Diesel Theory
10-22-2004, 05:54 PM
#1
Registered User
Thread Starter
Diesel Theory
Can anyone tell me why diesels don't lean out like spark motors would if you took out the throttle body? I just don't get this. I am well versed in gas motor tech, but neither I nor any of my friends can figure this one out. I totally get the differences between compression and spark ignition, but having a wide open air path freaks me out.
Anybody have a handle on this?
Mark
Anybody have a handle on this?
Mark
10-22-2004, 08:34 PM
#2
Registered User
Join Date: Jun 2004
Location: NW IL
Posts: 850
Likes: 0
Received 0 Likes
on
0 Posts
10-22-2004, 08:48 PM
#3
Adminstrator-ess
A gasser wouldn't lean out if you removed the throttle body, it would run wide open until it chucked an important part out the side of the block and quit running. Power/RPM on a gasser is controlled by the amount of air/fuel mixture allowed into the cylinders. Varying the air/fuel ratio of the mixture will change the RPM to some extent, but that's only due to a higher percentage of misfires than at the optimum mixture.
On a diesel power/RPM is controlled by the amount of fuel injected into the cylinder. The leaner you run a diesel, the less power it makes and the slower it runs. Obviously there comes a point where it's too lean (can't make enough power to overcome friction and pumping losses) and dies out, but even at idle it's nowhere near that point. Cummins 5.9's in medium duty trucks are typically set to idle about 150 - 200 RPM slower than our Dodges because Dodge cares more about low vibration than max economy.
Does this help any, I'm not sure I addressed your question directly.
On a diesel power/RPM is controlled by the amount of fuel injected into the cylinder. The leaner you run a diesel, the less power it makes and the slower it runs. Obviously there comes a point where it's too lean (can't make enough power to overcome friction and pumping losses) and dies out, but even at idle it's nowhere near that point. Cummins 5.9's in medium duty trucks are typically set to idle about 150 - 200 RPM slower than our Dodges because Dodge cares more about low vibration than max economy.
Does this help any, I'm not sure I addressed your question directly.
10-22-2004, 09:35 PM
#4
Registered User
Thread Starter
I hate to disagree. If you tried to idle down a WOT gasser the resulting leanout backfire would burn your face off!. I've done it, it stings....
Why doesn't a diesel go lean like that?
I'm going to read the link from above
Why doesn't a diesel go lean like that?
I'm going to read the link from above
10-22-2004, 11:55 PM
#5
Registered User
Join Date: Jun 2004
Location: NW IL
Posts: 850
Likes: 0
Received 0 Likes
on
0 Posts
http://www.engineersedge.com/power_t...sel_engine.htm
This one explains a little better the diffrences in how the engine speed is controlled. Also why the gasser has a throttle plate vs. none on a diesel.
Click on the power transmission tech menu button towards to top of the page. Won't let me link to it, for some reason, for more info,
This one explains a little better the diffrences in how the engine speed is controlled. Also why the gasser has a throttle plate vs. none on a diesel.
Click on the power transmission tech menu button towards to top of the page. Won't let me link to it, for some reason, for more info,
10-23-2004, 11:35 AM
#6
Registered User
Thread Starter
Thanks much for the replys. The article still didn't get at the root of my question, but I am starting to formulate my own hypothesis. It has something to do with the pre-ignition issues with gas motors. I have a feeling that if you could radically control the spark timing and variably control the compression ratio a gas motor could be throttled with fuel only.
I'm going to think about it some more...........................
I'm going to think about it some more...........................
10-23-2004, 03:01 PM
#7
Registered User
My CPL 2080, RV275, built as a complete engine by Cummins for a Peterbuilt 330, idles the same as the Dodge. It is 800 but shows 850 on the tach. Bigger engines idle slower.
Trending Topics
10-23-2004, 04:18 PM
#8
Adminstrator-ess
I think the heart of your question can be explained this way: On a diesel, there is never any fuel in the intake tract, fuel is confined to the cylinders. There can be no backfire unless a valve malfunctions.
10-23-2004, 05:03 PM
#9
Registered User
The main idea behind it is that in a gasser you "detonate" a charge of a homogenous mixture of air and fuel to frighten the piston. Hearing the loud bang it will hastily jump back towards the crankshaft.
On a diesel you have only air in there, the piston feels well until there's this crazy guy with a blowtorch firing towards it so it's forced to retreat gradually to stay away from the heat.
This should explain the original question and why diesels rev lower than gassers
AlpineRAM
On a diesel you have only air in there, the piston feels well until there's this crazy guy with a blowtorch firing towards it so it's forced to retreat gradually to stay away from the heat.
This should explain the original question and why diesels rev lower than gassers
AlpineRAM
Did my last (serious) reply clear up your question?
