Diesel rev limit factors 1

[murpia]

What's the limiting factor to rev-ing a diesel engine?
(I'm talking about the modern direct injection common rail fueled 4 valve per cylinder turbocharged designs).
Obviously for a given cubic capacity a diesel requires stronger (heavier) pistons & rods both for the increased compression and to accomodate the combustion chamber (compared to gasoline).
So, is the rev limit usually imposed by structural factors, airflow factors or combustion / emissions factors?
Regards, Ian

 

[TheBlacksmith]

I think ultimately combustion. Unlike gasoline, diesel burn rate is fairly constant regardless of speed/compression. All other factors can be overcome by materials engineering.

 

[SomptingGuy]

And combustion is limited by fuel mixing.

Remember the old days of IDI diesels? They could run faster than DI diesels of their time because of the (Ricardo Comet) prechamber, which boosted mixing (through high rates of swirl). But all that extra swirl led to poorer thermal efficiency than the DI diesels.

These days better fuel mixing is achieved via the massive (and I mean MASSIVE) fuel injection pressures available to common rail systems. These high pressures can be used with tiny holes to create a really fine mist, again giving better mixing.

 

[enginead]

I've measured combustion on a 2.0l DI diesel (fitted with an electroniuc Bosch distributer pump) and within it's preset constraints (max engine speed =4500rpm) the combustion rate scaled as a virtually linear function of engine speed if you removed some of the obvious conflicting factors (e.g. EGR).

I think the biggest constraint is the mechanical strength of the components. The maximum cylinder pressures in a modern diesel can be over 150 bar and the thermal gradients across things like the piston crown are extremely steep. All of which means the components (piston, gudegeon pin, conrod, big ends, crank, block etc..) have to be designed to be very strong and thus end up being heavy. All that mass going up and down, round and round generates a lot of force that increases exponentially as mean piston speed increases.

I've never seen it happen, but I'm told that if a diesel engine's piston rings fail and it starts to run on it's oil there are only two options:

a) Find a rag and stuff it in the airbox to starve it of air.

b) If a) doesn't work or you can't find a rag, leave the room/vehicle, shut the door/bonnet and wait for something to make a sharp exit though the side of the block (usually a rod).

The reason is without control (governer, injection pump) the engine speed will continue increasing until the one of the mechanical limits are breached and a component fails.

I've been wrong before, but certainly in my experience upto 4000rpm combustion showed no signs of slowing down.

Engine_ad

 

[franzh]

In the lab, I have never seen the piston rings fail so quickly as to be undetectable. Usually, the emissions show up first, or a decrease in measured fuel consumption, or the oil level warning sensor, or surging engine speed, blue smoke, and so on. Watching the rack position, or the pulse width on common rail injectors, and comparing the pulse to rpm is a good measure. When the pulse decreases and the rpm increases, there is another fuel source.

Ricardo has a V-10 F-1 type diesel engine, capable of 10,000 rpm. I'd pay a weeks salary to watch that one run! Just thinking of the sound brings my hair on my arm on end!
Sorry for the digress, back to the thread.

Mechanical limits of the physical components is the practical rpm limit. Connecting rod big ends go first, followed by the small end seperating from the piston. Valve float can happen anywhere, as the valve springs are just strong enough to keep the valves in profile with the cam at just over max rpm design, for max follower life. When the valves float, the close tolerances between the valves and pistons becomes inverse, two physical components cannot occupy the same space at the same time.
Been there, done that.
Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[murpia]

EngineAd, could you please expand on your statement that combustion rate scales linearly with rpm?
I'm interpreting this as the PV diagram is effectively following the 'classic' diesel cycle of constant pressure during combustion, and that it's shape is independant of rpm. If so this makes my modelling task much easier.
Or are you refering to something else?
Also, how does that relate to fuel injection duration, is that constant in time or crank angle, and therefore scaling injection pressure with rpm? Or, if injection pressure is constant, duration in time must be constant for equal volume and hence is longer in crank angle terms as rpm increases.
Thanks, Ian

 

[enginead]

murpia,

The combustion rate I referred to was the crank angle duration between 5% and 95% mass fraction burnt (measured using an incylinder pressure transducer). Essentially this crank angle duration stays more or less constant with a change in engine speed (hence speeds up or slows down in time terms). However it can move backwards and forwards in the compression and expansion stroke depending on injection timing and so it's difficult to say that the PV diagram looks the same at 4000rpm as it does at 2000rpm.

