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What's the difference between diesel and petrol cars?

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December 29th 2020 info review

For most drivers, you have two choices: diesel or petrol. Electric cars may be rising in popularity, but for the vast majority of people on the roads, they have to choose between two options.

Drivers may know petrol and diesel cars are different but do they know why? How do both engine types function and are the differences that substantial?

In the past, the difference between petrol and diesel engines was more pronounced. The gap is smaller now, but making the right choice is still important. Your chosen engine type can dictate issues such as running costs, road tax and even handling.

Below, we’ll discuss the difference between petrol and diesel cars, as well as the advantages of each. Hopefully, by the end, you’ll be able to spot more characteristics of each fuel type other than just the distinct “petrol smell”.

What is a petrol engine?

On the surface, diesel and petrol engines look the same. Popping up the bonnet of a diesel or petrol car wouldn’t throw up any obvious differences to an untrained eye. However, this changes once the engines are switched on.

A petrol engine is defined as an internal combustion engine with spark ignition. They are designed to run on petrol and other volatile fuel. A petrol engine pre-mixes fuel and air, which is its major differentiating factor compared to other engine types.

A petrol engine behaves in the following fashion:

  1. Pre-mixed fuel is combined with air via the intake stroke.
  2. A compression stroke then occurs which causes a piston to move upwards, catalysing the combination of fuel and air.
  3. The combined fuel and air is then combusted via a spark.
  4. The reaction causes a piston to move quickly, powering the exhaust and the exhaust valve.

The above is a simplification, but it is how a petrol engine works. So, what are the advantages and disadvantages of this process?

The advantages: why buy a petrol car?

A petrol car has three key benefits over its diesel counterpart. The advantages of petrol are:

  1. Price – petrol fuel is cheaper than diesel fuel. This is complemented by a petrol car’s price, too, which tends to be lower.
  2. Noise and handling – petrol cars are quieter than diesel cars, plus, for the most part, they feel a bit lighter and easier to handle.
  3. Emissions – petrol cars emit much lower frequencies of nitrogen oxide and other secondary and tertiary emissions compared to diesel engines. Read more about nitrogen oxide in our blog post: What Is Nitrogen Oxide And How Do Cars Produce It?

The disadvantages: why should I not buy a petrol car?

On the flipside, petrol engines have a few disadvantages too:

  1. Efficiency – while petrol cars do have cheaper fuel, they do use it at a faster rate. This makes them less efficient than diesel engines.
  2. Gear usage – petrol cars need more gear changes than a diesel model. This is to make the most of the engine and its power, making overtaking and acceleration a common part of driving.
  3. CO2 – petrol engines emit much more CO2 than a diesel engine. This isn’t great if you’re environmentally conscious, plus it leads to higher rates of road tax.

What is a diesel engine?

A diesel engine is powered by compressing just the air – it doesn’t premix like a petrol engine. The process is:

1)      The intake valve widens, allowing air to enter the engine. This causes the piston to move down.

2)      The air is compressed, which then moves the piston up. This stroke generates a lot of heat.

3)      Fuel is then infused during the combustion stroke, moving the piston down.

4)      The piston then reacts by moving upwards, powering the exhaust through the exhaust valve.

On the surface, this process appears to be simpler than a petrol engine. So, what are the advantages of this?

The advantages: why buy a diesel car?

The advantages of diesel are as follows:

  1. Fuel efficiency – diesel engines use fuel at a more efficient rate, as mentioned in the petrol disadvantages. Estimations of efficiency vary, but it’s safe to say that diesel engines are at least 15% more efficient than petrol engines.
  2. Power and towing – diesel engines tend to be more powerful, making them more suitable for larger cars and towing  thanks to their better torque.
  3. Low CO2 emissions – diesel engines produce less CO2. This usually places diesel cars in lower tax bands, which makes them more affordable to drive.

The disadvantages: why should I not buy a diesel car?

Diesel does have some disadvantages:

  1. Upfront costs and repairs – diesel cars cost more to purchase upfront. Fuel tends to be more expensive, but this is offset by the engine’s efficiency. Repairs can be more expensive on diesel cars, too, mostly due to their heavier frames.
  2. Other emissions – while CO2 is low, diesel cars produce other emissions in larger amounts.
  3. Noise – diesel cars are noisier, which may put some drivers off. Of course, this point is all down to personal preference, and most diesel drivers don’t even notice.

Should you buy a diesel or petrol car?

So, now that you have read the petrol vs diesel engine pros and cons, what’s your decision? While each engine has its characteristics, a driver’s choice is usually down to personal preference and other times price.

You’ll rarely find a driver who commits to just driving a petrol or a diesel – usually, other factors come into play, and both engine types have modernised to a point where their disadvantages feel small.

Wherever your preferences lie, we’re sure you’ll find a used car you love with findandfundmycar.com.

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