Direct and indirect injection

Submitted: Tuesday, Jan 14, 2003 at 17:22
ThreadID: 2961 Views:3704 Replies:2 FollowUps:1
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Can anyone explain the differance between direct & indirect injection on diesel motors?
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Reply By: OziExplorer - Tuesday, Jan 14, 2003 at 18:47

Tuesday, Jan 14, 2003 at 18:47
Craig, go to and enter in the search box:
direct diesel injection against indirect injection

There are arguments (read discussion) both ways. While direct injection is now more popular that indirect injection, there have been some significant design improvements lately on indirect injection that put it well back into the race for fuel efficiency.

Which do I personally prefer - both.
AnswerID: 11293

Reply By: Dion - Tuesday, Jan 14, 2003 at 22:41

Tuesday, Jan 14, 2003 at 22:41
Direct Injection: Injector nozzle protrudes direct into cylinder space through the head and therefore sprays diesel direct into the combustion space. Typically these types of engines require very minimum glow plug at start up.
Indirect Injection: Follow a smoky diesel vehicle and you will see why indirect injection is loosing popularity. The injector nozzle does not protrude through the head, rather injects into a separate chamber within the head, then the expanding gases spread from the precombustion chamber into the cylinder for the firing stroke. A very inefficient process. Some diesel engines also have precombustion nozzles in the precombustion chamber. These are more trouble than they are worth, working loose, falling into the combustion cylinder and destroying that cylinders piston, head, gudgeon pin, even known to bend con rods.
All things being equal, a direct injection engine will leave an indirect injection engine for dead as far as efficiency, servicability, power and torque. Indirect injection is used to power dinosours.


AnswerID: 11309

Follow Up By: Will - Wednesday, Jan 15, 2003 at 10:31

Wednesday, Jan 15, 2003 at 10:31

That is not entirely true, although it is true on older indirect injection, modern indirect injection technology has improved significantly.
What Dion explained is correct to a certain extent, indirect injection diesels are not THAT bad, the reasons why passenger cars are mostly indirect injection is as follows:
Direct Injection engines are noisier due to detonation but produce more power and require little if any preheating for cold starting.
They produce more power and therefore require heavier components making the whole engine heavier, they were also more expensive to build.
Indirect injection technology does not mean a smoky engine, a direct injection can smoke just as much if the injectors leak or the pump has been tampered with....
The differences are:
In a direct injection engine, the head is flat and the combustion chamber and swirl are in the piston, the injector sprays straigh to that chamber.
In an indirect injection the pistons are flat and the chamber and swirl are in the head, the injector injects to that chamber.
It is horses for courses and the reason why direct injection is becoming more popular in smaller diesels is cost and technology, with the aid of electronic controls they can now make them quieter.

FollowupID: 6254

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