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How does a diesel engine work?

Drivers may be familiar with the existence of diesel engines simply from seeing diesel fuel as an option at the gas pump. You may also have heard that many heavy-duty vehicles like trucks and tractors tend to favor diesel engines. But how does a diesel engine work, and why is it so well-suited for those big jobs?

Internal combustion engine basics

Cars that run on diesel and cars that run on gasoline are both examples of cars that have an internal combustion engine (ICE). This refers to an engine that generates power through the controlled combustion of a fuel and air mixture. As such, it may help to first understand the components of an ICE before diving into the specifics of diesel engines. Let’s identify and define a few key components of an ICE:

  • Combustion chamber: The cavity inside the engine’s cylinder where the combustion reaction of fuel and air takes place.
  • Pistons: Short cylindrical disks that fit closely within the engine’s cylinder and move up to create compression of the fuel and air mixture and move down as a result of the combustion reaction.
  • Fuel injection system: A system used by ICE vehicles to pump precise quantities of fuel (or a fuel and air mixture) into the combustion chamber.
  • Crankshaft: A mechanical component used to convert the downwards motion of the piston into rotational motion. This rotational motion is what gets transferred to the wheels via the drivetrain, turning the wheels and making the car go.

Diesel engine vs. gas engine

Now that you know the basics of an ICE, let’s examine the difference between diesel and gas engines.

Fuel type

One of the most obvious differences between diesel and gas engines is the fuel they run on. Gasoline and diesel are both refined from petroleum, but they’re quite distinct in chemical composition and even physical viscosity. The two fuels shouldn’t be used interchangeably — the engines that use them are designed differently and using the wrong fuel could result in potentially serious engine damage.

Compression ratio

Another difference between diesel and gas engines is the compression ratio used on either fuel. Gasoline engines utilize a fuel-air mixture that’s compressed and ignited by a spark. In diesel engines, diesel fuel is injected into highly compressed air. This extreme, much greater compression creates enough heat to ignite the fuel without requiring any external spark.

Engine construction

Gasoline engines and diesel engines have a similar look to them, but diesel engines tend to be significantly bulkier and larger. The higher compression rates of diesel fuel and the increased force of the combustion reaction need a sturdier build.

Pros and cons of diesel engines

Diesel engines come with their own set of advantages and disadvantages. Let’s examine some of these pros and cons:

Pros of diesel engines

  • Efficiency: Diesel fuel has a greater energy density than gasoline. Consequently, diesel engine efficiency tends to outpace gasoline cars. The U.S. Department of Energy found that diesel vehicles can get as much as 35% farther than their gasoline counterparts on a gallon of fuel.
  • Longevity: Gasoline is a solvent, which means that as it burns through your engine, it degrades and dissolves vital oils that keep your engine lubricated. Diesel, on the other hand, is an oil, which means it can gently lubricate the engine as it’s being processed. This, as well as a sturdier engine construction overall, helps increase diesel engine life expectancy well beyond that of gasoline engines.
  • Torque output: Diesel engines put out a lot more force during combustion, which translates to significantly higher torque output. Torque is the rotational force that gets your wheels turning from a dead stop, aiding with acceleration and carrying weight. This extra torque output is why diesel is the engine of choice for vehicles that need to haul large loads and perform heavy-duty work.

Cons of diesel engines

  • Usually more expensive: Diesel vehicles have a higher upfront purchase price than their gasoline counterparts. Diesel fuel is also often more costly at the pump than gasoline might be.
  • Availability: Not every gas station always has diesel fuel available to customers, so you may need to search around for your next fill-up.
  • Maintenance costs: Although diesel vehicles often require less frequent maintenance overall and tend to last longer, the cost of repairs when you do need them tends to be more expensive.

In summary

So, how does a diesel engine work? As you now know, it’s not that radically different from a standard gasoline engine. Both are internal combustion engines that utilize fuel to create a combustion reaction that powers the pistons and, ultimately, the vehicle. The major difference is that diesel fuel is distinctly different from gasoline and requires a much higher compression. That extra compression force, in turn, requires diesel engines to have a bulkier and sturdier design. It also creates a lot of extra torque, which makes diesel engines especially well-suited for heavy-duty work.

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