The Difference Between Diesel and Gas Engines

Gas and diesel engines have inherent differences in operation, cost and efficiency. Learn all about those differences, plus the pros and cons of each.

Gas and diesel engines look similar on the surface. Both use internal combustion to move pistons up and down inside cylinders. The pistons are connected to a crankshaft which converts their back-and-forth motion into the rotational movement needed to propel a vehicle. Both engines create energy by igniting fuel, but the biggest difference between the two is in the ignition process.

How Do Gas and Diesel Engines Work

Gas engines use a four-stroke combustion cycle. On the intake stroke, the piston starts at the top of the cylinder. The intake valve opens as the piston moves down, allowing the engine to draw in a blend of air and gasoline. During the compression stroke, the piston moves back up and compresses the gas and air mixture. At the top of the compression stroke, the spark plug is energized, creating a spark that ignites the mixture.

As the gas burns and expands, it pushes the piston down in the combustion stroke, completing the cycle. After that the piston moves back to the top of the cylinder, pushing the burned gases out of the exhaust valve.

Diesel engines follow the same four-stroke cycle as a gas motor, with one major difference. In contrast to a gasoline engine, air alone enters the cylinder during the intake stroke. As the piston moves back up and compresses this air, it gets hot. At the top of this stroke, fuel is injected into the cylinder. The heat of the compressed air is enough to trigger combustion, forcing the piston back down.

Differences Between Gas and Diesel Engines

Spark Plugs: A gas engine uses a spark plug to ignite the gas and air mixture in the cylinder. But diesel engines use the heat of compressed air, so they don’t need spark plugs.

Some diesel engines do have something called glow plugs, however. In cold temperatures, diesel engines can have trouble starting because the compressed air isn’t hot enough to ignite the fuel in a cold cylinder. Glow plugs use electrically-heated wires to warm the combustion chamber enough for the engine to start. Then combustion heat brings the engine up to operating temperature.

Efficiency: Diesel engines often boast better fuel efficiency numbers over their gasoline counterparts. Because diesel engines self-ignite, they’re designed to create much higher pressures during the compression stroke, leading to greater overall efficiencies.

With gas engines, by contrast, it’s critically important that the self-ignition temperature is never reached before the spark plug sparks. So compression in the cylinder must be lower, leading to somewhat lower overall efficiencies. Gas engines usually never have more than a 14:1 compression ratio. Diesels run at 18:1 or higher.

Life Expectancy: Roughly speaking, you can expect a diesel engine to last twice as long as a gasoline engine. Part of the reason for this is the nature of the fuel itself. Gasoline offers little lubrication action, while diesel fuel is more like a thin viscosity oil, lubricating every moving part it touches.

Pros and Cons of Gas Engines



  • Lower fuel efficiency;
  • Shorter life span;
  • Faster depreciation.

Pros and Cons of Diesel Engines


  • Great torque for a given engine displacement;
  • Longer life span;
  • Higher fuel efficiency.


  • More expensive;
  • Higher insurance premiums;
  • Higher repair costs.