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What Is a Diesel Engine and How Does It Work?

Masterpiece  ·  January 5, 2022

What Is a Diesel Engine and How Does It Work?

Besides petrol engines, diesel engines are the second-most common engine type that we see or hear about daily. But what is a diesel engine, and how does it work? Is it better or more efficient than a petrol engine? These are the questions we’ll answer in this guide.

A diesel engine works by compressing air until it becomes hot enough to ignite and combust the fuel. That's quite different from how a petrol engine works, and it does that without need of a spark plug to trigger the combustion process. 

As a result, diesel engines offer more power and better fuel economy. However, they don't provide high-speed performance as petrol engines do.

Keep reading as we explore the basics of how a diesel engine works and whether it's the best choice for you.

What Is a Diesel Engine?

A diesel engine, like its petrol counterpart, is a type of internal combustion engine, which means that it produces power by burning diesel fuel inside the cylinders. It differs from its petrol sibling not only in the type of fuel that it burns but also how it starts the combustion process – without a spark plug!

Instead, a diesel engine compresses air until it becomes hot enough to cause spontaneous combustion when fuel is sprayed. 

Let’s dig some more into the history of the diesel engine to understand it better.

A Brief Background on Diesel Engines

In the early days, diesel engines were mostly seen as too dirty, not quick enough, and unpleasantly loud. Fast forward to the modern era, diesel engines now use better technologies, making them efficient, versatile, and clean-burning.

When talking about heat engines, diesel engines are up there with the very best, becoming the preferred choice for a variety of operations, such as heavy-duty machinery, generation of power, and running vehicles.

A lot of people who know about diesel engines can only remember their appearance around the 1970s, when carmakers began using diesel engines in consumer cars. 

Back then, virtually every car manufacturer did their best to provide a passenger car that was powered by a diesel engine especially with the looming gas crunch. 

However, diesel motors go way, way back farther than the ‘70s. Rudolf Diesel, the man who invented the diesel engine, secured a patent for it in as early as 1892.

How Do Diesel Engines Differ from External Combustion Engines?

Petrol and diesel engines produce power by burning fuel inside their cylinders. For that, they’re called internal combustion engines as opposed to the external combustion engines used in steam locomotives.

For steam engines, a huge fire gets started at one part of its boiler, heating up water to produce steam. Next, the steam makes its way through long tubes to twin cylinders located at the other side of the boiler, moving the piston back and forth and causing the wheels to move. This is external combustion. 

Here’s how the diesel engine differs from it.

Rudolf Diesel built the diesel engine to get diesel fuel converted into mechanical energy right inside the cylinders where combustion takes place. This means that energy is conserved since there is no need for the heat to travel from its point of production to the cylinders. 

This is the reason that internal combustion engines have higher efficiency than external combustion engines.

How Does a Diesel Engine Work?

Before we go into details about how diesel engines work, let’s recap and simplify how a standard petrol engine works. 

It starts with air and fuel entering the combustion chamber, the mixture gets compressed, and a spark plug ignites it to set off combustion.

The process repeats continually, and all the energy released from that process transfers over to the crankshaft, making the car move.

Diesel engines differ from petrol because only air is compressed in the cylinders, heating high enough to spontaneously combust when fuel is introduced. 

Here’s how combustion in a diesel engine works:

  1. Air enters the chamber, and the piston compresses it to such levels that cause its temperature to rise.
  2. The fuel injector sprays a mist of fuel into the compressed air.
  3. As the fuel comes in contact with the hot compressed air, ignition results and sets off combustion.

As the combustion process goes on repeatedly, the energy generated turns the crankshaft and powers the car to move.

That’s the general idea behind the combustion process that takes place inside a diesel engine.

But how does that relate to you as the driver. What goes on as you insert the key, start the engine, and drive your car?

To better understand the ins and outs of a diesel engine, let’s talk about what happens between you and your diesel engine vehicle.

What Takes Place When You Start and Drive Your Car?

1. Turning the key warms it up

Once you turn the key, you initiate a process that builds up heat in the cylinders. You’ll notice the ‘Wait to Start’ light come on.

