However efficient the steam engine gets, it has an inescapable problem. The fuel is burnt in a boiler to produce steam, and that steam has to be introduced to a cylinder. These processes involve unavoidable heat losses. Heat transfer from burning fuel to boiler water can never be complete. Energy efficiency is also lost in converting water to steam. In addition, the steam inevitably cools when piped into a cylinder.

The oil engine: ignition in the cylinder

A way to obviate the losses inherent in the steam engine is to burn fuel right in the cylinder, where the energy it produces directly moves the piston. This is the principle of the internal combustion engine. The liquid fuel is first vaporised and then introduced into the cylinder. It is then ignited, either by an electrical spark in a petrol engine, or by compressing it strongly in an oil engine (often referred to as a diesel engine after its inventor, Rudolf Diesel). Although petrol engines are probably more familiar from their use in cars, the oil engine is the variety used in ships. This is largely because it can burn cheaper, low quality oil.

Oil engines have other advantages over steam. They burn liquid fuel which, unlike coal, can readily be pumped and thus can be stored in otherwise inaccessible spaces on the ship, such as the double bottom. This gives more quality space for carrying cargo. A further advantage flows from the liquid nature of fuel, in that firemen are no longer needed, reducing wage bills.

Reluctant owners

British shipowners were, on the whole, slow to embrace the oil engine, despite its promise of reduced running costs. There were many reasons given: the UK had plenty of coal but (then) no oil. Steam technology was tried and trusted. Plenty of engineers were available trained on steam, few on diesels. There were some honourable exceptions amongst British shipowners. For instance, the Royal Mail Group and the closely-associated shipbuilders Harland and Wolff of Belfast built many fine ships with oil engines. Other countries, notably Germany (which profited from its experience in developing submarine engines), Holland and Denmark were much more receptive to the oil engine. Some think that the reluctance of many British owners to build motor ships (a term for oil-engined vessels) was one factor in the decline of the British merchant navy.