What is it you're modelling? There is a very good approximation of diesel heat release described in Heywood (Internal Combustion Engine Fundamentals) using a 2 stage Webe function. This characterises the heat release rate during combustion and has two 'tunable' time constants to best fit the curve. With a little work it's relatively easy to get a good PV comparison with measured data. You can only use this for performance though, as it's zero dimensional you can't get any sensible emissions out of it. If you're intersted in fuel spray development there is an excellent 2D model by Hiroyasu (also described briefly in Heywood) which has enough detail to give trend changes in emissions and fuel spray development characteristics.

Fuel injection duration is not constant in either time or crank angle and varies depending on the required mass to be injected. Injection pressure will also vary depending on what kind of system is used (distributer, common rail, unit injector) and what it's operating condition is. Sorry, it's a bit vague but the pressures seen can vary between 700-3000bar, so the spread is quite considerable.

Enginead

 

[amorrison]

In gasoline engines there is a point when the dominant structural loading changes from combustion chamber pressure forces (low rpm) to mechanical component (piston+)inertia caused by high rpm.

I have often thought this transition point deserves its own name.

Anybody care to become famous?

It would sure be nice to see this point plotted on a power vs. rpm graph for various engines.

Does this information already exist - paper/text?

 

[SomptingGuy]

It's probably about the same point where 2nd order torsionals bottom out (for an I4 of course).

 

[murpia]

EngineAd,
Thanks again, if 5->95% burn rate could be assumed to be constant in crank angle terms then I can get somewhere. I am attempting to extrapolate the tuning potential of various configurations of diesels. A brief description:
1) take published values of bore, stroke, compression ratio, configuration (I4, I6, V6, V8 etc.) and power / torque curves, boost levels.
2) establish BMEP, estimate FMEP, hence estimate IMEP.
3) fit an ideal air Diesel standard cycle to the estimated IMEP and hence establish a load (cut-off) ratio for the combustion process and a peak cylinder pressure.
4) compare the 2 limiting factors of load ratio and peak pressure between engines to determine which are more highly 'tuned' as standard.
5) extrapolate the usual tuning factors of increased rpm, increased boost and increased fuelling for the various engines and see which has most potential.
6) estimate the likely fuelling requirements for the tuned engines.
Obviously a linear burn rate will allow me to extrapolate the increased rpm tuning factor with some validity.
Regards, Ian

 

[PeterLomax]

Hello,

EngineHead,

Could you provide us the injection duration, quantity, cylinder bore and stroke, bmep on the case you have studied ?

Regards.

Peter

 

[PeterLomax]

By the way, the lotus engine simulation help gives the following formula to medeling the diesel combustion duration :

0-100%(deg) = 30 + ( 50 / ( trapped_AFR x 0.06691 x 0.7))

There's no range specified for the applicable rpm range of this formula. Contrary to the gasoline formula, the RPM is not a parameter of the function..

 

[jejtopgun]

Hi,

I went to a convention a few years ago where most of the German Diesel manufacturer took part. The question was how to go over the 70 hp / litre region. Some people made a lovely presentation on the high performance diesel engine. A few parameters were interesting for the maximum RPM.

1 - combustion process is around 800 microseconds ( roughly the injection time ) for a common rail. This is still fairly fast and is taking place has the cylinder is moving downward. The limitation is therefore the moving time of the piston.

2 - the mechanical effort on the parts. Diesel parts like piston, conraod, etc are slighly bigger than in Petrol motors but still a bit of thinking make a Diesel motor runs above the 10 000 rpm region

3 - If the combustion and the machanical effort are not the problem then what ? well from the presentation I saw, the biggest problem they had with the engine ( and my personal experience concerning this subject prove this to be right ) is the valves ! Due to the effort made on the valve ( max 250 bar after combustion ) the valve train is fairly "eavy" and is not following the RPMs

Well, to bring up a positive note, it is possible to have a fairly fast burning process by bringing up some special injection system with higher pressure, a beeter design engine with some racing material for the pistons and conrods and a set of mechanical/pneumatic or electric/piezo valve train. It allow a Diesel to go to high RPM ( limits are unknown to me but in the 10000 rpm and above.

Ofcourse this technologie is not really cheap and therefore not for everyday engine for the moment but quite ok for racing purposes.

I hope I helped

Jerome Douay
Team74

http://www.team74.com