Diesel and gasoline engines are two of the most important types of engines used in vehicles and machines. Both are called internal combustion engines because they burn fuel inside the engine to produce power. These engines have changed the modern world. They allow people to travel long distances, transport goods, operate machines, build roads and buildings, work on farms, and move heavy loads. Cars, trucks, buses, boats, tractors, construction machines, generators, and many other machines have used internal combustion engines for more than a century.
Gasoline and diesel engines may look similar, and they have many of the same parts. Both use pistons, cylinders, connecting rods, crankshafts, valves, and other mechanical parts. However, they burn their fuel in different ways. One of the biggest differences is fuel efficiency. Diesel engines are generally more fuel-efficient than gasoline engines. In simple words, a diesel engine can usually travel farther or do more work using the same amount of fuel.
There are several reasons for this. Diesel engines use higher compression, diesel fuel contains more energy per unit of volume than gasoline, and diesel engines can work with a lot of air and a smaller amount of fuel. Modern diesel engines also use technologies such as turbochargers and direct fuel injection. However, diesel engines also have disadvantages. They are usually heavier and more expensive to build, and they need special systems to control certain harmful emissions. To understand why diesel engines are more efficient, it is useful to first understand their history and how both types of engines work.
History of Internal Combustion Engines
The Steam Engine
Before gasoline and diesel engines became common, steam engines were among the most important machines used to produce mechanical power. James Watt made important improvements to the steam engine during the 1770s. A steam engine is called an external combustion engine because the fuel burns outside the engine cylinder.
For example, wood or coal can be burned to heat water. The water turns into steam. The pressure of the steam then pushes a piston. The movement of the piston can be changed into mechanical movement. Steam engines were very useful, but they were large and heavy. They needed a boiler, water, fuel, and other equipment. Because of their size, they were mainly used in factories, trains, ships, and other large machines.
Development of Internal Combustion Engines
During the middle of the 19th century, inventors and engineers began working on engines in which the fuel would burn inside the engine itself. This was a major change. Instead of burning fuel outside the engine to create steam, an internal combustion engine burns fuel inside a cylinder. The burning fuel creates hot gases. These gases expand and push a piston. The movement of the piston is then changed into turning movement by the crankshaft.
Many inventors tested different engine designs during the 19th century, especially in Europe and the United States. The development of the Otto Cycle was an important step in the development of gasoline engines. The four-stroke engine system associated with Nicolaus Otto became one of the most important engine designs in history. The first Otto cycle patent was filed in 1887.
Another important inventor was Rudolf Diesel. He developed an engine that used a very different way of burning fuel. His first patent for his engine was filed in 1898. Diesel's engine was not immediately successful. Gasoline engines remained more common during the first half of the 20th century.
Diesel engines became more widely used after World War I. Their use increased even more after World War II, especially in heavy trucks, tanks, boats, and other heavy-duty machines. Today, gasoline and diesel engines are still used in many different types of vehicles and machines.
What Is an Internal Combustion Engine?
An internal combustion engine is a machine that changes the energy stored in fuel into mechanical power. The fuel burns inside the engine. The burning fuel creates hot gases that expand. The expanding gases push a piston.
The piston moves up and down inside a cylinder. A connecting rod connects the piston to the crankshaft. The crankshaft changes the up-and-down movement of the piston into turning movement. This turning movement can then be used to turn the wheels of a car or operate another machine.
The basic process can be explained simply. Fuel and air enter the engine, the fuel burns, the burning creates expanding gases, the gases push the piston, the piston moves the crankshaft, and the crankshaft produces useful power. Modern engines have many additional systems and electronic controls, but this basic process remains at the heart of an internal combustion engine.
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How Does a Gasoline Engine Work?
A gasoline engine normally needs three important things to produce power: gasoline, air containing oxygen, and a spark. Gasoline is mixed with air and brought into the engine cylinder. The mixture is then squeezed by the piston. At the correct moment, a spark plug produces a spark. The spark starts the burning of the gasoline and air mixture.
