or, if you want a constant acceleration, you could have a second variable to represent a rate of acceleration and use that. if you have taken physics(particularly kinematics and gravity), then this next bit may help spring a few ideas. if not, it may still help a little bit
in physics, velocity is calculated by a certain equation, velocitycurrent=velocityprior + acceleration*time.
for example, gravitational acceleration is 9.8 meters/second^2. so say at one second a ball is dropping at 10meters a second. one second later, the ball has accelerated 9.8 meters a second, to produce a velocitycurrent=velocityprior + accelration*time, which would be 10 + 9.8*1, or turn the velocity into 19.8 meters a second.
since fundamentally adding to the x coordinate or y coordinate by a certain amount is the same as a velocity( say if you add 1 each cycle to the coordinate, your "velocity" left or right would be 1 pixel moved each program cycle.), by constantly increasing the value added by a constant amount, that would produce an acceleration effect. say first i have
int coordx=0;(represents x coord)
int spdinc=0;(represents a speed increase)
at first maybe i want to add one, simple- coordx=coordx+1+spdinc;
since spdinc is just 0, nothing changed because of it. but to have it change next time, i should have
so next it would be again coordx=coordx+spdinc+1; but now that spdinc has changed to 1, now it has become coordx+2 effectively, which caused the "velocity"to "increase".
by constantly adding a "1" or whatever you want the second value to change by, it will mimic the example of real life of how gravity's acceleration is cumulative.
the reason this (or something incredibly similar) works is that there is not hardcoding in exactly how much the ball accelerates, the math to do it will be done for you through this
Hopefully this will help you out onto the right track.