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deisel -vs- gas engine combustion

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Dave in SV

04-07-2011 19:15:40




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I understand how combustion is timed in a gas engine, via a spark plug ignition... but how is it timed in a deisel engine, where there is no spark? I suppose I could Google this question... but I seem to understand you guys a lot better!




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MoMark

04-08-2011 04:33:19




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 Re: deisel -vs- gas engine combustion in reply to Dave in SV, 04-07-2011 19:15:40  
Even more intriguing to me is the fact that a diesel has no throttle plate to meter air - it gets full air flow all the time, so the speed is completely controlled by the fuel charge.

I currently have 2 diesels, one in my deuce & a half with a mechanical injection pump & the other is a 7.3 Power Stroke. Now that is a strange engine. The high-pressure oil pump pressurizes oil up to about 3000 psi, which pressurizes the fuel charge. The injector itself is 90 volts DC.

Unfortunately I don't have a big Deere diesel but I am trying to remedy that situation.

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David G

04-10-2011 15:45:05




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 Re: deisel -vs- gas engine combustion in reply to MoMark, 04-08-2011 04:33:19  
The newest Diesels use up to 30,000 PSI on the injection. I thought the 3000 PSI units were high pressure.



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buickanddeere

04-10-2011 17:58:18




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 30,000psi spray in reply to David G, 04-10-2011 15:45:05  
I wonder what the long term durability of a 30,000psi injection system will have? Tough to beat a Bosch P pump.



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onefarmer

04-07-2011 22:20:00




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 Re: deisel -vs- gas engine combustion in reply to Dave in SV, 04-07-2011 19:15:40  
Dave, Have you noticed how an air compressor gets hot when it runs? It isn't friction causing the heat, it's the air being compressed. The compression in a diesel is high enough to heat the air enough to ignite the atomized diesel fuel.

The injector pump serves two purposes. One to deliver a precise metered amount of fuel for the required speed and power of the engine. More fuel more speed, more power. It also serves as the distributor like that of a gas engine. Instead of distributing a spark to cause combustion, it's distributing the precisely metered fuel, into the heated air so it will combust at a precise time.

The injectors are like the carb of a gas engine. They, due to the injector pump making a high pressure, deliver the fuel through microscopic hole in the injector tip, causing the fuel to atomize.

Since I'm not a diesel mechanic this is all probably quite simplistic, but it's the general drift.

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husker44a

04-07-2011 19:55:54




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 Re: deisel -vs- gas engine combustion in reply to Dave in SV, 04-07-2011 19:15:40  
Dave a oil-burner is timed by the same principal as a gas engine but, the rattling you hear in a oil-burner is the cylinders firing. Gas has to have a spark to ignite. But, a deisel is ignited by compression and a lot of it comparred to a gasser. Also, on a gasser when you turn the key off it shuts the voltage off to the coil. Where a deisel is shut off by shutting off the fuel flow that goes to the injectors or sometimes a electric solinoid.

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Dave in SV

04-07-2011 21:26:27




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 Re: deisel -vs- gas engine combustion in reply to husker44a, 04-07-2011 19:55:54  
But to get the deisel fuel to ignite at precisely the right time, it's gotta be more than just compression that triggers it. No Horses said it's based on when fuel is shot into the cylinder. I guess I can't wrap my head around squirting fuel into a combustion chamber, which is under compression, at just the right time to advance the four-stroke cycle efficiently and with power. I understand compression equals heat, and enough heat will ignite the fuel... I'm just too stubborn or too simple-minded to understand how it all comes together (physically and timingwise). Heck, I'm probably spelling the word diesel wrong, to boot!

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buck eye al

04-08-2011 15:14:13




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 Re: deisel -vs- gas engine combustion in reply to Dave in SV, 04-07-2011 21:26:27  
Maybe this will help...

When the piston pressurizes air in a cylinder, the air temperature rises as others have correctly stated. How much it rises depends on the incoming temperature, the amount and the rate at which it is compressed. The quicker you compress it and the more you compress it, the hotter it gets. Well into the hundreds of degrees F. I don't have a chart handy but seems to me the shop manual for my 70D says as much as 500 F. Not real certain but I do feel safe saying in the hundreds of degrees.

Of course if it's only 10F and the cranking speed is low it's not going to be so high a temperature and that's why cold diesels can be hard to start.

The rest of the story comes from the ability of the pump with the injector's help to "time" the injection.

