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Old 08-15-2011, 07:35 PM   #1 (permalink)
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IDI vs. DI vs. Common Rail Clarification

Soooo I'm trying to understand the mechanics of diesels, I don't own one so I hafta do this all off of text. I'm going to be a senior in high school this year and I'm seeking a Automotive Technician as a career, and I like to say I'm very know how'y already with tech stuff, but i can never know enough information on these topics...so more to the point Indirect Injection Diesels is like a normal combustion engine with the fuel being premixed with air before getting into the Combustion chambers. Right? And Direct injection is fuel injection that actually shoot fuel into the combustion chamber. Right? and Common Rail is just a High pressure version of direct injection? So if I bought a pre 98.5 12 valve which to my understanding is just direct injected i could get a high pressure fuel rail, fuel pump, and piezoelectric injectors and that would be common rail then? bringing fuel rail pressure to above 15,000 Psi which from what I understand is what it needs to be Common rail. It seems to simple so I hope some one reads this and sets me straight.

Thank You for your time.
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Old 08-15-2011, 07:45 PM   #2 (permalink)
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read these two articles. it gives you a good
idea of the differences. basically, indirect
is fuel loaded into the intake runners. direct
is fuel in the chamber directly.

Indirect injection - Wikipedia, the free encyclopedia

Gasoline direct injection - Wikipedia, the free encyclopedia
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Old 08-15-2011, 08:46 PM   #3 (permalink)
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That really helped in clarification but in the first article about Indirect Injection was making it sound like that Direct Injection can only be used in bigger displacement engines. My question is why, is it because Direct Injection is a more violent burn than Indirect? And also What makes it Common Rail? because Everything Fuel Injected has fuel rails. Is it the having the Fuel Rail on the head rather than on the intake manifold or pre-chamber? The articles I've read make it sound like Common Rail has to due with Injectors and Fuel Pressure.
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Old 08-15-2011, 08:52 PM   #4 (permalink)
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"common rail" just indicates that there is a
common fuel rail that runs up to the head.
most high pressure common rails, or hpcr motors
run some sort of injection pump that runs very high
fuel pressure. the common rail cummins motor idles
at 6k, anad wot can run up to 26k psi, depending on
mods.

not sure about larger displacement for direct injection
only, as caddy has a 6 banger with direct injection
making some really nice numbers for a 6.
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Old 08-15-2011, 09:22 PM   #5 (permalink)
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I Just reread the Common Rail wiki page(i would be nothing without wiki) and it essentially said Common rail drives it name from the Fuel Rail which maintains a really high fuel pressure. But thank you very much for helping me figure that out.
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Old 08-15-2011, 10:35 PM   #6 (permalink)
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Quote:
Originally Posted by Josh429er View Post
Soooo I'm trying to understand the mechanics of diesels, I don't own one so I hafta do this all off of text. I'm going to be a senior in high school this year and I'm seeking a Automotive Technician as a career, and I like to say I'm very know how'y already with tech stuff, but i can never know enough information on these topics...so more to the point Indirect Injection Diesels is like a normal combustion engine with the fuel being premixed with air before getting into the Combustion chambers. Right? And Direct injection is fuel injection that actually shoot fuel into the combustion chamber. Right? and Common Rail is just a High pressure version of direct injection? So if I bought a pre 98.5 12 valve which to my understanding is just direct injected i could get a high pressure fuel rail, fuel pump, and piezoelectric injectors and that would be common rail then? bringing fuel rail pressure to above 15,000 Psi which from what I understand is what it needs to be Common rail. It seems to simple so I hope some one reads this and sets me straight.

Thank You for your time.
There are two injection systems IDI and DI.

direct injection is where the fuel is injected directly into the cylinder. (IDI is when it goes to the intake port just shy of the valve)

Now reguardless of injection method (IDI, or DI) the "feed system", can be of only two ways there are in dependent lines and the "common rail". On smaller medium and high speed diesels that use mechanical pressure they are usually independent because a mechanical common rail system is to big for small displacement motors.

Railroad diesels and marine engines are common rails but are mechanically injected via an rocker arm on the cam shaft. they are called low speed motors.


mechanical common rail injection is used on larger diesel engines because they can inject the fuel more reliability then a in depend line motor.


