Direct Injection & Indirect Injection Diesels

[Anonymous Poster]

DIRECT-INJECTION AND INDIRECT-INJECTION

The two fuel injection processes used in diesel engines, direct-injection (DI) and indirect-injection (IDI), are illustrated in Figures 6-2 and 6-3 (see below).

In a DI engine, fuel is injected directly into the cylinder above the piston.

In an IDI engine, fuel is injected into a small prechamber connected to the cylinder via a narrow passage that enters the prechamber tangentially. During the compression process, air is forced through this passage, generating a vigorous swirling motion in the prechamber. Then fuel is injected into the prechamber and ignition occurs there. The combination of rapidly swirling air in the prechamber and the jet-like expansion of combustion gases from the prechamber into the cylinder enhances the mixing and combustion of the fuel and air.

The more rapid mixing of fuel and air achieved in IDI engines comes at a price, however. The high velocity flow of air through the narrow passage connecting the main cylinder to the prechamber, as well as the vigorous swirling motion in the prechamber itself, causes the air to lose significantly more heat during compression than it does in a DI engine. Coupled with a pressure drop from the main chamber to the prechamber, this results in an air temperature in the prechamber after compression that is lower than that in a similar DI engine.

Since rapid fuel auto ignition requires a certain air temperature, an IDI engine needs a higher compression ratio to achieve the desired air temperature in the prechamber. IDI engines operate at compression ratios of about 20:1 to 24:1; while DI engines operate at ratios of about 15:1 to 18:1. The heat losses that necessitate these higher compression ratios have another, more important effect: they decrease the efficiency of the engine. IDI engines typically achieve fuel efficiencies that are 10% to 20% lower, on a relative basis, than comparable DI engines.

Even with the higher compression ratios, IDI engines may still be hard to start. Most IDI engines use glow plugs to heat the air in the prechamber in order to make starting easier. Glow plugs, which are small resistive heaters, are usually powered for only the first few minutes of engine operation.

With the negative attributes of harder starting and lower efficiency, one may wonder why IDI diesel engines are used at all. The answer is engine speed. As an engine gets smaller, generally it must operate at higher speeds to generate the desired power. As engine speed increases, there is less time per engine cycle to inject, vaporize, mix, and combust the fuel. As a result, the higher mixing rates afforded by IDI designs become necessary to achieve good combustion at higher engine speeds. IDI diesels most commonly are used in smaller automotive and light duty truck applications.

Chevron Entire Engine Article

 

[kerr]

this one is gonna hurt!

 

[geneP]

That's an informative article, John. Thanks for sharing it with us.

 

[JATO_RaT]

John, Cummins produces an article similar to this as well. Perhaps its worth mentioning that while it would appear that DI is the best way to go for large, slow turning diesels, it actually works quite well for the little diesels put in our compact tractors and light trucks. It demonstrates where this article needs a little updating. In the last 5 years, diesels have changed dramaticaly. Perhaps in New York you see little evidence of this, but here in California, its a constant reminder in the newspapers about the "Evil Diesel". The article you inserted suggests that a DI diesel is inherently superior to all IDI's. While this may be true for the most part, it fails to address some of the biggest advances in diesel technology. Here in CA. as I mentioned, diesels are bad, very bad. In an effort to combat this attitude, Cummins, Navistar, Kubota, Isuzu, Deutz, Perkins, John Deere, Catepillar, Detroit, Shibura, Yanmar etc. have struggled (yes struggled) to get their diesels to conform to the latest CARB (California Air Resources Board) requirements. CARB tends to think of themselves as the dictator of all that goes and expands its policies to the entire country it would seem. Cummins ups pump pressure from its electroniclly controlled Bosch system to make a better vaporized mixture of air and diesel. Other companys are designing diesels to do much the same.

Intoducing the first and ONLY diesel to meet and exceed all CARB requirements into the year 2004. Its going to shock you, its new or is it. Its super quiet, effiicient and very clean burning. It uses glow plugs when rarely needed. Its designed by a company that used to build old fashioned DI diesels almost exclusively and now build pretty much IDI diesels exclusively. Who is this ingenious new diesel technology leader, its Kubotas E-TVCS diesel (three vortex combustion system). Perhaps you have heard of it but figured since it was IDI it merited little attention, fact is, whats coming around the corner is very tight emission standards for diesels, even those on generators, tractors, pumps etc. Buses in CA are almost all CNG or propane, very few metro buses are diesel.

Perhaps the new 4X10 series diesels will use the new Kubotas so they too will have superior technology to match the expected triumphs they plan on the new series, if not, I'm sure they will settle for the same 3rd party vendor they have always settled for, the old fashioned, tried and true DI diesel from Yanmar, Rat...

 

[Anonymous Poster]

I wonder where the VW TDI fits into the mix... it's small (1900cc), high revving (much higher than Kubota), and direct injected.

 

[JATO_RaT]

I don't know about the TDI, what is its yellow line, red line RPM's. Out here in CA., most cars are not allowed to be diesel. There were and are exceptions. The restrictions for cars are so tight. I'm familiar with the diesel VW put in the VW Rabbit Pickup, that was a IDI, VW's feeble attempt to put together a fuel efficient ensemble that I don't think was very good. My dad had one. Mercedes gave it a stab as well and probably gave diesels some of the biggest black eyes around. While Mercedes makes a great product, the smoke pouring out of their turbocharged diesels was not one of them. Today, you see very few automobiles with diesels in California. About the only one left is Mercedes. They have cleaned up their act, of course they had to or CA. would never allow them to be sold. Today, very few trucks emit much smoke on the highways here, about the only ones are the old diesels and those are rapidly disappearing. Still, its a bad thing to be diesel here. Who knows where its going to wind up. Soot traps are coming for all diesels in CA. Snap throttle tests by the EPA for all diesel vehicles will be implemented as well. Some of it is welcomed, some of it is ridiculous. Rat...

 

[Boner]

Interesting info Rat. I have always had an interest in diesels. Always looking to learn more about them. I would like to find an old Toyota diesel for a Toyota truck that I have. ...something else to tinker with. They aren't as strick here in Louisiana as they are in CA.

 

[JATO_RaT]

I remember the diesel that Toyota put in their truck. It was a well layed out setup, very clean looking, other then that I'm clueless about how well it performed. Rat...

 

[Anonymous Poster]

The VW TDI has a redline up around 5k rpm, I believe.

As far as Mercedes diesels being junk, I would have to disagree. There are way too many of them running around with 500k to a million miles on them to be lumped in the category of GMs bastardized diesels. If anyone gave diesels a black eye in the US, it was GM.

 

[JATO_RaT]

No question about that, GM gave diesels a bad name, at least for a while. I don't think the Mercedes were junk, they were very well built, they just put out more soot then most small automobile diesels. For that reason, people saw an expensive car pouring out smoke and had negative impressions of the diesel, particularly the Mercedes built diesel. People almost expected it of trucks, but cars, especially when a perfectly good gasoline engine was available, that was to much. In high school, I really wanted to get an old 190D to fix up. Today I see a few new Mercedes diesel cars, but no where near what it once was. They are clean and fairly powerful. Rat...