Today turbochargers are the norm in diesel engines, rather than the exception. It's largely thanks to superchargers that diesels have become so popular on European roads (in 2006, more than 50% of new cars registered in Europe were diesels). Turbochargers have made diesels more powerful, more economical, more fun, more reliable and more efficient.
The first production diesel to make use of a turbocharger was a Mercedes-Benz 300SD - the first real S-Class - in May 1978. It was only available in America though, and in Europe the first turbodiesel was the Peugeot 604, released in 1979. Although the 604 was roundly considered a failure, it proved that turbodiesels were viable and kick-started the turbo diesel revolution in Europe; a revolution which still influences car manufacture today and which would lead to the manufacture of the stunning diesels which now dominate our roads.
In naturally aspirated diesel cars (an engine without a turbo is referred to as 'naturally aspirated'), there's a lot of torque available at low revs which allows for rapid acceleration, but the power available power diminishes as the speed increases. A turbocharger allows more power to be available to the engine as the speed increases, which means that there's more torque throughout the revs. Turbochargers are better suited to diesel engines than to petrol engines for this reason. Most diesel cars operate best between 1000 and 5000 rpm where petrol cars rev a lot higher, and these low rev ranges mean that a turbo has to change speeds less frequently, which in turn means that turbo-lag (which we'll come to in due course) is reduced.
How Turbochargers Work
Here's a brief outline of the way a turbine works:
- Diesel fuel and air are combusted in the cylinder.
- The exhaust gases are pushed through a turbine, which is mounted on a shaft.
- The shaft spins an air compressor, which draws in air.
- The air is compressed and blown into the cylinder.
It's not a perfectly efficient system, but it's not bad. It's certainly more efficient than making the existing cylinders larger, or adding new ones. Superchargers work a little differently, more like this:
- Fuel and air are combusted in the cylinder.
- The cylinder drives the crankshaft.
- The crankshaft spins an air compressor, which draws in air.
- The air is compressed and blown into the cylinder.
These processes are very simplified here, and they obviously don't take into account peripheral functions like inter-cooling, but these are the fundamentals.
What a Turbocharger Does
A turbocharger in a diesel engine can produce more power without requiring the same increase in fuel that a naturally aspirated engine would require to achieve the same increase in power. That's a desirable property, especially as fuel prices boom. What's more, the increase in weight from adding a turbocharger is negligible in relation to the power increase; to get the same power boost out of a naturally aspirated engine you'd have to add several new cylinders, which would eat far more fuel and add significantly to the overall weight.
The power is increased by forcing pressurised air into the cylinder, thereby allowing more fuel to be burned. Since diesel engines ignite fuel via compression, rather than with a spark plug like a petrol engine, diesel engines are naturally better equipped to handle the stresses which come from high pressure air being forced into the cylinders. Diesel engines are much better at harnessing the power the turbocharger has to offer than petrol engines.
The Difference Between Turbos & Superchargers
Turbochargers and supercharges do virtually the same job - they're both fundamentally air compressors. They work a little differently however, and as a result they each offer slightly different performance. They do the same job, but they do it differently. Where a turbocharger is driven by the pressure and heat of the exhaust gases, a supercharger is driven directly by the engine, usually via a belt, and can often consume up to a third of the engine's crankshaft power. This makes them less efficient than turbochargers, and are generally only used in vehicles designed purely to reach very high top speeds (like dragsters) or for very powerful engines (like tractor pulling competition vehicles) - in most other instances (like road cars) you'll probably find that a turbocharger will be used.
Reliability & Maintenance
While it's easy to think that a turbocharger adds a number of components which are simply waiting to go wrong, it's not really like that. It's true that a turbo adds a number of tubes, pipes, connectors and moving parts, and it's true that the turbofan is likely to be made from an exotic material of some sort (ceramic turbines are becoming popular), but modern turbochargers are exceptionally reliable.
There are a couple of things you can do to look after yours though. For example, when you finish your journey, let the engine idle for a couple of minutes before you switch it off entirely. This will allow the turbofan and rotating assembly to spin down instead of coming to an abrupt stop. When you kill the engine, the lubricating oil immediately stops flowing to the turbo, but the turbo doesn't necessarily stop spinning when the engine is turned off. If the fan continues to rotate without fresh oil, it will quickly cook the oil it received when the engine was running, and this will lead to the oil lines becoming blocked. The next time the turbo operates, now without effective lubrication, it will very quickly destroy its bearings and grind to a devastating halt.
Turbo Lag
Turbo lag is the name given to the slightly disappointing sensation when you stamp on the accelerator in a turbocharged car - and nothing happens. There's a pause between depressing the pedal, and the car accelerating (under the influence of the turbo at least; the car will still accelerate slightly under the power of the naturally aspirated function of the engine). Turbo lag is caused when the exhaust fumes which power the turbo don't immediately come to the high pressure necessary to initiate the turbo, and by the turbo overcoming its rotational inertia.
Since superchargers are powered directly by the engine and don't rely on exhaust fumes, they don't suffer from turbo lag. But modern turbocharged diesel producers have found several ingenious solutions to the problem of turbo lag. For example, major manufacturers like Toyota and Audi often use two, smaller turbos instead of one larger turbo in their diesel engines. One turbo is to be active across the entire rev range, and the other turbo is designed to activate only at higher revs. In this way, there's only very rarely any lag, and there is power constantly available to the engine.