in this arrangement the cylindrical spool 100 is held central by respective resilient means in the form of springs 102 104 as the back pressure of the nozzles 86 88 differs when the flapper 84 is in a non central position the flapper itself being electro magnetically driven by coils 106 108 the spool 100 is pushed from one side to the other against the centering force of the springs 102 104 by the pressure imbalance alternatively instead of the use of the centering springs 102 104 at each end of the spool 100 a centering feedback spring connected to the flapper may be used as will be appreciated there are various advantages and disadvantages of the different valve arrangements and combinations described in fig4 a 4 b and 5 which will usually dictate for particular applications which configurations will be most suitable referring now to the further embodiment illustrated in fig6 illustrated is a piston adapted to include exhaust valves in its head the exhaust valves being in the form of reed valves 33 associated with exhaust ports 35 in this form the piston mounted exhaust valve operating sequence is preferably as follows 1 as the piston travels downwards under the force of expanding gas above it as shown in fig7 a the pressure will gradually drop until the pressure differential above the exhaust port pressure is not sufficient to hold the reed valves closed at this point the reed valves will open which at full load operation will occur just before bdc it will be noted that opening of these valves is assured by exhaust ports 37 in the cylinder wall opening or becoming accessible just before bdc.