It means that the glow plugs are working to heat up the air in the cylinders before fuel is sprayed. Heating up takes around 15 seconds.

Most modern diesel engines, however, no longer rely on glow plugs to ignite diesel fuel but instead use injection pressure to build up heat in less than 2 seconds.

2. Start light is activated

When the dash indicator gives the Start signal, and you step on the accelerator and turn the key to start ignition, you’re telling your car to start pumping fuel. 

As fuel travels to the cylinders, it goes through fuel filters and the nozzles for fuel injection. Make sure that your fuel filters are replaced regularly because contaminated fuel can cause clogs in the injector nozzles.

3. Fuel pump pressurizes fuel and delivers to the cylinder

The fuel pump pressurizes the fuel (to 23,500 psi approx.), which is then delivered via the delivery tube, aka rail, ensuring that each cylinder is supplied with pressurized fuel at clockwork precision. 

How does the engine do this? 

The fuel injectors have ECU-controlled nozzles that spray right into the cylinder's combustion chambers. It is the engine control unit (ECU) that regulates the pressure and determines the fuel spray timing and duration. 

4. Pressurized fuel meets compressed hot air

As fuel makes its way to the chambers, another process takes place – air is supplied into the cylinders. Conventional diesel engines use air cleaners to supply air, but turbochargers have taken over the game.

With the right conditions, turbochargers can significantly reduce fuel consumption by 20% and boost engine power by 50%. 

That’s basically the principle behind how a diesel engine works, and the combustion process is repeated continually as you drive.

Main Parts of a Diesel Engine

We will cover only those components that are directly involved in the fuel cycle, specifically for a four-stroke diesel engine. 

Since the other components are not directly involved in this cycle, such as the alternator, we will not highlight them here. 

The key parts of a diesel engine are:

1. Cylinder Block Assembly

A cylinder block or cylinder engine is a metal block made of cast iron or aluminium alloy. The assembly is considered the ‘backbone’ of a diesel engine because it supports the combustion process. It consists of the following components: 

2. Cylinder Head Assembly

The cylinder head assembly is produced using cast iron or aluminium, which is preferred because it is lighter. The following parts make up the cylinder head. 

3. Piston & Connecting Rod

Pistons adjust the cylinder volume, which is important in compression. When the piston moves downward, it takes in a large volume of air. When the piston moves up, the cylinder volume retracts, compressing the air and increasing its temperature. 

The connecting rod ensures that the piston makes consistent upward and downward movements. To do its function, it has the following parts:

4. Crankshaft

The crankshaft converts the upward and downward movement of the piston into a rotary motion. On one end of the crankshaft is a timing belt that attaches it to the camshaft. On the other end is a flywheel that controls the power generated. Here are some parts of the crankshaft:

5. Oil Pan

The oil pan holds the engine oil. It is made from a durable material. 

6. Timing Chain Assembly

The timing chain provides an angular connection between the sprocket gears of the crankshaft and the camshaft.

7. Flywheel

The flywheel balances the speed of the diesel engine and serves as torque storage or energy reservoir, storing energy in its kinetic form. 

8. Fuel System Assembly

The first system goes all the way from the fuel tank to the engine’s fuel injector. Fuel systems are designed to supply fuel during strokes right inside the combustion chamber. 

All of the components that are discussed above play a vital role in ensuring that the diesel engine operates smoothly as required. If issues occur to any of the parts that have been discussed above, then, it will lead to a disruption in the working process of your diesel engine.

Many of these parts are also discussed in another article about parts of a car engine in this blog.

Pros and Cons of Diesel Engines

Diesel engines are fantastic machines, which is why they enjoy mainstream popularity alongside petrol engines. Still, they come with their fair share of pros and cons.

