A car accelerates from rest to $u \mathrm {~m} \mathrm {~s} ^ { - 1 }$. The energy spent in this process is $E \mathrm {~J}$. The energy required to accelerate the car from $u \mathrm {~m} \mathrm {~s} ^ { - 1 }$ to $2u \mathrm {~m} \mathrm {~s} ^ { - 1 }$ is $nE \mathrm {~J}$. The value of $n$ is $\_\_\_\_$.