Pumping losses
 

I was wondering what everyone here is trying to describe when they use the term "pumping losses". I understand that an engine basically moves air in much the same way as a compressor although less efficiently of course.

I am also aware that if you merely ground two opposing lobes on a camshaft a basic long block becomes a much more efficient compressor.

With the throttle closed "pumping losses" are greatly reduced since the force on the piston is 10X atmospheric (approximately depending on compression) with no throttle restriction, the same reason compression tests require wot to be accurate.

Pumping losses would probably be best listed in two cateogries.

First;
The actual losses due to moving the air into the engine, compressing the charge, and pushing out the exhaust.

Second;
losses due to continuous operation of the engine necessary in normal vehicle operation.

I will list a few, add anything you can think of.

All belt driven accesories
Reciprocation of pistons, pins and part of the connecting rod mass (include rings).
Oil pump
valve train
Flywheel
Increased load on water pump due to thermostat restriction to maintain operating temp
Exhaust restriction

regards
badger

With the throttle closed, the engine has to struggle to pull in air. It uses lots of power trying to pull air through a closed throttle. Try it yourself with a jumbo drinking straw, a regular drinking straw, and a coffee stirrer; you'll move a lot more air with a lot less effort through the jumbo straw than the coffee stirrer.

The bit about the force on the piston and compression tests is a logic failure. The force on the piston is not a loss, it's the actual source of the power. Everything else everywhere else is the loss.

Ok, after posting that it led me to this question that I've never quite understood: Why does a more free-flowing exhaust reduce torque? What I've heard, though not experienced for myself or learned the theory behind, is that backpressure is required to make low-end torque, but I have no idea why.

It's not backpressure it's velocity, too large an exhaust too soon can kill velocity thus impeding scavenging. A correctly sized exhaust, including header and downpipes will help scavenging. Once you get everything right with the header and downpipes, really you want the best flowing exhaust possible after that, which shouldn't harm anything. However when the pipes are badly sized at the header and downpipe end, the only thing keeping the velocity up maybe is the restriction in the muffler, so that's when a bigger muffler messes up your scavenging.

Back-pressure is ALWAYS going to hurt performance and mileage.

People used to think back-pressure was what caused the increased torque and fuel economy and to get that they put in smaller pipes. The smaller pipes increased EGV (exhaust gas velocity) which is what really made increased torque. Small pipes with straight through mufflers are optimal.

That's actually a perfect example of why the automotive world is stuck with cars that don't all get 40mpg or more. No offense intended.

That partially clears it up, but how does exhaust gas velocity help?

Keeping the velocity and thus momentum high will mean that there is a slight negative pressure in the chamber as the piston comes up to TDC and the intake valve opens, this helps transfer momentum to the intake charge. Also, during evacuation of the chamber gas that has slowed down in the exhaust system will need to be pushed from behind by the following gas, this push has to come from the piston, robbing power. Correctly phased pulse tuned headers will offer optimum extraction and scavenging due to the negative pulses from previous cylinders helping scavenge the succeeding cylinders.

Very well said, I think I understand it pretty well now.

A 3 inch diameter piston in a perfect vacuum (combustion chamber) would have just over 7 sqaure inches of surface area. Multiply that by 14.7 and you get 102.9 pounds of force.

The same piston at 185 pounds of compression has 1295 pounds of pressure on the top of the piston.

When you close the throttle (restrict airflow) in essence you are taking the engine to a high altitude instantly. Lets say you are restricting 90% of the potential airflow through the engine.

Now you have almost no compression and your vacuum is also reducing the suction forces as well.

When you open the throttle to test compression the engine cranking speed drops, due to increased compression even though your suction has almost disappeared.

It's similar to an engine that has jumped timing, but that affects vacuum and compression.

Without vacuum or compression there is a third force that causes losses (not friction) and thats the energy lost in the reciprocation of the mass of the pistons, pins, rings, and a portion of the connecting rod.

Each piston starts at TDC accelerates to 90 degrees, then decelerates to 180, reaccelerates to 270 then decelerates to 360. two complete cycles per psiton per combustion stroke which represents 90 degrees of the complete 720 degree otto cycle.

You are basically condemned to accepting those 8 violations of newtons law of inertia in order to achieve the benefit of a power producing combustion stroke, in every reciprocating engine.

