solar day not constant despite axial tilt

[jamrb]

Despite Earth's axial tilt effect on duration on solar day in one year (we all know winter days are shorter than summer's on northern hemisphere), I think there is another reason (I could not find anywhere ).

It is easy to demonstrate that in a pretty simplified model of Earth's circular orbit (with no tilt involved, i.e, axis is othogonal to ecliptic plane) with constant rotation angular speed w and also constant orbital angular speed W, sidereal day just depends (inverselly proportional to) on w (not considering precession either).

But the same way, solar day depends (inverselly proportional to) on (w-W).

Actually, this is the reason moon always shows the same side! Its rotation speed (w)is the same than its orbital (W), so "earthling" day on the Moon is infinite.

Now, due to Kepler's second law, the closer to the perihelion, the faster Earth moves, and viceversa.
This can interpreted as greater value for W when Earth is at perihelion, so corresponding solar days should be longer!
And then when closer to aphelion, they should be shorter.

But they are not, I really digged deep the internet to find out. Always 24 hours. Constant all year long...

What am I doing wrong?