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Multi Point Injection Diesel? 1

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SonyAD

Computer
Jul 29, 2008
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I was wondering whether it made any sense to have more than one injector per cylinder in diesel engines.

Instead of a centrally mounted injector fed by a fuel line from the common rail injecting fuel, for example, at a maximum of 1350 bar (like in my car, 90 PS/BHP? HDi, my 2000) why not have 2 or 3 in the hopes of improving spread and mixing and also speeding it up for better torque at high rpm.

Would it make sense to have 3 solenoid injectors per cylinder connected to 3 common rails fed by 3 injection pumps running at a peak of 1350 bar instead of 1 piezoelectric injector per cylinder connected to one common rail fed by 1 injection pump operating at a maximum of 1600 bar?

At least in terms of specific power if not of efficiency (running three 1350 bar pumps instead of one 1600 bar pump) or cost?

Or, if you can get it to work, use one injection pump feeding fuel at 1350 bar to one common rail with 8 or 12 outlets for two/three solenoid injectors per cylinder in an inline 4, for example.

You'd also be able to keep multi stage injection to high rpm with solenoid injectors.

I was thinking of placing the injectors so that they form an equilateral triangle inside, not necessarily near, the cylinder circumference and have them oriented centrally, towards the piston crown burn chamber.

Pls. don't lapidate or laugh. I'm not an engineer.
 
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Umm

About triple the cost for the fuel system.

The valves might get in the way.



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Injectors themselves are generally multi-hole already. Each is designed to shoot a jet into the fastest part of the swirling gas in the piston bowl.


- Steve
 
It comes down to cost. The fuel injection equipment already accounts for a large portion of a Diesel engine's cost.
 
Engine knocking is often caused by multiple flame fronts colliding. It would seem that multiple injectors, since the diesel fuel ignites shortly after injection, would lead to engine knocking.
 
Better mixing and charge preparation can only be a better thing in an IC engine - but as everybody has already said the cost and packaging would be a huge problem.

blacksmith

Not sure about that theory but I would suggest that you brush up on your combustion fundamentals. Diesels engines dont have the same combustion process as gasoline (laminar & turbulent combustion) they tend more to diffusion type flames and are in effect multiple flame fronts all burning away as the situation allows.


MS
 
Couldn't the injectors be moved to the engine block and be cheaper, solenoid injectors connected to a single common rail with 12 fuel duct outlets for all the injectors?

Maybe you could save money by having a lower pressure system able to deliver a max. injection pressure of only 1350 bar like my first gen. CR. And on solenoid instead of piezoelectric injectors.

I should think the nozzles would be simpler to machine as well, not needing as many holes.

3 nozzles instead of one would have a higher flow rate and main injection sequence would take less leaving time for post combustion at higher rmp and preinjection at higher rpms than would be possible with a single solenoid injector.

The micro droplets wouldn't have as far to travel in the same amount of time which would benefit high rpm torque through quicker dispersion and you could have lower injection pressures than with a single injector.

You could even fire the three injectors sequentially to promote better swirl for the second and third injectors.

There must be some application niche for this. Railroad or truck diesel, maybe.

At least a feasibility study.
 
I has always understood that too low a cetane fuel can result in delayed ignition and multiple flame fronts with knocking so I assumed mutliple points of injection would too.
 
If you could avoid flame collisions and cylinder wall collisions, while still maintaining the same injection pressure (I say this because reducing injection pressure would be a way to pack more injection points into the cylinder, but you don't want to do that for emissions reasons) then there could be some efficiency gains. However, they already have to back off of maximum efficiency (i.e. max peak cylinder pressure in this instance) for emissions purposes today.

Therefore, in theory it could work but in today's reality there are no gains available and the cost and engineering to achieve multiple injection points is significant. I wouldn't be surprised if this is done someday when we have better control of the combustion mix and can get the cylinder pressure up while still avoiding NOx (highly controlled homogenous EGR, during-combustion air injection, other exotic solutions).

To get the effect you want, why not just greatly increase the number of cylinders and make them smaller?
 
So, basically, it's a no go for most applications because it's unfeasible due to NOx emissions regulations and it would be susceptible to knocking because of flamefront convergence and contact with the cylinder walls.

Maybe someday or for really large bore applications.

Final question. Do ship engines use multi point injection?

I think using more cylinders would not achieve the same effect because of added crank inertia due to a longer crank, more friction points with a longer crank and more piston skirts and ring, crank flexing issues and more reciprocating weight.

Basically, more inertia, heat loss and friction for the same amount of burnt. More parasitic losses.

Thanks for the info.
 
I have been around some fairly large ship diesels (MAN 48/60; 48cm bore x 60cm stroke) and work closely with Pielstick PA6Bs (280mm bore and 330mm stroke) and they are all single point, the injectors are nearly the size of my forearm, but there's only one per cylinder.
 
Would multi point injection make sense for these, at least?
 
Scratch that. Stupid question. If there's no point for multi point injection in automotive diesels, there's certainly none in low speed and medium speed diesels.
 
Points that haven't already been made is that modern diesels:
A - Run at 1350 bar to get better atomization, not to get more fuel in faster.
B - Often shoot the fuel in multiple shots per combustion event (up to 5) for emissions and noise reductions. One big bang vs 3 little bangs.

ISZ
 
Just a couple of minor points to add.

Avoiding cylinder wall collision is not to avoid knock. It's to avoid massive soot generation, rapid oil degradation, and catastrophic engine failure due to ring failure and piston scuffing.

Flame front collision shouldn't cause knock, but it will cause localized oxygen depletion and excessive soot. Low cetane can cause flame front collision (late combustion) and independently knock (early combustion), and of course both in the same engine at different times due to unpredictability of the combustion.
 
JSteve, explain the low cetane/early combustion/knock scenario. I've only heard of low cetane/delayed combustion/knock.
 
Low cetane fuel does not self-ignite well. Therefore, it will not ignite predictably. It can ignite early if the engine is overcompensating for lost actual torque (due to misfire or late combustion), ramping up requested torque and fueling until the desired actual torque is achieved, at which point the amount of mass in the cylinder is such that the fuel ignites (in this case detonates) immediately upon injection. Since this is a different combustion mechanism than conventional diffusion flame, the effective torque release is early. Because you are nearer TDC and the cylinder volume is not expanding as rapidly, this is a more damaging condition than later detonation.

Of course, it can never be earlier than time of injection, so calling it early combustion is probably not as good of a description as unpredictable combustion - which is probably your point. In the late combustion case, I was focusing on the more catastrophic problem of fuel impinging on the cylinder wall, but detonation is bad anywhere as I think you are also pointing out.
 
Just had to say - great thread. I don't think we'll be installing multiple injectors soon (sorry SonyAD) but I've gained knowledge from reading the many posts.
 
This thread has been beat to pieces.. I think what the author meant to ask was if peripheral injection would offer any benefit? If so, and besides, Rube Goldberg would blush. Peripheral ignition offers some benefit with port injection SI and probably GDI.
 
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