Automotive Why engine efficiency drops for light loads?

[poe]

Jack, good call on English not being my first language. My usage of language is somewhat eccentric even in my mother tongue. Though no less effective at conveying my thoughts, I hope.

My "less than what the engine is putting out" statement was a poorly thought out and incomplete one. After Randy pointed it out the first time, I realized it made no sense. I want to go over the statement again in another post. Because while it is an incoherent statement, it is a good way to see how individuals construe the statement and where their focus isn't.

On another note, I spend a lot of time examining patents. Mechanical designs with specific attention to internal combustion engines. I'm going to start bringing some overlooked concepts to the foreground in this post. And I'm not fluent in math, although I understand it more than I can speak it. I would love and appreciate any work done on the mathematics.

Where I'm coming from is, I think most people who are interested in the subject of efficiency are interested from a design/form perspective, intuitively. They want to understand the structure/map of efficiency; at what points efficiency comes into play and what the causes of drops in efficiency are.

The experts of the field, mostly academics, talk from the narrower perspective of the given forms/designs with no regard to the form itself. It's a limited perspective that seems to me to come from being focused on the same form/design for too long and or with limited knowledge. These people have an excellent grasp of function, overwhelmingly function and poor grasp of form/design and its role. So formulas are brought into topics that have to do with form/design and the topic is hijacked by function and formulas. Form defines function.

So let's get into the nitty-gritty.
Plot me the graph that shows: of the constant and even pressure, say by way of hydraulics, exerted on the crown of a piston, what percentage of the constant and even pressure is translated to load output of the cranckshaft through what could be a power stroke? Don't think RPM, think single event. The graph should have TDC at origin on the X axis and BDC at other end of X axis. And load output is on the Y axis. This graph partially points to where focus is lacking.

Plot me another graph. One that shows the load/pressure on the piston crown during the power/combustion stroke of a diesel cycle from TDC to BDC. Again, TDC and BDC go on the X axis. Pressure, in your preferred unit obviously goes on Y axis.

Once we have these two graphs, let's look at how they relate. And there are other more important and more overlooked concepts I want to go over after this one.

I am still going to list the main categories of automotive engine efficiency and populate them, just waiting to see if anyone will. There are 4 main categories. Anything and everything that has anything to do with engine efficiency will fall into one or more of these categories.

 

[poe]

If I understand you correctly (english doesn't seem to be your mother tongue), you're talking about the force acting on the piston (the pressure inside the cylinder times the area of the cylinder) - what you call «the load» - versus the torque output - what you call «the engine is putting out». Of course there are losses in between, which are essentially going through friction and to power engine accessories, like the oil and water pumps. It is called «mechanical efficiency». It seems to be what you call «operation[al] efficiency» in post #33:

There is some misunderstanding. I'll respond later.

 

[poe]

You can look up the definition of thermal efficiency of an engine, and it is simply a ratio: the engine's work output divided by the energy of the fuel consumed in producing it. Usually the fuel's energy is quantified by its LHV (lower heating value). If the engine converts all of the fuel's energy to work (impossible of course), then the efficiency is 1.

I do have a problem here. I know, I know, it's how it's defined in the books. I use thermal efficiency in a different way, which will be illustrated soon in my future post regarding the categories of efficiency. And I will try to make the case that what is described as thermal efficiency above is better described as energy efficiency or something else, because we need to save thermal efficiency for the appropriate use. The way it is described now is confusing.

Indicated thermal efficiency is the most basic, measuring the effectiveness of the engine to utilize the air and fuel in the engine to push the pistons. It ignores the friction that is involved in the pistons' sliding up and down the cylinders, parts like the crankshaft rotating in the bearings, and whatever is required to turn the camshafts, water pump, oil pump, etc. The sum of these friction components is the difference between indicated thermal efficiency and brake thermal efficiency.

I think we have big disagreements here. I think this is an area where there is confusion. Give me time to think and I'll respond in a meaningful way.

