# Driving theory questions

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How much higher are the emissions of harmful particles from a diesel engine without a particle filter compared to a diesel engine with one?

A particle filter works similarly to the filter of a vacuum cleaner. The dirt gets caught in the filter and out comes air that is almost completely clean.

When choosing a diesel vehicle, one with a particle filter is recommended, as it reduces the amount of harmful particles with more than 95 per cent.” (Bilprovningen)

These filters have a long lifespan and can remove up to 90 per cent of the particles, including the smallest ones.” (BIL Sweden)

Different cars reduce the particle emissions to different extents. Modern cars sometimes reach 99.9%. We can use the figure 95%:

1. 100% - 95% = 5% that is still emitted with a particle filter.
2. The emissions with a particle filter is 5% and without one they are 100%. How many times more particles are emitted by an engine without a particle filter? 100% / 5% = 20 times as many particles. Convert 20 times into a percentage: 20 * 100 = 2,000%

2,000% is more than 1,000%, which makes this percentage the correct alternative. If you do the calculation based on a 90% decrease instead of 95%, the answer will be exactly 1,000 %.

It is perhaps easier to say that the emissions are reduced by 90–99.9%. However, there are indications to suggest that the Swedish Transport Agency will not always use the easiest and most common route in the tests, so turning the question around in this way gives you good practice for the actual test.

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