Traction-engines are usually steam locomotives dragging carriages behind them on ordinary roads; but the term may be here extended so as to include the steam-carriages for conveying passengers—attempted as early as the end of the 18th century (see below)—and the newer types of motor-cars.
Auto-cars or Motor-cars (Fr. voitures automobiles) of various patterns attracted much notice in France in 1894-97. Some of them were propelled by electricity, with storage batteries (see DYNAMO-ELECTRIC MACHINES, ELECTRIC RAILWAY), but the majority by some form of oil-engine (see GAS-ENGINE, p. 109). In Britain various locomotive acts, due to fear of accidents through the frightening of horses, had been imposed from 1861 to 1879, and seriously impeded progress in the use of road locomotives. Thus every such vehicle, besides paying a license of £10, required to have two persons in charge of it, and a man to go in front with a red flag; the speed even on country roads must not exceed 4 miles an hour, &c. Since November 1896, vehicles weighing, when not laden, less than 3 tons are exempted from these restrictions. Those weighing less than 1½ ton may go 12 miles an hour; from 1½ to 2 tons, 8 miles; over 2 tons, 5 miles. They must be able to back, must have a bell, and must not emit smoke or visible vapour. Generally the carriage resembles a horse-drawn carriage, is guided by a handle-bar, and has levers to vary the speed. In 1900 a 1000 mile trial run from London to Edinburgh and back occupied 11 days actual travelling; and 20 cars successfully completed the trial.
Steam-carriages.—It was to the conveyance of passengers by steam that all the plans of early inventors were devoted, and at the present day interest in them is chiefly historical. The main idea in all cases was to fit an engine and boiler to an ordinary carriage or coach. The boiler, usually carried at the back, was of some form possessing strength with little weight, and capable of rapidly raising steam; the engine also, either at the back or under the coach, usually worked a cranked axle, and the main axle carrying the two driving-wheels was driven from this by chain gearing; most of them ran on four wheels, though some had a fifth wheel as a steerer. Cugnot, a Frenchman, made a steam-carriage in 1769; it ran at 2½ miles per hour, and carried four persons; a capsiz in a street in Paris led to the imprisonment of the inventor, and put a stop to further trials. Murdock in 1782, Watt in 1784, Symington in 1786, and others made models, but the first one which actually ran in England was made by Trevithick and Vivian in 1803. This attained a speed of 8 or 9 miles per hour in the streets of London; but want of encouragement and difficulties produced by bad roads compelled Trevithick to turn his inventive faculties into other fields of work. Improvements in the main roads led to a great revival of interest about 1830, and during the years 1827-34 numerous more or less successful steam-carriages were built; one made by Sir Goldworthy Gurney ran for three months in 1831 with passengers between Cheltenham and Gloucester, while Hancock's steam-omnibuses (carrying 14 to 16 passengers) ran pretty constantly during the years 1833-36 in London, and attained often a speed of 10 or 12 miles per hour; some of his coaches ran long journeys, such as from London to Brighton; he was in fact the most successful of all these inventors. Scott-Russell also constructed and ran in 1834 six steam-coaches between Glasgow and Paisley. However, the great opposition placed in their way by road trustees, and the gradual spread of railways, led to the abandonment of all these schemes, and practically nothing further has been done (see TRAMWAYS). In 1891, however, Serpollet patented an ingenious and successful steam-carriage with a flattened coil boiler; superheated steam is used, and the engine is very economical. But the majority of the various patterns of motor-cars for roadways made in the decade 1890-99 used electricity or gas-engines rather than steam.
