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Diesel vs. Gasoline: Everything You Need to Know

As more and more foreign auto manufacturers are offering diesel models in the United States, many consumers have been asking themselves whether diesel vs gasoline is the better choice for their next vehicles.

As more and more foreign auto manufacturers are offering diesel models in the United States, many consumers have been asking themselves whether diesel vs gasoline is the better choice for their next vehicles. According to Bell Performance, Subaru, Audi, and Volkswagen currently sell cars with diesel engines in the United States. These engines offer improved efficiency over gas engines without using electricity.

Although gas cars are more popular than diesel in the U.S., diesel engines have nearly half the market share in Europe. Digital Trends notes that while many U.S. consumers consider diesel a dirty fuel, technology advancements have made this a clean, green option for drivers who want a high-powered engine without limiting efficiency. However, it can be challenging for car buyers to understand the difference between these two automotive options.

Gasoline vs. Diesel: What’s the Difference?

If you’ve ever wondered how different diesel and gas are, we have the answer for you.

At the most basic level, modern car engines work on the principle of four simple strokes: intake, compression, power, and exhaust. Repeat this cycle enough times per minute, and it has the ability to make any gear-head grin. Whether diesel or gasoline powered, the four engine strokes are the same, but there are key differences as to how the strokes are executed.

Engine Operation Differences

For gasoline engines, the intake stroke generally involves pulling air and fuel into the combustion chamber. At this point, a diesel engine is only pulling in air. Next is compression, where both engine types squeeze down the air into a small pocket. Ignition is controlled separately for each fuel type. Gasoline engines use a spark plug to time and start the power stroke. This small arc of electricity ignites the air fuel mixture and the powerful blast forces the piston down, churning out that highly craved horsepower. A diesel engine, on the other hand, only has a pocket full of hot air before the power stroke. As the piston approaches the top of its compression stroke, the air is hot enough from being squeezed down so small that when diesel fuel is injected, it immediately ignites. Thus, the timing of ignition for diesel engines is driven by the fuel injectors. Both engine types function similarly for the exhaust stroke, where a valve opens up and the piston forces the spent fumes out of the cylinder.

Efficiency Differences

While diesel engines continue to build a reputation of struggling with emissions, they actually have highly impressive fuel economy numbers in comparison to their gasoline counterparts. Indeed, diesel engines are significantly more efficient in most cases, especially when the throttle pedal is only partially depressed. Much of this is due to differences in the strokes described above, in combination with the self-ignition temperature. The self-ignition temperature is the temperature at which an air-fuel ratio will combust simply from heat alone. In gasoline engines, it is critically important that the self-ignition temperature is never reached during the compression stroke, as this would cause combustion to occur before the spark plug fires, potentially destroying the engine. As a result, gasoline engines have relatively low compression ratios (the amount the air and fuel is squeezed down during the compression stroke), because compression causes a rise in temperature. As diesel engines do not have fuel in the mixture during the intake stroke, they can compress the air much more, exceeding the diesel fuel’s self-ignition temperature. Higher compression ratios equate to higher efficiency, so diesel engines take advantage of this by injecting fuel into the air after it is compressed.

Another diesel efficiency benefit comes from the lack of a throttle body. When you press the accelerator pedal a gasoline vehicle, this opens a valve in your engine’s intake, allowing more air to enter the engine, and thus create more power. The engine’s computer realizes it needs to inject more fuel for the additional air, but you can think of the accelerator pedal as an “air pedal” in this case. For diesel engines, a throttle valve is not necessary. In this case, the accelerator pedal regulates how much fuel is injected. The throttle body used in gasoline engines acts as a restriction, especially when the throttle is only partially open, and this is part of the reason why diesel vehicles have better fuel economy in low load scenarios.

Air/Fuel Ratio Differences

What allows diesel engines to operate based on how much fuel is injected? One reason is the ability of diesels to operate within a very wide range of air/fuel ratios. Gasoline engines generally operate within a range of about 12 to 18 parts air to 1 part fuel (by mass). Usually, this ratio stays pretty close to 14.7:1, as at this ratio all of the fuel and oxygen is completely used up. A diesel engine, however, generally operates from 18:1 to as high as 70:1, and is capable of operating in super lean ratios. When you press the accelerator pedal in a diesel engine, this drops the air/fuel ratio. Significantly more fuel is injected relative to the air ingested, which increases horsepower. Soot is created when diesel engines operate in low air/fuel ratios, which is why you may see black smoke from diesel trucks when they floor the throttle pedal. Ultimately, while the four major strokes are the same for both diesel and gasoline engines, the finer details reveal the highly unique characteristics of each engine type.

