I wrote about a few news articles I'd read and how this was applicable. For instance, air travel was producing too many emissions compared to rail, so I found some data on it and wrote about how this could be reduced. I mentioned what I wanted to achieve (reduce the emissions by improving aerodynamics and engine efficiency) and how this course could get me there (studying those specific modules, doing those certain projects).
You could try and do some work experience or do something practical. How about building an aeroplane from different materials (paper, card, wood, etc) and investigate how different aerofoil shapes affect the performance of it? Is there a local rocket club you could get involved in? You could make a rocket, launch it and develop it.
For me, the most important thing was getting across that I understood how the principles of engineering worked. Get a brief, develop, construct (hands-on and construction skills will be very important too), test, redevelop, test, etc. How are you going to benefit them? What can you bring to the university and how is it applicable?
Note: I did not read a single book on engineering nor did any work experience. I just mentioned the stuff I'd done (product design technology, electronics, physics, maths, etc) and how that was applicable to the course. I wrote about simple research and where I wanted my career to take me and how this course was right for that. You don't need to have travelled to Mars and back to get onto the course, but there are plenty of opportunities to develop skills as you get your application underway.