Traction-engines for heavy loads.—The main difficulty with which builders of these engines have to cope has been the driving-wheels, the tires of which must be able to resist the great wear and tear and violent shocks caused by passing over bad roads or uneven ground, and must at the same time possess considerable adhesive powers, since the resistance to motion even on good macadamised roads is more than eight times as great as it is on rails. Boydell, who made the first successful engine in 1856, used a series of flat segments of rails, so jointed round the circumference of the driver that the engine ran, as it were, on an endless tramway; it was an extraordinarily noisy, clattering, unmechanical contrivance, still it gave great adhesion, and enabled his engines to drag heavy loads over exceptionally soft ground. A little later Bray brought out a wheel with rigid tires, fitted with short strong spikes projecting from the rim (these could be withdrawn when not wanted); this, again, had considerable adhesion, but was very destructive to the roads. R. W. Thomson of Edinburgh, in his so-called 'road-steamers,' adopted flexible india-rubber tires; the driving-wheels were light wrought-iron drums about 15 inches wide, with a flange at the sides; round the drum was stretched an india-rubber band 5 inches thick and 12 inches wide, protected on the outside with a flexible sheath of thin steel plates. This tire had the great advantage that it formed a perfect spring for the engine, and, flattening under the pressure, it gave a broad tread, and so increased enormously the adhesion; on paved roads with a gradient it had much better hauling power than any engine with rigid tires. The objection to the use of such tires is their heavy cost (those of the 'Ravee,' a road-steamer for service in India, cost £241), and the expense of renewals; they were, however, very successful, being employed in Glasgow and other towns hauling heavy castings, boilers, &c. from the workshops to the wharves. All these types and many others are now practically obsolete. The modern engine usually has rigid wheels, the rims are either cast-iron or wrought-iron, with diagonal wrought-iron cross strips riveted to them. Another kind of wheel, a good deal used, has rims of cast-iron with cells about 6 inches in the side, in which are placed hardwood blocks, projecting a little beyond the rim (an india-rubber pad between the block and the bottom of the cell acting as a spring); this gives a broad, flat tread, good adhesion, and does not damage the roads. The traction-engines now in use run on four wheels, the hind pair being drivers, the front pair steerers; the boiler is of the locomotive type, working usually at about 130 lb. steam pressure; the engine and attached parts are carried on the top of the boiler, cylinder at forward end, crank-shaft at firebox end. Motion goes from the crank-shaft to the driving-axle through steel spur-wheel reducing gear carried by other shafts. A differential compensating bevel gear is usually fitted, one driving-wheel being then loose on its axle; this enables the engine to turn corners without straining the axles. Springs are also now usually fitted, though their use somewhat complicates the method of supporting the gearing between the crank-shaft and driving-wheel axle; these, however, greatly reduce the shocks in passing over bad roads. Most of the engines now made have a winding-drum, which enables steep gradients to be overcome; the engine itself proceeds to the top of the incline, then a steel rope attached to the wagon carrying the load is gradually wound up on the drum, thus hauling up the load. To obtain increased power, the engines sometimes have two cylinders, and are also made compound; a stock of coal is carried in a bunker behind the foot-plate, and water in a tank under it. These engines are now most extensively used, and are much cheaper than horse traction; in one case the cost has been given as 2d. per ton per mile, including depreciation and interest on capital; where the roads are kept in good order they do no damage. The great obstacles to their more extended use were the legal restrictions in force till 1896. All locomotives for road use are liable to a carriage duty of from 15s. to £2, 2s., and after January 1897 to an additional duty of from £2, 2s. to £3, 3s., according as they weigh, not laden, less or more than 2 tons; the special license is not exacted for light locomotives.
Agricultural locomotives are essentially portable engines, fitted with various methods of self-propulsion—in some cases by pitch-chains, more usually by spur-gearing. They are much lighter in make, though otherwise very similar to traction-engines; they are employed mainly in drawing loaded wagons, thrashing-machines, &c. from farm to farm; by disconnecting the propelling gear and blocking the wheels, they can be used as stationary engines to drive by belting off the flywheel, with which they are always fitted, thrashing-machines, fans, and other farm plant. Fowler's steam-ploughs are locomotive; they are able both to pay out and wind up the steel rope moving the plough to and fro, and also to shift themselves along the field. Traction-engines are now made by all the chief firms of agricultural implement-makers (see Vol. VIII. p. 245); to the late Mr T. Aveling of Rochester, much of the present success of steam road-traction is due. In making macadamised roads, the rolling of the broken metal is now done by heavy steam-propelled rollers very similar in design to traction-engines.