The burning mixture produces hot gases. These gases expand and push the piston down. The piston is connected to the crankshaft. The crankshaft changes the piston movement into turning movement. This turning movement can eventually turn the wheels of a vehicle.
The Four Strokes of a Gasoline Engine
A normal four-stroke gasoline engine works through four basic steps called the intake stroke, compression stroke, power stroke, and exhaust stroke.
Intake Stroke: During the intake stroke, the piston moves down and the intake valve opens. The gasoline and air mixture enters the cylinder. The amount of air and fuel entering the engine is controlled by the engine's fuel and air systems.
Compression Stroke: During the compression stroke, the intake valve closes and the piston moves upward. The piston squeezes the gasoline and air mixture into a smaller space. This prepares the mixture for burning.
Power Stroke: During the power stroke, the piston reaches the correct position and the spark plug produces a spark. The gasoline and air mixture burns. The burning gases expand and push the piston downward. This produces the main power of the engine.
Exhaust Stroke: During the exhaust stroke, the fuel has already burned and waste gases remain inside the cylinder. The exhaust valve opens, and the piston moves upward and pushes the waste gases out through the exhaust valve. The cycle then starts again. These four strokes happen repeatedly while the engine is running.
How Does a Diesel Engine Work?
A diesel engine has many of the same parts as a gasoline engine. It has pistons, cylinders, valves, connecting rods, a crankshaft, a camshaft, and a fuel system. The main difference is the way the fuel is burned. A gasoline engine uses a spark plug to start combustion. A diesel engine normally does not need a spark plug. Instead, it uses very strong compression of air.
The Four Strokes of a Diesel Engine
A diesel engine also works through four basic strokes. These are the intake stroke, compression stroke, power stroke, and exhaust stroke. However, the way air and fuel are handled is different from a gasoline engine.
Intake Stroke: During the intake stroke, the diesel engine takes in air only. Unlike a traditional gasoline engine, fuel is not mixed with the incoming air at this stage. The piston moves downward while air enters the cylinder.
Compression Stroke: During the compression stroke, the intake valve closes. The piston moves upward and squeezes the air very strongly. Diesel engines use much higher compression than gasoline engines. When air is squeezed strongly, its temperature becomes very high.
Power Stroke: Near the end of the compression stroke, diesel fuel is sprayed directly into the hot, compressed air. Because the air is extremely hot, the diesel fuel catches fire. The burning fuel creates expanding gases. These gases push the piston downward and produce power.
Exhaust Stroke: During the exhaust stroke, the exhaust valve opens. The piston moves upward and pushes the burned gases out of the cylinder. The cycle then starts again.
What Is Compression Ratio?
One of the most important differences between gasoline and diesel engines is the compression ratio. Compression ratio tells us how much the engine squeezes the air or fuel and air mixture inside the cylinder.
A gasoline engine commonly has a compression ratio of about 8:1 to 12:1, although modern gasoline engines can have different values and some high-performance engines use higher ratios. For example, an 8:1 compression ratio means that a volume that takes up eight units of space before compression is squeezed into a space of one unit.
Diesel engines normally use much higher compression ratios, commonly around 14:1 to 25:1, depending on the engine design. The high compression is one of the most important reasons diesel engines can be efficient.
Why Does High Compression Make a Diesel Engine Efficient?
When air is compressed, its temperature increases. A diesel engine takes advantage of this basic physical fact. The engine squeezes the air very strongly until it becomes extremely hot. Then diesel fuel is injected into the hot air. The fuel can ignite because of the high temperature.
This means that a diesel engine does not need a spark plug to start combustion. The high compression also helps the engine produce strong pressure when the fuel burns. This contributes to the strong turning force, or torque, that diesel engines are known for.
In simple terms, more compression makes the air hotter, the hot air helps the fuel burn, the burning fuel creates expanding gases, the expanding gases push the piston, and the engine produces power.
Why Does Diesel Fuel Have an Advantage?
The fuel itself is another reason diesel engines can be efficient. Diesel fuel has a higher energy density than gasoline. Energy density simply means the amount of energy stored in a certain amount of fuel.
