What is the theoretically limiting factor on diesel RPM?

I believe that the strength required to hold things together with typical diesel cylindar pressures requires a heavy rotating assembly. The momentum of this assembly is what limits diesels from spinning higher RPM's like cars.

Another individual, "he who shall not be named", agrues the following:

"And rotating mass is not the limiting factor at all on diesels its compression ignition, ... I spin my 383 sbc with long stroke and a decent weight crank and pistons to around 8000k would go 9 if it made power. You can balance anything regardless of mass. You cannot start ignition in a diesel until around or after TDC because it relies on compression to raise temperature to allow radicals to form for combustion. If you are at too high of rpms combustion starts to late on the down stroke and you loose the thermal efficiency on the expansion stroke due to incomplete combustion during the key time during which power is made ( low s/v ratio). IE diesels are rpm limited, increasing boost to a certain extent can raise pressures to initiate combustion sooner which may buy you some rpms. What maybe 4 years ago they were playing around with 5000+ rpm Duramax's, it wasn't because of rotating weight that this idea was dropped fast."

I don't believe the diesel cycle itself is inherently RPM limited. Would any engineers or mechanics like to shed some light on this?

 

I'm not a diesel engineer. But I'm pretty good with a gas engine. Here's some thoughts:

1. Weight of the rotating mass will limit RPM's. A 5.9L I6 engine has some pretty big pistons and rods. I imagine the crak throws are pretty hefty, too. You could get around that by making a 5.9L V10.

2. Timing of the combustion is critical to power production and efficiancy. Chnaging it by as little as 2* can make a huge differance. That's why the distributer has an advance mechanism. You want maximum combustion power to be about 5-10* ATDC. Sooner than that and the piston and rod are trying to come down before the crank throw is ready to make use of the power.

Later than that and the piston is low in the cylinder. Because the crank throw is well past the mid point, you don't get much use out of the energy produced. Also, because the piston is low, the combustion area is huge. It would be like having a compression ratio of 4:1.

At low rpm's, you want the combustion event to start about 10-15* BTDC. As the rpm's climb, you have the start the combustion event sooner, so that the full power of the event occurs at the right postion of the piston and crank throw. At >3,500 rpm's, the initiation of the combustion event (spark) is as much as 40* BTDC.

In a diesel combustion chamber, the ignition event always occurs at about the same moment. As the rpm's climb, the combustion event does not advance. So the maximum power point comes later and later, as the piston is lower and lower in the cylinder. I don't see any way to advance the "spark" in a diesel.

 

You advance the injection event to advance ignition.

Diesel is a slower burning fuel than gas is. Longer burn times mean the faster it goes, the less is burned by the point that isn't doing work anymore.

 

But that only works to a certain point, doesn't it? My very limited understanding is that the "spark" event is controlled by pressure. Once the mixture is squeezed enough, it explodes. You can advance it a little by advancing the injection event. But doesn't it still have to be sqeezed?

Adding more boost early in the cycle could increase the pressure enough to make it work. But then your effective cylinder pressures would be huge.

And at 5K+ rpm's, the ignition event has to be fairly precise. It occurs nearly 7 times per second in each cylinder. Is the diesel explosion precise enough? I don't know.

 

There are guys with 12 valves that run over 5k rpms.

 

Also I believe the newer common rail Cummins motors run 5 injection events per power stroke.

 

Just for fun and going the opposite re the rotating mass principle, consider a very small CI engines such as used in model airplanes. They spin pretty fast and work quite well.

 

When you say "max" RPM it depends on what you mean by max.

At some point, higher and higher RPM will mean something breaks and self destruction follows. That's the max.

But if you rev an engine until it won't spin any faster and it still does not break, it's probably floating the valves and limiting itself by not being able to breath well enough to make enough power to go any faster. This is the most common "limit". The sound of the engine changes to a ragged sound and is easily recognizeable.

The higer an engine revs, diesel or gas, the worse it breathes when it is beyond it operating range. Finally, it takes more and more power to rev higher and it is producing less and less because of it's increasing speed. The speed and power output lines cross and it is putting out full power just to run itself. That's the max.

