Why is it difficult to operate Carnot Cycle?

As you will know, the Carnot cycle offers the highest possible thermal efficiency for any IC engine operating between two limiting temperatures. As such it is a tempting ideal! Its weakness is seen if the cycle is plotted, to scale, on P-V co-ordinates. This will show that a Carnot engine delivers a tiny power output (m.e.p.) for an acceptable maximum cycle pressure. In other words a Carnot engined car might provide a wonderful m.p.g. but wouldn