Diesel Engines Versus Gasoline Engines: Why Diesel Engines are Compression Engines and Why Gasoline Engines are Spark-Fired

Diesel engines versus gasoline engines, the differences between the two are both numerous and significant. The biggest difference between diesel and gasoline engines is the fuel ignition process. Inside their respective engine cylinders, gasoline and diesel engines ignite fuel completely differently.  

Diesel is compression-fired engines. Compression engines ignite the fuel, in the same manner, a firing pin ignites gunpowder. Compression engines — engines that burn diesel and fuel oil — ignite fuel by exposing it to exceedingly high temperatures generated by the compression of a gas. In the case of compression engines, air is the compressed gas that generates heat. 

Gasoline engines, on the other hand, are spark-fired. Spark-fired gasoline engines ignite fuel by exposing it to a spark. A spark-fired engine ignites fuel the same way an outdoorsman lights a campfire, with a flame. Spark-fired ignition is akin to exposing kindling to a match. Simply, spark-fired engines expose fuel to a flame in order to ignite it. Compression engines expose fuel to heat.

“The most significant difference is in the way the fuel is ignited in the combustion chamber. Either the engine is built to run solely on natural gas, requiring it — unlike a diesel — to use spark plugs for ignition, or it is dual-fuel, combining the natural gas with a small amount of diesel fuel, which is compressed in the combustion chamber until it ignites, a process known as high-pressure direct injection, or HPDI.”

Gasoline and Diesel Engines Distant Cousins

Because of the difference in how spark-fired and compression engines ignite the fuel, diesel engines and gasoline engines are dissimilar on a fundamental level. A gasoline engine has more in common with natural gas or propane powered engine than a compression diesel engine. 

Not only do gasoline and diesel engines fire differently, but there are also differences with respect to fuel efficiency and emissions as well. The differences most relevant to consumers is the fact that diesel engines are more fuel efficient than comparably sized gasoline engines. Relevant to everyone, not only vehicle consumers and drivers, is the fact that diesel engines pollute less and produce fewer toxic emissions than gasoline engines. As TheConversation.com explains, “So, while diesel fuel contains slightly more carbon (2.68kg CO₂/liter) than petrol (2.31kg CO₂/liter), overall CO₂ emissions of a diesel car tend to be lower. In use, on average, this equates to around 200g CO₂/km for petrol and 120g CO₂/km for diesel.”

Four Stroke Engines: Spark-Fired vs Compression

There are two primary types of internal combustion engines, spark-fired and compression. And of the two types, the vast majority are four-stroke engines. While there are two-stroke engines, most are small and generate far less energy than a four-stroke. The internal combustion engines found in almost all passenger cars, pickups, trucks, semis, and heavy equipment are four-stroke engines.

Four-Stroke Engine Cycle

As the name implies, a four-stroke combustion engine has four stages in a cycle. The first stage is the intake stroke. The second stage is the compression stage. The third stage is the combustion and power stroke. And, the last stage is the exhaust stroke. 

“A four-stroke cycle engine is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. The piston makes two complete passes in the cylinder to complete one operating cycle. An operating cycle requires two revolutions (720°) of the crankshaft. The four-stroke cycle engine is the most common type of small engine. A four-stroke cycle engine completes five Strokes in one operating cycle, including intake, compression, ignition, power, and exhaust Strokes.”

1) Intake Stroke of a Spark-Fired, Four-Stroke Engine 

During the intake stroke, the piston drops to the bottom of the cylinder. As the piston drops, a vacuum develops inside the cylinder. In traditional spark-fired engines, the vacuum would suck an air/fuel mixture out of the carburetor and into the cylinder. Today’s spark-fired engines, on the other hand, inject the air/fuel mixture into the cylinders. 

After hitting bottom-dead-center — at which point the cylinder is full of the air/fuel mixture — the piston begins the second stroke of the four-part cycle, the first upstroke. The first upstroke is the compression stroke. 

Intake Stroke of a Compression-Fired, Four-Stroke Engine

In a compression-fired, four-stroke engine, — just like in a spark-fired engine — the piston drops and creates a void in the cylinder. With respect to the action of the pistons, compression engines and spark-fired engines are the same during the intake stroke. However, unlike a spark-fired engine, only air fills the cylinder during the intake stroke.

In a compression engine, an air/fuel mixture is not sucked or injected into the cylinder during the intake stroke. In a compression engine, the addition of fuel occurs at the end of the first upstroke, the compression stroke. 

2) Compression Stroke of a Spark-Fired, Four-Stroke Engine 

The second stage of both spark-fired and compression engines is the compression stroke. During the compression stroke, the piston is pushed toward the top of the engine cylinder. As the piston moves toward the top of the cylinder, the gas inside the cylinder compresses. 

The compression of a gas — air, water vapor, fuel vapor, etc. — generates heat. The more pressure put on the gas, the more heat the compression generates. But, in a spark-fired engine, the heat generated from the compression of the gas does not fire the fuel. Instead, just before the piston hits top dead center, a spark plug sparks. 

Compression Stroke of a Compression-Fired Engine

In a compression-fired diesel or fuel oil engine, the compression stroke ignites the fuel. As the piston rises, the air inside the cylinder heats as the result of being compressed. Once the heat is enough to ignite the fuel, injectors spray fuel into the top of the cylinder head and it begins burning. Just like in a spark-fired engine, the expansion of the fuel as it burns inside the cylinder drives the piston down. 

