Diesel injector - # of holes 3

[jbthiel]

I would like to know why diesel injectors do not have large numbers of holes, 10 to 12, instead of only 2 to 8? (no limits due to emissions) My assumption is that if the fuel could be delivered faster, the pump timing could be retarded. This would create more power due to lower cylinder pressures BTDC.

Current fuel delivery systems seem to be limited on the amount of fuel that can be delivered per crank degree due to the injector. If you had more holes in the injector than you could get the total amount of fuel into the cylinder faster (assuming corresponding changes to the fuel pump delivery rate) while still being sufficiently atomized.

Am I off base and the real limitation is the rate of atomization and consequentially burn? Would a quicker delivery rate be too hard on the pistons and rods? (application is a custom built engine, ~7 Liters)

 

[PJGD]

With classic (conventional) diesel combustion systems in which all the fuel is put into the cylinder in one shot, establishing the number of nozzle holes, their diameter, length, cone angle relative to the cylinder axis, angular orientation, tip protrusion and other factors is a lengthy development exercise. With modern diesel fuel injection systems that are capable of splitting the injection event into 2, 3, 4 or even more discrete injections, there may be some slight latitude in certain respects.

You seem to want to put the same amount of fuel into the cylinder, but in a shorter space of time. To do this, you either need to keep the current nozzle flow area constant, and increase the (mean) injection pressure (expensive to do, typically), or keep within the current peak injection pressure limits and increase the nozzle flow area (a few large holes, or more smaller ones). As you move in this direction, and assuming you make no other changes, you will find that you are putting more fuel in during the "delay" period. This is the period at the start of injection when fuel is entering the chamber, coming up to temperature (circa 450°C) and becoming intimate with the oxygen in the combustion space. It takes a finite time for this to happen and thus for ignition to commence, but at the same time, fuel is continuing to be injected (and in your scenario, at a higher rate than normal). Once it does, the increasing temperature encourages the other fuel, now mixed with air, to ignite. The rapid burning of all this acuumulated fuel causes a big pressure spike and is what is responsible for the characteristic diesel combustion sound (the loud knock). Again, in your scenario that pressure spike (actually, the rate of pressure rise is the issue, not necessarily the peak pressure reached) will be greater, so will the sound pressure level, and the engine scantlings may have to be beefed up too for the same durability targets.

Modern flexible fuel systems like the latest common rail systems can mitigate this problem by injecting a small pilot amount of fuel (say 1 to 2 mm3/stroke) which will get the fire going, then they can feed additional fuel in small parcels at a rate that the engine structure can handle.

The number of holes in a nozzle is typically tied to the swirl level in the cylinder; higher swirl = fewer holes, and vice versa. Quiescent (no swirl) chambers typically have 8 or more holes.

Typically, one would expect that stoking the fire at too great a rate (high fuel flow rate and thus a shorter injection duration) would lead to higher in-cylinder temperatures, and thus more NOx emissions. The trend is to extend the injection duration to keep the temperatures and thus NOx down so that the EPA will allow you to sell your engines.

PJGD

 

[jbthiel]

Several questions in response:
1. Is there any reason that you couldn't run more injector holes in a high swirl engine? I would think that a highly atomized charge would burn more consistantly. And all else being equal, a highly atomized charge should burn faster as you are not waiting for the "stream" to atomize and burn from the outside in.

2. Is there any reasonable way to decrease the delay period? (non-computerized application) One thought that I had was mixing fuels, but I haven't been able to find the respective self-ignitions points and there is the potential for pump damage. Another thought was preheating the fuel between the injection pump and the injectors.

3. If the compression ratio was lowered sufficiently (~12:1) and the parts were beefed up the pressure spike due to a large amount of fuel igniting at once shouldn't cause a structural problem. My thoughts are that this would result in a larger pressure difference between just-befor TDC (energy loss) and just-after TDC.

Thanks for the response. JBThiel

 

[PJGD]

When the fuel comes out of the nozzle hole, it is already partially broken up due to cavitation that has taken place in the nozzle and as it travels through the orifice. Thus, as it leaves the orifice, there is a relatively solid core of fuel which has to force it's way through the dense air, but at the same time it is entraining air, getting broken up, and as it's energy is used up, it gets left behind by the main core which is still forging ahead.

This, basically is what creates the injection plume, and the plume typically has an included angle of about 20 degrees in still air. If you now add in the effect of swirl, the air motion, which varies from theoretically nothing at the center of the chamber, to relatively high velocity near the periphery, begins to deflect the lighter components of the plume, ie, the outer cloud of fuel vapor and moves it around the chamber so that the plume begins to look a little like a rooster tail. The plume now no longer occupies a 20° swath of the chamber but much more, and it's trailing edge now begins to interfere with the plume from the next orifice. That's the theory, anyway, but the bottom line is that if you want to avoid excessive smoke, then the swirl rate needs to go down as the number of holes goes up.

In general, I think you are right; more highly atomized fuel will ignite more readily and give a shorter delay period. As to whether it will burn faster and more consistantly, will depend on if it gets supplied with oxygen at the necessary rate. Don't assume that high swirl will necessarily provide that oxygen; near full load there is very little oxygen about, and plenty of burnt products to swamp your final parcel of fuel.

Like most products, diesel fuel injection systems are designed fairly close to their limits when running at rated conditions, however the durability requirements (800K + miles for heavy duty) do provide some cushion. But fuel lubricity is a big issue since the pumps depend on that for internal lubrication. Any alternative fuel needs to be examined for it's lubricity. Normal US diesel fuel is lousy stuff, particularly in terms of cetane rating. Adding cetane improver is likely to reduce the delay period.

