why vowel a is used before word eucalyptus 

'E' is a vowel but it does not always produce a vowel sound. 

For example: ( try to pronounce elephant and eucalyptus in Hindi)

• An elephant(sound produced by 'e' here is a vowel sound)
• A eucalyptus( sound produced by 'e' here is a consonant sound)

In cases where a consonant sound is produced by a vowel, we use 'a' instead of 'an'