I think you can find all of this in John Heywood's book on IC engines, if you hunt around.

Interesting Heywood is brought up. I see him as lacking structure in his understanding of internal combustion engines, as detailed and precisely as he understands the subject. But it wouldn't be right for me to say what I just said without backing it up. I have already started putting together the post that will illustrate my claim.

 

[poe]

You seem to call that «thermodynamic efficiency»

I'm sorry, but this is a fine example of I myself using the wrong word. My next post will clarify. And typing such long replies on this little phone screen causes me to error more than usual since I can only see so little of it all.

And I got to say, it is nice to be here. You guys are definitely tightening my usage of language regarding the very subject I'm talking about.

 

[poe]

That which effects the efficiency of the device but is outside the device and does not effect how the device is operated is a limitation factor, such as the efficiency drop that would occur the closer the temperature outside the device gets to the temperature inside the combustion chamber. This effects how the device is operating and not how it's operated. This is a thermodynamic limit, it is not thermodynamic efficiency.

Efficiency is the measure of an amount usable divided by total amount. Usable is the key word. Of the total amount, fuel in this case, some portion is used, some portion is wasted due to inefficiencies, and some portion is wasted due to thermodynamic limitation. That's why the term "thermodynamic efficiency" doesn't make sense when taking about engine efficiency. It's a limit and should not be combined with efficiency. When combined, which is too often, it is confusing and isn't meaningful.

That which effects the efficiency of the device and is from within the device is an efficiency factor, such as heat lost to the surfaces of the combustion chamber, which is a factor of thermal efficiency.

An automotive engine is a runaway device, and the most efficient way to regulate such a device is by loading it. This type of regulation falls in operational efficiency. As opposed to throttle regulating or fuel regulating, both of which fall under design efficiency.

Design efficiency is the most difficult category to deal with and is what largely determines the efficiency factor of mechanical and thermal efficiency. It is the base. Mechanical efficiency is next in line in terms of its effect on engine efficiency. And operational efficiency is separate from the engine, while still effecting engine efficiency because it effects the insides of the engine, how the engine is operated.

There are 4 categories of efficiency: design efficiency, mechanical efficiency, thermal efficiency, and operational efficiency. And now I have given some examples too, without having drawn a map yet. Who can populate the categories? How does what fit where?

It's not proper to haphazardly dip into different categories of efficiency and grab disparate concepts and mush them together and call it, for example, thermal efficiency or thermodynamic efficiency or whatever. There is a structure, and it's fixed. And efficiency must be understood under this structure. It is so confusing right now. Not in terms of mathematical formulations, but in terms of concepts of form/design.

I'll wait still before providing the map/picture of the categories and populating them. Just because thinking is good for us.

 

[jim hardy]

tightening my usage of language

reasoning is but language well arranged - Laviosier

 

[Randy Beikmann]

... I know, I know, it's how it's defined in the books. I use thermal efficiency in a different way, which will be illustrated soon in my future post regarding the categories of efficiency. And I will try to make the case that what is described as thermal efficiency above is better described as energy efficiency or something else, because we need to save thermal efficiency for the appropriate use. The way it is described now is confusing.

I don't necessarily agree with the way some quantities are defined in thermodynamics, but we need a common language in order to communicate outside this small group.

Richard Feynman actually derived many relationships in trigonometry and calculus before he saw them used in any book, and he thought that his notation was much more meaningful and appropriate. But even he had to adapt to the standard symbols for anyone else to understand his work.

 

[poe]

But even he had to adapt to the standard symbols for anyone else to understand his work.

I concede to this point. I'm going to need help adapting to standard symbols and terminology. But the language aside, I'll post the structure I've been referring to soon. I thought I could type it up, but I think I'll have to draw it, take a picture, and post it.

Please bear with me.

 

[jim hardy]

but I think I'll have to draw it, take a picture, and post it.

paint is primitive and painful

but it's great for annotating other pictures. Microsoft snipping tool is real handy.