For the same volume, diesel fuel contains roughly 10–15% more energy than gasoline, depending on the exact fuels being compared. The exact value can vary because fuel composition and measurement methods are different.
The higher energy content of diesel fuel is helpful, but it does not by itself explain the complete efficiency difference between diesel and gasoline engines. The engine's design is also very important.
Diesel Engines Can Use Lean Mixtures
Another major reason for diesel efficiency is the way the engine uses air and fuel. A gasoline engine generally needs to keep its fuel and air mixture within a relatively narrow range for normal operation.
A diesel engine works differently. A diesel engine normally takes in a large amount of air. The amount of fuel is then changed depending on how much power the driver needs. When more power is needed, more fuel is injected. When less power is needed, less fuel is injected.
This allows a diesel engine to operate with a lean mixture, which means there is a lot of air compared with the amount of fuel. Using less fuel compared with the available air can help the engine operate efficiently, especially when it does not need to produce full power.
Why Diesel Engines Do Not Need Spark Plugs
Gasoline engines normally need spark plugs because the spark starts the burning of the gasoline and air mixture. Diesel engines use a different method.
The piston squeezes the air so strongly that the air becomes very hot. Diesel fuel is then sprayed into this hot air. The fuel catches fire because of the high temperature. This process is called compression ignition.
Diesel engines may still have glow plugs, but glow plugs are not the same as spark plugs. Glow plugs are heating parts that help a diesel engine start in cold conditions. They are especially useful when the engine and surrounding air are very cold.
Why Are Diesel Engines Heavier?
Diesel engines are usually heavier than gasoline engines. The main reason is the high pressure created by their high compression. The engine must be strong enough to handle these forces. The engine block must be strong. The cylinder head must be strong. The pistons must be strong. The connecting rods must be strong. The crankshaft must also be strong.
Because these parts need to withstand high pressure and heavy loads, they are often thicker and stronger than similar parts in gasoline engines. This increases the weight of the engine and also increases the cost of making the engine.
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Why Can Gasoline Engines Run at Higher Speeds?
Gasoline engines are generally lighter than diesel engines. Their internal parts can be lighter because gasoline engines normally use lower compression. Lighter parts can move and turn at higher speeds. Engine speed is measured in RPM, which means revolutions per minute. Many diesel engines have a maximum speed below about 5,000 RPM.
Many four-cylinder gasoline engines can operate above 8,000 RPM. The exact speed depends on the engine design. The lower speed of diesel engines is not necessarily a weakness. Diesel engines are often designed to produce strong torque at lower speeds.
Torque and Horsepower
Two important terms used when talking about engines are torque and horsepower. Torque is a measure of turning force. Horsepower is a measure of how quickly an engine can do work.
Diesel engines are famous for producing high torque. This makes them useful for heavy vehicles. For example, a heavy truck carrying a large load needs a lot of force to start moving. It also needs strong pulling power when climbing hills. A diesel engine can provide this strong turning force at relatively low engine speeds.
Gasoline engines can produce high horsepower, especially at higher engine speeds. This can be useful for sports cars and other vehicles designed for high-speed performance. However, modern diesel engines can also produce large amounts of horsepower.
Why Are Diesel Engines Good for Trucks?
Diesel engines are widely used in heavy trucks because trucks often carry very heavy loads. A truck may carry thousands of kilograms of goods. The engine must provide enough force to move the vehicle and its load.
Diesel engines produce high torque and can maintain good fuel economy while working hard. Trucks also often travel long distances. For these reasons, the fuel savings of a diesel engine can be very valuable to a truck operator. Diesel engines are also common in buses, ships, tractors, construction machines, generators, and other heavy equipment.
Turbocharging and Diesel Engines
Modern diesel engines often use turbochargers. A turbocharger is a device that pushes more air into the engine. It uses energy from the engine's exhaust gases to help force more air into the cylinders. More air allows the engine to burn more fuel when more power is needed. This can increase engine power without needing a much larger engine.
Turbocharging has made modern diesel engines much more powerful than many older diesel engines. It has also helped diesel engines provide strong performance while maintaining good fuel economy.
