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]

another torque and power curve for an internal combustion engine (torque below, power above)

 

[SomptingGuy]

Ok, I'll bite then.

Familiar IC engine torque curves are shaped by breathing inadequacies and the limits of reliable combustion. For NA engines, the available airflow follows the peaks and troughs imposed by the restrictions and tuning of the intke system. For TC engines, the sky (or more often, the PMAX) is the limit.

Designers and calibrators can cut bits out of the available curve to suit applications.

A torque curve flat across most of the operating speed range is becoming more common among light duty automotive applications as TC becomes the norm.

- Steve

 

[SomptingGuy]

E.g, the Ford DLD 418 TDCI, which powers my little car.

http://www.docstoc.com/docs/74378014/DLD-Range-Leaflet

Torque is pretty much flat between peak torque to peak power speeds.

- Steve

 

[BrianPetersen]

Chrysler Pentastar (I have one of these)

http://jeephowto.com/wk2/engine/3.6_liter_pentastar_graph_01.jpg

Available torque is pretty close to constant through the entire useful rev range.

 

[JackAction]

I'm talking about the general concept of an ICE compared to other motor types (electric, hydraulic, etc.), not its fine tuning.

1.20 Characteristic speed power curves

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, in which the line marked 'bmep' is the product of indicated mean effective pressure (imep) and mechanical efficiency, and is known as brake mean effective pressure (bmep). 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. It is, of course, hardly necessary to point out that in practice a limit is imposed by the high stresses and bearing loads set up by the inertia of the reciprocating parts, which would ultimately lead to fracture or bearing seizure.

fig. 1.5​

Apart from this question of mechanical failure, however, there are reasons which cause the characteristic curves to vary from the simple straight lines of fig. 1.5, and which result in a point of maximum brake horsepower being reached at a certain speed which depends on the individual characteristics of the engine.

Characteristic curves of an early four-cylinder engine of 76.2 mm bore and 120.65 mm stroke are given in fig. 1.6. The straight radial lines tangential to the actual power curves correspond to the power lines in fig. 1.5, but the indicated and brake mean pressures do not, as was previously assumed, remain constant as the speed increases.

fig. 1.6​

On examining these curves it will be seen first of all that the mep is not constant. It should be noted that full throttle conditions are assumed - that is, the state of affairs for maximum power at any given speed.

At low speeds the imep is less than its maximum value owing partly to carburation effects, and partly to the valve timing being designed for a moderately high speed; it reaches its maximum value at about 1800 rev/min, and thereafter decreases more and more rapidly as the speed rises. This falling off at high speeds is due almost entirely to the lower volumetric efficiency, or less complete filling of the cylinder consequent on the greater drop of pressure absorbed in forcing the gases at high speed through the induction passages and valve ports.

When the mep falls at the same rate as the speed rises, the horsepower remains constant, and when the mep falls still more rapidly the horsepower will actually decrease as the speed rises. This falling off is even more marked when the bmep is considered, for the mechanical efficiency decreases with increase of speed, owing to the greater friction losses. The net result is that the bhp curve departs from the ideal straight line more rapidly than does the ihp curve.

[...]

If a suitable scale is applied, the bmep curve becomes a 'torque' curve for the engine that is, it represents the value, at different speeds, of the mean torque developed at the clutch under full throttle conditions - [...]

 

[gruntguru]

I agree with JackAction. It is a reasonable and probably useful generalisation to say that reciprocating IC engines produce approximately constant torque across their useful operating range. There are exceptions of course e.g. highly tuned and highly supercharged engines (esp' with centrifugal superchargers).

je suis charlie

 

[ivymike]

That generalization could lead to some baffling results in the context of a discussion of "what is engine load." If available torque is constant across the operating range of the engine, then our OP could leave here with the impression that his car requires the same amount of torque idling in the driveway as cruising the highway at 55 mph (27% load).

 

[BrianPetersen]

The available torque (pedal to the metal) may be approximately constant, but the driver-requested portion of it is not ...

 

[ivymike]

Not sure if you caught what I was saying or not ... since max torque at 800 rpm is much less than max torque at 2000 rpm (typically), if you're reading engine load electronically you might see the same figure (27%) at both idle/no load and at 55 mph constant speed driving. If you take torque to be constant for your engine, that ought to be a surprising result (same power torque!). In reality the low idle no-load torque is less than highway torque, but when expressed as a percentage of max at each speed, the resulting figure is the same.

 

[Tmoose]

some "full load" torque curves are "better" than others

http://www.motorcycle.com/blog/wp-c...6/Harley-Davidson-Iron-883-Star-Bolt-Dyno.jpg

 

[ivymike]

(*same torque!)

 

[JackAction]

Let me go back to the original problem and why I gave the answer I did.

