BMW Diesel

A DI (compression-ignited) diesel engine could operate at 18,000 rpm in theory, but in reality it's not possible due to several factors. Most importantly, the compression ignited combustion process, while very efficient, is also very sluggish. Even small bore diesel racing engines do not operate efficiently above about 5,000 rpm. The engine is physically capable of revving higher, but it will make less power at higher revs, since the rapid piston motion will cut short the combustion process before it is complete. The un-combusted fuel simply goes out the exhaust pipe as black smoke, and doesn't contribute to power output.

Another limiting factor with DI diesels is the time available for the fuel injection event. Even a 5,000 rpm diesel must complete it's fuel injection cycle within a few tenths of a millisecond (normally about 20 degrees of crank rotation max). A diesel racing engine operating at 3 or 4 times the speed (18,000 rpm vs. 5,000 rpm) of a normal diesel engine would not have a fuel injector capable of injecting an adequate fuel mass within the time available.

Diesel engines are more efficient than gasoline engines primarily due to the higher peak cycle pressures possible with the diesel (constant pressure) combustion cycle. The only limit to the max output possible with diesels is due to the mechanical and heat loads the engine is capable of sustaining. On the other hand, gasoline engines are limited by their resistance to combustion detonation or knock.

The fastest revving diesel I've heard of is the Renault DOHC Diesel at 6.500 rpm. There were rumours about the Audi R10 engine achieving 8.000 rpm, but it works in the 3.000 to 5.000 rpm range.

A key feature of these new diesel engines from BMW (and others like Fiat) is a common-rail fuel system with piezoelectric injectors rated at 29,000 psi (2,000 bar). The BMW 2007 model was capable of up to five injection events per combustion event. Fiat is running a new 2010 engine with capability up to 8 injections per event.

Needle lift time on these new injectors can be as fast as 100 microseconds depending on how they are driven. That is much faster than even the best solenoid systems by a factor of perhaps 4X or more. They are really limited by natural frequency of the spring/mass.

see: http://wardsauto.com/ar/bmw_diesel_us/ and http://fiat500usa.blogspot.com/2009/10/fiats-multijet-ii-engine.html

I belive that piezoelectic injectors operate 200 times faster than the solenoid operated injectors. VTG also plays an important role in the new generation common rail systems.

I wonder why/how model diesel engines achieve so easily their high revolutions (10-20,000 RPM)? Whereas car/truck diesel engines are restricted generally to under 5,000 RPM.

I realize that the fuel is slightly different and they do not use high pressure injection systems, but otherwise they are relatively similar in being compression ignition......

I always thought that the problem for large diesel engines was more to do with the fact that it is quite difficult to vary the timing of the injection with changing revs, similar in the way that a spark is adjusted with regard to the revs and load on a petrol engine.....but with common rail, surely this is not a problem anymore.......

This could be an interesting blog!!

Truck engines run at comparatively low rpm. While this makes them heavy for their output, they can be run at near full load for lengthy periods without unduly affecting reliability.

The high revving car diesels cannot run at full load for long (neither can a car petrol engine). This is OK because cars are run well below max power for almost all their life, using near max power for comparatively brief periods of acceleration.

For the truck, reliability is paramount so we see engines of large capacity, doing 2500 or 3000 rpm. These things last a million miles or so, whereas a car engine will need a lot of maintenance and repair to achieve that.

One of the reasons could be that revolutions in diesel engines were restricted by the delay period...........this is the time taken from when fuel is injected into the cylinder to when it actually starts to burn. You then had a period of uncontrolled combustion as the fuel that was injected into the cylinder during the delay period ignites spontaneously giving a very rapid rise in pressure......the rest of the fuel burns as it is injected.

Hopefully this diagram explains what happens during the compression and power strokes. including injection.

However in the latest common rail system with the piezoelectric injection the fuel injection is split up into seven smaller shots over a greater number of degrees of crankshaft rotation.....at least one being on the exhaust stroke. A smaller quantity of fuel is delivered over those seven shots in the common rail system than the fuel that was delivered by the old mechanical injection.

The common rail, piezoelectric fuel system, significantly reduces the delay period allowing faster engine speeds to be attained.

A lot of the low speeds that are prevalent in truck engines is due to the very heavy weight of the reciprocating components. When a piston is as big as a coffee can, with con rods to match, it becomes difficult to reverse directions 5,000 times per min.

Audi has been racing a diesel very successfully in the American Le-Mans series for the past 5-6 years. Jaguar may know more about that engine, I believe he follows that circuit.

I thank you all for your answers. I'm not an expert on engines (or anything else for that matter). This was the right place to come. Have a great Christmas and New Year.

Hey , where are you going so soon? Hang in here with the rest of us. I still have to make some stupid comment about adding an HHO generator to it.

Have a safe and healthy new year if you do decide to bail on us.