Diesel Engine
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How Does a Diesel Engine Work Exactly?

While 2017 and 2018 marked the all-time peak of global automobile sales at around 85 to 86 million units, there are still plenty of people in the market for a new car.

Today there are more choices for fuel, engine, and brand than ever before. You may be familiar with how a car engine works, but how does a diesel engine work? You’re about to find out, and Motor Era is going to help.

Gasoline and Diesel both use oil, but how they use it is very different. Even more different still is the electric motor, but we’ll save that for another day.

Strap in and get your drinks in their coasters, cause we’re revving our engines!

A Bit Of History

The history of the Diesel engine is an interesting one since the combustion engine had been around for about 16 years by the time it made its debut. First of all, the Diesel engine is named after its inventor, Rudolf Diesel.

In 1878, only two years after Nicholas August Otto patented his gasoline engine, young Rudolf was horrified to find out as an engineering student that only about 10% of fuel used in gasoline and steam engines was utilized for mechanical force.

The rest? Burned away into pointless heat, and exhausted during the Otto cycle, or retained in the engine’s components.

Rudolf Diesel thus began a fourteen-year journey of creating the Diesel engine. What are the benefits of diesel? This engine was more efficient, used less fuel, and got more bang for the buck. See what we did there?

What is a Diesel engine, and how does it work? You’re about to find out.

How Does a Diesel Engine Work?

Diesel engines can be 4-stroke or 2-stroke for engines that can be more vigorously cooled. 2-stroke engines are more efficient again, but cause more wear and tear because there is less time for the piston to cool before firing again.

But we’re getting a little ahead of ourselves.

Under Pressure

One of the biggest differences between gasoline engines and Diesel engines is that of pressure. The fuel pressure as it is injected into the piston chamber of a Diesel can be as high as 23,500psi or pounds per square inch. Compare this to gasoline, which is between 10 to 50psi.

The compression ratio of air in a diesel engine is much higher than gasoline, between 14-25:1 when fuel is finally injected. This compression causes the air to heat up to between 500-1000°C. This compression aids in two ways: First, it allows diesel to interact with more air molecules in a smaller space, and second you don’t need a spark plug to ignite the fuel.

No Spark

Speaking of compression and ignition, Diesel engines also have no spark plug. Instead, they rely purely on the heat generated by putting a gas (air) under extreme pressure and injecting the fuel thereafter. So while Diesel fuel might not have that certain spark, it certainly has a burning desire to get you where you need to be.

Some Diesel engines start that fire in a pre-combustion chamber with a glow-plug, but this is quickly diminishing as manufacturing quality and engine control unit (ECU) computers get better.

Glow plugs are basically a heating element that pre-heats the air in a “cold” engine, before turning it on. Once the engine is going, it has no need of a glowplug. It’s very similar in principle to a cigarette lighter.

Fuel Type

Gasoline and Diesel oil are very different compounds, though they both start out as crude oil. Gasoline is more highly refined and therefore has smaller hydrocarbon chains. Because it’s more refined, it actually has less energy density than Diesel oil.

Diesel oil or fuel is thicker, has a pungent odor, and has a higher boiling temperature than water, at about 200-300°C. This makes it safer than gasoline in many ways because it’s more stable, doesn’t evaporate as quickly, and is less likely to explode on accident.

It also has a longer shelf-life. It’s worth mention that Rudolf Diesel initially envisioned using vegetable seed oil as the base fuel of this engine design.

If you put diesel into a gasoline engine, not only would the fuel injector struggle to actually spray fuel into the cylinder, but the higher temperatures and pressures required to burn it would cause the engine to fail.

Putting It All Together

These are all great features of the engine, but how does it work?

Imagine a cylinder very similar to a gasoline engine’s cylinder. When the piston moves down, making the first stroke, a valve opens and air is sucked into the cylinder.

The valve closes and the piston drives upward for the second stroke, compressing the air to a very high level. As it does so, the air heats to as high as 500-1000°C.

As the piston reaches full compression, fuel is injected at extremely high pressures and ignites, pushing the cylinder down on the third, or power, stroke.

The last stroke, the fourth movement of the piston in the cycle, is the exhaust stroke. In this movement, as the piston moves up a second valve opens to push out the burned up fuel and air mixture, expelling it from the cylinder. Then the cycle begins again.

This happens hundreds or thousands of times in a second with various fuel-air mixes as determined by the ECU.

How Does a Diesel Engine Work? Now You Know

So now, when you and your buddies are out at a game or you tell the girls at the nail salon about your new Diesel vehicle and one asks you: How does a diesel engine work? Do you think you can answer?

We know you can answer with confidence, even if you don’t have a Diesel engine diagram to show them. That’s what Motor Era is all about. Not only to DIY your cars but so you can be informed to the best of your ability on purchases and maintenance. 

We know it’s tough to keep up to date with technology in cars or daily life, but we have you covered. Check out our DIY and other articles so you know what to do when it matters!