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What's the Difference Between Diesel and Gas?

By Autolist Staff | October 21, 2019

The difference between diesel and gas engines starts with the invention of the latter. In 1876, a man named Nikolaus August Otto invented the gas engine, which used four-stroke combustion. That is the basic idea behind modern car engines. At that point, the gasoline engine wasn't all that efficient. Only 10 percent of the fuel was being used to move the vehicle. The rest was merely producing heat.

Enter Rudolf Diesel

Two years later, in 1878, a man named Rudolf Diesel was in the era's equivalent of an engineering school called Polytechnic High School. He learned there about the poor efficiency of gas engines and resolved to come up with something more efficient. He eventually invented and patented the diesel engine in 1892. At that point, however, it was simply called the combustion power engine. Because of how efficient diesel engines are, they are often found on large pieces of equipment, like semis and container ships. However, the association with loud, big engines has made them less attractive to the average American driver. On the other hand, they are quite common in passenger vehicles in Europe.

Gasoline Engines Versus Diesel Engines

In a basic sense, both of these engines are very similar. They both use internal combustion to convert fuel into mechanical energy to move a vehicle. The fuel is converted through a rapid series of explosions within the engine. The primary difference between a gasoline engine and a diesel engine is the method by which these explosions occur. For example, in a gas engine, the fuel combines with air, which is compressed by pistons and given ignition from the sparks created by the spark plugs. However, in a diesel engine, the air gets compressed first, which makes it hot. When the fuel is injected next, it ignites from the hot air.

Four-Stroke Combustion In A Diesel Engine

A diesel engine uses the same combustion cycle as a gasoline engine. However, the stroke cycle is technically different since the diesel engine does not have spark plugs. Instead, it sucks in air and then compresses it. The fuel is directly injected into the combustion chamber where it ignites from the heated air.

Take a look at the four strokes:

  1. The intake stroke lets in air through the intake valve and moves the piston down.

  2. The compression stroke moves the piston back up while compressing the air.

  3. The combustion stroke injects fuel right when the piston reaches the top. The fuel ignites, and the piston is forced back down.

  4. The exhaust stroke moves the piston back to the top where the combustion exhaust is then pushed out through the exhaust valve.

Compression Ratio

Rudolf Diesel turned out to be correct that a higher compression ratio would yield higher fuel efficiency and more power. That occurs due to the concentrated air that results from the piston compressing the air inside the cylinder. The higher energy content in diesel fuel makes it more likely to react with the compressed air. In other words, the air molecules are so concentrated the fuel can respond with as many of them as possible. Rudolf's theory was correct because gasoline engines have a compression ratio between 8:1 to 12:1, while diesel engines have ratios between 14:1 to 25:1. That's twice as efficient in some cases.

The Injection Process Of Gas Engines

Another significant difference between diesel engines and gas engines is the process of fuel injection. There are two main types of injection in port injection or a carburetor. The port injection system works by injecting fuel right before the intake stroke, while a carburetor mixes both fuel and air well before the air gets into the cylinder. That means that fuel is injected into the cylinder before the intake stroke and then it's compressed. That limits the engine's compression ratio. Too much compression of the air causes spontaneous ignition, which results in knocking. This knocking is not good for the engine and may damage it eventually.

Diesel Engine Direct Injection

Diesel engines have the fuel directly injected into the cylinder. As this is a vital part of how diesel engines work, the diesel injectors are quite complicated. To deliver a fine mist of fuel, they must be able to resist the high temperature and pressure inside the cylinder. Making this system more efficient has yielded a lot of experimentation in diesel engineering.

Some ways to aid this process include the following:

  • Special induction valve

  • Glow plugs

  • Pre-combustion chambers

For example, the glow plug is a hot wire that quickly raises the temperature of the air in a cold engine to allow it to start more efficiently. Modern diesel vehicles have an ECM (engine control module) that communicates with an array of sensors to correctly time injection for the engine to start when cold.

Diesel Fuel Versus Gasoline Fuel

Both diesel and gasoline start out the same way, which is in the form of crude oil mined from the earth. Once it's processed and refined, it can be separated into various types of fuels. Diesel fuel is more substantial than gas and thus evaporates more slowly. It also has more energy density. These features, combined with the overall efficiency of the diesel engine, helps explain why the fuel economy is typically better than gas engines. As a passenger car driver, you might not realize just how many vehicles use diesel fuel. Aside from semi trucks, school buses, trains, construction equipment, emergency vehicles and power generators all use diesel engines.

Is Diesel Clean?

That has been a big con of diesel for a long time. It's heavy and oily and creates soot. However, in the last 40 years or so, significant improvements have been made to the cleanliness of diesel. The fuel is refined better, and computer monitoring within the engine helps reduce emissions. Soot has been reduced by catalytic converters and CRT particulate filters, which burn the residue. As recent emissions scandals have shown, however, diesel engines still have a harder time meeting emissions standards than gasoline engines.

A recent trend has been the use of biodiesel, a fuel that comes from plant oil or animal fat and has been chemically altered. Conversions for older diesel models were common. However, newer diesel models are sometimes unsuitable for biodiesel retrofits.