The term turbo-diesel, also written as turbodiesel and turbo diesel, refers to any diesel engine equipped with a turbocharger. As with other engine types, turbocharging a diesel engine can significantly increase its efficiency and power output, especially when used in combination with an intercooler. Turbocharging of diesel engines began in the 1920s with large marine and stationary engines. Trucks became available with turbo-diesel engines in the mid-1950s, followed by passenger cars in the late 1970s. Since the 1990s, the compression ratio of turbo-diesel engines has been dropping. Principle Diesel engines are typically well suited to turbocharging due to two factors: A "lean" air–fuel ratio, caused when the turbocharger supplies excess air into the engine, is not a problem for diesel engines, because the torque control is dependent on the mass of fuel that is injected into the combustion chamber (i.e. air-fuel ratio), rather than the quantity of the air-fuel mixture. The additional quantity of air in the cylinder due to turbocharging effectively increases the compression ratio, which, in a gasoline engine, can cause pre-ignition and high exhaust gas temperatures. However, in a diesel engine, fuel is not present in the combustion chamber during the compression stroke, since it is only added to the combustion chamber shortly before the piston reaches top dead center. Thus, no pre-ignition can occur. As per turbocharged petrol engines, an intercooler can be used to cool the intake air and therefore increase its density. History The turbocharger was invented in the early 20th century by Alfred Büchi, a Swiss engineer and the head of diesel engine research at the Gebrüder Sulzer engine manufacturing company. The turbocharger was originally intended to be used on diesel engines, since Büchi's patent of 1905 noted the efficiency improvements that a turbocharger could bring to diesel engines. However, the first production turbocharged engines to be manufactured did not occur until 1925, 10-cylinder turbo-diesel marine engines used by the German passenger ships Preussen and . The turbocharger increased the power output from to . In 1925, Büchi invented sequential turbocharging, which according to Helmut Pucher (2012) marks the beginning of modern turbocharging technology. By the late 1920s, several manufacturers were producing large turbo-diesels for marine and stationary use, such as Sulzer Bros., MAN, Daimler-Benz, and Paxman. Subsequent improvements in technology made feasible the use of turbochargers on smaller engines that ran at higher engine speeds, so turbo-diesel locomotive engines began appearing in