A diesel's high end performance drops off sooner than a gasser because the fuel burnes slower. And because it has much higher reciprocating mass for a given displacement. So, it's high speed power curve drops off sooner as the RPM raises.

Another kind of max is simply full HP against a load. Like pulling a grade, while heavily loaded, at full throttle. There is a final speed the truck is capable of with the engine in it's best gear for that moment. It might be 2000 RPM or whatever. At that moment it is at the max RPM. If the grade steepens, the engine slowes and you must downshift. Max is dropping off.

 

The 'spark' is when the air in the cylinder has been compressed enough for it to have reached the auto-ignition temp. of the fuel. There is a formula to figure it out from compression ratio, initial pressure and temp.

 

much higher than that in the sled pulling world.... 6k+ is not unheard of on a balanced and blueprinted engine...

injection timing can be extremely advanced... on my Ppumped 24valve, I'm running 22* of timing BTDC... sledpullers are in the 30's and sometimes even higher

 

The beauty of discussing theory is that none of us is ever wrong

The actual amount of time it take to burn diesel fuel is different than the time it take to burn gasoline and nitromethane or whatever other fuels have been mentioned in this thread.

Compression ignition is NOT what limits rpm.

Diesel engine have a key difference from gas engines that's not the ignition source. There's simply no fuel in the cylinder before the ignition event. It is the introduction of the fuel that initiates the combustion event.

Introducing fuel takes time. Now we can see how injection pressure is one of the main factors influencing maximum diesel RPM.

I assume that "max rpm" means the maximum rpm at which the engine will still make good power. The sled pullers are revving the engines way up there. They are injecting tons of fuel with low(ish) injection pressures, and this takes a long time to do if you have a nozzle small enough to atomize even remotely well. Their combustion is horribly inefficient, as the radically timing advance means they end up compressing a flaming fireball (bad on engine parts). Only at high rpm can you get away with this.

If you had a HPCR engine running at, say, 3000 bar injection pressure with high flow injectors, then you could inject even massive amount of fuel in probably < 10 degrees crank rotation at 2K rpm.

Assuming for the moment that you have infinitely strong and light supporting parts (crank, rods, pistons, valves, etc), the maximum RPM for a diesel will simply be a function of the quantity of fuel injected and the time available to do so.

More injection pressure= more RPM capability
Less power (fuel)= more RPM capability
More cylinders for the displacement= more RPM capability.

On the last point, I'll explain why simply having 12cyl for a 6L engine vs 6 cyl makes the rev capability higher (never mind the crank and piston lightness and all that).

With more cylinders, you subdivided the total quantity of fuel you need to inject. Twice as as many cylinders means half as much fuel per cylinder for the same power (fuel rate). Because each injector is passing half as much fuel, it is only firing for half as long. That means it takes half as much time to inject the fuel and presto-- you can rev that engine much higher.



So while the rest of the engine will often hit its limits before the fuel system does, if you optimize the supporting cast, the fuel system becomes the last limit, and those limits boil down to injection pressure and nozzle flow rate.

(currently sitting at test cell at Cummins Technical Center)

Justin

 

I thought it was generally piston velocity and acceleration/deceleration forces that limit most high performance engines.

At some point you reach the limit of what available materials can do, when it comes to adding strength without adding weight (and thus also increasing load).

 

What Hohn posted makes sense, but you'd think that the Duramax 3450RPM Redline would be higher. Wonder what hinders their Redline? Ford is 4200.

 

that might be because he works for Cummins...

 

This is my thought too. Most modern engines run many events, common rail is the best for this because there is no cam operating an injector lobe.

Multiple injection events, pilot injection helps to light the main injection. Early E model c15 Cat engine had pilot injection too but was limited to cam profile. Common rail eliminates this because the pressure is always there, the ecm just has to command the pilot valve. Also able to change fuel maps and customize the application.

As far as mass goes I believe there is a limit too but if it is a modified race engine with lightend components this would open the door to higher rpm.