Gasoline vs Diesel

In China, diesel cars are not favored by people due to the so-called shortcomings of diesel fuel, such as emitting black smoke when burning and the loud noise of diesel engines. However, in foreign countries, diesel cars are very popular.

Today, the editor will introduce diesel engines to car enthusiasts and compare them with gasoline engines.

Firstly, let's understand the difference between diesel and gasoline from a chemical and physical perspective.

Diesel

There are two types of diesel. One is light diesel, with a boiling point of approximately 180 to 370 degrees Celsius, and the other is heavy diesel, with a boiling point of approximately 350 to 410 degrees Celsius. Generally, cars use light diesel. Diesel is mainly composed of complex hydrocarbon mixtures (such as alkanes and cycloalkanes), which have relatively high boiling points and are difficult to ignite. It's not impossible to ignite diesel, but certain conditions must be met. Generally speaking, at room temperature and normal atmospheric pressure, even if an open flame like a lighter is thrown into liquid diesel, the flame will be extinguished instead of igniting the diesel. To make diesel burn, it must be heated and pressurized; otherwise, it is difficult. Although the boiling point of diesel is higher than that of gasoline, the self-ignition point of diesel is lower than that of gasoline.

Gasoline

Gasoline is not classified as light or heavy like diesel, but like diesel, it has different grades. The main component of gasoline is a mixture of hydrocarbons, but unlike diesel, gasoline has strong volatility and is very easy to mix with air to form explosive mixtures. So even if gasoline is spilled inside a gas station, the smell of gasoline can still be detected outside the station. Due to its volatile nature, gasoline is more dangerous compared to diesel, especially when it spills all over the ground after a vehicle collision.

Next, let's explore the differences between diesel engines and gasoline engines in cars.

To understand the difference between these two engines, the first thing to know is what "compression ignition" is. Compression ignition refers to the phenomenon where a fuel ignites due to being compressed, heated, and pressurized. Diesel engines rely on compression ignition.

1. Combustion Method

The editor believes that the biggest difference between diesel engines and gasoline engines is the way they ignite the fuel. Gasoline engines rely on spark plug ignition, while diesel engines rely on compression ignition. Due to the low self-ignition point of diesel and the ability of diesel engines to compress the air in the cylinder to 1/16 of its original volume or less, according to the Ideal Gas Law, the higher the pressure, the higher the temperature. So, the temperature of the compressed air is higher than the self-ignition point of diesel. At this point, injecting diesel fuel particles into the cylinder will cause the diesel to self-ignite and explode, generating strong pressure to push the piston and drive the crankshaft. From the figure below, it can be seen that after the air is compressed to a high temperature, the fuel injector injects particles into the cylinder, and the high-temperature air ignites the diesel particles.

However, since the self-ignition point of gasoline is often higher than the temperature of compressed air, gasoline-air mixtures can only be ignited by spark plugs. In the picture below, the green part is a mixture of air and gasoline, which is ignited by a spark plug.

2. Compression Ratio

More professionally, compression ratio refers to the ratio of the volume of the piston at the Bottom Dead Center to the volume at the Top Dead Center. Simply put, it is the ratio of the total volume of the cylinder to the volume of the mixture at the time of explosion.

Due to their different working principles, the compression ratios of the two engines are different. Generally speaking, the compression ratio of gasoline engines is about 10:1, meaning that air entering the cylinder will be compressed to one-tenth of its total volume. However, the compression ratio of diesel engines can reach 16:1 or even higher.

3. Throttle Valve

Firstly, it should be noted that gasoline engines have throttle valves, while diesel engines do not.

The function of the throttle valve on gasoline engines is to control the total amount of air entering the cylinder to regulate the engine's output. The more air there is, the more fuel can be fully mixed with air, the more complete the combustion, and the higher the horsepower. This is the way to control the output horsepower of gasoline engines. However, the way diesel engines control horsepower output is completely different.

The amount of air inhaled by a diesel engine is the same every time, and the computer controls the engine's horsepower output by injecting a certain amount of diesel fuel particles into the cylinder. Since the horsepower of the engine is proportional to the amount of fuel in the cylinder and the amount of air remains constant, there is no need to use a valve to control the intake of air.

4. Air-Fuel Ratio

Although the optimal air-fuel ratio for both gasoline and diesel is approximately 15:1, meaning that when 15 grams of air is mixed with 1 gram of diesel or gasoline, combustion is theoretically most complete. However, the air-fuel ratios of diesel engines and gasoline engines are significantly different in actual operation.

The air-fuel ratio range of gasoline engines is approximately 6:1 to 25:1, which is a range very close to the optimal air-fuel ratio. If it exceeds this range, the fuel cannot be ignited. However, the range of air-fuel ratios for diesel engines is indeed very wide, ranging from 10:1 to 75:1, and can still burn fuel. What exactly is the reason for diesel engines to have such a wide range?

The reason is actually very simple. Since diesel engines inhale the same amount of air every time, the computer controls the engine's horsepower by controlling the amount of fuel. When the engine is idling, only a small amount of fuel is injected into the cylinder, and at this time, the air-fuel ratio may be close to 75:1. When the engine requires high horsepower output, there is a large amount of fuel mixed in the cylinder, and at this time, the air-fuel ratio may be close to 10:1.

5. Engine Braking

The braking system of a car actually consists of three parts: 1. Mechanical braking; 2. Air braking; 3. Engine braking. For example, if we are pushing an oil drum uphill and we stop exerting force, the oil drum will push us downhill. This is the principle of engine braking. Stepping on the accelerator can push the car forward, while releasing the accelerator can slow down the car.

Although both engines have engine braking, the reasons for their occurrence are different.

The engine braking of gasoline engines is due to the complete closure of the throttle valve after releasing the accelerator, which creates a vacuum in the cylinder during the intake stroke and generates a counteracting force on the piston. It is this force that slows down the engine, which means it is the source of engine braking. In the figure below, due to the throttle valve being closed, no air can enter the cylinder, causing a vacuum to form inside the cylinder when the piston moves downwards, generating a force opposite to the direction of piston movement (black arrow) (red arrow).

However, since diesel engines do not have a throttle valve, this function is achieved by controlling the exhaust valve. When the accelerator is released, the exhaust valve is closed, so that the exhaust gas is trapped in the cylinder. During the exhaust stroke, since the exhaust gas cannot be compressed, it generates a counteracting force on the piston, which produces engine braking. In the figure below, due to the exhaust valve being in a closed state, the combustion exhaust gas cannot be discharged, resulting in high pressure in the cylinder and generating a force (green arrow) opposite to the original direction of piston movement (black arrow), thereby slowing down the engine.

6. Torque

Many car enthusiasts may say that diesel cars have high power, but what exactly is this power? Is the power of gasoline engines not strong enough?

In fact, the force that everyone is referring to is torque, which is the force used to accelerate. The statement that diesel engines have greater torque than gasoline engines is partially true. To prove that this statement is correct, we must keep some variables constant.

We must first determine the displacement of the two engines because a comparison cannot be made if the displacements are different.

When the displacement of the engine is fixed, the reason why diesel engines have greater torque than gasoline engines is mainly due to the difference in stroke ratio.

Previously, the editor also mentioned that diesel engines have a higher compression ratio than gasoline engines, which implies another point. Since we have already specified the displacement of the two engines in advance, under these conditions, the stroke of a diesel engine must be longer than that of a gasoline engine. I still remember when introducing F1 engines, I mentioned that the shorter the stroke, the greater the horsepower, and the longer the stroke, the greater the torquetorque. This is the reason why diesel engines generate more torque than gasoline engines of the same displacement.