AER have made no secret of the fact that they are unwilling to reveal the cubic capacity of the engine, they have (indirectly) declared their unwillingness publicly from the outset (
link to DSC article from March 2014). Trackside, it didn't sound like a small capacity engine, but if Trussers has had 2.4l confirmed and is prepared to state that over the airwaves, that's good enough for me.