carrera

The term “boxer” engine comes from the layout of pistons that lay horizontally toward one another, similar to two opposing boxers touching gloves at the outset of a bout. The pistons in a boxer/flat engine form two banks—one on each side of a single crankshaft.

The boxer engine does more than sound intimidating; it allows for a lower center of gravity than inline/straight and V-type engines, improving handling. (There’s a reason Porsche uses the boxer engine in their 911, 718 Boxster, and 718 Cayman sports cars.) Boxer engines, however, tend to be bulkier and more awkwardly-shaped, making them difficult to fit in a front-mounted engine compartment. (Subaru—the only other carmaker currently using a boxer engine—manages to do so quite successfully, however.)

Diesel Engines

Examples of Diesel Engines
V-6 Turbodiesel: Ram 1500 EcoDiesel
V-8 Turbodiesel:Ford F-250 Super Duty

Get rid of the old notion of smoke spewing out of raucous 18-wheelers; modern, clean-burning diesel engines found in passenger cars are far less gross. The combustion that occurs in a diesel engine doesn’t require a spark; rather, high-energy diesel fuel ignites due to the high compression of the pistons: air is compressed, heating it to very high temperatures; the fuel is injected, and the mixture ignites.

While diesel engines come in various numbers of cylinders, they differ from their gas counterparts specifically because they use compression rather than a spark to ignite the compressed fuel/air mixture. But it’s more than just how combustion occurs that sets these powerplants apart: By virtue of the fact that higher pressures are required for combustion, a diesel engine has to be built like a tank to withstand the abuse. As a result, they tend to last longer than standard internal combustion engines. Diesel engines are also more efficient; they extract more energy from their fuel than gasoline.

And finally, diesel engines provide one benefit many enthusiasts love: more torque at lower engine speeds, which makes them feel zippier off the line.

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