By the Numbers

All About Diesel Engines

Gas engine alternative

From semi-trucks and heavy equipment to passenger cars and trucks, you can find millions of diesel-powered vehicles sharing the road with traditional gasoline engines. Since its invention in 1892 by Rudolf Diesel, the diesel engine has been a formidable alternative to the gas-powered engine.

How much do you know about diesel engines? If your knowledge begins and ends with knowing that diesel fuel is sold at the gas station, we’ve got you covered. Read on to learn how diesel engines differ from gas engines, their advantages and disadvantages, and how to determine if a diesel-powered engine may be right for you.

How a diesel engine works

Like a gas-powered engine, a diesel engine is an internal combustion engine that uses a series of small explosions (combustions) to power the vehicle. What makes a diesel engine different is how the combustions occur. In a diesel engine, the air is compressed followed by fuel being injected into the cylinder. Because the air is so hot, the fuel ignites without the use of a spark plug as in the case of a gasoline engine.

Here is what the four-stroke process looks like in a diesel engine:

  • Stroke 1 – Intake: Inlet valve opens to bring in air while pulling the piston down.
  • Stroke 2 – Compression: Inlet valve closes and the piston moves back up to compress the air, heating it up.
  • Stroke 3 – Combustion (power): When the piston is at the top, fuel is injected and ignites. The combustion pushes the piston back down.
  • Stroke 4 – Exhaust: Exhaust valve opens and the piston returns to the top, expelling exhaust out of the cylinder. 

The cycle repeats itself in each cylinder hundreds of times per minute to power the vehicle.

Diesels on US roads

In the US, diesel vehicles accounted for just 3% of total passenger auto sales in 2014. Medium and heavy-duty trucks made up the majority of sales while cars, SUVs and minivans tallied just 1.5% of sales1. With diesel fuel costing up to 50 cents more a gallon, most Americans are happy to stick with their gas-powered vehicle.

While most Americans haven’t made the switch to diesel for their personal vehicle, diesel is the engine of choice for semi-trucks, construction equipment and buses thanks to diesel’s ability to efficiently move large loads at low speeds.

Diesel is much more popular in Europe where close to half (49.5%) of all vehicles sold had a diesel engine in 2016. In some countries like Italy, Portugal and France, the percentage is even higher2. With its better fuel economy and lower CO2 emissions, diesels have long been favored in Europe.

Advantages of diesel


Better mileage – Improved fuel economy is a big selling point for diesel engines. Diesels can achieve 25-30% better mileage than a gas engine due to the higher efficiency of diesel fuel, while the direct fuel injection during the combustion process wastes little fuel.


More power – More torque and a better power-to-weight ratio allows diesel engines to handle heavy loads, which explains why it’s used in big rigs and heavy equipment. The improved towing power is why some truck buyers look to diesel.


Lower emissions – Modern diesel engines emit lower levels of CO2 and carbon monoxide than gas engines.


Less maintenance – With no spark plugs and less stress, diesel engines can run longer before requiring maintenance.


Long lasting – Thanks to higher compression, the parts of a diesel engine suffer less wear and tear than a gas-powered engine. It isn’t uncommon to hear of diesel engines making it to the 500,000 mile mark.


Safer – Diesel fuel is less volatile and the vapor is not as explosive as gasoline. This reduces the chance of the fuel igniting and causing a vehicle fire.

Disadvantages of diesel


Higher fuel costs – The higher cost of diesel fuel has been a hurdle to diesel vehicles being widely accepted. Diesel fuel can be up to 50 cents more per gallon than regular unleaded fuel.


More expensive purchase price – A vehicle with a diesel engine is typically more expensive to purchase than its gasoline counterpart. Built to handle more punishment, the heavy-duty parts add to the sticker price.


Noisier – While modern diesels aren’t the noisy engines of years past, they still are noisier than gas engines. Listen to both vehicles side-by-side to see if the noise level is a deal breaker for you.


Pollution – Despite advances, diesel fuel still produces some harmful emissions like nitrous oxide and soot. However, today’s low-sulfur diesel fuel has greatly reduced these emissions.


Cold starts – Diesels can have difficulty starting in cold temperatures since they don’t have spark plugs and instead ignite spontaneously. When it’s cold, the air may not be hot enough to ignite the fuel. Devices like heating elements and block heaters are used to counter the problem.

Is a diesel right for me?

To determine if a diesel vehicle is right for you, you need to examine your driving habits. If you drive a lot of highway miles, a diesel could help you save money in fuel costs. Diesels typically deliver better highway mileage numbers than city driving. However, if you don’t put a lot of miles on your vehicle, it will be difficult to make up the higher purchase price of a diesel in gas savings.

If you find yourself towing a boat, camper or trailer on a regular basis, diesel’s low end power could be right for you. The additional torque can help you efficiently haul a bigger load. Definitely take a close look at diesel if you’ll be doing a lot of your driving in the mountains. Again, diesel’s low end power is a big help on those steep climbs.

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Additional sources: 1Diesel sales in the US2Diesel sales in Europe

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