Diesel Engine Theory

[jlgurr]

Came across an interesting phenomenon which may simply be the norm for a diesel engine. Will try to explain it and see what the diesel experts think... This happened on a Massey-Ferguson GC1705 but I'm guessing that doesn't matter as it may occur on most if not all SCUT's with a diesel engine.

Was tilling the garden last week and it was pulling hard on the engine. Was running about 2000 RPM or so and thought I'd give her a little more so I nudged the throttle up. No response, so I nudged a little more. No response, so I let off the HST and the engine RPM increased almost immediately, as if a brake had been released.

Being an engineer and curious (sometimes a dangerous combination) I tried to repeat the scenario and get the same results. Voila, it worked. Soooo, I pondered this while puttering around and recalled reading that a diesel engine air intake is always wide open with only the delivered amount of fuel affecting the engine RPM.

With the engine loaded and nearly falling off on RPM, is it plausible that nudging the throttle up did not increase the RPM because the engine could not get any more air; until after the load was relieved by letting off the HST? I don't think what happened is a problem or of any concern, just curious.

Thanks,
Jeff

 

[Heywood Jannockitov]

Nothing to do with the engine. Letting off a bit on the HST is like down-shifting a gear transmission to a lower gear. That is why your rpms increased. With HST, if engine begins to lug, ease off on the HST pedal to "lower the gear"

 

[Scott65]

You are correct that there is no air metering. Some vehicle applications used a butterfly plate to create a vacuum, but only to draw exhaust gases through the emissions system, so that doesn't really apply here. Air/fuel ratio is not nearly as critical on a diesel engine. The more you cram in, the more power you get. To a certain point. Hence the introduction of forced induction (turbos mainly). When you ask for more throttle, your just supplying more fuel. If it's mechanical injection, it's opening up the pump. If it's electronic, the computer is holding the injectors open longer and ramping up rail pressure.

You were probably just at the limits of the engine. It was holding at the power and load that you had, but couldn't overcome the demand for more until you removed some load. If it was in fact running out of air, it would be due to restriction. Turbo not spooling, clogged filter, poor intake design, etc. If none of these are the case, and you feel you shouldn't have been at its limits, it would seem there's a problem elsewhere. Given the above info, one would first check lack of fuel next. Filter, transfer pump, injection pump, sensors if electronic...

 

[buickanddeere]

A lugging engine can't increase the Rpms because it's being lugged to the max. The injection pump was already at max output.
To make more power the engine needs to move more air/fuel per minute through it. That would be raising Rpms , raising boost pressure etc.
Why were you operating the tiller at less than rated Rpms and were lugging the engine ? It's small tractor, not a steam locomotive .
Reduced Rpms also reduces coing air flow and will cause a partly loaded machine to overheat.

 

[dusty3030]

Were you running it at 2500 rpms or so before you started moving forward with the tiller or did you start at 2000 rpms and as you were moving tried to add fuel and rpms would not increase?

You need to run it up to PTO speed before you put a heavy load on it and adjust your speed with hydro pedal to keep it up to speed - ie if it starts lugging down let up on the pedal and let the tiller / engine catch up.

Don't start at low rpm doing heavy work like tilling or mowing, run it at PTO speed on your tach and adjust your speed with hst pedal to match the load.

 

[jlgurr]

Were you running it at 2500 rpms or so before you started moving forward with the tiller or did you start at 2000 rpms and as you were moving tried to add fuel and rpms would not increase?

You need to run it up to PTO speed before you put a heavy load on it and adjust your speed with hydro pedal to keep it up to speed - ie if it starts lugging down let up on the pedal and let the tiller / engine catch up.

Don't start at low rpm doing heavy work like tilling or mowing, run it at PTO speed on your tach and adjust your speed with hst pedal to match the load.

Started the row at proper PTO RPM but the engine slowed down because my ground speed was obviously too fast. That was a little bit intentional to "see what she could do." With my gas powered lawn mower I just pour on more throttle and it helps, to a certain extent. With the diesel it just flat lined until I let off the HST. Which relieved some of the load as Heywood mentioned.

 

[truckdiagnostics]

Sound like you have mechanical pump, in theory if you drop or decrease the load pump should keep rpms the same, there could govenor, linkage issue going on....

 

[Scott65]

I believe as he's described it, he was at x amount of throttle, say 50%. But loaded the tractor to 100% for that rpm range. He raised the throttle without relieving any load, now calling for 75% throttle, but can't overcome. Once he removed the load, it jumped to the 75% throttle like it was trying to do

 

[jlgurr]

I believe as he's described it, he was at x amount of throttle, say 50%. But loaded the tractor to 100% for that rpm range. He raised the throttle without relieving any load, now calling for 75% throttle, but can't overcome. Once he removed the load, it jumped to the 75% throttle like it was trying to do

Yeah, that's it exactly. Thank you Scott.

 

[jlgurr]

Sound like you have mechanical pump, in theory if you drop or decrease the load pump should keep rpms the same, there could govenor, linkage issue going on....

Not sure if I understand that correctly... Are you saying that the throttle lever position sets the RPM and a governor adjusts the fuel flow to maintain that RPM setting?

My small and simpleton knowledge of diesel engines led me to believe the throttle lever was effecting the amount of fuel supplied to the injectors.