All Cummins B series diesel built before 2003 are all mechanical injection they feed their fuel independently using an injection pump.

The injection pump meters the fuel so that enough fuel is in the rail when the injector is "popped" open when the fuel pressure falls below the "pop" open pressure it closes, the injection pumps are fixed to the engine rpm's and usually run at either 1/2 or at 2x the crank shaft speed.

starting in 2003 the B series motors moved to electrical common rail injection, the common rail uses electrical injection, where an electrical coil is energized causing the injector to open and close when the signal is cut off. because of this you have the ability to inject fuel as many times as you want.

The more times an injector opens per power cycle the quieter the motor gets.


It is this reason why the motors on the CR's dodges are so quiet compared to the mechanical dodges. However port pressure(pressure at which the fuel is injected) raises the bang factor goes up, the VP-44 trucks run around 4,000-5,000psi and as such you can hear then from a mile away compared to their older brethren the P-7100 (aka 12v'ers) which run lower.

the Common rails in 2003 came out with i believe 3 injection events, but i know that the 6.7L dodges have either 4 or 5 depending on if it is in regen.

the added injection events can cause cylinder wash which is somethi9gn that needs to be monitored because fuel draining into the oil is a bad thing.





Now here is a shocker, many people think that "Common Rails" are a fairly new technology, they are not. Common rail diesel and common rail gas engines were used during world war 2 by the Germans to great effect. and were in-fact also direct injection.

This allowed the Germans to down many allied plains because they couldn't launch down they would have to invert to drop altitude and as such they could escape allied fighters this way. (allied fighters used a single head carburetor until midway through the war when they were able to use dual headed carbs that could allow them to drop altitude without the need to "roll over" first.
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Old 08-16-2011, 02:09 AM   #7 (permalink)
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You summed up everything I read today. lol Thanks for the clarification Wiki did say that everything from DI and CR is old as dirt being invented around the 20's which is Insane when you think about. Makes you wonder what where discovering now. And why the hell everyone looks to the Prius for the answer there green needs when all you need is a DI engine.

Maybe I need to re-read the article but I don't remember if wiki said that its the mechanical DI system that cant be made for small engines but that makes sense. I think it said in general it cant be done. Wiki said that the mechanical lift pumps are reliable in the sense that it keeps fuel flow in check but its not for making power mostly because it cant keep up irregular speed changes, or high engine speeds. Also Do DI diesels need glow plugs? Because they made it seem that it didn't.
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Old 08-16-2011, 03:13 AM   #8 (permalink)
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You summed up everything I read today. lol Thanks for the clarification Wiki did say that everything from DI and CR is old as dirt being invented around the 20's which is Insane when you think about. Makes you wonder what where discovering now. And why the hell everyone looks to the Prius for the answer there green needs when all you need is a DI engine.

Maybe I need to re-read the article but I don't remember if wiki said that its the mechanical DI system that cant be made for small engines but that makes sense. I think it said in general it cant be done. Wiki said that the mechanical lift pumps are reliable in the sense that it keeps fuel flow in check but its not for making power mostly because it cant keep up irregular speed changes, or high engine speeds. Also Do DI diesels need glow plugs? Because they made it seem that it didn't.
typically you cannot achieve the best thermal efficiency with a direct injection system for a very small motor. that is not to say it is not being done now a days, but it would have to be electrical. most of your smaller engines are infact IDI, unless turbo charged. the reason is because of the design, IDI engines require good quick airflow, rather wise DI systems use slow moving air and fast burning fuel.(back to thermal efficiency) a mechanical system cannot really meter the proper fuel amount on such a small engine, if you think about it most diesel run lean so lean that they are running 50-60:1, now think if your whole Kubota tractor engine is say 23cid that means your burning .46units of fuel, the metering would be to difficult, not to say it couldn't be done, but such small set up could pose problems down the road.


