The two 'Great Engines' at Kempton Park, numbers 6 and 7, are of the inverted vertical triple expansion type. The three cylinders, in which the steam is expanded in succession before passing to a condenser, are arranged in line directly over the crankshaft, with the plunger pumps below it. The layout originated in the USA where Edward P. Allis introduced it to a waterworks in 1886, though ships' engines on a similar plan had been used many years earlier.

The Kempton examples are thought to be the biggest ever built in the UK and date from 1926-1929. Efficiency and reliability of engines of this type were hard to beat though the initial cost, including the tall engine house, was high. At one time about a third of all waterworks pumping engines in the UK were of this type though the Kempton engines were the last working survivors when they stopped in 1980.

Mechanically the arrangement is attractive by virtue of the 120° crank setting, which not only helps to balance the moving parts but provides a steady input of power from the engine and a steady discharge of water into the pressure main. There are never less than two steam cylinders driving and one plunger pumping at any time.

Each engine stands 62 ft tall from basement to top of the valve casings and weighs over 800 tons. They were supplied with steam by a battery of water-tube boilers at a pressure of 200 psi, at 150°F of superheat.

The engines were designed for 24-hour, 7 days-a-week operation, though in practice they were run overlapped (9 months on; 3 months off). Coal consumed amounted to a maximum of 13 tons per day, equal to the best electricity generating stations of the day.

This page will display a very complex animation of the engine and pump operation.