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

 

[Panther140]

gruntguru, I am not sure how you would use load to describe engine's power output in a way that totally contradicts what I just got done saying. What are your thoughts? Just to make sure my post was clear, Ivymike's posts about load earlier in this thread are another way of saying what I am trying to say

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[gruntguru]

Your post suggested that "load" describes a "force" only.

je suis charlie

 

[JackAction]

Ever heard of a "load bank". "Load" is often used to describe power output.

Power only describes load in an electrical context. In a mechanical context, it is always a force or a torque. Load (disambiguation)

But as you said earlier:

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.

When comparing 2 engines, if the rpm is the same, power is proportional to the torque so it doesn't make a difference to use one or the other WHEN you compare it to a reference value (i.e. max torque or max power).

 

[LionelHutz]

A lot of pissing and moaning that depends on a definition. Load generally does refer to torque in mechanical systems vs power in electrical systems. But, the OP is asking about fuel consumption so maybe power should be used?

I'm sorry, in my previous post there is an error. The following statement is false whether you define engine load as power or torque:

The engine load is directly related to the tractive effort

If you increase the tractive effort, the power of the engine can stay the same if the vehicle speed is also reduced;
If you change the gear ratio, you will change the tractive effort without changing the engine torque.

Funny, even though I've admitted making a mistake, I wasn't struck by lightening.shadeshappy

JackAction - I consider both of your statements wrong. I think there is confusion between tractive effort and tractive force when using the generally accepted automotive definitions. Tractive force is the force produced at the road surface. Tractive effort is the force the engine produces to create the tractive force. So, effort is the output torque of the engine. I would take more issue with the second part of ivymike's statment than the first part, because he was correct and then says there are other loads to consider which are already included in the effort.

I would also never say an ICE is constant torque. This can be reasonable over a certain rpm range but it will not work over the complete engine operating range.

You are wrong about an electric motor being constant HP and having infinite torque at zero speed. They are basically constant torque machines from 0 speed until rated speed (or until you run out of voltage). This is the range electric motors are operated in probably 99% of the time. You can clearly see this constant torque portion of the curve in the graph you posted. They then switch to constant power machines once you run out of voltage.

 

[JackAction]

Tractive force is the force produced at the road surface. Tractive effort is the force the engine produces to create the tractive force.

Traction, or tractive force, is the force used to generate motion between a body and a tangential surface, through the use of dry friction.

In automotive engineering, the terms [tractive force & tractive effort] are distinctive: tractive effort is generally higher than tractive force by the amount of rolling resistance present

[Tractive effort applies] to different operating conditions, but are related by common mechanical factors: input torque to the driving wheels, the wheel diameter, coefficient of friction (μ) between the driving wheels and supporting surface, and the weight applied to the driving wheels (m).

The driving wheel torque is not the engine torque. An engine can produce any value of torque and still produce the desired wheel torque through a proper transmission.

I would also never say an ICE is constant torque.

[...]

You are wrong about an electric motor being constant HP and having infinite torque at zero speed.

When I state these facts, it is on a theoretical point of view.

In theory, with an ICE, the same quantity of fuel mixture goes inside the engine (it depends on the engine's displacement). Once this fuel mixture has burned, it produces a certain force, creating a certain torque. So the same torque is produced by every cycle, no matter the rpm. Thus the rpm of the engine determines the power output. So only a certain defined amount of torque is produced.

In theory, with an electric motor, only a certain amount of power can be accepted (current X voltage). Since an electric motor is machine that convert electrical power to mechanical power and that power is always conserved, therefore the torque is inversely proportional to the motor rpm. For example, with a DC motor, torque depends on the input current and speed on the input voltage. Both of these can easily be set to any value with the help of a transformer, but their product will always remain the same, i.e. same power input.

In practice, with an ICE, fuel mixture quantity can vary depending if the cycle is at a rpm or another. The combustion quality can also vary depending if the cycle is at a rpm or another, affecting the force available on top of the piston. Of course, there are no free lunch, and an engine cannot produce an infinite amount of power, just by indefinitely increasing the rpm. Flow restriction and mechanical failure comes into play at one time or another.

In practice, with an electric motor, torque will not be infinite at zero rpm. Limits are imposed depending on their construction and, as with an ICE, mechanical failure. If you have a big enough motor, of course you can have your rpm range within that upper torque limit.

 

[Panther140]

gruntguru - What else would you call a force that an engine is applying torque to?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[LionelHutz]

Wow, I can quote Wikipedia too.

Tractive effort is the force generated by a vehicle's engine or motor in order to generate motion through tractive force. Tractive effort differs from tractive force, which is the actual force applied at the road surface, by the amount of rolling resistance present.

