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Why can't diesels rev as high as spark ignition engines?

Why can't diesels rev as high as spark ignition engines?

(OP)
It is my understanding that diesels generally rev slower than gas engines for the following reasons.

1. Diesel burns/combusts more slowly than gasoline.

2. With the diesel cycle, and clever injector pulsing, combustion can be prolonged by continuing to inject fuel after combustion has started. A longer stroke helps to exploit this fact. Longer strokes generally lead to lower operating rpm.

3. While exploiting diesel fuel's stability, engine designers can use relatively high compression ratios, and higher precombustion pressures, resulting in significantly higher combustion pressure, as compared to gas engines. This leads to a need for higher strength pistons and connecting rods, which adds more mass, and reduces useable rpm range.

If I am correct on the above, would it be possible to inject fuel for a shorter period of time, presumably less fuel, lower pre and post combustion pressures and making a higher operating rpm range possible? (while making less power). I'm not saying this would be desirable in the real world. Just trying to understand the limitations on a diesel engine.

In short, could you make a small-ish diesel, say, 2.0L, run at 8500rpm and actually work?

RE: Why can't diesels rev as high as spark ignition engines?

There is a delay between when the injection occurs and when ignition happens. If the "window" in which the temperature that the compressed air is above ignition temperature is too short, ignition becomes unreliable. Additionally the diesel engine is limited by the rate at which the air and fuel can mix. When the revs go up, the combustion period gets extended later and later into the power stroke (no matter what you try with the injection duration), efficiency goes down, smoke goes up, exhaust gas temperature goes up.

RE: Why can't diesels rev as high as spark ignition engines?

Diesel combustion is a complex process. In nutshell, it is a diffusion process that is rate limited by the ability of the injector to deliver fuel droplets in the required size and penetration into the chamber, as well as the ability of charge motion combined with the kinetic energy of the injection to mix fuel and air into a locally combustible ratio, everywhere that the fuel reaches.
Therefore, there is only so much chemical energy that can be released in a given trapped air mass, in the desired window starting around TDC and ending roughly between 1/8 to 1/4 of a revolution later. Where this window exists for a given application depends a lot on the operating constraints such as NOx, and allowable peak cylinder pressure, which will limit how much the bulk of this process can be prioritized toward TDC, which is preferred for efficiency and power density. Conversely, smoke and exhaust temperature constraints will limit how much this process can be delayed after TDC, given the acceptance of lower power density and efficiency in a trade-off against NOx and/or peak cylinder pressure.

The above is very simplified. If you really want to learn about diesel combustion, take a class or buy a suitable textbook.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why can't diesels rev as high as spark ignition engines?

Another factor is that diesels must have very "mild" camshaft timing so they can be started and operated at low revs reliably - the cam timing limits their ability to rev as it would a petrol engine.

RE: Why can't diesels rev as high as spark ignition engines?

(OP)
Thanks for the replies. Those answers make sense.

"If you really want to learn about diesel combustion, take a class or buy a suitable textbook"

Take a whole class just to answer this one question? How will I know ahead of time if the course will answer it?


Thanks for the help though!

RE: Why can't diesels rev as high as spark ignition engines?

The Audi R10 TDI which won Le Mans supposedly reved to 8,000 RPM. It was a 5.5 liter V12. So you can get a diesel to rev that high, it just depends on the preconceived goals and limitations. Since most diesels are used in applications where specific fuel consumption is a high priority, they are not going to place empathizes on extreme RPM.

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RE: Why can't diesels rev as high as spark ignition engines?

8000 rpm in diesel, pretty high, would be interesting to know, how VAG engineers do that :) Diesel engine in normal car with highest redline, i think is new Mazda Skyactiv-D, 5200 rpm, this diesel engine also have very low compression ratio - 14:1, they claim, that it's world's lowest diesel engine compression ratio :)

RE: Why can't diesels rev as high as spark ignition engines?

I doubt if that Audi R10 race winning engine would be capable of cold starting at -30 C. So, it could use more aggressive cam timing and lower compression ratio and so forth.

That engine also certainly uses a high boost pressure, which facilitates more time and pressure in the "window" of sufficient temperature and pressure in the cylinder.

It also uses common-rail injection, which is capable of some trickery, such as injecting part of the fuel shot well before TDC so that it is partially premixed before the cylinder reaches ignition conditions.

It is also no doubt designed to run near full load a lot of the time, which keeps the pistons hot, which also should help with maintaining better ignition conditions. It's quite possible that if you took that engine and coasted down the side of a mountain with it under overrun-coast conditions at 8000 rpm for some time (let's say a couple of minutes, which would never happen under racing conditions), thus cooling off the pistons, it might take some trickery to get the engine to take up the load again with it already spinning that fast and having cooled off its pistons.

And, I have no idea if the fuel that they were running it on is the same as the stuff you get at the filling station around the corner from home.

RE: Why can't diesels rev as high as spark ignition engines?

There are a few traditional hurdles as well. Rotating and reciprocating masses are higher to cope with higher pressures and localised piston crown temperatures. Longer strokes are desirable, to acchieve a compact combustion chamber and high compression ratio.

I wonder if the Audi racecar combusts any fuel in a "pre-mix" state? Early injection of some fraction of the total fuel.

je suis charlie

RE: Why can't diesels rev as high as spark ignition engines?

Number 1 is not correct.
The whole deal of high octane is to resist combustion, if gasoline burned faster then detonation would not be suppressed.
Diesel in a si engine will cause detonation because it burns faster. Gasoline in a diesel is difficult to burn because it resists combustion especially with out a spark.

