man working on diesel engine

A Short But Complete Guide to How Diesel Engines Work

You’ve probably heard a lot of talk about the advantages of having a vehicle with a diesel engine. 

They’re longer-lasting and more reliable. They’re much more fuel efficient. They’re more powerful, etc. and etc.

There certainly are many reasons to consider looking into getting a diesel. This is whether it’s a pick-up truck for hauling or simply an efficient, everyday driver.

But why exactly do diesel engines produce these advantages? 

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This article will explain how diesel engines work for those looking into getting a diesel vehicle to drive, those looking to learn more for their own repairs, or simply the mechanically curious.

A Brief History

The invention of the diesel engine came on the heels of the 1876 invention of the (four-stroke) gasoline engine by Nicolaus Otto. This engine was both in power and efficiency a great upgrade to the predominantly used steam engine.

Still, these early motors were by no means efficient machines. It was this fact that led to the development of the diesel engine as we know it today. 

It was in 1878 that a 20-year-old Rudolf Diesel, the inventor of and namesake to the diesel engine, learned of this inefficiency of these early machines. This information, along with his experience working with early refrigeration which taught him some of the principles of thermodynamics, is what inspired him to begin working on a higher efficiency internal combustion engine.

In 1892, he patented many of the ideas and practices that went into this early development. One such patent was for a compression-ignition engine. It’s this that forms the foundation for what makes a diesel engine work.

How Gas and Diesel Engines Work Similarly

There are many similarities between the process for producing energy between a gas-powered and diesel-powered engine. This is because these are both internal combustion engines. 

In these types of engines, the ignition of fuel produces chemical energy. This chemical energy acts upon the mechanical parts of an engine to generate mechanical energy.

It’s this mechanical energy which, through an engine’s crankshaft is transferred to rotary motion. This motion, in turn, is what drives the wheels of your vehicle.

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Just about every vehicle on the road today, whether gas or diesel, is powered by an engine which follows a four-stroke combustion cycle.

The first of these four strokes is the intake stroke. Here an intake valve opens allowing air to flow into the cylinder by means of a vacuum created by the downward movement of the piston.

Next is the combustion stroke. During this stroke, the piston within the cylinder moves back up, compressing the air that was let in during the intake stroke.

After this comes the combustion stroke. This is (for a diesel engine) where the fuel is injected into the cylinder, at this point in the process called the combustion chamber. As you may have guessed, this fuel is ignited and combusts generating chemical energy which forces the piston back down.

On the way back up from the downward motion created in the combustion stroke comes the exhaust stroke. During this final stroke of the cycle, an exhaust valve opens. The upward motion of the piston forces the exhaust created during the ignition phase out of this.

How Diesel Engines Work Differently

While the four strokes of diesel and gasoline engines are the same, the way they go about this cycle are a bit different. 

The differences here primarily come during the fuel injection and ignition processes. 

In a gasoline engine, a fuel-air mixture is introduced into the cylinder during the intake stroke. After compression takes place, this mixture is ignited via a spark plug, creating the combustion phase.

In a diesel engine, however, there are no spark plugs. After air in intaken and has been compressed, fuel is directly injected into the cylinder of compressed air. It’s the heat generated during this compression phase, rather than a spark plug, which ignites the fuel within the combustion chamber.

It’s this injection process which is the greatest difference between a gasoline and diesel-powered engine, outside of the fuel itself.

In a diesel engine, this is no simple process. The exact amount of fuel needs to be injected in just the right place at just the right time.  In modern engines, this process takes place through electronically-controlled direct fuel injectors. These are linked up to the central vehicle computer, known as the ECM (engine control module). 

Because of the complexity of this injection process, different manufacturers have experimented in many different ways with it. This could mean adjusting the location of the fuel injectors or adjusting when during the compression process the fuel is actually injected.

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Each unique system requires different ECMs programmed for their specific injection processes. Different diesel engine manufacturers have their own ECMs programmed according to the process followed by their specific engine build. Allison engines using ECMs programmed for their engines, a Cummins engine using a Cummins ECM, etc.

Gasoline Versus Diesel Fuel 

The other great, most overt difference in these two types of internal combustion engines is the fuel itself.

Gasoline and diesel are both products of crude oil. They are simply produced at different phases of refinement of this nonrenewable resource. 

Diesel fuel is actually a less-refined product of crude oil than gasoline is. As you know if you’ve ever had diesel and gas on your hands, it is also much oilier than gasoline. It is also significantly denser, which means it is heavier and evaporates at a higher temperature than gas.

The reasons for these characteristics has to do with the chemical makeup of the fuel.

Both products are made up of molecular chains of hydrogen and carbon atoms. Diesel, however, is made up of larger chains. It’s this that gives diesel its heavier composition.

This denser composition leads to diesel have a higher chemical energy potential. It’s this potential that is released upon ignition and what leads to diesel engines having that greater power and efficiency you’ve heard so much about.

Diesel Pros and Cons

While diesel engines are indeed more efficient and often produce a greater power output, they do come with some cons. 

Although more environmentally-friendly overall, the greater amount of unburned particles produced by diesel engines is one of the negatives of their operation. 

Still, overall there are many advantages to owning a diesel engine.

In addition to the greater efficiency and power already talked about, diesel engines can also be easier to do repair work on. This is due to how diesel engines work in that they require less moving parts to run.

Do you like to do your own vehicle repairs? Whether you’re a long time diesel owner or newly converted, feel free to get in contact with me if you have any repair questions.