I continue to learn a bunch about my new diesel truck. As a first time diesel owner, I learn new things almost every day, from friends, and forums such as this one.

I do have a very basic question. And, I’m not ashamed to ask it, because I asked several friends, and they didn’t have an answer. So, I’m not the only one who is ignorant.

Here’s the quesation. I know my truck doesn’t have spark plugs. And, I remember reading in this forum that Cummins doesn’t have glo plugs. So, how does the engine fire up? Where does the fire come from to ignite the fuel. Is it just tremendous compression, or what?

Sorry for such a simple question, but I am interested in learning the answer.

 

You're right, ignition of the very finely atomized diesel comes from the high compression - roughly 17 to 1 on a 2002 H.O.

The engine intakes air and compresses it, and then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine.

The injector on a diesel engine is its most complex component and has been the subject of a great deal of experimentation -- in any particular engine it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process.

One big difference between a diesel engine and a gas engine is in the injection process. Most gasoline engines use port injection or a carburetor rather than direct injection. In a gasoline engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine -- if it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking. A diesel compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power is generated.

All functions in a modern diesel engine are controlled by the ECM communicating with an elaborate set of sensors measuring everything from R.P.M. to engine coolant and oil temperatures and even engine position (i.e. T.D.C.). The ECM senses ambient air temperature and retards the timing of the engine in cold weather so the injector sprays the fuel at a later time. The air in the cylinder is compressed more, creating more heat, which aids in starting.

 

Like Dr. Evil said, heat plus compression=bang. My truck runs 16.2:1 compression, and with the heat in the combustion chamber it explodes the diesel air mix. Laymens terms.

 

Short and simple.

The heat to ignite the fuel comes from the fact that the piston compresses the air enough to ignite the fuel. When you compress a gas it gets hot. You may have also noticed the inverse happening when a liquid/gas is allowed to decompress, an LP tank or aersol can.

http://auto.howstuffworks.com/diesel1.htm

 

Excellent answers.

Thanks very much.

I appreciate it!!

 

Good write up Doc.. Gives a very good overview of the diesel process.

 

[quote author=Lil Dog link=board=8;threadid=23571;start=0#msg222196 date=1071179495]
Good write up Doc.. Gives a very good overview of the diesel process.


[/quote]

Actually, I really cant take credit...I pulled it from www.howstufffworks.com - should have mentioned that. Getting too l a z y to type... ;D

 

Just a little bit of clarification to the above good stuff. When the diesel/air mixture ignites it is designed to burn and NOT to explode. Exploding is bad. Burning at a controlled rate is good. Probably the easiest way to show this is to go to a gasser. When the engine 'pings' the mixture is exploding and not burning as it should.

 

[quote author=DodgeRamCO42 link=board=8;threadid=23571;start=0#msg221829 date=1071118446]

And, I remember reading in this forum that Cummins doesn’t have glo plugs. So, how does the engine fire up? Where does the fire come from to ignite the fuel. Is it just tremendous compression, or what?

Sorry for such a simple question, but I am interested in learning the answer. [/quote]

Just for your information. The glow plugs in diesels - such as Power Stroke - do not ignite the fuel. In simple terms it pre-warms the air in the cylinder, making it much easier for the fuel to ignite in the compressed air. Our trucks have heater grids in the air intake, and having had both I can say ours is the much, much better system.

Don't ever be sorry for asking a question. Every one of us had to learn somewhere and these forums are the best - this one being the best of the best.

Stan

 

Hence the term "compression ignition".

 

Hence??? are we even allowed to use that word on here.....the next post I read Ill prolly see - "ergo" and "henceforth"

 

[quote author=Dr. Evil link=board=8;threadid=23571;start=0#msg222265 date=1071187565]
Hence??? are we even allowed to use that word on here.....the next post I read Ill prolly see - "ergo" and "henceforth"
[/quote]

I knew when Chris used the word "Epiphany" last week, there it was all down hill from there Blasted sailors anyway!

Practices drinking tea with my pinky finger pointed skyward

 

Also just so you know th pressure from which a injector sprays is anywhere from 5,000 psi to 25,000 psi depending on motor size. 5,000 psi is more for our trucks and 25,000 psi is what alot of the big rigs will spray at. So you have to take that into effect. While that air is being compressed to a very high psi the injector will spray around 5,000 psi of diesel into the cave part of that piston. So that is also why a diesel "knocks" "rattles" etc...

 

I knew an Epipheny once and let me tell you, OOPS, wrong forum..... :-[ :-X

Henceforth and heretowith,

 

[quote author=Lary Ellis (Top) link=board=8;threadid=23571;start=0#msg222285 date=1071189338]
Practices drinking tea with my pinky finger pointed skyward
[/quote]

Don't forget those crumpets... :