Heating the fuel at the injector, prior to injection will give you hypergolic combustion which is essentially a condition with no delay period. Potentially, this should give many benefits and has been explored in the past (see a 1985 SAE paper from researchers at Eaton Corp.). However, it is very difficult to make work in practise due to nozzle carboning and other issues.

As for your third question, I am not qualified to provide a good answer. I have seen diesel engines operate at that sort of CR, but the pressure spike did make it very noisy. If you go the low CR route, it runs counter to the short ignition delay objective. You will certainly need the cetane improver!

PJGD

 

[tbuelna]

I have seen DI diesel nozzles with up to 9 holes in them. Actually more small holes are useful in smaller bore engines to prevent overpenetration of the injector spray. The smaller holes (with a proper L/D ratio), combined with high injection pressure, promote rapid mixing and burning.

Most DI diesel now days use the "mexican hat" chamber shape with a radially symmetric set of nozzle holes. The limit to the number of holes in a particular tip design is generally a matter of how many can be physically fit into the given space, without causing stress problems in the tip. There is also the issue of being able to produce the very small diameter holes required, in a production environment.

 

[aorangi]

Many thanks to PJGD for his very informative and highly competent posts.

To my knowledge, "Mexican hat" combustion chambers are typical of large medium and low speed Diesels while automotive modern direct injection cars and most trucks ones use toroidal chambers.
I think the reason is the ratio inflamation delay vs revs. A smaller part of the fuel is injected at the end of the delay on huge slow speed engines and since their combustion chamber is larger their gradient of pressure increase against rotation angle of the crank is much lower. Direct injection for high speed Diesels became practically achievable with the appearance of pilot pre-injection, and toroidal chambers allow more squish induced air turbulence than open Mexican-hat ones, so that 6 holes injectors are sufficient. Please correct me if I'm wrong.

About compression ratio, it must be decreased with an increase in boost pressure. Since a higher boost increases the pressure and temperature of the charge, it shortens the delay. Some engines for military fast boats have two-stage turbocharging with inter and aftercooling. Their CR is reduced sometimes down to 8:1 and it becomes insufficient for starting. They must be pre-heated or have a specific starting system such as the Hyperbar which launches the turbocharger by the means of an auxiliary combustion chamber in the exhaust before the turbine. An electric high-speed motor-generator on the turbocharger shaft can also be used.

On gas-generators, the CR was about 6:1 and all the power of the Diesel was absorbed by their own supercharger, output power being provided by a geared exhaust turbine. Strange animals, most of the times without any crank and rods, the opposed pistons of the diesel being directly connected to the compressor larger pistons, return compression stroke being done by air-cushions located on the back side of the compressor pistons. It's said they run very smoothly.

Cheers
Aorangi

 

[PJGD]

Although "tbuelna" is generally correct in his other statements, I would concur that the appropriate terminology for today's high speed DI diesel combustion chambers is Toroidal. Essentially, it is an exagerated, highly re-enterant Mexican hat . It has a large cone in the center to fill the otherwise inactive dead space, and the re-enterant lip attempts to keep the combustion activity inside the chamber.

It is often stated, and generally true that for a diesel combustion system, you need orderly air motion. That is to say that you are working with a fixed geometry nozzle which projects the fuel into the chamber with a known orientation, with a known plume shape, and a known pattern (assuming a multi-hole nozzle). You now want that fuel to meet up with as much oxygen as possible, and to do so it is much easier if you can make the air move in the direction that you determine is best to achieve that objective. This implies an orderly, controlled air motion in terms of speed and direction, with the minimum of uncontrolled swirls, eddys or turbulence in the conventional SI sense.

The combustion chamber options available to the engine designer are severely limited by the flexibility of precision manufacture of the nozzle geometry on the one hand, and manufacturing issues of the piston and cylinder head on the other. Over the years, nozzle geometries have rationalized down to three primary designs: Pintle nozzles (a single-hole with variable geometry) typically used for divided chamber engines; the hole type nozzle (typically fixed geometry, multi-hole) used on DI engines (and crazy mixed-up engines like the '79 Oldsmobile diesel!); and outward opening poppet nozzles which have been used on both types of engine. The multi-hole nozzle is strongly in the ascendancy today, with variable (flow) area designs on the near horizon.

Mexican-hat chambers imply no re-entrancy of the side walls, and I would claim that the term is generic rather than specific. It can extend from small diameter but deep chambers, to those that are wide but shallow. The latter are associated with quiescent chambers which work well on engines that run up to about 2200 rev/min; above that, chambers start to move towards the toroidal shape. The squish provided by the re-entrant lip also results in an orderly air motion; it imparts a cork-screw rotation to the torus which already has swirl due to the intake port geometry.

The combination of controlled high air motion along with controllable high pressure fuel injection which has driven the trend towards small holes (around 0.150 um today), has enabled the modern high speed low emissions diesel. Remember, diesels are not detonation limited as are SI engines. Typically they are peak cylinder pressure limited, which has driven their robustness historically. Now, with the advent of injection systems which can divide the injection event up into multiple discrete injections, you can control the heat release and thus the rate of pressure rise which makes the combustion event much less harsh. Directionally, this should result in lighter, more powerful diesel engines in the future.

PJGD

 

[enginebob]

I'll try this one with out writing a book.
The more the holes the more the pressure drop in the nozzle. The more the pressure drop,the less the atomization. Unless you meant just add more holes and reduce their size to keep the same nozzle pressure? Then comes the question how would you machine such small holes? How do you keep them clean? There may even be a point where if the hole where too small then all they would do is coke up. And their pattern would most likely not be very consistant.