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Diesel vs. Gas Engines: What’s the Difference?

Aug 29, 2019 ·

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Whether you’re buying a new car soon or are thinking about becoming a technician, you might be wondering, “what’s the difference between a gas and diesel engine or is diesel better than gas?"  

Both gas and diesel engines are internal combustion engines that convert chemical energy into mechanical energy. The mechanical energy moves pistons up and down inside cylinders, which creates a rotary motion that turns the wheels of a car forward.

Gas and diesel engines both convert fuel into energy through a series of small explosions or combustions. However, these engines have their differences as well. Keep reading to learn all about what makes gas and diesel engines unique, the four-stroke combustion cycle, common misconceptions and more.

Gas vs Diesel Engine

Before getting into what makes these engines different, it helps to know what the difference between diesel and gas fuel is. They’re both mined and begin as crude oil, but are separated during the refining process. Diesel fuel is thicker than gas and has more energy density.

One of the most important differences between gas and diesel engines is the thermal efficiency of diesel engines, or the work that can be expected to be produced by the fuel put into the engine. A diesel engine is about 20% more thermal efficient than a gas engine. This directly relates to a 20% increase in fuel economy. 

Diesel engines are used in a wide variety of equipment and vehicles, anywhere from medium duty trucks to overseas cruise ships and power generators. Diesel engines are used because of their increased fuel efficiency, which results in lower operating costs. They are also used because of their reliability — a diesel engine runs at a much slower RPM (revolutions per minute) than a gas engine, therefore there is less wear and tear and the life cycle of the engine is longer. 

Increased thermal efficiency also translates to more power and torque. Essentially, diesel engines provide a better bang for your buck for the fuel put into them. A diesel engine’s high torque application is very beneficial for hauling, as it helps with carrying heavy loads. 

Gas engines, on the other hand, have a much higher volatility point but a lower flash point. The combustion of a gas engine is controlled by a spark. The fuel is compressed with fresh air and once the piston is on top of its stroke and the compression is at what the manufacturer wanted, a spark ignites the fuel and air mixture, which is what causes the engine to run.

A diesel engine does not use a spark. Rather, it is what’s called a compression combustion engine, and there is a much higher compression ratio on a diesel engine than there is in a gas engine. The air fuel mixture is squeezed so much that it explodes on its own. 

Essentially, a gasoline engine is a spark-fired combustion, and a diesel engine utilizes compression.

Diesel vs Gas Engine Life Expectancy

Another differentiator between gas and diesel engines is life expectancy.  

When gasoline engines hit the 120,000 to 150,000 mile mark, the cylinders will start to show some wear, which decreases efficiency. Diesel engines, on the other hand, are known for their long life cycle — there are some that are a million miles plus motors. 

While diesel engines are designed similarly to gas engines, the cylinder on a diesel engine has a removable liner. When the engine reaches 200,000 or 300,000 miles, the liner can be replaced without having to replace the entire engine.

Diesel engines are low RPM, but high torque. A gas engine in a car will turn twice as many revolutions per minute going down the highway as a diesel truck, meaning the diesel truck wears at half the rate of the car, resulting in a longer life expectancy.  

Finally, diesel engines are built very robust, which allows them to last longer. The castings are built thicker, the cylinder walls are built thicker and the oiling system has a higher volume. Diesel engines are really built for the long haul.

The Four-Stroke Combustion Cycle

Gasoline and diesel engines use a four-stroke combustion cycle: 

  • Intake stroke: The piston travels from TDC (top dead center) to BDC (bottom dead center). As the piston moves down, this creates a low pressure inside the cylinder, and air is drawn into the cylinder through the open intake valve and fills the cylinder with oxygen rich air.
  • Compression stroke: The piston travels from BDC to TDC. The piston is compressing a large volume of oxygen rich air (and in some cases a 14.7:1 ration of fuel), preparing for combustion. As the piston approaches TDC, a spark is induced causing an explosion (combustion).
  • Power stroke: The piston travels from TDC to BDC. The enormous cylinder pressures created from combustion push the piston downward, transferring the power of combustion to the crankshaft.
  • Exhaust stroke: The piston travels from BDC to TDC. As the piston rises in the cylinder, exhaust gasses are pushed out of the cylinder through the exhaust valve and enter the emissions or after treatment system before being released to the atmosphere.

Keep in mind that diesel engines have no spark plug to ignite the fuel—they create combustion by taking oxygen rich air and fuel and compressing the mixture until the point of combustion. The heat of the compressed air ignites the fuel in a diesel engine.

Common Misconceptions

There are a variety of misconceptions out there when it comes to gas and diesel engines. However, as technology advances, many of these misconceptions have proven to be untrue. 


