What is Engine Load? 2

[U6an]

Dear all,

Hello I'm a new guy to both this forum and engines

I have a question (maybe a stupid one) about what is defined by Engine Load and how it affects the fuel consumption of the engine?

Having found no exact definition, I assumed the Load to be dependent on the % of the Throttle being open and, thus, also dependent on the ratio of mixture volume to the full cylinder volume.
WOT causes this ratio to be equal to 1 (or higher, if we have a volumetric efficiency > 100%), % of Throttle being open less than 100 causes the volume of mixture be smaller than the cylinder volume. Right?

But RPM depends on the Throttle opening as well. Does it also mean that the RPM is proportional to the Engine Load?

I feel I have a mess in my head.

Regards,
U6an

 

[ivymike]

Engine load is the power that the outside world takes away from the engine. An engine connected to nothing can have essentially no load, regardless of throttle opening or RPM. If you connect an engine to a dyno or a machine, then you can load the engine.

 

[ivymike]

Further to the above, an engine produces more power and can therefore accommodate more load when its throttle is open wider, and less power, load, when its throttle is less open.

The amount of power available from the engine at 100% throttle will vary over the operating speed range (rpm range) of the engine. At any given speed, the maximum power that the engine can output is "full load."

If the output power of the engine is less than the external load, the engine will decelerate.

If the output power of the engine is greater than the external load, the engine will accelerate.

 

[U6an]

ivymike,

Thank you for your response, not clear for me still.

If we have a car that accelerates, can we then say that the Engine Load is the Tractive Effort?

 

[ivymike]

The engine load is directly related to the tractive effort ... there are losses between the engine and the road which make them diverge a little bit.

Engine load % is normally a comparison of the actual engine output torque to the maximum engine output torque at the current engine operating speed.

 

[Tmoose]

Just going down the level road is likely to require HP according to a curve like the red or blue curves here.

http://i1072.photobucket.com/albums/w362/OldTeleman/PRIUSHPgraph.gif

A chart like in the lower right corner here shows which transmission gears are available at various road speeds during maximum acceleration.

https://www.flickr.com/photos/mr38/5784805058/in/set-72157626856774562

This chart looks to show road load vs speed, and HP available in each gear at full throttle.

http://i237.photobucket.com/albums/ff47/SuperL98new/TopSpeed.jpg

 

[U6an]

ivymike,

Thank you. I will assume then the Engine Load to be approximately equal to the Tractive Effort.

Do you know how to find out the effect of the Engine Load on the fuel consumption?


Tmoose,

I'm not quite sure what you meant..

 

[ivymike]

If efficiency was constant, then engine load would be proportional to fuel consumption. Unfortunately efficiency is not constant, so you'll need a BSFC map to relate engine load&speed to fuel consumption.

see thread71-363722

 

[JackAction]

The engine load is the torque output of the engine.

An internal combustion engine is - approximately - a constant torque motor, meaning it can produce the same maximum torque at any rpm. The fact that it happens at a rpm or another, dictates how much power it produces.

Torque is proportional to the amount of force put on the piston, which is proportional to the amount of air being burnt in the combustion chamber, which is why you can relate the engine load to the quantity of air going into the engine, knowing the maximum amount of air that go in at a particular rpm, i.e. at WOT.

RPM does not really depends on throttle opening per say. When an engine is at constant speed, there is only the friction force going against the engine (from the bearings) and some others to run pumps and such, such that the engine can run. This represents the load at idle.

If you give enough heat (fuel combusted) to produce enough torque to go against this load, the engine will idle at constant speed.

If you give more heat, it will produce extra torque which will be transformed into acceleration of the moving parts (F=ma or T=Iα in rotation). Of course, if you give less heat, there will be a deceleration of the rotating components.

But you can increase the load by coupling an external torque to the shaft of the engine (a pump for example or a vehicle that needs to be accelerated). In that case, if you don't increase the heat in the combustion chamber, the engine will begin to decelerate. But if you increase it enough such that the torque converted by the engine equals the torque required by the load, then the speed of the engine will stay constant.

 

[BrianPetersen]

I wonder if the original poster is referring to the parameter that the on-board-diagnostics call "load", if you are watching the engine parameters with a Scangauge or other such tool.

