Oil & Fuel General diesel question

[dodge man]

I'm going to show my general ignorance of how tractors work. How is the fuel and throttle governed on a diesel tractor? I can picture a common small engine and a governor opens up the throttle blade as needed. Of course in your car you open up the throttle with your right foot. How does this work on a tractor? You normally set the throttle by hand. Then does the throttle stay open the same no matter the load? How does the fuel injection work? As far as I know most tractors still don't have computers to tell them how much and when to inject the fuel. Its a mechanical set up I assume but as the engine hits a heavy load, how does it know to deliver more fuel? Or do tractors have a govenor to open up the throttle when it hits a heavy load?

Thanks in advance for the answers.

 

[tallyho8]

Yes tractors have governors that allow the injection pump to pump more fuel when necessary to maintain a constant speed.

 

[N80]

I think a lot, maybe most (?) bigger, newer Ag machines have computer controlled fuel systems. I don't know about valve timing.

My very simple tractor might have a governor, I don't know. But when the throttle lever is set, if you approach a hill, or aquire more load, you have to adjust the lever accordingly.

 

[Mickey_Fx]

Fuel/air systems are quite different between a gas and diesel engine.

In a gas engine air and fuel are regulated to control engine speed. The "throttle" controls the air flow and fuel is metered trying to maintain a relatively consistent fuel/air ratio.

In a diesel, air flow is not controlled, the engine always gets a full measure of air. Engine speed is controlled by varying the amount of fuel injected. In diesels the so called "throttle" is actually controlling only the fuel supply.

On any engine with a governor, the operator sets the speed of the engine via the governor and as loads vary, the governor varies either the air or fuel in an effort to try and maintain the set speed.

Tractor engines do have governors.

 

[N80]

Tractor engines do have governors.

I'm assuming the governor can only adapt so much. Is that right? In other words, I put my tractor in gear, set the throttle and putt along. Come to a small hill and it will just keep going along about the same speed. But, hit a bigger hill and it might start to bogg a bit. Same thing with mowing. cruising along with the throttle set just right but then hit a big deep patch of grass and brush and it will start to bogg down. So the governor, at least mine, does not seem capable of miantaining a constant ground speed or power output for all conditions. Does that sound right?

 

[Egon]

I'm assuming the governor can only adapt so much

It may be about response time!

No lead and the response is lousy!

 

[john_bud]

I'm assuming the governor can only adapt so much. Is that right? In other words, I put my tractor in gear, set the throttle and putt along. Come to a small hill and it will just keep going along about the same speed. But, hit a bigger hill and it might start to bogg a bit. Same thing with mowing. cruising along with the throttle set just right but then hit a big deep patch of grass and brush and it will start to bogg down. So the governor, at least mine, does not seem capable of miantaining a constant ground speed or power output for all conditions. Does that sound right?

That's about right at low rpms, but that's not where most engines operate.

Say you set the lever fix the engine at 2500 rpms, with no load the fuel injection pump is giving 10% of maximum volume. Based on the rpm set, the pump can give up to 80% of it's maximum volume - that's based on the lever position + the fuel delivery curve and more fuel would just produce black smoke. Now you engage a 6' bush hog, the engine bogs down and the fuel injection pump is commanded (either electronically or mechanically) to give more fuel until the rpms come back up to the set point of 2500. Now the pump is injecting 20% of potential volume. You start mowing and the engine again bogs and the pump gives more fuel and is at 65% fuel volume as you cut the knee high grass. Then you get into a low spot with much higher thicker wetter grass and the engine bogs. The pump responds by going to the maximum it can based on the lever position (80% of maximum in this case), but the load is still too high and the engine drops more rpm. You can either slow down to reduce the load or stop and move the lever down more so the pump is allowed to speed up the engine, give more fuel and make more hp to match the new load. If you only slowed down, when you're out of the thick stuff you can just speed back up and never have to touch the lever. If you pulled the lever down, the engine will raise the rpms as the load decreases to a higher set point. Maybe 2800 rpm. You then have to hand regulate the lever to the 2500 rpm initial set point or you just keep on at the new higher set point with the higher potential maximum fuel volume.

So the only time you have to manually change the lever position is when the load is higher than the power the engine can produce at the maximum set rpm. If you think of the lever as an rpm set point device and not a throttle, it may make more sense. Mentally tally up the engine rpm and the maximum hp it produces at that rpm. If the load exceeds the hp, you have to either reduce the load or increase the rpm to get to a higher hp region of the engine's power curve.


And yes, I made up the numbers to support my point. But, they are probably in the ball park.

jb

 

[RobJ]

I'm assuming the governor can only adapt so much. Is that right? In other words, I put my tractor in gear, set the throttle and putt along. Come to a small hill and it will just keep going along about the same speed. But, hit a bigger hill and it might start to bogg a bit. Same thing with mowing. cruising along with the throttle set just right but then hit a big deep patch of grass and brush and it will start to bogg down. So the governor, at least mine, does not seem capable of miantaining a constant ground speed or power output for all conditions. Does that sound right?

Nope you are correct, when you set the speed control (throttle), you are basically saying I want 25hp....or 20hp, etc. The engine sees this a speed (rpm) If you go over that hp the engine will fall flat. It may or may not end up at WOT, in either case it's trying to hold that speed you wanted..say 1800rpm. WOT might be 2500rpm. But it will never maintain the exact rpm under load. On the gen sets we used to built, standard us 60 hertz is 1800rpm gen speed. We always had to start at 1850-1875 to get the gen to hold 1800rpm under full load.

 

[dodge man]

So do diesels have a throttle body or some other such device that has a butterfly valve that is opened and closed depending on the throttle position? From what I just read from the above posts, I'm assuming the air/fuel ratio is a little less critical on a diesel engine.

 

[SFish]

Dodge:

There is no throttle in a diesel, as Mickey_Fx said, and the amount of diesel injected into the cylinder determines power/engine speed, while timing is based on when diesel is injected relative to top dead center.

Anybody interested in diesel engines, and doesn't already know about them, should check out 2-cycle diesels.

Steve