MIDI dynamics

[Hilary Snaden]

I have been trying to get LilyPond to generate MIDI versions of some 
piano music. The music script renders nicely, and there are no errors or 
warnings, but the MIDI file plays with no dynamics at all. What am I 
doing wrong? (LilyPond 2.18.2.)

\score {
  \unfoldRepeats
  \new Staff
  <<
    { \tempo 4=86 }
    \new Voice = "v1" {
      \set Staff.midiInstrument = "Acoustic grand"
      \set Staff.midiMinimumVolume = #0.2
      \set Staff.midiMaximumVolume = #0.8
      \upper
    }
    \new Voice = "v2" {
      \set Staff.midiInstrument = "Acoustic grand"
      \set Staff.midiMinimumVolume = #0.2
      \set Staff.midiMaximumVolume = #0.8
      \lower
    }
    \new Dynamics \pianodynamics
  >>
  \midi { }
}