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When it comes to speed, grease monkeys would know what to look for一V6s and V8s. These engines are kings of the street, and you can always expect an exhilarating ride from vehicles that sport these power sources.

But what about the classic i4 engine?

A Closer Look at the Old-Fashioned i4

An i4 is an inline, four-cylinder engine. It has all the cylinders lined up with one crankshaft. It’s also called an L4 (longitudinal 4).

A four-cylinder engine is one of the most common types you’ll see in passenger vehicles. It’s known for its fuel efficiency, but power isn’t exactly its strongest suit.

An i4 can be naturally aspirated (NA), which means it solely relies on atmospheric pressure to draw air into the cylinders, or it can be turbocharged (forced induction).

An inline four-cylinder engine is a classic example of a flat-head engine. It’s an older type of engine design that has its valves in the block.

In a flat-head engine, the valves are next to the cylinders and air-fuel mixture. The position of the valves is the reason why the cylinder heads are flat, hence the flat-head design.

Putting i4 Engines On the Map: The Mercedes AMG A45S

The i4 isn’t the most powerful engine, and every motorhead knows it. So when Mercedes-Benz rolled out the AMG A45S in 2019, the industry had to take a good look at it.

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The M139 engine on the 2019 Mercedes AMG A45S is arguably the most popular i4 on the market. The 2.0L M139 was capable of churning out 372 lb-ft. of torque and reached a top speed of 168 mph.

mercedes amg m139 wikimedia
The 2.0L M139 was capable of churning out 372 lb-ft. of torque and reached a top speed of 168 mph. Image credit: Wikimedia.

The AMG A45S can go from a 0 to 60 sprint in 3.9 seconds, which is quite fast for a hatchback. The monstrous M139 also came with twin-scroll, twin-turbos, and a seven-speed double-clutch.

Another reason why the AMG A45S became the dream i4 is because of its Nanoslide piston linings, which were developed by the automaker itself.

These piston linings are thin, low-friction coatings inside the cylinder that can save several kilos per engine and reduce friction by as much as 50%.

How Does an i4 Work?

The basic principle of an engine is to produce enough mechanical force to propel the vehicle. To do this, chemical energy from fuel must be converted to heat energy, and heat energy must be converted to mechanical energy.

Four-Stroke Cycle Operation

An i4 engine works the same way as V6s and V8s in terms of kickstarting the combustion process. All of these engines use the four-stroke cycle for each cylinder.

A four-stroke cycle includes the following:

  • Intake stroke
  • Compression stroke
  • Power stroke
  • Exhaust stroke

Are i4 Engines Good?

Although the i4 isn’t the most powerful engine out there, it still offers a lot of benefits that even racetrack-ready engines get jealous of.

First, four-cylinder engines are more fuel-efficient than other types because they have fewer pistons. This also makes them lighter than V6s and V8s.

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Another benefit is that they produce fewer emissions than other types, so they’re not as harmful to the environment compared to larger engines.

Lastly, four-cylinder engines are among the easiest engines to maintain, so you can expect to save a couple of bucks down the road (with proper vehicle maintenance, of course).

Leveling Up the i4

As small as it might seem, an i4 can deliver the same amount of power (or even more) as a V6 when turbocharged.

A turbocharger is a performance upgrade that uses exhaust heat to power a turbine wheel, converting heat energy into mechanical energy.

A turbocharger is designed to provide additional pressure on top of the atmospheric pressure in the intake manifold. The additional force pushes more air into the combustion chamber, increasing overall engine power.

Types of 4-Cylinder Engines

There are two other types of four-cylinder engines aside from the inline setup: the V4 and boxer-four.

V4 Engines

The V4 engine is more powerful than an i4, but it’s more popular in motorbikes than cars.

A V4 sports a shorter crankshaft than other four-cylinder engines, making it stronger and less susceptible to twisting under torque. A V4 also has a crankcase that’s better at managing airflow, which lets more energy be transferred to the drive wheels.

However, a V4 engine is a rare configuration in cars for a couple of reasons.

The V4 is a rattly engine that often requires a balancing shaft to stop excessive vibration. It can also take up a lot of room in the engine bay because it requires two of everything (cylinder heads, cam gearing, exhaust manifolds, etc.).

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Boxer-Four Engines

A boxer-four engine features a squat and rectangular setup. This type has a lower center of gravity, which results in better lateral stability and decreased roll compared to other engines.

In a boxer engine, the pistons move horizontally instead of vertically, producing a smoother acceleration with less vibration.

Boxer engines are common in Subaru vehicles, but they can also be found in cars like the Toyota GR 86, Porsche 718 Boxster, and 718 Cayman.

Popular Vehicles With an i4 Engine

The Mercedes AMG A45S isn’t the only i4-backed car that made the headlines. Here are other vehicles that sport the classic four-cylinder engine:

Wrapping Up

An i4 engine is a four-cylinder engine that’s arranged in a straight line.

Although it’s not as powerful as a V6 or V8, an i4 offers several benefits, including impressive fuel economy, less carbon footprint, and easy maintenance. An i4 engine can also be as powerful as a V6 when turbocharged.

Aside from the i4, four-cylinder engines also come in V and boxer types. V4 engines, however, are rare in cars and mostly found in motorbikes.

About The Author
CarParts Research Team
Written By Research Team

Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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