From shipping trucks and school buses to yachts and cargo ships, diesel engines power many of the world’s vehicles. Diesel engines in boats, trains and trucks transport goods and products all over the world — roughly 94% of freight is shipped in diesel-powered vehicles. Heavy-duty equipment on farms and construction sites is typically powered by diesel engines. Military vehicles and industrial facilities employ diesel engines, as well. Diesel engines are also common in passenger cars, especially in Europe.

What makes diesel engines so popular in these applications? Not that long ago, a major answer to that question was price. Historically, diesel fuel was cheaper than gasoline as it is less refined. Its price has risen in recent years for a variety of reasons, most notably increased demand from industries in China and the United States.

Fuel price wasn’t the only reason for diesel’s popularity, though. Diesel engines offer high fuel efficiency and exceptional torque, making them preferable to other combustion engines in many applications. Part of the reason why diesel engines are so efficient is that diesel is a more energy-dense fuel than gasoline, containing about 10 to 15% more energy.

How is it that diesel engines can outperform gasoline engines when it comes to torque output and fuel efficiency? To answer that, we’ll have to dig into the details about how diesel engines work.

What Is the Difference Between a Diesel Engine and a Gasoline Engine?

Diesel engines and gasoline engines are both examples of internal combustion engines — they’re powered by tightly-controlled explosions inside of the engine that move the pistons. Like any other combustion, the combustions inside these engines require fuel, oxygen and something to ignite the reaction.

There are many steps involved in translating the explosive energy of combustion into mechanical energy — say, to spin the wheels of a truck or the propeller of a boat. Combustion happens inside of cylinders in the engine blocks. In these cylinders, oxygen and fuel ignite, and the force of their combustion pushes a piston inside the cylinder down. Control rods attached the piston spin a connected crankshaft. The spinning crankshaft can then move a vehicle, turn a propeller or generate electricity, depending on the application.

While diesel and gasoline engines operate on similar principles, there are significant differences in how a diesel engine functions. A close look at a diesel engine vs. a gasoline engine, though, shows how a diesel engine provides certain advantages.

What are the differences between diesel and gasoline engines?

1. Diesel Engines Throttle Fuel, Not Air

Modern gasoline engines use a fuel injection system that mixes air and fuel before it enters the cylinder. The amount of air and fuel brought in to the cylinder — and thereby the speed or power of the engine — is controlled by the throttle.

In a diesel engine, on the other hand, fuel and air aren’t mixed until they meet in the cylinder. The same amount of air is drawn into the cylinder every time and is compressed by the piston rising into the cylinder. Just as the piston reaches to its maximum height in the cylinder, a controlled amount of diesel fuel is injected into the top of the cylinder.

The injection system for a diesel engine is part of the reason for its fuel efficiency. Since only the fuel is throttled, diesel engines have finer control over how much fuel is used at any point in time. When users need more power from an engine, the engine simply injects more fuel into the cylinders.

2. Diesel Engines Use Superheated Air to Ignite the Fuel

In a gasoline engine, spark plugs use a quick jolt of electricity to ignite the fuel/air mixture in the cylinder. The sparks need to be timed precisely with the motion of the pistons in the engine. These elements all need to be coordinated in an ignition system.

Diesel engines, on the other hand, use the motion of the piston in the cylinder to ignite the fuel. As the piston travels up the cylinder, it compresses the air inside. Rapidly compressing air like this increases the temperature of the air until it is superheated.

At high temperature and pressure, diesel fuel will ignite spontaneously without the need for an externally-provided spark. For this reason, diesel engines don’t need the spark plug ignition system used in gasoline engines. The air-compressing action of the piston — which is already necessary for the operation of the engine — starts the combustion reaction on its own.

Compression ignition is one reason why diesel engines are easy to maintain. Since ignition happens in diesel engines without the necessity of a separate system with its own parts, diesel engines have fewer parts that might wear down and need to be replaced.

You might remember a diesel engine that required an extra step or two when you started it up. That’s because it can sometimes be difficult to start a diesel engine when it’s cold. If compression alone is not sufficient to raise the air temperature high enough for the diesel fuel to spontaneously ignite, users will struggle to start the engine.

Manufacturers sometimes solve the problem of cold diesel engine starts with the use of glow plugs, small electric elements that pre-heat the head of the cylinder immediately before engine ignition. Glow plugs operate a lot like old-fashioned light bulbs — electricity runs through a filament, and it gets hot. Larger engines today usually rely on the engine control module (ECM) to monitor ambient temperatures and delay fuel injection accordingly so the air can be compressed until it reaches the necessary temperature.

3. Diesel Engines Use Different Fuel

There’s a good reason why diesel is marked very differently than gasoline at gas stations. You certainly don’t want to put the wrong type of fuel into your vehicle. The two fuels are very different, and their differences help explain why diesel engines operate differently than their gasoline-fueled counterparts.

Both diesel and gasoline are refined from crude oil, but they are noticeably different in both smell and texture. Diesel is heavier and more oily. Sometimes, you’ll hear it called “diesel oil.” Its boiling point is higher than that of water, so it evaporates more slowly than gasoline does.