That "load" is more-or-less the percentage of air/fuel delivery going through the engine relative to what it would be at wide-open-throttle (100% "load").

It's not unusual for "load" to be reported in the 25% - 30% range with the engine idling in neutral (zero "actual load" being output), because that's how much it's taking to spin the engine to overcome friction, oil pumping, water pumping, throttling, etc.

The fuel injection system uses this to calculate the correct amount of fuel delivery and also the correct ignition timing, variable-valve-timing actuation (where so equipped), etc.

It is correlated to the output torque, but not directly so.

 

[gruntguru]

Engine "load" in technical literature is a dimensionless parameter used to fix any operating point of the engine. The other parameter required is engine speed. Looking at the characteristic map of an engine, the x axis will usually be speed and the y axis may be in units of torque, power or load. The upper limit of the map, the "full power curve" represents 100% load at each speed. Lower values of load at any speed will simply indicate that power and torque are reduced to that percentage.

"Tractive effort" is the linear force produced at the drive wheels. If gear ratio is held constant, "tractive effort" will be proportional to load, but it is not accurate to say that tractive effort = load.

je suis charlie

 

[SomptingGuy]

One more view...

"Load" is a poorly defined and ambiguous term used far too much to mean different things to different people. To me it has one of two usual meanings:

1) Dimensionless. A combination of engine input settings to provide a proportion of the available shaft torque for a given engine speed. Correlated with pedal position (driver demand) in a vehicle. Although continuous, "zero", "part" and "full" are the three most common values used. Modern engine control systems may attempt to make the correlation linear, or at least feel linear.

2) Dimensioned. BMEP. Precisely defined. Normalised for engine size & type, but its range is speed-dependent.


- Steve

 

[gruntguru]

Agree with that. "Load" seems to be used interchangeably to describe Power, Torque or BMEP which is OK at a fixed engine speed where all three are proportional.

je suis charlie

 

[BigClive]

Good question. In the engine testing I have done I usually regard that at a certain fixed RPM the maximum torque (or engine load) that can be developed is proportional to the manifold vacuum and thus the fuel consumption is likewise proportional to the manifold vacuum.

 

[ivymike]

Nobody is going to correct this one, eh?

An internal combustion engine is - approximately - a constant torque motor, meaning it can produce the same maximum torque at any rpm. The fact that it happens at a rpm or another, dictates how much power it produces.

LOL

 

[JackAction]

Nobody is going to correct this one, eh?

An internal combustion engine is - approximately - a constant torque motor, meaning it can produce the same maximum torque at any rpm. The fact that it happens at a rpm or another, dictates how much power it produces.

LOL

What needs to be corrected?

ICE have a "constant torque" because - approximately - the same maximum force can be applied on the top of the piston no matter the rpm. Consequently the power is zero at zero rpm and increases as the rpm increases.

On the other hand, electric motors are considered "constant power" because the same power input (voltage X current) is - approximately - available at any rpm. Consequently, the torque is infinite at zero rpm and decreases as the rpm increases.

 

[IRstuff]

"ICE have a "constant torque" because - approximately - the same maximum force can be applied on the top of the piston no matter the rpm. Consequently the power is zero at zero rpm and increases as the rpm increases."

I guess all that work that goes on to tune the power curve of a car is pretty noob, then

TTFN
faq731-376

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[JackAction]

Thank you for that last graph. It shows really well that the torque is at least 75% of the maximum torque throughout the rpm range and that the power varies linearly from idle to 5500 rpm (which means torque is [keyword] approximately [/keyword] constant). Here's another one which compare a gasoline engine with an equivalent electric motor:

Can you tell which one is «constant-torque» and which one is «constant-power»?

 

[ivymike]

torque, power, and bsfc from another internal combustion engine here:

http://s7d2.scene7.com/is/content/Caterpillar/C15_ACERT_354_bkW_475 bhp

 

[ivymike]

I don't suppose it helps the original poster any, but one reason I completely disagree with the statement that "torque is approximately constant" is that the shape of the torque curve is one of the primary factors that makes one IC engine better than another one for a given application. If they're all "approximately equal" then there's really nothing that makes a diesel better than a gasoline engine for pulling a semi up the hill, or one diesel better for marine propulsion and a second one better for a tractor and a third better for racing ... they're all about the same!