The science behind diesel engines (no spark plugs required) – Jalopnik

Every certified Gearhead knows that diesel engines do not use spark plugs. But why not? If the power in a motor is created by combustion or explosion of flammable liquids, will a spark plug in a diesel engine not help to ignite fuel faster and improve combustion efficiency? The answer lies in the chemical properties of the Diesel Fuel, the way it is inflamed, and the alternative of the spark plug it uses. Let us go through how a diesel engine works.

A modern diesel engine runs on a combustion cycle with four stroke inlet, compression, strength and exhaust. The inlet cycle consists of a piston that moves past a cylinder, causing air to suck in. Then comes the compression stroke where the piston rises again, so that the air in the cylinder is compressed. This act of compression or pressure on the air heats it to a temperature high enough to ignite diesel. Then diesel is sprayed into the cylinder where it comes into contact with the overheated air and automatically ignites. The resulting inflammation causes a mini explosion that pushes the piston through the cylinder. That is a force. The last (exhaust) cycle moves the piston up and expels the burnt or spent gas from the cylinder.

The difference between gasoline and diesel engine cycles is the way in which air and fuel are mixed and inflamed. That said, there are other differences between a gasoline (spark inflammation) and diesel (compression exclusion) engine. Air and fuel are mixed in a gasoline engine before the cylinder arrives. During the power stroke, a spark plug in the cylinder ignites the mixture, causing combustion. A spark plug delivers a precisely timed spark that ignites the air fuel mixture. Why is a spark plug used in gasoline engines and no diesels? Firstly, diesel fuel is more difficult to ignite with an external fire source such as a spark. Instead, it is easier to ignite diesel fuel by introducing it in higher temperatures such as overheated air.

Although diesel engines do not use spark plugs, they use glow plugs, heating elements that are placed in a diesel engine cylinder or combustion chamber. They are used to start diesel engines in cold weather by heating the air in the cylinder. This is a useful component in a diesel engine, because during cold weather the air in the cylinder may not heat up to the temperatures needed to ignite the diesel-air mixture, even when compressed.

Although diesel and gasoline fuels are based on petroleum, they have various chemical properties. Even diesel engine oil differs from the oil from the gasoline engine. The most important distinctions are their flash points and car inflammation temperatures. The minimum temperature in which a combustible liquid emits sufficient vapors to form an inconvenable air fuel mixture is the temperature of the flash point. Auto inflammation temperature is the minimum temperature at which a fuel automatically ignites due to the surrounding air temperature, but without the presence of an inflammatory source such as a fire or spark. This concept can also cause a runaway diesel engine.

In comparison with gasoline, diesel is a more stable fuel. The temperature of the flash point, or the temperature at which the vapors emits is high at 126 degrees Fahrenheit. This means that you have to hold a diesel fuel match for longer to ignite. That is the reason that spark plugs do not work in a diesel engine. In comparison with gasoline, diesel fuel has a lower car inflammation temperature of 410 degrees. It is therefore easier to ignite diesel if you heat the air around it to 410 degrees. In a diesel engine this air is heated by compression. That is why diesel engines are also called compression engines.

The more you compress the air in a diesel engine cylinder, the hotter it is before diesel is introduced, and the more efficiently the resulting air-fuel mixture will burn. That is why diesel engines have a higher compression ratio of 14: 1 to 25: 1. The higher the compression ratio, the greater the thermal efficiency of the engine or the possibility to convert the heat of the fuel into usable work, such as moving the piston down and creating power and torque. A high compression ratio also translates into more torque. That is a reason why diesel engines offer more torque than comparable gasoline engines and also offer better fuel consumption. These are good reasons why modern diesel engines surpass gas engines in heavy trucks.

Modern diesel engines also use fuel injectors that change diesel fuel from liquid to attress particles by means of pressure. These injectors spray a precise amount of attress fuel particles in the combustion chamber or cylinder at high pressure. This allows the attributed fuel particles to be evenly distributed over the cylinder, so that they can burn more efficiently.