Direct Fuel Injection
Modern diesel engines use advanced direct fuel injection systems. Direct injection means that the fuel is sprayed directly into the cylinder where combustion takes place. Modern fuel injectors can spray very small amounts of fuel at very high pressure. The engine's computer can carefully control when the fuel is injected and how much fuel is injected.
This helps the fuel mix with the hot air and burn more effectively. Better fuel injection can improve fuel economy, power, and emissions. However, modern diesel fuel systems are expensive and complicated.
Why Do Diesel Engines Use Less Fuel During Some Driving Conditions?
Diesel engines mainly control their power by controlling the amount of fuel injected. When the driver needs less power, the engine can reduce fuel injection. When the driver takes their foot off the accelerator and the vehicle is slowing down, modern engines can often reduce or stop fuel injection under certain conditions. This means the engine may use almost no fuel while the vehicle is slowing down using the engine.
Modern gasoline engines can also stop fuel injection during certain types of deceleration, so this is not an advantage that belongs only to diesel engines today. Still, the basic difference in how the two types of engines control power is important when understanding their design.
Diesel Engine Efficiency and Thermodynamics
Another reason diesel engines can be efficient comes from thermodynamics, which is the study of how heat and energy move and change. A diesel engine uses a high compression ratio. This allows the engine to turn a large part of the fuel's chemical energy into useful mechanical energy.
Diesel engines can also operate with a very lean mixture when they are not under heavy load. The combination of high compression, controlled fuel injection, and lean operation helps reduce wasted fuel. This is why diesel engines are often especially efficient when they are working steadily for long periods.
The Disadvantages of Diesel Engines
Diesel engines are efficient, but they also have disadvantages. One important problem is air pollution. Diesel engines can produce relatively low carbon dioxide emissions for the amount of useful work they produce because they are efficient. However, diesel combustion can produce higher levels of nitrogen oxides, commonly called NOx.
NOx is harmful air pollution. Diesel engines can also produce particulate matter, which consists of very small particles in the exhaust. Older diesel engines were known for producing visible black smoke and soot. Modern diesel engines are much cleaner because of improved engine design and exhaust-cleaning systems.
Diesel Particulate Filters
Modern diesel vehicles often use a diesel particulate filter, commonly called a DPF. The DPF catches very small particles from the exhaust. Over time, particles collect inside the filter. The engine must then burn these particles away. This cleaning process is called regeneration. The DPF helps reduce the amount of particulate matter released into the air. However, it adds another part to the vehicle and can increase cost and maintenance requirements.
Diesel Exhaust Fluid
Many modern diesel vehicles also use diesel exhaust fluid, often called DEF. DEF is used in a system that helps reduce nitrogen oxide emissions. The system changes harmful nitrogen oxides into less harmful substances before the exhaust leaves the vehicle. This technology allows modern diesel engines to meet strict pollution limits. However, the system adds more parts, sensors, and costs to the vehicle.
Why Are Diesel Engines More Expensive?
Diesel engines are generally more expensive to manufacture than similar gasoline engines. There are several reasons. First, diesel engines need stronger internal parts because they use higher compression. Second, modern diesel engines use complicated fuel injection systems. Third, modern diesel vehicles need advanced systems to control emissions.
These systems can include particulate filters, special exhaust parts, sensors, and diesel exhaust fluid systems. All of these parts increase the cost of the vehicle. Therefore, a diesel vehicle may cost more to buy even though it can save money on fuel.
The Efficiency Trade-Off
The benefits of diesel engines come with some costs. A diesel engine can use less fuel, but it may be heavier and more expensive. It can produce strong torque, but its maximum engine speed is usually lower.
A diesel engine can be very efficient, but its pollution-control system can be complicated. It can be excellent for heavy-duty work, but the extra cost may not make sense for every small car. This is why manufacturers have to balance fuel economy, price, weight, power, emissions, and reliability when choosing an engine.
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Why Were Diesel Engines Popular in Europe?
Diesel engines became especially popular in Europe. In 2015, about one-third of passenger cars in the European Union were diesel-powered. One important reason was fuel prices. Fuel has historically been expensive in many European countries. When fuel is expensive, people have a greater reason to choose cars that use less fuel.