The original question is What is Engine Load? and the OP was attempting to relate somehow the engine load with rpm. A big error that needs to be corrected.

ivymike rather explained well the concept of engine load, except that he constantly used the term power instead of torque. It might seem trivial since if we know the rpm, power and torque are related. Unfortunately, people often use these terms like they means the same thing (like when ivymike says same power torque!), but they don't.

That can be confusing because in electricity, a load is a measure of power. But in mechanics, it is a measure of a force or a torque.

Torque is torque, rpm is rpm and power is the combination of both. Since the OP was tempted to relate load and rpm, I thought the distinction was important to point out, i.e. that torque is independent of rpm:

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.

Sorry if my choice of words did not emphasize that fact.

Here is an example to show why the distinction is important and why power is not a mechanical load:

You have 2 cars, one with a gear ratio of 10:1 between the wheels and engine and the other with a gear ratio of 5:1. Both cars have a load of 650 lb.ft at the wheels. What is the engine load?

Answer: The one with the 10:1 gear ratio has an engine load of 65 lb.ft and the other one has an engine load of 130 lb.ft. Note that it is of no importance at what speed the cars are going (or even if they are moving at all) or what are the rpm of the engines. Both engines may produces the same power or they may not. They may even produce 0 hp if the car is not moving (since an engine is not necessarily an internal combustion engine as we know it, we can assume an engine can produce a torque at 0 rpm).

Now let's try with power instead:

You have 2 cars, one with a gear ratio of 10:1 between the wheels and engine and the other with a gear ratio of 5:1. Both cars have a «load» of 25 hp at the wheels. What is the engine load?

If you define the engine load as power, your answer would be 25 hp in both cases. But then again, if both cars have the same speed, the engines will not be at the same rpm. But are they at the same speed? We don't even know.

So, if the cars are not moving (initial acceleration of a dragster at the starting line for example), the power at the wheel is 0 hp, so it would mean that the engine load is zero? I don't think so.

This is why the distinction between torque and power is important and why stating that torque and rpm are independent of one another is also important.

 

[ivymike]

oh good grief .. we have twins
thread71-380667

 

[ivymike]

btw, I did mention "Engine load % is normally a comparison of the actual engine output torque to the maximum engine output torque at the current engine operating speed."

Nice straw man argument "if the cars are not moving (initial acceleration of a dragster at the starting line for example), the power at the wheel is 0 hp, so it would mean that the engine load is zero" .. I don't think anyone here would have made that mistake.

 

[gruntguru]

If P=0 at an rpm where Pmax=0 what is the load percentage?

Hint: 0/0 does not equal 0

je suis charlie

 

[JackAction]

ivymike,

If we define engine load as torque, these following statements are false and misleading because they all relate engine load to power:

Here you literally define engine load as power:

Engine load is the power that the outside world takes away from the engine.

Here you compare power with torque (How do you know if 25 hp is more or less than 100 lb.ft of torque?):

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.

Here, it is a tricky one: Fuel consumption is proportional to torque output, but the torque output is not proportional to fuel consumption as fuel consumption is related to power not torque:

If efficiency was constant, then engine load would be proportional to fuel consumption.

But these statements are true because they all relate engine load to torque:

The engine load is directly related to the tractive effort

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

Naturally, if we assume engine load is power, the first statements are true and the last ones are false.

This is where I chime in the discussion - respectfully and without pointing finger at anyone - and reinforce the position that engine load is in fact a measure of torque, not power. The reason I did not point out your mistake is because I know that you understand what engine load is and I know that by using the term power, you meant torque, an error that many people do, including me.

You then - literally - laughed at me.

I tried to explain myself - quite respectfully, I think - but you are responding with condescending arguments («oh good grief .. we have twins», «Nice straw man argument»).

So, ivymike, here is an easy question that can be answered with only one word and that may help someone who is reading this thread in hope of defining what is engine load:

Is engine load a measure of the engine output torque or power?

 

[JackAction]

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

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

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

 

[ivymike]

Is engine load a measure of the engine output torque or power?
yes. both, or either.

Load % is typically calculated from torque, but "load" can be interpreted a few ways. Power is easier to explain to someone who just plain doesn't know what "engine load" is.

No, I didn't compare power to torque, that's your assumption based on a narrowly chosen view of what "load" means which is inconsistent with my earlier statement. Same story on the fuel consumption.

(Load) and (load %) are not the same ... and (load %) doesn't have units.

If you don't like being called out for straw man arguments, there's an easy solution: don't make 'em.

http://en.wikipedia.org/wiki/Straw_man

... if you think you've been very helpful and informative, then good for you.
I'm going to have to concede that you've won the internet today. (I've got an engine test to prep for. The subject engine will be loaded to max power for the intended application, so that I can confirm the sizing of the cooling system.)

 

[Panther140]

Load is any force that the engine is working against. period.

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

 

[gruntguru]

Period?

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

je suis charlie