The government is continually putting stricter requirements on diesel manufacturers when it comes to pollution. 25 years ago, semi-trucks on the road were billowing black smoke out of tailpipes. However, thanks to today’s technology and engineering, diesel engines run cleaner than ever before. In fact, many joke that the air coming out of a diesel engine today is cleaner than the air going in!


One of the first things that might come to mind when you think of a diesel engine is the noise it makes. Modern technology has made it possible to eliminate the loud rumble heard when starting up a diesel truck. In fact, from the inside of many diesel-powered vehicles, you wouldn’t even know if it was using a gas or diesel engine.


Even though diesel fuel often costs more than gasoline, it contains more potential energy. Therefore, less diesel fuel is required to accomplish the same amount of work. Because of this, the overall cost for gasoline and diesel fuel over time comes out to be nearly the same. However, this is dependent on the vehicle, driving style and several other factors.


Turbochargers have become huge in the performance market for both gas and diesel engines. A common misconception is that you can go out and buy a bigger turbo and make your vehicle faster, whether it be a gas powered car or a diesel powered truck. However, it’s important to note that a bigger turbo alone will not create any more power. A series of other improvements must be completed to make the turbocharger work efficiently and actually produce more power.

The Role of a Technician

If you’re considering becoming a technician, you might be wondering if you need to know how to work on both gas and diesel engines. While this isn’t a must, technicians who can work on both have great advantages when entering the workforce.

By having knowledge on both gas and diesel engines, you can become a great asset to a dealership or wherever you decide to work in the transportation industry. Even automotive dealerships have cars with diesel engines, which means they need qualified technicians who can work on their vehicles. 

When applying for jobs, having knowledge of gas and diesel engines can really set you apart and give you a competitive advantage. Additionally, it can provide job security due to the demand for technicians who are trained to work on different types of engines.

Gas and Diesel Engines at UTI 

So how does one train to work on gas and diesel engines?

Working on diesel engines requires a specific type of training, which is why many technicians choose to complete a training program such as Universal Technical Institute’s Diesel Technology program

In this 45-week program, you can learn the basic mechanical functions of combustion engines. Foundational courses build your base knowledge from the ground up, teaching you how to diagnose, repair and replace gas and diesel engines. From here, you can move on to specialized advanced training on trucks and engines from leading brands. 

In the program, you’ll learn:

  • Truck Brakes and Chassis: Training on complete trucks in a real-world shop environment, you’ll learn to perform service, repairs and electronic diagnosis on foundation and anti-lock brake systems.
  • Truck Preventive Maintenance: Receive instruction on basic truck wheel alignment procedures and learn how to perform original equipment manufacturer recommended maintenance and Department of Transportation annual safety checks.
  • Diesel Engines: Learn the principles of operation of four-stroke engines including diesel combustion fundamentals and other characteristics unique to diesel engines.
  • Diesel Fuel Systems: Discover how to safely diagnose and repair mechanical and electronically controlled diesel fuel-injection systems.
  • Transportation Refrigeration: Learn to service and repair major components on a transport refrigeration unit.

Additionally, UTI’s Diesel Technology program is accredited by the ASE Education Foundation, a non-profit organization that evaluates technician training programs against standards developed by the automotive, truck and collision industries.

Graduates of the program are well prepared to complete ASE examinations and can substitute their training for one of the two years of work experience required to become ASE certified. Having specialized training, real-world experience and valuable certifications can really make you stand out to employers.1

Overall, gas and diesel engines have similarities and differences. As the transportation industry continues to evolve due to changes in technology, a new generation of technicians who can work on both types of engines will be needed to step up to the challenge.

Learn About UTI’s New Diesel Program 

UTI offers several diesel specialized training programs, which were created in collaboration with leading brands like Cummins, Freightliner and Peterbilt.15 Recently, we launched a program for those interested in becoming an agricultural technician — the Fendt Technician Academy Manufacturer-Specific Advanced Training program!

In just 8 weeks, students accepted into this program can gain knowledge and skills that employers look for in their agricultural and diesel equipment technicians. They can also earn agricultural technician qualifications that could help them as they start working in the field.

To learn more, request information today. We’re here to help you get on track toward achieving your career goals!

UTI Campuses That Offer Diesel Mechanic Training

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1) UTI is an educational institution and cannot guarantee employment or salary.

2) For program outcome information and other disclosures, visit

15) Manufacturer-paid advanced training programs are conducted by UTI on behalf of manufacturers who determine acceptance criteria and conditions. These programs are not part of UTI’s accreditation. Programs available at select locations.

Universal Technical Institute of Illinois, Inc. is approved by the Division of Private Business and Vocational Schools of the Illinois Board of Higher Education.


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