3) Combustion Stroke, A.K.A., Power Stroke of a Spark-Fired, Four-Stroke Engine 

When a combustion engine fires, contrary to a common misconception, the fuel inside the cylinders does not explode. The fuel inside combustion engine cylinders burns, albeit extremely fast. And ideally, the fuel burns uniformly. In the combustion stroke of a combustion engine, the fuel ignites at a predetermined location. 

In a spark-fired engine, the spark ignites the fuel, the flame spreads, and the fuel expands as it ignites. The expansion of the burning fuel drives the piston down and the cylinder fills will exhaust as the fuel burns. 

Combustion Stroke, A.K.A., Power Stroke of a Compression-Fired, Four-Stroke Engine

In a compression-fired engine, the heat generated from the compression of the air in the cylinder heats the inside of the cylinder. “The compression stroke begins when the piston travels up the cylinder, compressing the trapped air. The pressure raises between 32 bar-50 bars and the temperature to 600 degree Celsius. [Injection of the diesel or fuel oil] begins somewhere near the TDC of the compression stroke, fuel is sprayed into the hot air, ignites, and burns in a controlled manner due to the heat of compression, resulting in the power stroke”

As in a spark-fired engine, the combustion of fuel in a compression engine drives the piston down. And, the cylinder fills with exhaust gases. 

4) Exhaust Stroke of Spark-Fired and Compression-Fired, Four-Stroke Engines

The exhaust stroke in both spark-fired and compression-fired engines is the same. Once the piston hits bottom dead center and the cylinder is full of exhaust, the piston rises back to the top forcing the emissions out the exhaust manifold. 

Once the exhaust stroke finishes, the four-stroke cycle is complete and the process begins again. 

Why Compression Engines Can’t Run on Gasoline

Gasoline will not function to power a compression engine. “Automotive engineers have spent decades trying to build a [gasoline-powered compression] engine because diesel provides better fuel economy than gasoline engines,” explains Wired.com. Gasoline does not have sufficient energy density nor sufficient compression resistance to operate a compression engine.

The problem with the fact that gasoline cannot power a compression engine is that there is a direct correlation between compression ratio and fuel efficiency,  “Automotive engineers can improve fuel efficiency and fuel economy by designing engines with high compression ratios. The higher the ratio, the more compressed the air in the cylinder is. When the air is compressed, you get a more powerful explosion from the air-fuel mixture, and more of the fuel gets used.”

In relation to diesel, gasoline is a light, highly-volatile fuel. The maximum compression ratio gasoline can sustain before autoignition occurs is between 8:1 and 10:1. Diesel compression engines have a compression ratio anywhere from 18:1 to 25:1. In many cases, the compression ratio is even higher. 

Importance of Compression Resistance to Fuel Efficiency

For the same reason, gasoline cannot power a compression engine, gasoline engines are less fuel efficient than diesel engines. Compression resistance is the reason. Compression resistance is one of the two most important factors in fuel efficiency. The energy density of a fuel is the other. 

The compression ratio range of an engine determines its thermal efficiency. Thermal efficiency is the amount of energy that goes into an engine compared to the amount of that energy the engine converts to mechanical work. It is energy input vs energy output. 

Increasing the compression ratio of an engine increases its thermal efficiency. The reason being, the higher the compression ratio, the more heat the compression of gas creates. In the case of combustion engines, air or an air/fuel mixture constitutes the compressed gas. 

Diesel Has Capacity for Greater Fuel Savings

The most limiting factor with respect to gasoline fuel savings is gasoline itself. Since gasoline is not particularly energy-rich and gasoline has a low compression resistance, there is only so much technology can do to improve the fuel efficiency of gasoline vehicles. Diesel, on the other hand, is limited by technology. Diesel engine technology still does not take full advantage of the high energy potential of diesel. Nor, do diesel engines take full advantage of the fact that diesel has very high compression resistance.

And, there is a third quality of diesel that modern technology has yet to address, oxygenation potential. The biggest issue to date with diesel is that diesel is so dense and energy-rich that it is difficult to oxygenize. Fuel combustion is the oxidation of hydrocarbons. Hydrocarbons will not oxidize if they are not oxygenated. And, the density of diesel makes oxygenizing it difficult. 

There are means of increasing the oxygenation of diesel fuel and increasing fuel efficiency. The Rentar Fuel Catalyst is an aftermarket, pre-combustion fuel catalyst that increases the oxygenation potential of diesel. 

Rentar Fuel Catalyst

Because of the lengths and sizes of the hydrocarbons in diesel fuel, they bind together into clusters. Why hydrocarbon molecules cluster together, the molecules on the inside of the clusters have no exposure to oxygen. As a result, the hydrocarbons in the middle of a fuel cluster either go unburned or partially burned.

Hydrocarbon clusters are the result of the positive and negative charges that are inherent in molecules. “Most fuels for internal combustion engine are liquid, fuels do not combust until they are vaporized and mixed with air. Most emission motor vehicle consists of unburned hydrocarbons, carbon monoxide and oxides of nitrogen. Generally, fuel for the internal combustion engine is a compound of molecules. Each molecule consists of several atoms made up of a number of nucleus and electrons, which orbit their nucleus. Magnetic movements already exist in their molecules and they therefore already have positive and negative electrical charges.”

The Rentar Fuel Catalyst neutralizes the molecular charges that draw hydrocarbon molecules together. Once the charges clustering the hydrocarbons together is neutralized, the hydrocarbon molecules drift apart. Separated, hydrocarbon molecules have the exposed surface area necessary for oxygenation. 

In field and laboratory tests, the Rentar has proven to reduce fuel consumption by between 3 and 8 percent. The fuel savings for off-the-road vehicles is even greater. And, the Rentar Fuel Catalyst reduces emissions by between 15 and 55 percent, depending on the emission type. 


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