This paper describes a new approach to controlling hydrocarbon (HC) fuel combustion in internal combustion (IC) engines. When minute quantities of an ultrahigh molecular weight polymer are introduced into the fuel charge of an IC engine, the fuel becomes viscoelastic. This results in improved combustion efficiency and reduced knock. The polymer achieves this by modifying three physical properties of gasoline molecules: the variable vapor pressures of the fuel constituents, the size of the fuel droplets, and the size distribution of the fuel droplets, whether carbureted or injected. Controlling these physical properties of gasoline has been the greatest challenge to achieving an optimal air/fuel charge to the spark plug at ignition. The polymer provides this control by greatly reducing the population of submicron-size droplets, while reducing the average droplet size, and narrowing the droplet size distribution. By controlling these variables, the polymer permits a more uniform air/fuel mixture and, thus, more efficient combustion. This, in turn, produces lower overall temperatures, antiknock performance, higher peak pressure, increased torque, greater fuel economy--especially during transients--and a dramatic reduction in harmful emissions. Physical control of the fuel charge is, moreover, more environmentally sound than the chemical solutions to the knock problem that have predominated since the 1920s, when Thomas Midgley discovered the antiknock property of tetraethyl lead .