In Great Britain petroleum spirit or a mixture of petroleum and benzol are the principal fuels. Paraffin is not in much favour for use on road vehicles but is much used for agricultural tractors. From 1914 to 1918 many vehicles were run on coal-gas, which was stored in fabric envelopes at low pressure or in steel cylinders under high pressure. An alternative was to generate gas from coke or charcoal in a producer carried on the vehicle. Alcohol benzol-petrol mixtures were also used during the same period, but found little favour. On the continent, however, the use of alcohol and producer gas has continued to find adherents, but so long as petroleum remains available at commercial prices alternative fuels are likely to receive little encouragement. The Diesel or heavy-oil type of engine makes steady progress in commercial use.
5 and 7. Lorries used in such services as road building sometimes are equipped with gear-boxes having two reverse speeds, a low reverse for safe manoeuvring in confined spaces, and a high re verse for use when it is necessary to run the lorry backward a considerable distance. Sideways or clash-engagement gears are most common, and although open to many objections, are re liable and efficient. Constant-mesh transmissions, in which one member of each gear train is free on its shaft and can be coupled thereto by means of a jaw clutch, have come into use recently, and are sometimes provided with synchronizing clutches which facilitate engagement of the different gears. When a shift is made, the synchronizing, cone-type friction clutch engages first, and increases or reduces the speed of the gear with which it is combined until it is substantially equal to that of the shaft on which the gear is mounted; further motion of the shift lever then causes the jaw clutch to engage without clashing. The draw ing herewith shows a section of a five-speed truck transmission with synchronizing clutches on the driving members of the two pairs of first-reduction gears and the driven members of the second-reduction gears. Gear-boxes of the constant-mesh type usually have helically-toothed gears, which are more silent in operation than spur gears. Various types of self-changing gear boxes based on the planetary principle are now in use in Great Britain, though the old shifting type remains popular.
There are two chief methods of driving-axle construction, e.g., the solid-forged and built-up types. In the former the imposed load is borne by a forged-steel axle shaped like a double-handled banjo, the driving and differential gears being mounted in a casing fitting to the banjo portion, whilst the wheels are mounted on the ends and the driving shafts pass through the hollow ends. This type is adopted extensively. The built-up type, in which the load and gearing are carried entirely by cast casings, finds little favour in Great Britain, but is much used by Continental builders. For taxicabs and the lighter lorries the final drive is generally by bevel gearing with a ratio of up to 7 to I, but for other types, and particularly for large passenger vehicles, axles are mostly worm-driven. For heavy-duty lorries, however, the double reduction gear, by bevel and spur gears, continues in favour among some makers. Axle gear ratios vary from 61 to 1 for small vans up to 12 to I for heavy goods vehicles. For large passenger vehicles the ratio ranges between 71 and 81 to 1, according to the class of service. Front axles are invariably of the Ackermann type, consisting of a load-carrying beam with pivoted stub axles on which the wheels are mounted. The stub axles are inter connected with each other, and to the steering gear, by levers and connecting rods. When front-wheel brakes are fitted the beam must be made strong enough to resist the braking torque in addition to the load carried on the axle.