Direct injection systems depending on the set up you can use either glow plugs or grid heaters. grid heaters heat the air in the intake manifold so that it is already warm by the time it gets to your cylinder.

rather wise a glow plug system tries to heat every cylinder, the major problem is you can get bad engine knock, and even then trying to heat 8 cylinders the heat is dissipated to quickly.

there are advantages to both, but i personally like grid heaters as they tend to stay on During the cranking process ensuring a constant flow of warm air into the cylinder area, rather wise diesel and hot glowing metal do not mix

This is crucial as a cold diesel engine literally saps any warmth around it, (like try wrenching on a cold diesel engine and your hand (regardless of gloves), will freeze.) A cold diesel engine usually will not hit the compression temperature for auto ignition of diesel fuel injected. the use of grid heaters and block heaters was necessary a long time ago because the injection systems had static timing, (ratherwise starting with the VP-44 and go newer), they were dynamic, allowing for colder starting operations.

For example there are stories of a 24v(Vp-44) cummins starting down to -20 below zero and some, rather wise a old 12v'er will struggle to even run.

now newer common rails can advance the timing so much that they can achieve the most reliable start because they can inject when the air is the most compressed, but once again you have the basic problem of cold dense air is colder then warmer air.

regardless diesels much use their heating systems to generate heat at idle in the cold, as diesel are cold idlers, meaning that they drop below combustion temp at idle, it can be 100F out and my 99 dodge will idle at 200-250F, now when it is -10F it will idle in the 150-180F, unless i kick in high idle and even then it is only running 250ish. At these temperatures the block is cooling down and absorbing the heat from the motor, without a load the engine will cool down progressively, unlike a gas engine which will overheat if given the chance.

this is why having a working heating system is important for cold weather operation, typically for all 24v trucks it is only needed down below 10-20F, both of my trucks will start down to 10-15F without heaters that is not to say you should always do it.

For IDI engines they must use glow plugs and then the glow plug is tuned on and is located in the prechamber, (where the injector is), rather wise on a DI system the glow plug is located inside the cylinder.
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Old 08-16-2011, 05:08 AM   #9 (permalink)
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typically you cannot achieve the best thermal efficiency with a direct injection system for a very small motor. that is not to say it is not being done now a days, but it would have to be electrical. most of your smaller engines are infact IDI, unless turbo charged. the reason is because of the design, IDI engines require good quick airflow, rather wise DI systems use slow moving air and fast burning fuel.(back to thermal efficiency) a mechanical system cannot really meter the proper fuel amount on such a small engine, if you think about it most diesel run lean so lean that they are running 50-60:1, now think if your whole Kubota tractor engine is say 23cid that means your burning .46units of fuel, the metering would be to difficult, not to say it couldn't be done, but such small set up could pose problems down the road.


Direct injection systems depending on the set up you can use either glow plugs or grid heaters. grid heaters heat the air in the intake manifold so that it is already warm by the time it gets to your cylinder.

rather wise a glow plug system tries to heat every cylinder, the major problem is you can get bad engine knock, and even then trying to heat 8 cylinders the heat is dissipated to quickly.

there are advantages to both, but i personally like grid heaters as they tend to stay on During the cranking process ensuring a constant flow of warm air into the cylinder area, rather wise diesel and hot glowing metal do not mix

This is crucial as a cold diesel engine literally saps any warmth around it, (like try wrenching on a cold diesel engine and your hand (regardless of gloves), will freeze.) A cold diesel engine usually will not hit the compression temperature for auto ignition of diesel fuel injected. the use of grid heaters and block heaters was necessary a long time ago because the injection systems had static timing, (ratherwise starting with the VP-44 and go newer), they were dynamic, allowing for colder starting operations.

For example there are stories of a 24v(Vp-44) cummins starting down to -20 below zero and some, rather wise a old 12v'er will struggle to even run.

now newer common rails can advance the timing so much that they can achieve the most reliable start because they can inject when the air is the most compressed, but once again you have the basic problem of cold dense air is colder then warmer air.

regardless diesels much use their heating systems to generate heat at idle in the cold, as diesel are cold idlers, meaning that they drop below combustion temp at idle, it can be 100F out and my 99 dodge will idle at 200-250F, now when it is -10F it will idle in the 150-180F, unless i kick in high idle and even then it is only running 250ish. At these temperatures the block is cooling down and absorbing the heat from the motor, without a load the engine will cool down progressively, unlike a gas engine which will overheat if given the chance.

this is why having a working heating system is important for cold weather operation, typically for all 24v trucks it is only needed down below 10-20F, both of my trucks will start down to 10-15F without heaters that is not to say you should always do it.