Suction forces are significant, but compression forces are greater. Reciprocation forces are generally not considered because the assumption is they are inevitable, but that is not the case in the WW1 rotary aircraft engine.

If you take a radial engine, bolt the crankshaft to a fixed structure and allow the engines block pistons, cylinders and cylinder heads to freely rotate you have a rotary engine (not exactly but fairly close). The pistons are not reciprocating, instead they are rotating around the fixed big end crank journal along with the connecting rods, unlike a recip where the rods never rotate around the crank journal.

Google animated engines Gnome and you will see a moving illustration of a rotary engine. The motion of the pistons in the cylinders is a function of the different rotational axes of the block and connecting rod big ends. Combustion forces push the head away from the piston exactly the opposite of any reciprocating engine
Without compression you do not have a pump, even though with compression it's not a very efficient pump, all you need is intake and exhaust for a pump.

Even in a perfect Vacuum with perfect lubrication (no loss) you would still have to use energy to change the inertial state of the pistons 4 times per revolution. The only way to eliminate that loss would be to have no mass in your reciprocating components.

At idle speed (600 rpm) you have 10 revolutions per second. say there are 4 cylinders. Thats 160 inertial reversals per second of all the reciprocating mass of the components involved.

Each inertial reverses energy cost is increased by the fact that it goes from acceleration to deceleration. that force is greeater than acceleration to a stop or acceleration from a stop.

regards
gary

Gary:
I suggest you spend some time looking at a college text with regards to piston engine design - pumping losses are well-documented.

"Along with friction forces, an operating engine has pumping losses, which is the work required to move air into and out of the cylinders. This pumping loss is minimal at low speed, but increases approximately as the square of the speed, until at rated power an engine is using about 20% of total power production to overcome friction and pumping losses."

Diesels do not have a valve in the intake, which is one component of it's superior efficiency over spark-ignition engines.

And if you think about it, take a look at the Otto cycle thermodynamic graph. Do you see anything in there about changing pressure due to a throttle? If you add the effects of the throttle to the Otto chart, and then take into account the losses associated with it, I think you'll clearly see what is going on.

And for what it's worth, the Gnome engine had less in the way of pumping losses due to the fact that it had no throttle - the engine was on or off. As I recall, Bentley was the first rotary manufacturer to unstall a progressive spark cutoff that killed a certain amount of cylinders, a "makeshift throttle" as it were.

gary: "With the throttle closed 'pumping losses' are greatly reduced"

I think you have it backwards. Pumping losses are greatest when the throttle is closed.

As you said, you can think of your engine as an air compressor. But every air compressor is also a vacuum pump. It's just a question of which end you look at.

When intake is restricted (that is, when you are at any setting other than WOT), there is a vacuum in the intake manifold. It's obviously not an absolute vacuum, but it's a vacuum relative to atmospheric pressure. A lighter throttle setting means more restriction and more vacuum.

Your engine has to do work to create that vacuum. This work is wasted. It's energy that is not heading toward your wheels. That's why this graph looks the way it does:

https://www.autospeed.com.au/cms/gall...0&a=110216&i=6

WOT means you are wasting little or no energy creating a vacuum. That's why WOT produces optimal BSFC (at least on that graph; I think on some engines, in certain conditions and for specific reasons, the optimal setting might be 70-90%, instead of WOT).

I think proper use of heavy throttle can be a critical element of maximizing FE. This is perversely counterintuitive, and therefore I think it's one of the most challenging concepts in the world of hypermiling. In particular, I think it's the reason why extreme P&G can be so effective. In my opinion, the essence of P&G is to maximize throttle angle and minimize throttle duration. These two go in hand-in-hand; you can't do one without the other (assuming you want to keep your speed within a reasonable range). And note that this is the exact opposite of what most of us were taught to do, and still do: use light throttle, and maintain this close to 100% of the time. So for most of us, throttle angle is low and throttle duration is high. Trouble is, that's the opposite of what the BSFC graph is trying to tell us to do.

"losses due to continuous operation of the engine necessary in normal vehicle operation"

Those other losses (like the cost of running various accessories) are not normally called pumping losses. The term is normally used specifically to refer to the work involved in drawing ("pumping") air into the engine, and getting rid of it.

"A 3 inch diameter piston in a perfect vacuum (combustion chamber) would have just over 7 sqaure inches of surface area. Multiply that by 14.7 and you get 102.9 pounds of force."