Once again - definitions....

As for the rest of your post, good for you and your theories. Even theory has different meanings. Apparently, your definition is abstract. Mine is more scientific and says that a motor design has a safe current limit. This current limit will limit the possible torque the motor can produce. This current limit makes your "in practice" statement completely wrong too.

You first made your statements without any "in theory" attached and now you're justifying both by using "in theory". Big surprise.


Panther140 - yes, I would agree that the load on a ICE is always the force it's working against. An ICE produces a twisting force at the crankshaft so it's rather hard for the load to be anything else.

 

[JackAction]

Mine is more scientific and says that a motor design has a safe current limit.

In practice, with an electric motor, torque will not be infinite at zero rpm. Limits are imposed depending on their construction and, as with an ICE, mechanical failure.

That seems pretty much the same to me.

You first made your statements without any "in theory" attached and now you're justifying both by using "in theory". Big surprise.

From the reference in my earlier post (7 Mar 15 02:07):

Theoretically there would be no limit to the horsepower obtainable from the engine, as any required figure could be obtained by a proportional increase in speed.

As for a reference for an electric motor, you are right, I did not provide one. There is one I had in mind since the beginning of the thread, but couldn't find a copy of the extract I needed online and my paper version is not close by (Then again, I never thought I would have to explain myself that much). Luckily, it did show up on the Google preview this time, so you have an extract from Theory of Ground Vehicles, p. 227 in the image attached to this post (Sorry. p.228 was not shown on Google preview so fig. 3.20 wasn't available, but from the context you can imagine it looks like fig. 3.19).

As you can see, I can quote more than Wikipedia. And when I do quote Wikipedia, I used text that is referenced. The reference for my quoted text stating that it is input torque to the driving wheels (as opposed to engine torque), clearly relates the traction effort to engine power, not torque. Even in your link, it says (although unreferenced) that the traction effort available is limited by the available power; which is inconsistent with relating it to engine torque.

Funny thing. I read p.255 of the previous Wikipedia reference - which talks about fully electric haulage locomotives - and it says:

At low speeds the traction system is limited by current so tractive effort is applied proportionally to throttle notch level. Tractive force delivered in this region may be independent of speed, or reduce with speed, depending on the locomotive characteristics and control. At higher speeds the system is limited by power so the tractive effort available decreases at increased speeds according to force velocity product P = F_t/dbv.

Look at the figure going with the text showing typical tractive effort performance curves: The «high speed» range is at least 70% of the whole speed range. I would consider the «constant power» section being more predominant than the «constant torque» section.

By the way, theory is often abstract. That is how you differentiate it from practice.

 

[JackAction]

Sorry, it seems only one link can be attached to a post. Here's the file for the Theory of ground vehicles reference.

 

[LionelHutz]

Yes, a theoretical limitless hp and an ICE being constant torque are exactly the same thing. How could I have missed that point.

Of course traction effort is limited by available power. There is nothing inconsistent about that statement.

You posted that complete nonsense about electrical motors being constant HP and having limitless torque at low rpm's as if it was a fact. You backpedaling with this "in theory" BS to justify your claims. Now you're saying they are constant torque up to a certain speed, but that doesn't matter because it's only a small portion of the operating range? I can't even follow what you're claiming anymore.

The fact that the locomotive specific application runs a motor well above the voltage limit doesn't change anything about what I posted. The motor is constant torque until you can't increase the applied voltage. The locomotive is a rather specialized application. I work with electric motors every day and 99.99% of the applications I deal with never take the electric motor above it's constant torque operating range.

You should really be more careful with your broad generalized use of "electric motors". The series wound DC locomotive motor in not the only type of electric motor.

 

[JackAction]

Yes, a theoretical limitless hp and an ICE being constant torque are exactly the same thing. How could I have missed that point.

I can't believe how condescendant you are, but I'm sorry if I'm not sure what you don't understand. This might be a quote of interest from the same earlier post:

If the mean effective pressure (mep) and the mechanical efficiency of an engine remained constant as the speed increased, then both the indicated and brake horsepower would increase in direct proportion to the speed, and the characteristic curves of the engine would be of the simple form shown in fig. 1.5

And before you say anything, torque is directly proportional to mep (in theory; in practice, I know that you have to considered friction mep as well). So, in theory, the torque also remained constant as the speed increases. The rest of the post (not me, the reference) explains why practice differs from theory.

You posted that complete nonsense about electrical motors being constant HP and having limitless torque at low rpm's as if it was a fact. You backpedaling with this "in theory" BS to justify your claims. Now you're saying they are constant torque up to a certain speed, but that doesn't matter because it's only a small portion of the operating range? I can't even follow what you're claiming anymore.

