Diesel Engine Theory

[TSO]

Everything seems to have been answered....

And just a point to consider with HST and that tractor:

The engine is powering the PTO shaft, implement pump, secondary pump for steering (if applicable), flywheel/belt accessories, and finally the internal HST pump. With a gear transmission, you have all of the above except the HST pump. This is why HST is rated for less HP than the gear model equivalent... The engine powers motion thru the internal pump, not direct linkage. Though I suspect that the rating, if measured at the PTO, would be closer in HP rating if both tractors were stationary and in neutral, than they would be while under load and moving... As the HST system draws power.

People say that lugging a diesel engine is not harmful, though I try not to do it too often personally.

 

[MHarryE]

Here is a power curve typical of a CUT or SCUT. There is very little torque backup. Also note the top end of the curve. There is no power output at high idle. Power increases to peak as the governor increases fuel flow to meet the power demand. This is why you will see the tachometer quickly drop when you move, engage the pto, etc. For a mechanically controlled engine this speed drop from high idle/zero output to max rated power is 7%. Tighter governing is available for special applications such as generators where engine speed control is needed to maintain 60 Hz (North America, 50 Hz most of the world). The overrun area can be decreased on FADEC engines (Full Authority Digital Engine Control) found on larger engines.

When you set your engine speed control to maintain a lower RPM, you shifted this curve to the left, at least the RH portion. In this curve, full rated power is 33 kW at 3600 rpm. Shift the power point from 3600 to 3000 and the max power is 31 kW but due to 7% governing, the no load speed will be 3210. Assuming you are operating at this point and you need more power, changing the engine speed control will not give you a boost because there is no torque reserve. Note how the torque actually decreases from 3600 rpm to 3000 rpm.

A larger agricultural engine like the one in our 7720 John Deere which has FADEC has a much different curve. Engine horsepower actually increases as the speed drops below rated engine speed due to a very large torque backup. PTO horsepower is 130 rated at 2100 rpm but 160 HP at 1650 rpm. However it will act the same way if running part engine speed and I run into a need for more power. Push the engine speed control slide to the max and it won't speed up. Push the speed control up and drop the power shift down a gear and it will speed up.

As for the engine power rating difference between the standard transmission and the hydro, the advertised difference is due to the parasitic load due to the hydro charge pump. SAE and ISO power ratings specify power with all parasitic loads. The hydro charge pump is something not found in the standard transmission. It is an open center pump putting out a continuous flow and whatever the charge pressure relief setting is. The standard specifies subtracting the theoretical parasitics. For example charge pressure of my L5740 is 350 psi. If I assume 6 gpm pump flow and 85% efficiency, its about 1.5 HP, the difference in PTO HP between a Glide Shift or Direct Drive and Hydro in the Grand L series (not the L5740 because it is only HST).

 

[Jerry/MT]

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...

You got it correct.Scott65. If you got no response to a command for more power and then removing load allowed the rpm's to increase, your fuel flow was being limited at the initial condition.

 

[CalG]

Perhaps it is only the matter of terms, but " If you got no response to a command for more power........Your fuel flow was being limited...) doesn't really consider the realities of displacement.

There is a reason the larger displacement engines can develop more horse power. It's not just cramming more fuel into any given space. There is stoichiometry to consider.

 

[Scott65]

Perhaps it is only the matter of terms, but " If you got no response to a command for more power........Your fuel flow was being limited...) doesn't really consider the realities of displacement.

There is a reason the larger displacement engines can develop more horse power. It's not just cramming more fuel into any given space. There is stoichiometry to consider.

I don't quite understand what your saying in the first part. I don't know if your suggesting the op had a lack of fuel, or a lack of displacement. I think we've pretty well determined a lack of displacement was the cause.

Stoichiometry is a consideration, but a stoic mixture is not nearly as important on a diesel. They can certainly run too rich or too lean, but it has to be considerably more extreme than a gasoline application. That's why air metering isn't necessary. It's also the reason why a runaway can happen. If you dump unmetered, large quantities of gasoline into an engine it loses tremendous power and eventually dies. A diesel gains power and accelerates until it runs out of fuel or blows apart.

Larger engines make more power, not because of cramming more fuel into "any" given space. It's cramming more fuel into a larger space. Bigger pistons have more crank drive force, turbo drive pressure, etc. as long as the fuel is there to drive it

 

[buickanddeere]

I don't quite understand what your saying in the first part. I don't know if your suggesting the op had a lack of fuel, or a lack of displacement. I think we've pretty well determined a lack of displacement was the cause.

Stoichiometry is a consideration, but a stoic mixture is not nearly as important on a diesel. They can certainly run too rich or too lean, but it has to be considerably more extreme than a gasoline application. That's why air metering isn't necessary. It's also the reason why a runaway can happen. If you dump unmetered, large quantities of gasoline into an engine it loses tremendous power and eventually dies. A diesel gains power and accelerates until it runs out of fuel or blows apart.

Larger engines make more power, not because of cramming more fuel into "any" given space. It's cramming more fuel into a larger space. Bigger pistons have more crank drive force, turbo drive pressure, etc. as long as the fuel is there to drive it

Take the throttle out of the gasser and it will run away just as well as the diesel as the fuel flow is increased.
Toque on the crankshaft is a function of average combustion chamber pressure, piston diameter and stroke length. Factor in the rpm and HP is known. A 350 cu" engine moving 300 cu ft per minute of air fuel mixture is not going to make any more power than 200 cubic inch engine moving 300 cu ft of air/fuel mixture through it.