Diesel Engine Theory

[truckdiagnostics]

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.

The throttle sets the rpm, the Governor will control the fuel amount to maintain a set rpms. As load increases the Governor will increase the fuel to the injectors/nozzles keeping the RPMs the same. At some point you can lug the engine down to when the RPS decrease, However, they should not rise past the throttle setting. Scott65 has the description correct. If the engine is lugged down to max HP for a fixed RPM then you raise the throttle to say bump it up against max governed speed, then relive the load or decrease it will go to the max governed speed. I apologize I did not realize you where increasing throttle while under load. I thought you removed load and it went past preset throttle position, which it should not do. That will teach me to answer these things on my smart phone.

 

[joeu235]

I don't know anything about operating a HST. But for a geared tractor, I was taught by my dad that under a heavy load, the tractor should drop 100 RPM when the the clutch is released. If less or more, a different gear needs to be selected.

 

[Jay4200]

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.

I think it's simpler than that - the engine governor was likely wide open, but RPMs were less than the "throttle" (not really a throttle) setting was looking for - so the engine was simply producing as much power as it can make.

 

[Winmac]

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? .

In a nutshell yes, your understanding is correct.

 

[Mace Canute]

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.

As a load increases on the engine the governor will increase the pumps fuel output in an attempt to maintain engine rpm at the level set by the throttle. The governor will keep increasing the pumps output until it is at it's maximum possible level and if more load is added then the only other possibility is dropping rpms. There is a phenomena called "torque rise" which is manifested as an increase in torque output as the rpms drop as load is applied. This is the reason your engine pulled down to 2000 and held there without falling further. The power output required to do all the work was equaled by the power available at that rpm. When you decreased ground speed the total power requirement was reduced and engine rpm increased to the equilibrium point once again.
This diagram illustrates the relationship between rpm, torque output and horsepower output.

 

[SPYDERLK]

As a load increases on the engine the governor will increase the pumps fuel output in an attempt to maintain engine rpm at the level set by the throttle. The governor will keep increasing the pumps output until it is at it's maximum possible level and if more load is added then the only other possibility is dropping rpms. There is a phenomena called "torque rise" which is manifested as an increase in torque output as the rpms drop as load is applied. This is the reason your engine pulled down to 2000 and held there without falling further. The power output required to do all the work was equaled by the power available at that rpm. When you decreased ground speed the total power requirement was reduced and engine rpm increased to the equilibrium point once again.
This diagram illustrates the relationship between rpm, torque output and horsepower output.

Wow. 1200 seems low for max torque point. What engine is that Mace?
Thanks,
,,,larry

 

[CalG]

By definition, "Lugging"* an IC engine is the condition where increasing the "throttle" does not result in an increase in engine RPMs.

*Fuel delivery in the case of a compression ignition engine, fuel and air for the otto cycle.

 

[jlgurr]

Gee Whiz Wally, I feel like I've stayed at the Holiday Inn Express with all this new knowledge!

Thank you everyone!

Sent from my iPhone using TractorByNet

 

[Mace Canute]

Wow. 1200 seems low for max torque point. What engine is that Mace?
Thanks,
,,,larry

I have no idea, it was just a diagram from a site I found here...

Frontier Power Products - Making sense of diesel engine specs

 

[Baby Grand]

I want to say one of the two quad box Macks I drove had operating RPMs at 1200 - 1800 RPM.
So 1200 would be about right for max torque on that machine.