Here is what you need to know:

The Benefits of Diesel Engines

  1. Energy density: Diesel is an incredibly energy-dense type of fuel. That means you’ll get more energy out of one tank of diesel than you would a tank of petrol. It means that you can expect much better fuel economy, even from a small diesel engine or generator.
  2. No spark plugs or distributors: Diesel engines have no spark plugs (and no distributors). That means you won't have to regularly spend money repairing or replacing those parts when they wear out. Plus, there's no need for any ignition tuning when it comes to diesel engines.
  3. Longer lifespan: We also saw earlier how diesel engines rely on high air pressure for their combustion process instead of spark plugs. Diesel engines are much tougher since manufacturers design them to withstand those high pressures. A nice side effect of that is that diesel engines last much longer, which also adds to the vehicle's resale value as a whole.

The Drawbacks of Diesel Engines

  1. Specialised repairs: Diesel engines are clearly different from standard petrol engines that most cars have, meaning, different set of skills and knowledge is needed to maintain and repair those engines. Unfortunately, like any other type of repair in the car world, specialised skills mean higher costs. You can quickly locate truck & diesel mechanics near you through CarPart Au.
  2. Less speed: If you’re looking for adrenaline-packed, high-speed performance, diesel engines aren’t the ones for you. These engines are much better at delivering more power, yes, but they’re not going to give more speed. That’s why diesel engines are more often used in industrial applications like utes, trucks, and trailers and less so in passenger cars.
  3. Fuel costs: Diesel fuel costs the same or more than regular petrol. Remember: there’s a higher demand for diesel because passenger cars aren’t the only ones that need them. Industrial vehicles like utes, trucks, and trailers need diesel too, and they need it in larger volumes than everyday drivers do. Simply put, there’s more competition for diesel, which is why it often costs more.

Bottom Line: Is Diesel Engine Better Than Petrol Engine?

It’s clear that electric vehicles are taking over internal combustion engines. Still, diesel engines remain the pillar on which many machines are designed, including buses, trucks, heavy equipment, and farm machinery.

More and more car makers turn to EVs and petrol cars, but it doesn’t mean that diesel engines are going out of style. It’s still very much an option, but is it a better option than petrol?

Let’s suppose you’re an everyday Aussie driver who wants to buy a new family vehicle. The question boils down to this – Is a diesel engine better than a petrol engine for your daily use? Well, it depends.

We’ve explored everything to know to answer the question “How does a diesel engine work?” Diesel engines are better for you if you prioritise power over speed. 

For some, that might mean people who plan on towing small recreational vehicles or boats. Or it could also be handy for people who transport a car full of adults daily. For these drivers, what they need are heavy-duty power generators, and that’s what diesels are. 

In other words, it all depends on your priorities and intended use for the vehicle.

To learn more about how a diesel engine works and what components it needs, check out our Blog at CarPart AU!

FAQs about Diesel Engines

1. Do diesel engines have spark plugs?

No, diesel engines do not use spark plugs.

When you explore the question, “How does a diesel engine work,” you’ll eventually realise that one crucial component is missing: the spark plug. 

That's because the combustion process in a diesel engine doesn't need a spark plug to work. Remember: the piston compresses the air to such a level that it becomes incredibly hot. So hot that it's enough to ignite the diesel fuel without an electrical spark.

2. Why do diesel engines use glow plugs?

Diesel engines don't have spark plugs, but you'll find that they do have glow plugs instead. These components work similarly, meaning they also receive electrical power and are crucial for the combustion process. But that's where the similarities stop.

As we’ve seen above, the fuel inside a diesel engine can only combust if the air is hot enough. Unfortunately, that can be challenging to accomplish in cold climates, especially since the engine block is made of solid metals that get incredibly cold.

So, glow plugs save the day by generating heat inside the engine’s cylinders. These plugs glow red hot (hence the name) to heat air enough to ignite the diesel fuel.

Each cylinder will have one glow plug. So, a two-cylinder diesel engine will have two glow plugs, and so on.

3. Can you use petrol in a diesel engine?

No, you must never pour petrol into a diesel fuel system. Not only does petrol function differently compared to diesel, but it can also be harmful to the fuel already inside the system. Pumped the wrong fuel? Here’s what to do.

For example, petrol will take away the lubrication properties that diesel has. That will damage the diesel fuel pump and any valve along the way by launching metal debris throughout the fuel system.

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