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The Differences Between Petrol and Diesel Engines

We all know that you do not put diesel in a petrol car or vice versa. But have you ever wondered why? Or are you in the market for a new car and currently comparing fuels? Whether you are just curious or need the information to make an informed vehicular decision the answers are here.

How do petrol and diesel engines work?

Both engines work by harnessing explosions, using the 4-stroke combustion cycle. The energy produced by these explosions is captured by the cylinders and pistons in the engine and converted into the circular motion of the wheels. This drives the car forward. The reason that petrol and diesel require different engines becomes clear when we consider how these explosions are produced.

Petrol requires ignition to explode. Petrol mixed with oxygen is injected into the cylinder and the spark plugs produce a perfectly timed electrical spark to ignite the petrol. The resulting explosion pushes the piston away, in a linear motion, and that energy is then used to move the car.

Diesel does not require an ignition source. Diesel will self ignite at a much lower temperature than petrol. The increase in temperature produced by the compression as the piston moves back is enough to cause the diesel to explode. Diesel engines do not have spark plugs. For this temperature increase to occur the compression produced by the piston is much higher than that in a petrol engine.

Why can’t diesel be put in a petrol engine and vice versa?

Putting diesel in a petrol engine is the less serious mistake of the two to make. The diesel will coat the spark plugs and will lead to the car misfiring. Diesel is used as a lubricant in a diesel engine and so will cover the inside of the petrol engine. The lower volatility of the diesel means that the spark plugs, if they fire, will not be sufficient to ignite the diesel and the engine will not run.

Petrol in a diesel engine can cause more damage. Petrol acts as a solvent and dissolves the diesel still in the car. This reduces the lubrication in the system and damage will be caused as the metal parts of the pump start to come into contact with each other. Allowing this mix to spread through the rest of the car could lead to the whole fuel system having to be replaced.

How do petrol and diesel engines compare when looking for a new car?

Have a look at the handy guide below to the differences between petrol and diesel engines when comparing cars.

Diesel Engines

  • Increased torque and able to pull heavier loads
  • Tend to be heavier due to engine parts needing to be more rugged
  • More economic due to higher fuel efficiency and lower fuel costs
  • Longer engine life
  • Lower carbon dioxide emissions

Petrol Engines

  • Cheaper to buy than diesel engines
  • Lightweight engine
  • Lower maintenance costs
  • Low levels of nitrogen oxides produced
  • Higher horsepower – higher top speeds
  • No issues starting in cold weather

The fundamental difference between petrol and diesel engines is simple – petrol engines require ignition, in the form of spark plugs and diesel engines do not. This translates into a whole host of considerations when you look at the performance, cost and environmental impact of the two engines.

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