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What Is Compression Ignition?

Explaining what diesel’s “big bang” is all about.

Mar 23, 2021
Let's discuss something that is well known by diesel mechanics and hard-core enthusiasts but may not be fully understood by new diesel owners or people who may have heard the term "compression ignition" when engines are being discussed but ultimately are clueless about the descriptor's true definition. What is compression ignition?
Compression ignition basically is the term that is used for describing the manner in which diesel engines function. It happens when intake air—which often is combined with recirculating exhaust gas—is compressed at extremely high pressure, creating the extreme heat that's necessary for igniting diesel fuel (approximately 1,400 degrees Fahrenheit), which has a higher density than gasoline. Gas engines rely on spark plugs to create the heat.
As fuel is injected, it mixes with the hot air in the cylinders and explodes. In this event, which promotes the rotating assembly's movement, ignition at a constant pressure enables a diesel to generate its fabled torque. Yes, the process is somewhat similar to combustion in a gasoline engine, but there are two important differences. With compression ignition, it all happens without spark. And the fuel-air union happens spontaneously in the diesel's combustion chambers, as opposed to gas and air mixing and being introduced to the engine through the cylinder head(s).
Diesel performance—power, if you will—is all about how a compression-ignition engine is fueled. A diesel will make great torque with a richer (more volume) fuel supply. And, while enhancing airflow through turbocharging increases efficiency and contributes to improved performance, impressive gains definitely can be achieved through fueling improvements only.
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Benefits of Compression Ignition

If you've just purchased a diesel-powered pickup, or especially if you are considering buying one, you might be wondering how you'll actually win with compression ignition. What are its benefits versus gasoline propulsion?
Efficiency is the first notable gain. Diesel engines do not consume as much fuel as gas engines. While diesels use an air/fuel mixture with 40 percent more air per fuel that's burned, a gasser uses a stoichiometric mixture of 14.7:1. Diesel's leaner fuel mixture results in higher efficiency because combustion temperatures are also lower. When the engine retains heat, more energy acts on the pistons.
Compression-ignition engines also are easier and more cost-effective to maintain since they are not dependent on spark plugs or wires. And oil burners operate optimally in a lower rpm range, which makes them durable and adds to their longevity (provided they are maintained). No, diesels are not big-time horsepower generators (unless they're very highly modified) like gassers are. Oil burners don't rev as fast or high, because of the big difference in the two fuels' burn rate (gas burns faster).
While the initial cost for a compression-ignition-powered rig is higher than prices for gas pickups, the difference in capability (towing, hauling, climbing steep grades, etc.) and monies saved in fuel economy, upkeep, and longevity (when they're well maintained, newer diesels can be driven 300,000 miles and more), definitely make diesels the trucks to consider if you're in the market for a new rig.
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