It is no secret that Americans are more familiar with gasoline engines than diesel engines. R.L. Polk compiled statistics to support this assertion. R.L. Polk has confirmed this with statistics showing that only 2.8 percent of registered passenger cars (cars and SUVs) used number 2 diesel fuel in 2013. Most Americans expect to see spark plugs or coils when they open the hood. This is in contrast to turbochargers (which are two of the most critical elements on almost every diesel engine, hence the name “turbodiesel”)

To understand the differences between gasoline and diesel engines, let’s start with the commonalities between them. The engine’s overall makeup doesn’t change based on the type of fuel used. A crankshaft is spinning, connecting rods, pistons moving up or down, the air being pumped into, and exhaust filtering out. The basic architecture of a diesel engine is almost identical. However, what happens in the diesel cylinder is different from what you will find in its gas-powered counterparts.

It is easiest to explain the differences between gasoline and diesel engines using “air” or “fuel.” Airflow is everything in a gasoline engine. You are throttling the air. Diesel mills are the opposite. It works by throttling fuel injection. The air follows the same pattern. There is no need for you to throttle the airflow. Diesel engines also do not create the vacuum.

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Take Air

To illustrate the flow of fuel and air in modern diesel power plants. The compressor housing (intake) is where fresh air is introduced. It is then compressed in the compressor wheel, which creates a boost. This causes the air to become denser and also warmer.

A charge air cooler, also known as an intercooler, cools down the compressed air before reaching the cylinder heads. An air-to-air intercooler is the most common type. It is a heat exchanger. The intercooler reduces the temperature of intake air on the journey to the engine and results in a minimal loss of boost. You can check out more from Track Tech.


Once the compressed air has been forced into the cylinder, things get exciting. The intake stroke is when the piston drops to the bottom of its range. This causes the intake valves to open, which allows “unthrottled air” to fill the cylinder. This is different than a gasoline engine because 1) gasoline engines inject fuel and air during their intake stroke, and 2) diesel engines draw air only during the intake stroke. The intake valve(s) then closes, and the compression stroke begins. The piston moves upwards, and the air that filled the cylinder once now occupies only 6% of its original area. Under tremendous pressure, this air instantly heats up to 400 degrees, which is enough heat to ignite the diesel fuel. That’s what happens at the top end of the piston’s stroke. Once the super-heated air has reached the desired temperature, a shot diesel fuel is released into the cylinder via its respective fuel injector. This happens just before the piston reaches the top dead center. A diesel engine burns its fuel using heat compression. Spark plugs are used in gasoline engines.

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