For IDI engines they must use glow plugs and then the glow plug is tuned on and is located in the prechamber, (where the injector is), rather wise on a DI system the glow plug is located inside the cylinder.

First off I would like to say that you are most likely the smartest person i have ever talked to. lol That took a little bit to understand thought that Glow plugs where like Spark Plugs but with less spark.http://www.cumminsforum.com/forum/images/smilies/rofl[1].gif I mean that Glow Plugs are much like spark plugs they don't spark they just get hot and help create high compression for the diesel when it cannot do it other wise. And I guess Einstein is right. It doesn't take genius to make something larger, but it does to make it smaller. So could the Kubota be common Rail, You said that Common Rail lacks the bulky mechanical injection systems but its also more precise with injections.
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Old 08-16-2011, 06:15 PM   #10 (permalink)
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First off I would like to say that you are most likely the smartest person i have ever talked to. lol That took a little bit to understand thought that Glow plugs where like Spark Plugs but with less spark.http://www.cumminsforum.com/forum/images/smilies/rofl[1].gif I mean that Glow Plugs are much like spark plugs they don't spark they just get hot and help create high compression for the diesel when it cannot do it other wise. And I guess Einstein is right. It doesn't take genius to make something larger, but it does to make it smaller. So could the Kubota be common Rail, You said that Common Rail lacks the bulky mechanical injection systems but its also more precise with injections.

Glow plugs are like spark plugs but think of them more like an electrical stove, the element that you put the pot on will heat the water up, the difference is by heating the air it will raise the temperature up, hopefully as close to 400F as it can get, normally a 5.9L Cummins is about 500F when it has reached TDC. they don't create the high compression but by injecting heat into the cylinder they can try to decrease the air density and by doing that they can raise the temperature because warm air tends to be less dense and will expand temperature wise, similar to as when you take a bottle with air in it and a bit of water and throw it into a freezer.


Our kubota is a 2006 tractor(i think either 06 or 07) and uses an IDI mechanical system, similar to a 12v it is an inline plunger type pump.

the kubota could be common rail, but the cost to build it in common rail would quite expensive, plus it would be harder to map for it considering many tractor are abused beyond belif. The biggest reason i think would be that it would still have to be IDI injection, as direct injection on such a small motor would be even harder to produce then an IDI version. If such is the case a common rail system would be a little to big for such a mall motor.

After all our kubota's 25HP motor is a meer 15 inches long, there would be no room to put the CP3 pump, and all the accessories, so in the end the mechanical system would be cheaper in the long run, IMO.
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Old 08-17-2011, 03:23 PM   #11 (permalink)
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Glow plugs are like spark plugs but think of them more like an electrical stove, the element that you put the pot on will heat the water up, the difference is by heating the air it will raise the temperature up, hopefully as close to 400F as it can get, normally a 5.9L Cummins is about 500F when it has reached TDC. they don't create the high compression but by injecting heat into the cylinder they can try to decrease the air density and by doing that they can raise the temperature because warm air tends to be less dense and will expand temperature wise, similar to as when you take a bottle with air in it and a bit of water and throw it into a freezer.


Our kubota is a 2006 tractor(i think either 06 or 07) and uses an IDI mechanical system, similar to a 12v it is an inline plunger type pump.

the kubota could be common rail, but the cost to build it in common rail would quite expensive, plus it would be harder to map for it considering many tractor are abused beyond belif. The biggest reason i think would be that it would still have to be IDI injection, as direct injection on such a small motor would be even harder to produce then an IDI version. If such is the case a common rail system would be a little to big for such a mall motor.

After all our kubota's 25HP motor is a meer 15 inches long, there would be no room to put the CP3 pump, and all the accessories, so in the end the mechanical system would be cheaper in the long run, IMO.
Alright That helps a lot. Thanks for helping me out.
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Old 09-21-2011, 06:26 PM   #12 (permalink)
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All that reading just confused me, there was words in there I couldnt pronounce.
This is what I got, Please correct me if Im wrong. I could write a elabirate response, but it this the basic Idea?

IDI - Fuel goes into a pump and then is sent through injectors to power the vehicle.

DI - Fuel goes directly into the injectors to power the vehicle.
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