Comparison Engine/PTO HP and "rated RPM"

[Threepoint]

[snip]
One small nitpick here: RPM means Revolutions Per Minute. It is already plural. There is no such thing as RPMS. Putting an S after RPM would mean that "RPM" was a singular event that had to be made plural. A revolution is singular. Revolutions per minute, is plural. RPMs means you are discussing various engine speeds, not a particular engine speed. Such as: "What are the two different RPMs you like to run your tractor at while plowing or discing?"

Ha! A nitpick, maybe, but you raise another very interesting technical point. You've highlighted how linguistic awkwardness can come from strict adherence to rules of grammar. Especially when it comes to acronyms.

Consider the sentence: "When you have several gear ratios to choose from, engine revolutions per minute don't matter as much."

If I instead substitute the acronym, but strictly follow the rule of grammar on singular vs. plural verb form following a plural noun, the sentence becomes: "When you have several gear ratios to choose from, engine RPM don't matter as much."

Now, I myself am fine with the second construction. However, I'm pretty sure that Ms. Holcomb, my 12th grade English teacher, would have told me to switch to the singular form of the verb and say "...RPM doesn't matter as much". :laughing:

So, if the rule in that instance is honored in its breach, I don't see why we can't all just add an "s" to RPM where it sounds better, and call it good. That way we don't have to use the wrong form of the verb. We can just say "...RPMs don't matter as much." :dance1:

 

[Threepoint]

Threepoint,

In diesels they don't control the air intake. The boost is a direct result of HP being produced, which drives the turbo with exhaust energy. Boost pressure is limited by the waste gate in the turbo, which opens to bypass exhaust around the turbine, to limit the intake manifold pressure. Fuel is controlled, but it's not the mixture they are controlling, it's the fuel volume. They can keep ramping up the fuel quantity as long as there is enough air and time to burn the fuel efficiently. The fuel map knows the amount of air entering the engine, or the intake manifold pressure and controls the fuel to give the HP desired at that throttle position and RPM, or the max fuel that can be burned with that amount of air, or the max HP they will allow the engine to produce, or to limit the temperature in the cooling system.

The turbo is a feedback loop. The more power you make, the more air the turbo pumps in. Which means you can make still more power, which spins the turbo even faster, etc. But it's not just the quantity of air that determines the maximum amount of fuel, the fuel takes time to inject and time to burn. The power stroke is very short, so eventually, fuel is still being injected as the exhaust valve is beginning to open. Lots of smoke and no more power produced.

The advertised HP and torque specs are decided on, I guess, for various reasons. The engine has to survive at the rated HP, without any failures, and has to be happy to work long hours at it's rated torque, at the PTO RPM, or max HP RPM. All this while meeting emissions standards.

The turbo simply makes a small displacement engine act like a large displacement engine. But it's still physically small, with small bearings, pistons, etc. So it needs to have excellent cooling and lubricating systems to put up with all the strain. Turbos and large fuel delivery can easily overload engines. Before turbos, engine were much heavier for their HP and the limit was how much atmospheric air was available.

Great explanation! Thank you, again, John! :salute:

 

[Raspy]

So, if the rule in that instance is honored in its breach, I don't see why we can't all just add an "s" to RPM where it sounds better, and call it good. That way we don't have to use the wrong form of the verb. We can just say "...RPMs don't matter as much." :dance1:

Well, you make a good point. But consider that the true meaning of RPM is plural, and the common mis-use of that term is not related to the use of either don't or doesn't, but more to a misunderstanding of what the term means. So, I prefer to use the correct, RPM and apply the wrong form of the verb. Besides, there is nothing wrong with simply saying "revolutions" or "revs", and let the "per minute" be assumed. Whenever I am in discussion about engines, and someone says RPMS, I can only assume they are not familiar with the term and what it means. So, their credibility is weakened in any explanation they may offer. I may seem like an old fuddy-duddy, but technical explanations about the internal physics of engines is very interesting. Subtleties, abound. Hidden forces affect performance. Acceleration equations and the relationship between surface area and volume are always at play. So, the reasons they do what they do, that makes us all study them with interest, need to be addressed as carefully and accurately as we can. If we use words incorrectly in our discussion, and a literal listener applies the correct meaning of the word to the given explanation, he will come away with an unintended answer.

I am always struggling with the language to do the best I can with it. It is always a limiting factor in story telling or technical explanations, partly because we all have different experiences with each word. Or because the running movie in one's head is hard to narrate in a way that causes the same emotion in the reader that was felt by the writer. Whew!