Along with the differences outlined above, there are also major distinctions in how each fuel type is capable of slowing down a vehicle, known as engine braking. For more information, check out the video below.

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The price difference

Diesel fuel, which is syrup-like compared to watery gasoline, packs more of a punch in terms of energy per unit of weight than gasoline (say, per gallon or liter). Even though it often costs more than gasoline, it contains more potential energy, so less diesel is required in terms of fuel to accomplish the same amount of work (in this case, driving distance) as gasoline. That’s why diesel cars tend to get mileage in the 30s, 40s, or 50s — or more — depending on the engine and vehicle type. In terms of fuel comparative costs, it’s either a wash or a slight advantage for either fuel, depending on the price difference, vehicle, and driving style. But overall, the final cost for using either fuel over time is going to be nearly the same.

Diesel owners also have another fueling option: Biodiesel. Biodiesel is made from non-petroleum sources such as fry oil or vegetable oil. Indeed, the first diesel engines ran on peanut oil-derived diesel fuel. And while making your own gasoline at home is very complex, dangerous, and ill-advised, it is possible to safely make your own biodiesel using ready-made kits.

Getting your diesel vehicle to run on biodiesel may require some small modifications — especially if it’s older — but it depends on several factors, including what percentage of your fuel is biodiesel, which can be blended with regular diesel. There is a large biodiesel community online that can answer questions about the popular alt-fuel. The per-gallon cost of bio-diesel may also be higher than pump fuel, but since you can make it yourself, it really depends on aggregating the cost of the equipment, the oil source (which can be free), and your time.

Engine, Gasoline, (Water Cooled)

Gasoline Engine (Water Cooled) is constructed to give continuous, economical operation. It is easy to start and operates with a minimum of vibration.

The 2, 3, and 4 H.P. engines are of the horizontal single cylinder type with over-size Timken roller main bearings, a high tension magneto, noned cylinder, and a hopper cooler. Cylinder, hopper and crankcase are all cast in one piece. Operates economically on gasoline and special equipment is available for adapting engines to use tractor fuel, kerosene, distillate or natural gas.

The 4 and 5-6 H.P. single cylinder vertical engines are self oiling and water cooled from either tank or radiator. These engines are precisely controlled by a sensitive governor and equipped with a high tension magneto and friction clutch pulley. Bearings and gears are fully enclosed in a dust and moisture proof crankcase and are readily accessible when side plate is removed.

In higher horsepower the 8, 10, 10-12, 15, and 20 H.P. engines are vertical double cylinder type with combination pump and splash oiling, high tension magnetos, taper roller main bearings and automobile type carburetion controlled by a throttling governor. These engines can be equipped with flat or V-belt pulleys or chain sprockets and with or without outboard bearing for heavy belt work. Friction clutch or pulley and long or short base optional. Equalized bearing load gives long bearing life. Water cooled from either tank or radiator.

Source: This article is a reproduction of an excerpt of “In the Public Domain” documents held in 911Metallurgy Corp’s private library.

Do most gasoline engines need the additives in most diesel oils?

Nyholm: Diesel and gasoline oils are formulated with a variety of additives that improve wear protection, corrosion protection, resistance to foaming, viscosity retention and more. Whether the application is gasoline-powered or diesel-powered, many of the same additives are used based on what we are asking them to do in the formulation.

There are additives designed to manage the byproducts of combustion, and some of those byproducts change depending on whether you’re burning gasoline or diesel.

If you’re running a gasoline engine, it’s best to use an oil that contains the correct additives to handle the byproducts of gasoline combustion. The same holds if you’re operating a diesel engine.

2 Answers 2

Diesel engines don’t rely on spark plugs, but they still work by igniting the fuel to generate a force that moves a cylinder in the engine. The air/fuel mixture in a diesel engine is ignited by the compression of the same cylinder during the «compression» stroke. That is, like most gasoline engines in cars, diesel engines are four-stroke engines.

The compression stroke, in a gasoline engine or in a diesel engine, causes heating of the cylinder contents as the compression happens. Diesel engines are designed to have a more severe compression of the cylinder contents during the compression stroke. This results in much higher heating of the contents; in fact the temperature becomes so high that the ignition of the fuel and air becomes spontaneous.

You may ask, where does the energy to power the compression stroke come from? It comes from inertia in the engine as it is running. Another way to say it is that the previous power stroke provides the energy for the compression stroke. (The power stroke is when the exploding air fuel mixture pushes the cylinder out, doing useful work.) There is so much work released in the power stroke that there is enough to move the car forward as well as provide inertia for the next compression stroke.

Here are some more web links with helpful explanations:

The diesel engine (also known as a compression-ignition or ‘CI’ engine) is an internal combustion engine in which ignition of the fuel that has been injected into the combustion chamber is initiated by the high temperature which a gas achieves when greatly compressed (adiabatic compression). This contrasts with spark-ignition engines such as a petrol engine (gasoline engine) or gas engine (using a gaseous fuel as opposed to gasoline), which use a spark plug to ignite an air-fuel mixture. The diesel engine has the highest thermal efficiency of any standard internal or external combustion engine due to its very high compression ratio and inherent lean burn which enables heat dissipation by the excess air.

In theory, diesel engines and gasoline engines are quite similar. They are both internal combus­tion engines designed to convert the chemical energy available in fuel into mechanical energy. This mechanical energy moves pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the up-and-down motion of the pistons, known as linear motion, creates the rotary motion needed to turn the wheels of a car forward. Both diesel engines and gasoline engines covert fuel into energy through a series of small explosions or combustions. The major difference between diesel and gasoline is the way these explosions happen. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it’s compressed, the fuel ignites.

Be sure to watch the animation at How Stuff Works. It’s quite helpful!

Basics of diesel engines

A diesel engine is an engine that converts the energy stored in the diesel fuel into useful mechanical energy.

How diesel engines operate

The energy conversion actually takes place in two steps. First, the fuel reacts chemically (i.e. burns) and releases energy in the form of heat in a cylinder equipped with a piston. Then the heat causes the hot gases entrained in the cylinder to expand, and the expanding gases push the piston. The reciprocating motion of piston is then converted into rotational motion through the crankshaft.

Most diesel engines are multi-cylinder engines and their cylinders are typically arranged either in-line or a ‘V’ type configuration, although other combinations are also available. In an in-line engine, as the name implies, all cylinders are instlalled in a row. In a ‘V’ type engine, cylinders are arranged in two rows of cylinders, set at an angle relevant to each other and interconnected with a common crankshaft. Each group of cylinders making up one side of the «V» type configuration is typically referred to as a ‘bank of cylinders’.

Figure 1 — Typical diesen engine cross section during air intake (piston-cylinder shown at the top, crankshaft shown at the bottom)

Differences between Diesel Engine and Gasoline Engine

• Unlikegasoline engines, diesel engines don’t require an ignition system or a spark plug to ignite them. Diesel fuel is injected in the cylinder as the piston rises to the top of its compression stroke. When fuel is injected, it vaporizes and self-ignites due to the heat created by the compression force.

• Unlike gasoline engine, which can limit the amount of air entering the engine through the butterfly valve of the carburator, the air amount entering the diesel engine is always maximum. Therefore, the diesel engine speed is limited only by the amount of fuel injected in the cylinders.

Types of diesel engines

Two types of diesel engines are in use nowadays: two-stroke cycle engines and four-stroke cycle engines (the word cycle refers to any operation or series of events that repeats itself).

A stroke in the engine refers to the distance travelled by the piston from the top of the cylinder to the bottom. For a diesel engine, the highest point of travel by the piston is called top dead center (TDC), whereas the lowest point of travel is called bottom dead center (BDC). There are 180 0 of travel between TDC and BDC, or one stroke.

Despite their different characteristics, both types of diesel engines must go through the following four (4) stages: intake, compression, fuel injection-combustion, exhaust.

Intake: The piston is located near BDC. The intake valve or port open up and the fresh air is taken in. The exhaust valve or port remains closed while air intake is completed.

Compression: Once the piston passes BDC, the intake valve closes and the piston starts going upwards up to TDC.

Fuel injection-ignition: As the piston approaches TDC, the fuel is injected through dedicated fuel injectors. As the piston moves towards TDC air and fuel mixture is compressed leading to heating and combustion of gases inside the cylinder. Gases expanding due to the combustion heat force the piston to move downwards towards BDC. This reciprocating motion of piston between TDC and BDC is converted to rotating motion by the crankshaft.

Exhaust: As the piston passes BDC, the exhaust valve or port opens and exhaust gases are discharged from the cylinder.

The basic difference between two-stroke and four-stroke cycle enginers is that a two-stroke cycle engine requires only two strokes of the piston to complete one full cycle. Consequently, it requires only one rotation of the crankshaft to complete a cycle. In the case of a four-stroke cycle engine, the engine requires four strokes of the piston to complete one full cycle consisting of — air intake, compression, fuel injection and gas exhaust. Therefore, it requires two rotations of the crankshaft or 720 degrees of crankshaft rotation (360 x 2) to complete one cycle.

Figure 2 — The four (4) stages-events of a typical diesel engine