RE: Why can't diesels rev as high as spark ignition engines?

My 82 Isuzu I mark 1800 cc diesel could turn 5500 rpm with lots of black smoke:)

RE: Why can't diesels rev as high as spark ignition engines?

I think that you can theoretically run the diesel much faster, as Audi has clearly proven. See also attached link to SAE paper written by Steve Weinzierl of AVL.

http://papers.sae.org/2002-01-0160/

And I second the comment from Dicer. The difference in the speed of heat release is more dictated by the physical nature of the fuel introduction versus the chemistry of the fuel. Can we get a still smaller droplet size, yet get the penetration necessary to use all the available air?

RE: Why can't diesels rev as high as spark ignition engines?

the engine speed is mainly affected by rotational mass - the piston/conrod/gudgeon pin assembly on a Rover L series diesel (a fairly common 2L diesel engine) weighs in at around 1.6kg - for comparison, a "race spec" piston/conrod for the equivalent petrol engine by the same manufacturer, comes in at around 0.8kg.

A VW tuning specialist, Darkside Developments, have a 500Bhp 2L diesel, which they've had up to 7000rpm. The stroke is stock.

http://www.darksidedevelopments.co.uk/arosa-mk2-2-...

RE: Why can't diesels rev as high as spark ignition engines?

Gen 2 Dodge 426 hemi V8 from the 1960s rod weighs about 1150 grams. The piston weighs about 850 grams. Not sure if that include the wrist pin or not. Add 50 grams or so for the rod bearings, and about the same for rings maybe. That's about 2100 grams. And there are 8 of them.

Power curves' peaks exceed 6000 rpm pretty easy. It took some development to make them reliable enuff for NASCAR races. Most folks today consider those heavy Hemi pieces un-necessarily heavy, and even that strategic lightening and redesigned geometry would have netted even better reliability with much lighter parts. All that weight probably slowed the acceleration a little bit, but in general the power effectively overcame that.

RE: Why can't diesels rev as high as spark ignition engines?

@chairchild, I suspect there is a bit of a chicken and egg thing going on...
since the combustion is (a) rate limited, and (b) peak combustion loads are quite high, compared with typical Otto engines, a typical diesel engine (a) doesn't need to be designed for high rpm, and (b) does need to be designed for high combustion forces; so there is both an incentive to make parts "stout", and little penalty for doing so.
As both you and Tmoose have pointed out, when necessary, it is possible to make engines that support both high combustion forces and high rpm reliably, given appropriate development.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Why can't diesels rev as high as spark ignition engines?

I stand to be corrected but the audi mentioned the fuel pressure was in the region of 45,000 psi and that info is about 5 years old, am going to rush off and see if i can find my info source... that kind of pressure tells me that atomisation would be the key to faster burn and then ability to rev higher.. comments.

RE: Why can't diesels rev as high as spark ignition engines?

(OP)
That makes sense to me!

RE: Why can't diesels rev as high as spark ignition engines?

Sounds about right. Link

je suis charlie

RE: Why can't diesels rev as high as spark ignition engines?

thanks to carwhisperer and the link from carguru; getting myself further into trouble......running on memory [sometimes fatal] the article alluded to the fact that double that figure, of 3000 bar 45 kpsi was anticipated shortly, and that was approxomately 5 years ago.All THE BEST GOLFPIN

RE: Why can't diesels rev as high as spark ignition engines?

1. Diesel actually burns faster than gasoline. Octane (used for gasoline) is a measure of ignition delay, and cetane (used for diesel fuel) is its inverse. The numbers are essentially rigged for consumers so the higher number represents "better" performance for a given fuel. The gas engine desires long ignition delay (high octane/low cetane) because it can develop more compression, and hence greater efficiency, before TDC with confidence the bulk heat release will occur after TDC. The Diesel desires short ignition delay (low octane/high cetane) because it injects fuel shortly after TDC into the hot air created by compression, and the longer the fuel takes to start bulk heat release, the lower the heat and less expansion stroke available.

2. Yes, a Diesel *could* continue to inject fuel over the entire power stroke, but doing so would be inefficient because constant volume combustion is more efficient than constant pressure combustion. Few real engines are truly constant volume or constant pressure, but those that light off their fuel very quickly are closer to constant volume than those that light off their fuel slowly.

3. I believe you are generally correct in your assessment insofar as you qualify your statement "assuming the same materials and a given bearing load."

There is nothing inherent to the Diesel cycle that limits its RPM as long as you can get fuel into the cylinder fast enough. Yes, running at higher compression means you might end up with higher reciprocating mass (assuming equal materials) and would thus need beefier bearings. At the same time, however, the better efficiency of high compression means more work is being extracted from the fuel, so displacement can likely be reduced. It's just one big basket of trades.

Engine designs, like all designs, reflect trade offs in pursuit of objectives. Torque and RPM are interchangeable insofar as the application has a transmission. People generally find torque easier to drive with a manual transmission, but an automatic transmission tends to level the playing field. Friction losses climb substantially with RPM, but assuming we can pump as much fuel into the cylinders as we want, the real limitation in power is how much air we can move through the engine, and that tends to favor high RPM. Long stroke diesels are very frugal with fuel, but tend to be larger and heavier due to the associated strokes and loads. Thus, there are few Diesels in racing and aircraft applications where power to weight is king, but the automotive engines with the best fuel economy tend to be diesel. Bottom line... the best motor is one that best satisfies the requirements associated with its intended use.

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