Thanks for throwing a wrench in my Nitpick. :thumbsup:

 

[finn1]

Threepoint,

In diesels they don't control the air intake. The boost is a direct result of HP being produced, which drives the turbo with exhaust energy. Boost pressure is limited by the waste gate in the turbo, which opens to bypass exhaust around the turbine, to limit the intake manifold pressure. Fuel is controlled, but it's not the mixture they are controlling, it's the fuel volume. They can keep ramping up the fuel quantity as long as there is enough air and time to burn the fuel efficiently. The fuel map knows the amount of air entering the engine, or the intake manifold pressure and controls the fuel to give the HP desired at that throttle position and RPM, or the max fuel that can be burned with that amount of air, or the max HP they will allow the engine to produce, or to limit the temperature in the cooling system.

The turbo is a feedback loop. The more power you make, the more air the turbo pumps in. Which means you can make still more power, which spins the turbo even faster, etc. But it's not just the quantity of air that determines the maximum amount of fuel, the fuel takes time to inject and time to burn. The power stroke is very short, so eventually, fuel is still being injected as the exhaust valve is beginning to open. Lots of smoke and no more power produced.

The advertised HP and torque specs are decided on, I guess, for various reasons. The engine has to survive at the rated HP, without any failures, and has to be happy to work long hours at it's rated torque, at the PTO RPM, or max HP RPM. All this while meeting emissions standards.

The turbo simply makes a small displacement engine act like a large displacement engine. But it's still physically small, with small bearings, pistons, etc. So it needs to have excellent cooling and lubricating systems to put up with all the strain. Turbos and large fuel delivery can easily overload engines. Before turbos, engine were much heavier for their HP and the limit was how much atmospheric air was available.

Another way to look at it is that Diesel engines run at a wide range of air/fuel ratios at a given engine speed, frame somewhere around 100:1 at rated speed/no load to around 15-16:1 at rated power. The turbo match, including compressor and turbine end and wastegate (or variable geometry) designs are chosen to match the desired power output across the lug curve while staying within cylinder pressure and other engine thermal and mechanical design limits.

Remember that, on modern engines, the turbo is also electronically controlled in many instances.

 

[Raspy]

Or, even a better way to look at mixture, which is kind of a silly concept in diesels anyway, is to look at it as how much excessive air there is. "Mixture" is different than "ratio". The burn never happens at 100:1, it happens at around 14.7, give or take, and the rest of the air is just along for the ride to make compression. Of course, the real world mixture is not precisely stoichiometric, because of the imprecise nature of injection systems, droplet size, turbulence, time, etc.

A good example of mixture vs ratio is to imagine a bucket of fuel sitting outside. You can light that fuel and it will burn at a "ratio" to air. It will vaporize, mix with air, heat to the combustion temp and ignite. All in a continuous process until the fuel is gone. At a ratio of somewhere around 14.7:1, give or take. But if you compare the amount of fuel in the bucket to the amount of air in the entire atmosphere, it would be far too lean to burn. Comparing the fuel in the combustion chamber, to the amount of air in the combustion chamber, doesn't always make sense if you are looking at fuel/air ratios with respect to burning efficiency. A lot of the air is just along to make compression, under normal power output, and the fuel/air ratio varies from the tip of the injector out to the cylinder wall.

It is true that there is always an excess amount of air, under normal designed HP conditions, but we still see smoke because of injection inefficiencies. On mechanical injections systems the fuel can be ramped up faster then the boost, causing smoke, or huge injectors let in more fuel than can be burned before the exhaust valve opens. Or cold cylinder walls (which are always cold when compared to combustion temps) inhibit the flame near the cold wall. Atomization is never really very good, and is actually a fog of droplets. Each of those surrounded by air, instead of a pure evaporated liquid.

Striving for high HP is the opposite of striving for combustion efficiency. We have to settle for smoke and a hot exhaust in order to get useful power from an engine that is a practical size to carry around in our trucks.

Internal combustion engines are very inefficient and use a lot of their energy just to pump the exhaust out, and pump the fresh air in, all while overcoming a huge amount of friction, driving a parasitic valve train, and trying to run at a temperature that is designed to allow water cooling. For example, the exhaust valve opens way before BDC on the power stroke to dump the pressure. All waste. But it makes the engine run better. About a third of the fuel energy goes into the cooling system and is expelled to the atmosphere while producing no useful work. Over and over, as you look at the typical four stroke engine, you spot inefficiencies. It works, and it has been refined for 150 years, but it is so inefficient. Various improvements come along and are celebrated as fine improvements, such as variable valve timing and common rail injection, but they are just small upgrades. Other improvements are waiting in the wings, like electronic valves, or opposed pistons 2 strokes.

Two interesting and incremental improvements are coming soon, I hope. One is the Mazda Skyactive-X and the other is the opposed piston Achates engine. I'll take the Achates. Eventually, we may have electronic valves and that will be a game changer.