While petrol and diesel engines essentially do the same thing, they mechanically operate differently and produce different outcomes. What is the difference between diesel and petrol engines?
When we think of diesel cars, we think of run-down noisy trucks that constantly belch black smoke. While this perception is justified, those so-called trucks are usually of the older generation from yesteryears, driven by commercial drivers whose companies refuse to invest in newer vehicles.
Unfortunately, that’s the prevalent image many of us conjure when discussing diesel vehicles. However, with powertrain technology advancing a lot in the past two decades, they have made diesel engines sufficiently smooth and refined, which can be used in private passenger vehicles and even luxury ones.
But what is the difference between Petrol and diesel engines? Why are some cars fitted with it, and what are the general misconceptions regarding diesel engines?To get to the bottom of all these questions, we must first understand the main difference between a diesel and a petrol engine.
The main difference between Diesel and Petrol engines
Picture credit: Valvoline
In a broad sense, diesel and gasoline engines are similar. They are both internal combustion engines (ICEs), created precisely to convert their fuel into mechanical energy.
The key difference between them is how these combustions occur. In petrol engines, fuel is first mixed with air and then compressed by pistons, where sparks from spark plugs then ignite it. In diesel engines, the air is compressed before it gets mixed with the fuel. Since the air heats up when compressed, it makes the fuel ignite.
Why are some cars fitted with diesel engines?
Traditionally, vehicles that are considered a workhorse, such as trucks and lorries, are fitted with diesel engines. They are fitted with diesel engines because slow-combusting diesel engines are proven to be fuel efficient and can achieve higher torque at lower speeds.
Since they are heavy cars which get heavier through the load that they carry, they need a lot of torque to get them going. They also move around a lot, which is why it makes sense to go with an engine that is fuel efficient.
Diesel VS Petrol Pros and Cons
If these workhorses are fitted with petrol engines, they would need to be engines with larger displacements to get the same amount of torque, and the con of that is that they would drink more fuel and also raise road tax costs.
The primary disadvantage of diesel engines is that they are expensive to manufacture, and the emissions they create are worse than petrol engines. However, modern diesel engines are better when it comes to emissions, which we will cover down below.
Generally, some cars are fitted with petrol engines because they are more potent by way of horsepower. This is why most high-performance cars, such as Ferraris and Lamborghinis come exclusively with petrol engines. Not only can they produce more horsepower, but they are easier to tune. Another advantage of petrol engines is that Petrol is also usually cheaper than diesel. However, a petrol engine’s lifespan is usually shorter than a diesel’s.
Common perceptions of diesel engines
Diesel Engines are Dirty: True in the old days, but not anymore. Introductions of turbocharging and direct injection technology have revolutionized what’s possible with a diesel engine. In addition, further refinement of the computer-controlled injectors has meant more complete and efficient fuel combustion, resulting in better and cleaner performance in modern diesel engines.
Better Fuel Consumption: Manufacturers usually publish 25% better fuel economy for diesel engines than a petrol motor of equivalent performance. This is mainly true in the real world, but with the caveat that you are hardly likely ever to match the claimed lab-tested figures. Of course, driving style plays a part too; the more aggressive you are with the throttle, the more fuel you burn – but you do burn less driving a diesel.
Maintenance Cost: The maintenance cost of diesel engines are traditionally higher than petrol vehicles, but the gap has shrunken considerably over the years. Oil changes cost more in diesel vehicles, and there is also the inevitable servicing cost of the turbocharger and diesel particulate filter, which you don’t worry about in a petrol engine. That said, you don’t have to bother yourself with changing spark plugs in a diesel.
Diesel Engines are Noisy: The moment you fire up a diesel engine, you are greeted by that familiar clattering sound. It is an inherent characteristic of a diesel engine that you have to live with. That being said, carmakers have been working very hard to refine the behaviour of diesel engines on the move. So, whilst your diesel car sounds a little like a lorry on idling, rest assured that most of them are as quiet and refined as you wish for when you get going.
Diesel Emissions – NOx, Diesel Particulates: Diesel engines emit high levels of nitrogen oxides, but sophisticated emission systems and filters in the modern car trap these dangerous particulates and prevent them from being released into the atmosphere.
Why I daily a diesel
A diesel-powered vehicle can be an ideal daily driver. If you spend as much time on the road as we do, a diesel vehicle could make sense as it is fuel efficient.
The other reason why I a daily a car with a diesel engine is because I live in a hilly area. Since diesel engines can give you plenty of low-down torque, it is ideal for me, as the climbs are not so arduous.
But truthfully, the main reason why I daily a diesel car is because it’s purely out of choice. Since my beloved vehicle only came from the factory with either a 2.5-litre diesel or a 4.0-litre petrol engine, it was wiser to choose a diesel engine as the road tax and fuel cost of the petrol car with big petrol engine would leave me out of pocket.
Yes, I could have gotten a smaller car with a smaller diesel engine like the BMW 320d, but once again, I drive what I drive because I adore the car. Sometimes, it’s not all about facts and figures, but more of “would you look back at your car when you are walking away from it” feeling.
Unfortunately, in my case, the lust is strong for a Land Rover Discovery, especially one with a 5-cylinder diesel engine, which in itself is a unique bit of kit. It also has seven seats, big cargo space and rugged four-wheel drive capabilities, making it the perfect runabout vehicle for daily duties.
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