The energy density of diesel is higher than that of gasoline. A gallon of diesel has about 147,000 British thermal units (BTU), compared to the 125,000 BTU of a gallon of gasoline. This helps diesel engines be more fuel-efficient, as there is simply more energy in each gallon of diesel fuel.

An alternate form of diesel is called biodiesel. Instead of being refined from crude oil, biodiesel is made from a mix of ingredients like waste animal fats, soybean oil and used cooking oil. Broad market availability of biodiesel is relatively new, but the idea of it is as old as the diesel engine itself. Rudolf Diesel, the inventor of the diesel engine, recognized that his engine could run on different types of fuel, and operated some of his exposition models on fuels based on ingredients like peanut oil. Biodiesel was relatively rare as early as the 2000s, but more than 2.8 billion gallons of this biofuel were produced in 2016.

How Diesel Engines Work

Understanding diesel engines might start with learning differences between diesel and gasoline engines, but there’s more to the story. How do diesel engines run?

The “combustion” part of an internal combustion engine happens inside of sturdy cylinders in the engine block. A diesel engine’s block is heavier than the block of a gasoline engine and has more support webbing. Internal combustion engines can have different numbers of cylinders. For example, four-cylinder engines are common for passenger cars.

At the top of each cylinder, called the head, there are valve-controlled openings that allow oxygen-rich air to enter and exhaust to exit. The cylinder head also has the fuel injector nozzles that spray diesel into the combustion chamber.

A piston fills the lower part of each cylinder. The piston is precisely machined to fit snugly into the cylinder while still being free to move up and down. Control rods connect the pistons to the engine’s crankshaft. As the explosive power of combustion pushes the pistons downward, they turn the crankshaft. This is how an internal combustion engine functions — how it converts energy from a fuel into rotational energy that can move a vehicle or boat or produce electricity.

The operation of a diesel engine is described in “strokes.” A stroke is a full movement of a piston, either from top to bottom or bottom to top. Typically, diesel engines have four strokes per cycle:

  1. Intake stroke
  2. Compression stroke
  3. Combustion stroke
  4. Exhaust stroke

It takes much longer to explain each stroke than it does for each stroke to happen. The entire four-stroke cycle only takes milliseconds to complete.

1. Intake Stroke

Combustion can’t happen without air. The cylinder fills with air during the intake stroke, when the intake valve is open, and the piston moves down.

A diesel engine can generate more power and operate more efficiently if it can supply its cylinders with a larger number of air molecules. Many diesel engines accomplish this with a turbocharger. A turbocharger has a high-velocity fan that compresses air before it enters the cylinder, packing the oxygen molecules close together.

With more oxygen in the cylinder, the diesel engine is more efficient because there is a greater likelihood that the injected fuel will react with as much oxygen as possible. Additionally, the increased amount of oxygen allows the engine to inject more fuel into the cylinder in the combustion stroke. This generates more torque, making the diesel engine more powerful — or allowing a smaller turbocharged engine to do the same work as a larger engine without a turbocharger.

2. Compression Stroke

The piston moves back toward the top of the cylinder during the compression stroke. The air intake valve closes, and the air inside the cylinder compresses rapidly with the movement of the piston.

As the oxygen molecules are pressed close together, the temperature of the air inside the cylinder rapidly increases until it is superheated. These high temperatures are necessary for the combustion of the diesel fuel, as there are no spark plugs in a diesel engine.

3. Combustion Stroke

At the top of the compression stroke, the fuel injector forces diesel fuel into the cylinder. Fuel injectors vary quite a bit from engine to engine. The fuel needs to be distributed as evenly possible, and the injector needs to accomplish this task in a high-temperature and high-pressure environment. Sometimes fuel is injected into a small, specially-shaped chamber immediately connected to the cylinder that swirls the fuel and air.

Like the air in the cylinder, the diesel fuel is also often kept under high pressure. While the fuel pump in a gasoline engine might only pressurize gasoline to 10 to 50 pounds per square inch (PSI), fuel in a diesel engine can be pressurized up to 23,500 PSI. The spray of diesel fuel through the fuel injector nozzles is monitored carefully by the ECM. When the throttle is opened up, the fuel injection system increases the amount of fuel going into the cylinder.

What happens to the pressurized diesel fuel when it meets the highly-compressed, superheated oxygen in the cylinder? Just what you’d expect — it explodes. The fuel spontaneously combusts, and the force from this reaction pushes the piston back down the cylinder. The movement of the piston downward is translated by the control rods to rotate the crankshaft of the engine.

4. Exhaust Stroke

After combustion, the oxygen in the cylinder has been consumed, and the air inside the cylinder needs to be exhausted so the four-stroke cycle can restart. As the piston moves back up toward the top of the cylinder, the exhaust valve opens and the gases are exhausted from the cylinder.

In a turbocharged diesel engine, the exhaust isn’t done working after leaving the cylinder. Instead of going straight to the tailpipe, the exhaust is directed to the turbocharger, where it spins the turbine that draws in fresh air for compression. In this way, a turbocharger is largely a contained system. It uses the energy of the gases expelled from the cylinders to power the turbine that draws in and compresses more fresh air.

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