Turbodiesel

Turbodiesel refers to any diesel engine with a turbocharger. Turbocharging is the norm rather than the exception in modern car and truck diesel engines.This type of engine was first introduced in a production car in May 1978 in the Mercedes-Benz 300SD (series W116, engine OM617.950), only produced for the United States. In Europe, its first application was in the Peugeot 604 in early 1979 (model year 1978).

Characteristics

These improvements in power, fuel economy and Noise, Vibration, and Harshness in both small- and large-capacity turbodiesels over the last decade have spurred their widespread adoption in certain markets, notably in Europe where they (as of 2006) make up over 50% of new car registrations. [http://www.economist.com/business/displaystory.cfm?story_id=8082092] Turbodiesels are generally considered more flexible for automotive uses than naturally-aspirated diesels, which have strong low-speed torque outputs but lack power at higher speeds. Turbodiesels can be designed to have a more acceptable spread of both power and torque over their speed range or, if being built for commercial use, can be designed to improve either torque or power at a given speed depending on the exact use.

Turbochargers are in many ways more suited to operation in diesel engines. The smaller speed range that Diesel engines work in (between 1000 and 5000 rpm for a private car, and as little as 1000-2500 rpm for a larger unit in a commercial vehicle) mean that the turbocharger has to change speed less, reducing turbo lag and improving efficiency. Diesel engines do not require dump valves (see the turbocharger article for more information). Perhaps most significantly, the diesel engine is immune from detonation because the fuel is not injected until the moment of combustion, so the compression ratio does not have to be reduced, or other anti-detonation measures taken, as would be necessary for a turbocharged spark-ignition engine.The turbodiesel engine can also help with the amount of torque it can give out. Commonly used in trucks, it helps improve the towing capacity of a truck, as well as fuel economy.

Turbochargers v. Superchargers for Diesel Engines

A turbocharger is generally more desirable than a supercharger unless outright power is required. Turbochargers offer increased power without the same decrease in fuel economy. In both a turbo- and a supercharged engine, power is increased by providing air under pressure to the engine's cylinders. This allows an increased amount of fuel to be burnt, producing more power. However, this inevitably increases fuel consumption. A supercharger is driven directly from the engine and thus its boost output is directly related to engine speed. A turbocharger is more directly controlled by the pressure of the exhaust gases which, as well as increasing with engine speed, also vary significantly with engine load.

When a diesel engine is put under a load there is greater resistance to the expansion of combustion gases in the cylinder. This increases combustion pressure and temperature which, in turn, increases the pressure and temperature of the exhaust gases. A turbodiesel engine under a heavy load will thus drive its turbocharger at a greater speed than if the same engine is run at the same RPM under little or no load.

This has the effect that a turbocharger delivers boost, thus increasing power (and fuel consumption) only when such a power increase is demanded by putting the engine under a heavy load. A turbodiesel-powered vehicle accelerating from rest, for example, will put its engine under a heavy load, thus causing high boost pressures to be delivered by the turbocharger. This is detected by the fuel injection system which delivers more fuel to provide more power. Once the vehicle reaches a constant speed and constant engine RPM load decreases significantly, the pressure of the exhaust gases through the turbo drop, boost and fuel delivery decrease, thus lowering fuel consumption to near the same levels as a naturally-aspirated diesel engine. If, say, the vehicle starts climbing a gradient, the engine load increases and the turbocharger and fuel system provide more power. Extra fuel is delivered only when needed.

A supercharger delivers near-constant boost pressures, and so fuel consumption suffers. Superchargers have the advantage of having no boost threshold (an RPM level below which a turbocharger does not operate effectively) and almost no lag. Superchargers only need to be connected to the engine's intake system, thus making installation easier and reducing to some extent the increase in internal temperatures that occurs with turbocharging.

Even in engines operating under a constant load (such as electrical generators), turbochargers have advantages over superchargers. The main advantage is that a turbocharger does not "rob" power from the engine to the same extent that a supercharger does. A supercharger takes power directly from the engine's crankshaft to drive it- large units can draw up to 10% of the engine's total power when at full boost, although of course, they provide a power increase much greater than this. Turbochargers are driven by the engine's exhaust gases. A relatively smaller power loss is caused by the turbocharger's turbine restricting the flow of exhaust gases and increasing back-pressure. In a gasoline engine this power-loss is much more pronounced. It is commonly referred to as turbo-lag and is experienced at lower engine speeds. However, since these speeds are where a diesel is most efficient, the turbo spools (spins) very quickly and lag is almost non-existent. The diesel's torque output is increased and a broader range of engine speeds can be used.

Turbodiesels in the United States

During the 1990s, turbodiesel engines were mainly used in the United States for light trucks. An example is the Ford Power Stroke engine series, mounted on Ford F-Series Super Duty pickup trucks, the E-series vans and the Excursion sport utility vehicles.

As diesel sold in the United States had a high rate of sulphur, modern engines developed in Europe would have emissions problems so very few sedans, station wagons, and hatchbacks were sold with diesel engines.

After ultra low sulphur diesel was introduced in the United States in October, 2006 automakers began to develop turbodiesels which could take advantage of it to reduce emissions. Manufacturers like Volkswagen have been releasing cars with four and six-cylinder turbodiesels.

Mercedes reintroduced turbodiesel technology with the 2005 E320 CDI. Boasting a 0-100 km/h (0-62 mph) time of 6.6 s and fuel consumption of 37 mpg, the full year production estimate of 3000 CDIs were ordered within five months. Even more recently, however, Mercedes introduced the BlueTec diesel engine. Offering a 3.0 liter V6, as well as urea injection and particulate filters, the E320 BlueTec has been named the cleanest turbodiesel offered in the United Statesfact|date=June 2008.

ee also

*Common rail
*Diesel engine
*Injection pump
*Intercooler
*Turbocharger
*Variable geometry turbocharger