The Differences between Diesel and Gasoline Engines


The Differences between Diesel and Gasoline Engines

Many 2019 trucks come in gasoline and diesel variants, such as the 2019 Chevrolet Silverado 2500HD available at Zeck Chevrolet near Oklahoma City. Diesel and gasoline engines are similar but different. They are similar in that they both combust fossil fuels to use pistons to drive a crankshaft. Driving the crankshaft ultimately provides power to a drivetrain for some purposes such as propelling a vehicle forward.

In vehicle engines at least, both also use a four-stroke combustion cycle. There is the Intake Stroke where intake valves open to let in fuel and air mixture as the piston moves down, much like pulling down a syringe to let in a liquid. Next is the Compression Stroke where the piston moves back up, compressing the fuel and air mixture. Following the Compression Stroke, the Combustion Stroke marks where the piston reaches the very top. The fuel and air mixture are then ignited propelling the piston forcefully down and providing power to the crankshaft. Finally, in the Exhaust Stroke, the exhaust valves are opened as the piston moves back up forcing out the fumes created in the Combustion Stroke.

The main difference between diesel and gasoline engines occurs in the Combustion Stroke.

The Combustion Stroke

In the Combustion Stroke, gasoline engines use spark plugs to ignite the gasoline and air mixture. A spark is provided electrically to essentially cause the mixture to explode much like applying a match to gunpowder.

Diesel engines don’t use spark plugs or any other direct-flame approach. They use an entirely different process that was invented in the late 1800s.

The namesake of diesel engines, Rudolf Diesel, a 19th-century German inventor and mechanical engineer, was very unhappy with the fuel waste associated with steam engines. As much as 90% of a given fuel’s energy is wasted when powering a steam engine. He went about designing a more efficient engine that used diesel or diesel oil. Faced with the fact that diesel oil isn’t readily combustible to an open flame or spark, Rudolf Diesel sought a different method. After a great deal of research and effort, he used the principles of thermodynamics and the Carnot cycle to form the basis of the diesel engine.

In short, in diesel engines, the diesel fuel and air mixture is compressed to the point that it explodes.

Diesel engines don’t use spark plugs for ignition, they use high compression ratios. This is the main difference between diesel and gasoline engines. This significant difference spawns a host of other distinctions.

Diesel Engines Are Really Compressed

A compression ratio is the ratio of the maximum to minimum volume in a given internal combustion engine’s cylinders. Put another way, it’s the ratio of the volume of an engine’s cylinder when the piston is all the way down to the volume when the piston is all the way up.

The compression ratios between diesel and gasoline engines vary greatly. Since gasoline engines don’t rely on compression to ignite their fuel, a typical compression ratio for them is about eight-to-one (i.e., 8:1). Diesel engines are usually about 14:1 but can go as high as 25:1.

It is because of this high compression that diesel engines have that characteristic “knocking” sound when they are running.

Direct Fuel Injection

The knocking sound isn’t the only characteristic difference as the diesel engine’s ignition method and high compression ratio create other issues that need to be solved.

Diesel engines inject fuel into the cylinder. There are few other options for diesel engines. Gasoline engines, on the other hand, have more options for getting fuel to the cylinders. For example, they can use carburetors as well as multiple forms of fuel injection (e.g., injectors in a throttle body or individual cylinders).

The injectors used by diesel engines differ as well. Diesel fuel injectors have to be able to withstand the higher pressures inside the cylinder while still delivering the fuel in a mist.

A Glowing Difference

Given the thermodynamic properties of the combustion process in a diesel engine, the temperature is a big consideration. When a diesel engine is in lower temperatures, the compression process may not raise the temperature of the air enough. One might even say diesel engines are a bit cold-blooded. To counter this, many diesel engines have a “glow plug.” A glow plug is a wire or filament heated similar to the heating elements one may find in a toaster. They heat the combustion chambers and raise the air temperature so the engine can start and begin heating itself.

While diesel and gasoline engines both get the job done in converting fossil fuels to mechanical energy, there are notable differences in how they do it. In the end, to most consumers, the real difference probably lies with the price per gallon at the fuel pumps. This economic discussion is another topic entirely.

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