 

[poe]

I don't necessarily agree with the way some quantities are defined in thermodynamics, but we need a common language in order to communicate outside this small group.

Something to start getting the idea across as I try to put more information together.

 

[poe]

Design efficiency is: usable energy out divided by (thermodynamic and mechanical and thermal energy losses minus total energy put in).

Design efficiency has a theoretical limit of up to but not including %100. It sets the limits on thermal and mechanical efficiency and usable energy, which is total energy minus the thermodynamic limit. Nature is obviously way efficient. What is the efficiency of photosynthesis?

This is the best I can do from my phone... does it make sense?

mechanical efficiency is: total mechanical energy out divided by total mechanical energy created, created/generated being key here. So while fuel that didn't combust during the combustion/power stroke falls in the design efficiency category, fuel that did combust falls in the category of mechanical efficiency.

Mechanical energy here is the pressure on the piston head, I picture atoms bouncing off the piston head. During and at the end of the power stroke whatever pressure that could have done any kind of work, but didn't, is a mechanical loss. And remember, the shorter the time interval is for the power stroke, the more loss there is during the stroke. F1 cars have short strokes because the loss that occurs during the stroke gets worse at the end of the stroke.

Thermal inefficiency is the measure of loss of heat from one body or medium to another. It is about heat transfer. So heat ejected through exhaust does not count, because it's the medium itself being transferred.

 

[jim hardy]

Always test your concepts by seeing whether they lead you to the correct equation. Human mind is quite capable of believing things that Mother Nature disallows.
That's the trouble with words - what sounds plausible isn't necessarily so.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

Thread re-opened.

 

[jack action]

mechanical efficiency is: total mechanical energy out divided by total mechanical energy created, created/generated being key here.

Where does the lost energy go? This definition is true if you think the lost energy linked to mechanical efficiency goes into friction and powering the accessories.

So while fuel that didn't combust during the combustion/power stroke falls in the design efficiency category, fuel that did combust falls in the category of mechanical efficiency.

The fuel that doesn't burn would go into the thermal efficiency (which you incorrectly refer to as «thermodynamic efficiency»), i.e. when you divide the amount of energy found in the PV diagram and divide it by the potential energy you can get out of the amount of fuel going into the engine.

Mechanical energy here is the pressure on the piston head, I picture atoms bouncing off the piston head. During and at the end of the power stroke whatever pressure that could have done any kind of work, but didn't, is a mechanical loss.

No, whatever pressure that could have done any kind of work but didn't, is included in the thermal efficiency found with the PV diagram.

F1 cars have short strokes because the loss that occurs during the stroke gets worse at the end of the stroke.

No. The only reason to have a shorter stroke is to have a smaller engine.

Thermal inefficiency is the measure of loss of heat from one body or medium to another. It is about heat transfer. So heat ejected through exhaust does not count, because it's the medium itself being transferred.

This loss is also included with the thermal efficiency, i.e. found with the PV diagram. A heat loss during the process will inevitably induce a pressure loss. It would be more evident looking as temperature-entropy (TS) diagram, which gives the same energy output as the PV diagram.

The heat ejected through the exhaust is certainly included into the thermal efficiency as well.

Design efficiency is: usable energy out divided by (thermodynamic and mechanical and thermal energy losses minus total energy put in).

So what you sum up as «thermodynamic and mechanical and thermal energy losses» is in fact officially thermal efficiency.

I think you think that a PV diagram is just for perfect, theoretical, processes and this is where you come up with «thermodynamic efficiency». It is not. The processes can be evaluated with their expected losses or the PV diagram can be measured on the engine itself, which will include all losses.

What you call «design efficiency» is what is officially called mechanical efficiency, i.e. the one determined by the energy at crankshaft output divided by the one found with the PV diagram.

So, unless you can find an official source for your efficiency definitions, you should stick with the traditional definitions to avoid confusion.