If you want to understand the difference between gasoline engines and diesel engines, you first need to understand the difference between gasoline and diesel. Petroleum is mainly refined through fractional distillation: crude oil is heated first to obtain different vaporized substances, and then the vapors are cooled to produce different products, such as gasoline, diesel, kerosene, lubricating oil, asphalt, and so on.

Gasoline has a low molecular weight, so its boiling point is only 20-220°C, but its ignition point is above 400°C. Diesel has a higher molecular weight and a longer carbon chain, so its boiling point is higher, while its ignition point is only 220°C.

These differences in fuel characteristics also lead to major differences between gasoline engines and diesel engines.

1. Different ignition methods

Because gasoline has a high ignition point, after the engine piston compresses the mixture, it must be ignited by a spark plug

A diesel engine, by contrast, does not require a spark plug. It relies on the temperature rise caused by compressed air to directly ignite the diesel fuel

This compression-ignition effect also gives diesel engines many advantages, such as no need for an ignition system and therefore a lower failure rate. Compression ignition also allows for a much higher compression ratio: a compression ratio of 13:1 is already high for an ordinary gasoline engine, but a diesel engine can easily reach 17:1

By using a longer piston connecting rod to increase piston stroke, engine torque is greatly improved, and with the same displacement, diesel engines deliver better fuel economy

2. Different engine speeds

Gasoline engines can premix the combustible mixture, meaning gasoline and air are mixed before being injected into the cylinder, or they can use direct injection, where a high-pressure injector atomizes the gasoline so that it mixes and is compressed inside the cylinder at the same time

When the engine piston reaches top dead center, the spark plug ignites it, in short, gasoline and air are mixed first and then burned

However, diesel has a high viscosity and cannot mix effectively with air, so a diesel engine first draws in air, then compresses and heats the air, and only injects diesel when the piston is close to top dead center, using the high-temperature air to ignite the diesel

The entire combustion process is also divided into the ignition delay period, rapid combustion period, slow combustion period, and afterburning period, and this method has an obvious drawback: it requires a longer combustion time, which is also why piston stroke is increased as mentioned earlier

Therefore, diesel engine speed cannot be too high. If the speed is too high, not only will the diesel fuel fail to burn completely and produce excessive black smoke, but torque attenuation will also occur

For example, the world's largest engine is a diesel engine installed on a ship, and its rated speed is only 102r/min

The engine speed of many diesel versions of passenger vehicles is also far lower than that of their gasoline versions

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