Q about hi-pressure injector

[TheNeedForZ]

I heard somewhere that diesel engines use high pressure injector to atomize fuel. Some of those injectors use VERY high pressure like 20000psi IIRC?

 

I recall a 30-06 generates 50000psi of peak pressure in the gun chamber and a .22 rimfire generates around 20000psi....It seemed impossible to generate that kind of rapid response pressure via mechanical means. So how does a diesel injector produce that amount of pressure? Or did I remember the figures wrong?

 

Thnx

[baddriver]

I don't know first hand what the pressure in a diesel fuel injector is, but it would certainly have to be significant. Diesel engines typically use compression ratios of around 30:1, meaning that the un-heated air in the chamber would have to be around 30*15 psi = 450 psi. Add to that the effect of heating the air, and I could believe well over 1000 psi in the chamber before the start of combustion.

 

Then, remember that combustion starts as soon as the injection pulse begins. This means that the temperature and pressure starts climbing while the pulse is being injected, and to complete the injection, the pressure in the injector has to be more than pressure in the chamber even after a significant quantity of fuel has burned.

 

As far as how mechanically to generate this pressure, remember that moderate forces delivered over a small area can produce remarkable pressure. Try figuring the pressure on the head of a nail when you drop a 20 lb hammer on it.

 

We have an airless paint gun at work that sprays paint at over 3000 psi. Before you get to use it you have to watch a video showing what can happen to your fingers if you put them in front of the bright-orange tip. (It jost pokes a tiny little hole, you see, and it doesn't hurt a bit,....until the toxic solvent in the paint starts to spread throught the tissue over the next few hours and you get to have your finger amputated) 3000 psi is necessary to atomize the paint being sprayed into the air. If we were spraying into a high-pressure chamber, it might very well require 10x as much pressure.

[BrandonsZ]

On tractor trailers, i'm not sure how they get the pressure that high, but it's true, and the valving is not a solenoid like in passenger cars and trucks, they actually have a roller cam follower to open the valve for fuel intake completely seperate from the intake and exhaust vavles per cylindar, but all on the same cam. Just saw it on "Trucks" Saturday.

[Moridin]

I don't know first hand what the pressure in a diesel fuel injector is' date=' but it would certainly have to be significant. Diesel engines typically use compression ratios of around 30:1, meaning that the un-heated air in the chamber would have to be around 30*15 psi = 450 psi. Add to that the effect of heating the air, and I could believe well over 1000 psi in the chamber before the start of combustion.

 

Then, remember that combustion starts as soon as the injection pulse begins. This means that the temperature and pressure starts climbing while the pulse is being injected, and to complete the injection, the pressure in the injector has to be more than pressure in the chamber even after a significant quantity of fuel has burned.

 

As far as how mechanically to generate this pressure, remember that moderate forces delivered over a small area can produce remarkable pressure. Try figuring the pressure on the head of a nail when you drop a 20 lb hammer on it.

 

We have an airless paint gun at work that sprays paint at over 3000 psi. Before you get to use it you have to watch a video showing what can happen to your fingers if you put them in front of the bright-orange tip. (It jost pokes a tiny little hole, you see, and it doesn't hurt a bit,....until the toxic solvent in the paint starts to spread throught the tissue over the next few hours and you get to have your finger amputated) 3000 psi is necessary to atomize the paint being sprayed into the air. If we were spraying into a high-pressure chamber, it might very well require 10x as much pressure.[/quote']

 

I believe my friend's Cummins in his Dodge is around 16:1 compression. As for how the injectors deal, there is a mechanical box with plungers and stuff to produce that amount of pressure. I believe the injectors just open and close to let the pressurized diesel in. The bigger the holes, the more fuel of course (which is exactly what makes a lot more power from a diesel). Now the new diesels, like those from VW don't use an injector box if I remember correctly. That's where most of the noise comes from. I'm not sure how they would come up with that kind of pressure, but I think I heard from somewhere about a Piezo style injector.

[Dave240Z]

Read this

 

http://www.vwvortex.com/artman/publish/audi_news/article_1357.shtml.

 

If I had the $$$ and if it was available in the US, I would pick this up in a hearbeat. 4.2L V8, twin turbos @ 37Psi MAX, 326bhp/480lb-ft, AWD, 0-60 in 5.9sec all while getting an average of 25mpg!!!

 

Here's the relevant part for this discussion if you're lazy.

 

Common rail technology with piezo injectors

 

The Bosch EDC16 CP system, with fuel delivery, trigger-start, boost pressure and exhaust-gas recirculation control, is responsible for engine management on the V8 TDI. As on the A8 with a petrol engine, this electronic system now means that it is possible to start the engine automatically at the push of a button.

