Oil & Fuel Diesel for Dummies

   / Diesel for Dummies #11  
Compression pressure @ TDC in a modern Indirect Diesel Injection system like Kubota's when the engine is cold is around 450-500psi.

The inline fuel injection pump is providing pressure of around 3,000 psi to a pintle fuel injector nozzle which is approximately 6X higher pressure than the air inside the combustion chamber.

The compressed air in the combustion chamber temperature has been raised above the ignition temperature of #2 diesel fuel so that when the injector opens, the fuel immediately ignites as it is sprayed into the hot compressed air in the combustion chamber. The injector nozzle is situated inside the precombustion chamber leading into the combustion chamber as it is an indirect diesel injection system (IDI) as opposed to a direct injection (DI) system where the injector nozzle sprays directly into the combustion chamber on top of the piston crown.

The inline pump is mechanically driven and timed by the engine crankshaft, similar to the camsaft.

The timing of the injection event into the diesel engines 360* of crankshaft rotation is specifically important as the injector begins to spray after the intake and exhaust valves have been closed and the piston is still rising upwards. This crank shaft angle of injection is a very short period of time and the diesel fuel spray coming out of the injector into the combustion chamber must first mix and then ignite to provide power. The degrees of crankshaft rotation when the injection, mixture, and ignition of the fuel into the hot air is very short and the timeframe is also extremely short on the order of a couple of milliseconds.

In gasoline engine, the fuel and air are already mixed prior to entering the combustion chamber, however in diesel engines the fuel and air have to mix while inside the combustion chamber, prior to ignition.
Attached is a photograph of a Denso Pintle Injector nozzle.
 

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   / Diesel for Dummies #12  
As you can see in the photograph of the single hole pintle nozzle of that Kubtoa diesel fuel injector, there is only one outlet of around 200 microns diameter. That is due to the lower rpm operation of this design of engine and the lower power output.

A high speed modern direct injection fuel injector nozzle has 5 or 6 holes with super tiny laser cut fuel passages in the tip, each about 150 microns for a different wider spray pattern that increases the speed with which fuel-air mixture occurs. The injection pressure in a HSDI engine is also about ten time higher than the injection pressure in your Kubota engine.

If you'd like to know more about what is happening, purchase this book, Bosch Diesel Engine Management, it is a good read:

http://www.bentleypublishers.com/product.htm?code=h011

Robert Bosch

Diesel Engine Management, 3rd Edition

Your Complete Guide to Diesel Engine Components & Systems

Diesel-Engine Management maintains the BOSCH hallmark of presenting complex technical information in a clear and concise format. Diesel-Engine Management's primary focus is fuel-injection systems and components—topics range from engine efficiency, turbochargers and timing devices to high-pressure delivery lines, nozzle holders and temperature sensors—and is fundamental reading for automotive mechanics, technicians, educators, instructors, students and enthusiasts.
Systems covered:

* PE inline fuel injection pumps and governors
* All variants of VE..F, VE..EDC, and VE..MV (VP29, VP30) distributor injection pumps and VR (VP44)
* PF single-plunger fuel-injection pump
* Unit Injector System (UIS)
* Unit Pump System (UPS)
* Common-rail fuel-injection system
 
   / Diesel for Dummies #13  
With the compressed air in the Kubiota's IDI combustion chamber being around 500 psi prior to the injection event, and the fuel injection event being around 3,000 psi, it is apparent that the fuel will enter the combustion chamber instead of the air entering the injector! /forums/images/graemlins/blush.gif

However, once the injection event is initiated and fuel-air mixture and concurrent ignition occurs, the combustion chamber pressure then rises to over 20,000 psi @ 1,200*C, so the closing and sealing the injection nozzle is just as important as closing the valves! /forums/images/graemlins/wink.gif
 
   / Diesel for Dummies
  • Thread Starter
#14  
Great info, and thanks to everyone.

Skypup, I'm having a hard time visualizing the pre-combustion chamber. Where is it in relation to the valves and combustion chamber? Is it just a scalloped out area within the cylinder head?
 
   / Diesel for Dummies #15  
Many times, a precombustion chamber is a tall and narrow extension of the main chamber. On cars, they were usually in the head. Honda was early in using them, calling the design stratified charge, and using a rich mixture in the prechamber coupled with a leaner main charge to reduce emissions.
 
   / Diesel for Dummies #16  
Sir Harry Ricardo patented the "Ricardo IDI PreCombustion Chamber for diesel engine head design in 1931. There are a couple of different variations of the original design in use today, but they all are basically the same thing.

Here is a link to a site with some info on him and a pic of his IDI diesel head design which is still in use today:

http://www.oldengine.org/members/diesel/Misc/Ricardo.htm


Kubota's design is the same thing with some variation of the intake tract leading up to the IDI chamber to improve "air swirl" and decrease emissions by resulting in better high speed mixture of the fuel charge with the intake air.
 
   / Diesel for Dummies #17  
Sir Harry Ricardo was an extremely fascinating person with an extremely inquisitive mind.

His personal description of the combustion event as it occurs inside a diesel engine combustion chamber is one of my personal all time favorites, be sure to download the attached PDF file and read his stream of consciousness description, it is first class!!! /forums/images/graemlins/wink.gif

Let me know if you can see his point of view in your mind?
 

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   / Diesel for Dummies #18  
Thanks for posting a link to Sir Ricardo's description. It was a very interesting read. I really enjoyed seeing it from Sir Ricardo's perspective.
 
   / Diesel for Dummies #20  
<font color="blue"> Skypup, I'm having a hard time visualizing the pre-combustion chamber. Where is it in relation to the valves and combustion chamber? Is it just a scalloped out area within the cylinder head?
</font>

Normally, although there are many different similar designs, the IDI pre combustion chamber is a small 50cc or so in the head where the fuel injector is located. The actual combustion chamber is below this, similar to an automotive gasoline engine combustion chamber, ie the actual combustion chamber is on top of the piston's flat crown.

A direct injection diesel engine is completely different as there is no pre combustion chamber, instead the entire combustion chamber is contained within a depression into the top of the piston crown, sorta like a small bowl, and the injector is located directly over in the center of the cylinder head.
 

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