@all
Sorry to crash in again but here goes...
I'd like to make myself clear on a couple of points:
1. Injection pressures: When I talk of higher injection pressures, what the manufacturers are trying to attain is better atomisation of fuel. The higher the pressure, the smaller the droplets of fuel generated, the smaller the droplets, the faster they vaporise and burn. Thus one can decrease the Air / Fuel ratio futher without producing smoke as compared to a lower - pressure injection system.
2. The ability to use more fuel without making more smoke does NOT mean that you will be using more fuel all the time. Here, I'm touching a fuzzy area.
Basically car manufacturers are trying to DECREASE engine sizes all across the board, why? Because smaller engines have lower frictional losses. And these losses are especially significant at low load driving. If you can make the same power with a smaller engine, you will use less fuel in low load conditions.
3. Engines prepared for competition, or special duty (truck pulls, diesel drag races, high powered military marine engines etc.) use very low compression ratios of between 8.5 and 12.5 to 1. As can be expected, starting is a problem, they use ether vapours to start the engine and then when the engine starts up, it keeps running. The Turbo pressures are humongous however, with twin series turbos you are looking at upto 6 bar of boost. They do this to limit maximum cylinder pressures to between 150 and 200 bar. Fuel consumption for these engines is terrible a good turbodiesel can touch .250 kg/kW Hr of diesel consumption, whereas these engines do no better than .50 kg/kW Hr.
4. As for the TDI vs. common rail battle, well keep in mind that in the TDI system (VW / AUDI), each cylinder has its own injector pump and electromagnetically operated injector mounted in the cylinder head. Since there are no pipes to carry this pressure, they can produce higher pressures than comparative common rail systems. However, being phased to the camshaft, they produce pressure only when the cylinder is about to fire. Producing pressure for any longer will only waste power, as the fuel not injected has to be bled off. Also, unlike in a common rail fuel system, you cannot decide on fuel pressure you want, you get what the system gives you (it changes with respect to the rpm). In a common rail system, a separate pump and pressure regulator keep pressure maintained in a 'rail' shared by all injectors and pressure can be varied by the regulator.
Thanks for reading me out guys!
If anyone's REALLY interested, I'll try to scrape up some coursework material a Bosch Engineer gave us students during our course on Diesel engines. It's mostly promotional stuff, but the pictures and the theory is fairly interesting. The ones I've got are in French