 

Mixture preparation is by means of a latest-generation common rail system. It has a high-pressure pump – located inside the V of the engine – and one pressurized pipe or "rail" for each cylinder bank. The maximum injector pressure is now 1,600 bar, 250 bar more than with previous common-rail systems, a value that corresponds with the weight of a medium-sized saloon on the surface area of a fingernail.

 

This high injection pressure results in even finer atomisation of the fuel and therefore in better mixture preparation and more efficient combustion. The consequence is not merely extra power and torque, but also reduced fuel consumption and emissions.

 

The piezo inline injectors are undoubtedly the most important innovation of the new common rail system. Electrically excitable piezo crystals undertake the triggering of the injectors. As soon as a voltage is applied to the crystals,

they contract and, assisted by a hydraulic element, mechanically trigger the opening of the injector needle.

 

The piezo injectors offer multiple advantages compared with conventional solenoid valves. The moving masses on the injector needles can, for instance, be reduced by no less than 75 percent, from sixteen to four grams in each case.

 

This enables considerably smaller and more precisely metered injection quantities. At the same time, piezo technology permits a higher injector needle speed. With a value of 1.3 metres per second, the piezo system responds twice as fast as other common rail injectors.

 

The number of injection processes per combustion cycle can thus be varied almost at will. In the case of the 4.2 V8, the Audi TDI development team decided on up to four injections. In the lower engine speed range there is a double injection in addition to the main injection, and a single pre-injection in the mid-range.

 

This strategy produces a smoother combustion process, the benefits of which are most immediately apparent in the engine's acoustic behaviour. The 4.2 TDI engine has once again become significantly quieter and smoother-running, even compared to its already highly cultivated predecessor, and without doubt it sets the benchmark for the class in this discipline.

 

The fuel is injected into the combustion chamber via eight-hole jets. They too play a major part in making the combustion process ultra-efficient, and thus improving the quality of emissions, by distributing the fuel homogeneously throughout the combustion chamber.

[Moridin]

You should check out the Touareg with the V10 TDI. That thing is awesome. 553 lb ft of torque at 2000RPM. Im pretty sure they used a derivative of this motor at Le Mans last year. The motor made so much torque it ripped through the clutch(es).

[SleeperZ]

Even my old tech Mercedes has injectors that pop at 135 Bar (2000 psi). It has 20.1:1 compression, so you've got to have that kind of pressure to inject into the cylinder at the peak of the compression stroke.

[TheNeedForZ]

Dave240Z,

 

thanks for the great info, gotta love the piezo injectors.

[Dave240Z]

You should check out the Touareg with the V10 TDI. That thing is awesome. 553 lb ft of torque at 2000RPM. Im pretty sure they used a derivative of this motor at Le Mans last year. The motor made so much torque it ripped through the clutch(es).

 

The Touareg V10 TDI is certainly sweet, but the A8 4.2 V8 TDI suits my taste a bit more. Of course I could be easily persuaded to ditch the A8 4.2 if it was available with V10 TDI as an option.

 

The only engines used in LeMans by VAG are derivitives of their 4.2L V8 gasser. They are incredibly fast though, and have dominated their class for several years now. This was their last year of racing that configuration I believe.

 

Audi does have a gasser V10 engine available, although it doesn't share much, if anything with the V10 TDI just as the 4.2L V8 gasser doesn't really have much in common with the 4.2L V8 TDI. This engine currently provides the motivabion for the Lamborghini Gallardo and is rumored to be used in an upcoming Audi supercar deemed the "LeMans Quattro"

 

 

 

Even my old tech Mercedes has injectors that pop at 135 Bar (2000 psi). It has 20.1:1 compression, so you've got to have that kind of pressure to inject into the cylinder at the peak of the compression stroke.

 

This is true, diesel tech has always required very high fuel pressures to overcome the high cylinder pressure. The difference today, is that modern diesel tech use electronically controlled injectors fed by a common rail which allows extremely precise fuel delivery while significantly reducing the typical diesel clatter that most people are familiar with. The end result being a more efficient, powerful and quieter diesel.

 

It's worth noting that the modern TDI engines experience very high dynamic compression ratios when you combine the static ratio of 16:1 with turbos pushing in excess of 37Psi which will result in cylinder pressures way in excess of 20:1.

 

No matter how you slice it, diesel tech has come a long way recently. Just 10 years ago I wouldn't have even remotely considered driving a diesel car on a daily basis. They were always slow, noisy and rather dirty albeit slightly more efficient at the pump. Today however, you are able to get as good if not better power while achieving incredible efficiencies with hardly any of the negative side effects previously associated with diesels.