Portable Energy and Propulsion Technologies
To provide locomotion for transport vehicles involves the availability of two inter-dependent components: (1) A portable fuel (stored chemical energy) or a portable energy source in the form of a battery (stored electrical or flywheel energy); and (2) an engine or motor that consumes stored portable energy and converts it into mechanical motion. Various portable fuels and energy storage devices have been developed in the last century, the champion fuel being petrol because of its low cost and availability, and the champion battery being the lead-acid device because of its ruggedness and recharge ability. Engines can be divided into three categories: (I) internal combustion engines (ICEs) fed by portable chemical fuels that can react with oxygen in the air, (II) motors driven by the electricity from chemical or mechanical storage batteries, and (III) fuel-cell engines (FCEs) fed by chemical fuels that react with atmospheric oxygen. ICEs in category (I) can convert the heat of fuel combustion into mechanical motion via piston action with an efficiency of 30–40%, while motors in category (II) can convert electricity from a storage battery via induction into mechanical motion of the wheels of an automobile via a gear-train with an efficiency of 85–95%. FCEs in category (III) utilize an electrochemical reaction between atmospheric oxygen and a fuel that takes place on special electrodes with an efficiency between 45% and 85%. The generated electricity then drives a motor as in (II). The quoted efficiencies are ratios of delivered energy of mechanical motion divided by energy extracted from a fuel or battery. Although FCEs are more efficient than ICEs, because of problems discussed below, the ICE has so far won out in the automotive field. It is presently the most developed device for propelling cars. Motors driven by batteries alone have also lost out against ICEs because of driving range limitations discussed below. In what follows, we first review portable fuels and energy holding batteries, and then discuss ICEs and FCEs.