10-24-2004, 03:23 PM
#13
DTR's 'Wrench thrower...' And he aims for the gusto...
Join Date: Oct 2003
Location: Smith Valley, NV (sometimes Redwood City, CA)
Posts: 2,668
Likes: 0
Received 3 Likes
on
2 Posts
mhuppertz,
It's not really useful to talk about a diesel in terms of lean or rich. At least not in the sense of total air in the cylinder vs. total fuel in the cylinder. In a gasser all the air/fuel mixture in the cylinder is already at the proper mixture for burning. It was mixed outside the cylinder or on the way in. If you put in a little less fuel you get a leaner mixture, but remember, it is already mixed and you can only change the ratio so much before it won't run at all. The key here is "already mixed". In a diesel only air comes into the cylinder and is compressed. When the injection starts to take place, (imagine you are looking very closely at the injector tip) pure fuel emerges from the injector into pure air. Right at the injector tip the fuel is liquid and too rich to burn, out away from the tip , the air has no fuel and is too lean. Somewhere in between these two places the fuel/air mix is about right and combustion begins. Combustion continues until the available fuel is burned. At this stage the mixture is again too lean, as the fuel is mostly gone, and the fire goes out. There is almost always too much air but the mixture varies in the injection area. So, unlike a gasser that has only one mixture ratio throughout the cylinder volume, a diesel has both too rich and too lean depending on where you look at it. It doesn't matter how much air is in the cylinder or how little fuel is injected, it will burn the amout of fuel that is injected, as long as there is sufficient air, because it gets to the right mixture somewhere near the injection point as it is sprayed in. Again, the "mixture" is not really a useful term in the sense of total cylinder volume. At full load the engine will begin to smoke as the volume of fuel becomes so much that there is not enough air to burn it all. At this point the mixture is rich but it is also putting out full power and to get more you would have to add more air, meaning more boost or less intake restriction, or less exhaust restriction. Or if you wanted less smoke you could back off on the throttle and have less power. When a gasser is smoking it is too rich and the power is lower than it could be no matter where the throttle is set. And the economy is bad.
Forget about rich and lean and look for smoke signals.
Wetspirit
It's not really useful to talk about a diesel in terms of lean or rich. At least not in the sense of total air in the cylinder vs. total fuel in the cylinder. In a gasser all the air/fuel mixture in the cylinder is already at the proper mixture for burning. It was mixed outside the cylinder or on the way in. If you put in a little less fuel you get a leaner mixture, but remember, it is already mixed and you can only change the ratio so much before it won't run at all. The key here is "already mixed". In a diesel only air comes into the cylinder and is compressed. When the injection starts to take place, (imagine you are looking very closely at the injector tip) pure fuel emerges from the injector into pure air. Right at the injector tip the fuel is liquid and too rich to burn, out away from the tip , the air has no fuel and is too lean. Somewhere in between these two places the fuel/air mix is about right and combustion begins. Combustion continues until the available fuel is burned. At this stage the mixture is again too lean, as the fuel is mostly gone, and the fire goes out. There is almost always too much air but the mixture varies in the injection area. So, unlike a gasser that has only one mixture ratio throughout the cylinder volume, a diesel has both too rich and too lean depending on where you look at it. It doesn't matter how much air is in the cylinder or how little fuel is injected, it will burn the amout of fuel that is injected, as long as there is sufficient air, because it gets to the right mixture somewhere near the injection point as it is sprayed in. Again, the "mixture" is not really a useful term in the sense of total cylinder volume. At full load the engine will begin to smoke as the volume of fuel becomes so much that there is not enough air to burn it all. At this point the mixture is rich but it is also putting out full power and to get more you would have to add more air, meaning more boost or less intake restriction, or less exhaust restriction. Or if you wanted less smoke you could back off on the throttle and have less power. When a gasser is smoking it is too rich and the power is lower than it could be no matter where the throttle is set. And the economy is bad.
Forget about rich and lean and look for smoke signals.
Wetspirit
10-24-2004, 04:53 PM
#15
Registered User
Thread Starter
Wetspirit,
That actually makes the most sense of anything I have read. I guess because of the lower RPM range there is time for most of the fuel to burn before the exhaust valve opens. I a gas motor you get colliding flame fronts which cause burn consistency issues, but in a diesel the burn expands accourding to it's optimal mixture path over time.
I need to think about that to see if it makes total sense!
Thanks,
Mark
That actually makes the most sense of anything I have read. I guess because of the lower RPM range there is time for most of the fuel to burn before the exhaust valve opens. I a gas motor you get colliding flame fronts which cause burn consistency issues, but in a diesel the burn expands accourding to it's optimal mixture path over time.
I need to think about that to see if it makes total sense!
Thanks,
Mark