Diesel cars were attractive because they could provide good fuel economy, especially for people who drove long distances. However, diesel passenger cars became less popular later. The growth of electric vehicles was one important reason. Stricter pollution rules also affected the diesel car market. The exact diesel share has changed over time and varies between European countries.
Diesel Engines in the United States
Diesel engines have not been as popular in American passenger cars as they have been in Europe. However, diesel engines have remained very important in American heavy trucks and other large vehicles. One reason for the poor reputation of diesel passenger cars in the United States can be connected to the problems experienced with Oldsmobile diesel engines in the early 1980s.
Oldsmobile introduced diesel engines to American car buyers. However, those engines had serious reliability and design problems. The experience damaged the reputation of diesel passenger cars among many American buyers. Modern diesel engines are very different from the diesel engines of the early 1980s, but the old experience affected public opinion for many years.
Diesel Engines and Electric Vehicles
The car industry is changing because electric vehicles are becoming more common. Electric vehicles use electric motors instead of gasoline or diesel engines. As electric vehicles become more popular, the market for gasoline and diesel passenger cars is changing. However, diesel engines continue to be useful in many areas.
Heavy trucks, ships, construction equipment, tractors, and other large machines have different needs from passenger cars. These machines often need high torque, long operating ranges, and the ability to work for many hours. Diesel engines are still well suited to these jobs.
Are Diesel Engines More Powerful Than Gasoline Engines?
It is not correct to say that every diesel engine is more powerful than every gasoline engine. The power of an engine depends on its size, design, turbocharger, fuel system, engine speed, and many other factors. Diesel engines are generally known for high torque. Gasoline engines are generally known for their ability to operate at higher speeds and produce high horsepower.
Modern diesel engines can produce both high torque and high horsepower. Turbocharging and improved fuel injection have made modern diesel engines much more powerful than older diesel engines. Therefore, diesel and gasoline engines should not be compared only by their maximum power. Their design and intended use are also important.
Why Are Diesel Engines More Efficient?
There is no single reason why diesel engines are more efficient. Several features work together.
Higher Compression: Diesel engines squeeze air much more strongly than gasoline engines. This makes the air very hot and helps the diesel fuel burn without a spark.
Higher Energy Content of Diesel Fuel: Diesel fuel contains more energy per unit of volume than gasoline. This gives the engine more energy to work with from the same amount of fuel.
Lean Operation: Diesel engines can use a large amount of air with a relatively small amount of fuel. This can improve efficiency, especially when the engine is not working at full power.
Direct Fuel Injection: Diesel fuel is sprayed directly into the cylinder. Modern fuel injectors can control fuel delivery very accurately.
Strong Engine Design: Diesel engines are built to handle high pressure. This allows them to use high compression and produce strong torque.
Turbocharging: Turbochargers allow modern diesel engines to use more air and produce more power without requiring a much larger engine.
Efficient Fuel Control: Diesel engines control power mainly by changing the amount of fuel injected. This can reduce fuel use when the engine does not need much power.
When Is a Diesel Engine a Good Choice?
A diesel engine can be a useful choice when a vehicle travels long distances, carries heavy loads, needs strong pulling power, works for many hours, uses a large amount of fuel, needs good fuel economy, is used for commercial transportation, or is used for heavy work.
This is why diesel engines remain common in trucks, buses, ships, tractors, construction machines, generators, and other heavy equipment. Diesel engines can also be useful in passenger cars when the driver regularly travels long distances.
When Is a Gasoline Engine a Good Choice?
A gasoline engine can be useful when the vehicle needs to be lighter, a lower purchase price is important, high engine speed is useful, the vehicle is relatively small, the vehicle is mainly used for shorter trips, or a simpler engine is preferred.
Modern gasoline engines can also be very efficient. Engineers have improved gasoline engines through technologies such as turbocharging, direct fuel injection, better valve control, hybrid systems, and improved computer controls. Therefore, gasoline engines should not be considered inefficient simply because diesel engines are generally more fuel-efficient.