I said in theory electric motors have infinite torque at zero rpm. Of course, there is an actual limit in practice (as I have stated). And that limit will apply within a certain speed range, i.e. when you will reach rpm = P_max / T_max. I think I clearly demonstrate, with appropriately referenced real world examples, that it is a small portion of the rpm range (Of course, one can use an oversize motor for their needs and use it only in the lower rpm range). You have to be of bad faith to twist my words that way.

The fact that the locomotive specific application runs a motor well above the voltage limit doesn't change anything about what I posted. [...] The locomotive is a rather specialized application.

I'm sorry if the theory that I did not create - that I can provide references from other quality sources - do not match your unreferenced statements. I'm sorry if electrical motors used in cars and locomotives do not match you electric motor experience.

You should really be more careful with your broad generalized use of "electric motors". The series wound DC locomotive motor in not the only type of electric motor.

I'm not specialized in electrical motors, so sorry if I did not mentioned «series-wound» electric motors as was specified in one of my source. One would think that someone specialized in that domain would of understood what I was referring to.

I just brought up the electric motor to explain to another person how we can approximately generalize a power curve for different power sources, which was not even an important statement by itself.

Because, in the end, my point is that engine load is a synonym for engine torque. Period (as would say Panther140).

Now, if some people want to twist my words by selecting them one by one and then taking something out of their head with absolutely no reference other than «I'm right because I know», have your fun at it. But if you will accuse me of being wrong and laugh at me, I will defend myself with arguments and references.

 

[Panther140]

"Because, in the end, my point is that engine load is a synonym for engine torque. Period (as would say Panther140).

Now, if some people want to twist my words by selecting them one by one and then taking something out of their head with absolutely no reference other than «I'm right because I know», have your fun at it. But if you will accuse me of being wrong and laugh at me, I will defend myself with arguments and references."

This is a good thread with opposing viewpoints, not adversary ones! This is engineering stuff its supposed to be fun!!

Just to clarify what I mean when I say that Engine load is the sum of all forces acting against the engine:
- An engine that makes 100 ft/lbs of torque at XXXX RPM under WOT would be under 100% load if it were spinning something that required 100 ft/lb of torque to move at whatever RPM it is at.
- Load can be independent of available engine torque
- Load and available Torque at XXXX rpm are independent variables
- Acceleration and deceleration of the system compensate for differences in Available torque and engine load.. until other limiting factors come in to play.
- An engine's own parts can add to the load it faces (picture a 2 ton crankshaft in a 4 cylinder engine, or a massive flywheel)

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[missinglink]

The best I can say,or what I am acqainted about it,is when I had installed megajolt onto one of my cars. I've known it to be the plenum vacuum

 

[ivymike]

I swore to myself I wouldn't peek at this thread again... but I'm feeling ill to my stomach and thought this might help resolve it.

Not that it really matters, but those who would say that "engine load" can only mean "torque" might struggle to explain why SAE J1995 would offer "a method for determining gross full load engine power with a dynamometer." If "load" can only mean torque, then there's no single value for "full load engine power" on a variable speed engine, is there?

In fairness, that was the original version (1995 - 2013) and in the revised version (2014) the omit the term "load" altogether. Guess some folks found it confusing.

 

[JackAction]

but those who would say that "engine load" can only mean "torque" might struggle to explain why SAE J1995 would offer "a method for determining gross full load engine power with a dynamometer." If "load" can only mean torque, then there's no single value for "full load engine power" on a variable speed engine, is there?

I don't have a copy of J1995, but J1349 (2004) - which is the same standard but for net values instead of gross values - mentions also «A method for determining net full load engine power and torque with a dynamometer».

Looking at the definition section:

3.1 Net Brake Power and Torque

The power and torque produced by an engine at any speed [...]

So these values are measured with respect to a given rpm. There is not just a single value for an engine (As opposed to «Rated Net Power and Torque» also defined in the standard and means «the peak Net Brake Power and Torque produced by the engine within the operating speed range»).

So, according to the standard definitions, «net full load engine power and torque» means «net engine power and torque @ full load condition» which also means that all 3 values (power, torque & load) are rpm dependent.

There are no definition for «load power» or «load torque» in the standard.

 

[Panther140]

You have a MJLJ running on the MAP sensor (I also have one). You can get a MJLJ that uses a TPS to find laod, in which load is measured as a percentage based on Throttle Position. If you boost your car with the MAP sensor, then it will tell you that your engine is operating with 230 kpa in the intake manifold. Does that mean your engine is at 230% load? How could it when the TPS version simply says its at 100% load?

Conclusion: Manifold pressure is just a sypmtom/effect. Engine load is the cause of it.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin