Mechanics and Maintenance

Petrol VS Diesel

3 min read

With petrol prices continuing to be constantly variable, some days it’s $1.20/L, and the next it’s $1.70/L, many people are looking to diesel powered vehicles. The price of diesel usually remains fairly consistent and you often get better mileage from them, but what is the actual difference?

Is it better for the environment and do you get better performance out of a diesel powered vehicle? In this article, we discuss the benefits and differences of both options.

What Is The Difference Between Petrol And Diesel?

On the outside petrol and diesel engines look the same. They both operate using chamber, pistons, and a crankshaft, and run on an internal four-stroke system. What sets them apart is how they ignite the fuel source on the compression stroke. A petrol engine uses a spark plug to ignite the fuel/air mix when the piston has reached top-dead centre and achieves maximum compression.

The compression and ignition stroke of a modern diesel engine works in a very different manner. During the intake and compression stroke only involve air. Once maximum air compression is achieved, the fuel is then injected into the chamber. The pressure of the compression actually heats the air and this heat actually ignites the fuel when it is injected into the chamber.

Both engines then have the power stroke, when the piston is pushed down into the chamber, and then expels the exhaust gases on the returning stroke. Then the process begins again and repeats.

Considering that, it is clear that the main differences are that a petrol engine relies on spark plugs, and a diesel engine does not; and the difference in the fuel injection processes. Other differences are that most commonly diesel engines are more durable and have been designed to deal with tougher conditions than standard petrol powered vehicles, which is probably why most diesel engines are seen in big cars and larger trucks.

As a side note even though diesel engines don’t require spark plugs, many come equipped with glow plugs that aid in the heating of the compressed air when the engine warms up to proper operating conditions. Diesel engines typically have smaller pistons and a longer stroke.

What Are The Differences Between Petrol And Diesel Fuels?

Both of these fuels are made from oil, however with the specific chemical makeup of diesel, it is capable of offering 18% more energy than petrol. This is because of the way that diesel is combusted; it uses less fuel to get out the same amount of power. The explosion inside the engine chamber is bigger and results in an increased power stroke.

Therefore, diesel engines are able to produce more kilowatts of power, due to the strength of the power stroke. Although they are not able to operate as fast as petrol engines, this makes them ideal for carrying heavy loads at lower speeds. Which is why you’ll most often see the engines in working vehicles, where their main function is moving equipment or goods from place to place. What diesel engines offer in power makes up for their inability to match the speed and acceleration of petrol powered vehicles.

As far as environmental concerns, diesel engines are not perfect, as diesel fuel is denser than petrol and the unburnt diesel contains particulates, heavy metals and carcinogens. The fuel when burnt creates more nitrogen oxides, leading to a higher production of ozones. Which are not great for living organisms.

Currently, diesel engines are subjected to emissions controls that try to trap these problems before they escape out of the vehicle's exhaust. These measures are often effective but can create extra maintenance, as they require changing of filters and fluids more often than petrol powered vehicles.

What Are The Costs?

Diesel engines cost more to produce as they have a few more systems when compared to a petrol engine, and this is why you often see more petrol engines inside that everyday car. Even though the fuel saving can be good, many people balk at the price tag. Basically you will spend around 5% more on the price of a vehicle when looking for a diesel engine.

But as many of these are used for work purposes the savings of fuel costs outweigh the initial cost of the vehicle.

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