GEARING ENGINES FOR ECONOMY

Modern vehicle engines are using up to 10 per cent less fuel than they did a decade ago through more efficient consumption. This has been achieved mainly by matching turbochargers to faster, higher pressure fuel injection so that slower engine speeds give the same output level. Maximum torque speeds are now commonly about 1300rpm instead of 1500rpm in earlier engines. In practice, however, it is difficult for drivers to restrict the engine speed to maximium torque when cruising where wind resistance tyre drag and other parasitic losses often effectively restrict level road speeds. Various devices that have been developed to encourage drivers to change up sooner and to change down later include a dual-rate return spring resisting accelerator movement. The author suggests that simple fuel or governor settings cannot be used to limit engine speed because the power will be adversely affected in all gears. Power-to-weight ratios are important in restricting the amount of gear changing required to improve fuel consumption by keeping an engine in its most economical speed band. (Author/TRRL)