How Does a Diesel Engine Work? December 23, 2014

There’s nothing like the deep throaty sound of a diesel truck chugging along. The while of a turbo, the rumble from the engine, and the inevitable black smoke make a diesel truck a very visceral thing indeed.

While you probably know the basics of a diesel engine like the fact that they take diesel fuel and that they usually offer more torque, the details of what truly makes a diesel engine unique aren’t widely known. Diesel and gas engines may both run cars and trucks, but how they both function is pretty different. Diesel engines are amazing pieces of technology and understanding how they work is an exercise in understanding your truck. Here’re the basics on how a diesel engine works.

What is a Diesel Engine?

At its core, a diesel engine is an internal combustion engine. This means that fuel is fed into the engine where it combusts and creates power. This power is then transformed into forward motion and just like that you’re driving.

To contrast this, an EXTERNAL combustion engine is something in which the fuel and combustion happen outside of the engine, like you would find in older steam engines. In other words, a diesel or gasoline engine has combustion happen inside of the cylinder while external combustion engines have combustion, usually in the way of a wood or coal-fed fire outside of the cylinder and usually up to 20 feet away.

While we don’t deal with external combustion engines much at all today, it’s worth understanding the basic difference.

How Does a Diesel Engine Work?

Diesel engines are simpler than their gasoline-drinking brethren. First, air is pulled into the cylinder where it is compressed by the piston. This compression is far higher than is found in a gas engine. With the air compressed up to 25 times smaller than its original volume, fuel is sprayed into the cylinder to mix with the compressed air.

When a gas is compressed it becomes hot. The more you compress it and the quicker it is compressed the more heat that is produced. In an automotive diesel engine this compression results in temperatures anywhere from 1,000°F and higher.

The fuel is sprayed by an electronic fuel injection system that expertly controls the volume and timing of the fuel. Since the air in the cylinder is now at least 1,000°F it combusts instantly. In a gasoline engine the combustion is driven by a spark delivered from a spark plug and ignition system but in a diesel, the heat of combustion is all it takes.

This explosion drives the piston back down completing the cycle and delivering it’s contribution to the engine’s power output. The crankshaft, with it’s alternating piston positions drives the next piston up while the one in question is driven down by the explosion. This process continues for the entire time the engine is operational. While there is no spark plug, a diesel engine completes the same four strokes that a typical gasoline engine completes. Since there is higher compression during each stroke, the torque generated is higher, which is why diesel engines are so common with higher weight vehicles. The other reason is fuel economy.

Diesel Engine Efficiency Explained

If you’ve ever looked into diesel engines before, you probably noticed they offer far better fuel economy than a gasoline engine does. This is due to a few key points that diesel engines offer, but what you might not realize is that a diesel engine can be up to 40% more efficient than a similar gasoline engine.

The biggest reason for this is the lack of a spark plug. Since the diesel engine uses compression to generate the heat for combustion, the air is more evenly combusted unlike a gasoline engine where some fuel and air is often missed. Diesel engines also require less fuel when working at lower power, unlike a gasoline engine that requires far more fuel to idle. The other two reasons for better fuel efficiency with diesels has to do with the fuel. First, diesel fuel carries more potential energy than gasoline, meaning it has more power per gallon available. Second, diesel fuel is a better lubricant than gasoline is. This lubrication helps the engine run with less friction, thereby allowing for better overall fuel economy.

Disadvantages of Diesel

So if diesel engines are so great, why doesn’t every car and truck have one? Well, there are a few downsides to diesel engines. First, since diesel engines operate at far higher compression ratios than a gasoline engine, they must be larger and heavier to account for this. This also means there is some risk of injury with diesel engines due to overpressure and explosion, and while this is rare, it is still a possible concern.

Pollution is a major concern for diesel engines. The black smoke you see coming from a diesel truck under load is mostly unburned soot particles, and while these aren’t common for daily use, they are still a possibility. Another big drawback to diesel adoption is the cost. The upfront cost for a diesel engine is higher than a gasoline engine, but over time the maintenance and fuel cost savings more than make up for it.

Finally, diesel engines are difficult to operate in cold weather. Since diesels rely on the heat in the cylinder, a cold engine doesn’t operate nearly as well as a warm one. The colder it is the harder a diesel is to start and keep running, which is why many use cylinder heaters and block heaters that keep the engine at a steady temperature while not running.

Diesel Engines Today

Today, diesel engines are far cleaner than ever before and offer higher torque and huge fuel economy improvements. While diesel trucks and cars have been around for quite some time, the need for better fuel economy with CAFE standards being implemented throughout the country, many companies and people are turning to diesel.

What was once a dirty and loud necessity is now a clean, efficient, and low-noise alternative to the standard gasoline engine.

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