When the fuel comes out of the injector nozzle it is not a squirt of liquid or really much of a fine mist. It's more like a fog. We all know it doesn't matter what the fuel, the smaller the particles, the easier it is to burn it up because it more easily combines with oxygen and burns. That's why we split firewood into little pieces and call it kindling. Each fuel has what is called a flash point. That is the temperature at which it burns. Raise its vapors to this temperature in the presence of oxygen and it will burn. May not continue to burn until more vapor is produced but the vapors will light up. See an easy to understand table of flash points at www.engineeringtoolbox.com/flash-point-fuels-d_937.html.

On that table the flash point of #2 Diesel is listed as 125F.

Now put all those parts together into the engine. The injector pump squirts liquid #2 into the injector. When the injector can hold it back no longer because the pressure has reached it's "breaking point" the fuel bursts out the nozzle into the combustion chamber as a vapor or microscopic droplets of fuel. Those microscopic droplets have a flash point of 125 F and there is oxygen in this chamber already heated to maybe 350 to 500F well above their flash point and that temperatiure is probably still rising. Those droplets of fuel BURST into flame!

Then the typical diesel "knock" is heard, smoke comes out the stack & we're ready to do some work!

Doesn't make any difference if it's a 4 stroke, a 2 stroke or a pile driver. They all work the same. Squirt a little #2 through a small enough nozzle at high enough pressure into hot enough oxygenated air and fire will happen.

Helpful? Hope so. Later.

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Dave in SV

04-08-2011 19:33:36




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 Re: deisel -vs- gas engine combustion in reply to buck eye al, 04-08-2011 15:14:13  
VERY helpful info! Posts by all others on this subject were also very helpful. Bottom line, as No Horse stated it, timing is controlled by when the fuel is injected into the cylinder. Now I'm going to start chewing on just HOW the injector/pump/fuel distribution system can so specifically time the injection of fuel.



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Bob Harvey

04-08-2011 17:42:37




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 Re: deisel -vs- gas engine combustion in reply to buck eye al, 04-08-2011 15:14:13  
I gotta pipe in. Is the injector/pump TIMED to each cylinder? I suspect it is, need a definitive answer.



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buickanddeere

04-08-2011 18:00:16




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 Re: deisel -vs- gas engine combustion in reply to Bob Harvey, 04-08-2011 17:42:37  
Each cylinder is individually supplied with a precision puff of fuel each time a piston raises close to TDC on the compression stroke. Injection event initiates 16 to 28 degrees prior to top dead center depending..... ..... .. Amount of fuel delivered depends on how long the injector sprays. The identical amount of fuel injected as a short blast while the piston is just prior to TDC. Starting at 24 degrees btdc. Is more efficient that the same amount of fuel starting at the same time eg 24 degrees btdc. Delivered at a lower flow rate. And the injection event continuing after TDC. Deere for example found two cylinder power and economy going up to 13mm pumps from 10mm pumps.

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Bob Harvey

04-08-2011 19:03:47




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 Re: deisel -vs- gas engine combustion in reply to buickanddeere, 04-08-2011 18:00:16  
Thanks for the reply. I think it will answer the original posters question.



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Dave in SV

04-08-2011 19:24:23




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 Re: deisel -vs- gas engine combustion in reply to Bob Harvey, 04-08-2011 19:03:47  
Yes it did!



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buickanddeere

04-08-2011 15:54:12




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 Re: deisel -vs- gas engine combustion in reply to buck eye al, 04-08-2011 15:14:13  
Pretty much but flash point and ignition temp are two different values.

http://hypertextbook.com/facts/2005/EileenTang.shtml



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730d se

04-08-2011 04:47:05




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 Re: deisel -vs- gas engine combustion in reply to Dave in SV, 04-07-2011 21:26:27  
Well, to make it even more complicated, some engines actually "fire" or squirt fuel more than one time in a cycle. It is nano seconds apart but it helps with noise and polution.
A fluid such as diesel is not compressable like air. Therefore, the cylinder pressures, albeit they may be 400 PSI and over, are not high enough to force fuel backwards through the injectors.



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JMS/.MN

04-07-2011 23:35:20




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 Re: deisel -vs- gas engine combustion in reply to Dave in SV, 04-07-2011 21:26:27  
Works as long as the injector pump creates more pressure than what is in the cylinder. Beyond that, timing is critical.



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No Horses

04-07-2011 19:19:47




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 Re: deisel -vs- gas engine combustion in reply to Dave in SV, 04-07-2011 19:15:40  
It is timed by when the diesel is squirted into the chamber.



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