I think you're making a major mistake in overlooking this essential fact: when the throttle is open, atmospheric pressure is operating equally on both the top and the bottom of the piston.

"When you close the throttle (restrict airflow) in essence you are taking the engine to a high altitude instantly."

No. You're only taking the top of the piston to a high altitude. The bottom of the piston is still at sea level. That means as the piston travels toward the ground (Subaru and Porsche owners please make the appropriate mental adjustment), at a small throttle setting, it's fighting against atmospheric pressure. It has atmospheric pressure below it, and a vacuum (relatively) above it. You are requiring the engine to create a vacuum. This wastes energy.

"Now you have almost no compression"

That makes no sense. Of course you have no compression. The piston is moving down, not up. (So I suppose you could say it's compressing the air below it.)

"and your vacuum is also reducing the suction forces as well"

I wonder what suction forces you're talking about. The vacuum itself is a suction force, and the piston is fighting it. More throttle means less vacuum (suction), and less work for the piston to do.

"When you open the throttle to test compression the engine cranking speed drops, due to increased compression even though your suction has almost disappeared."

You have it backwards. When you open the throttle to test compression, cranking speed doesn't drop. It increases. That's the whole point of opening the throttle: to achieve a higher cranking speed, and get a more realistic, accurate compression reading. And the cranking speed went up because you relieved the motor of the burden of creating a vacuum. So the piston has atmospheric pressure both at the top and the bottom, instead of just at the bottom.

"Reciprocation forces are generally not considered because the assumption is they are inevitable"

Good point. But sometimes there are exceptions, like Wankel/Mazda.

holy: "Why does a more free-flowing exhaust reduce torque?"

I think it has to do with the concept of resonance. The exhaust isn't moving in a steady flow. Rather, it's a series of pulses. Gas behavior inside an exhaust pipe has some important similarities to gas behavior inside a pipe organ (or any other musical tube, like a saxophone or a flute). This same concept explains why a high-performance header gives each cylinder a pipe that is equal in length to the others, even though it's not the shortest path to follow. I think there are similar issues on the induction side.

I think road is saying the same thing, in different terminology, when he talked about how the pulses move more efficiently when they are in phase with each other.

It's not an accident that we talk about "well-tuned" headers. It's a lot like tuning a musical instrument.

Gasoline Direct Injection to the rescue! Available now in the Cadillac CTS and soon in the new base model v6 Camaro, coming to a Chevrolet dealer near you... :cool: No throttle, just like a diesel. Almost like a diesel engine with spark plugs and low [for diesel] compression.

This is a concept I've been using since I began hypermiling. You can even have long duration if you manage to keep your RPMs low. At low RPMs, with near-WOT, you're looking at no restriction, totally free flow of all the air it can gulp. I accelerate at near-WOT, using early shifting to limit my acceleration; by varying my shift points I can accelerate as slow or fast as the rest of the traffic, generating the same amount of power in a higher gear with less pumping loss.

holy: "You can even have long duration if you manage to keep your RPMs low."

Just to be clear, I was saying that long throttle duration is generally a good thing to avoid. But as long as you're maximizing BSFC, by having WOT in a high gear (i.e., low RPM), then it's OK to stay in this mode as long as you need to. Because the only choice that's more efficient is staying home! I think you understand this, but I wanted to try to avoid confusing people.

Anyway, what you described is basically identical to the way I drive. The one thing that interests me is that you use near-WOT, instead of WOT. I can guess certain reasons why you might do this, but I'm curious about your views on this.

I've done some experimenting both ways, but I don't have enough data yet to reach a conclusion.

Guys I post here to contribute information not to be insulted by assumptions about my background knowledge.

To prove my point beyond question here is a test scenario

Same road same speed same car.

Accelerate to a certain speed:

1. Coast in neutral to a lower speed, note time and distance
2. Decelerate using the exact same shift points you used in acceleration, downshifting with your foot off the gas with engine off at exactly the same points you upshifted
3. Decelerate using same shift points as 2 with accelerator at WOT, of course with engine off.
4. Remove spark plugs push car to same speed and repeat 3.

Scenario 1 demonstrates total energy losses without engine as a factor.
Scenario 2 demonstrates total energy losses with throttle closed-engine included (your maximum pumping losses although they are not moving the maximum amount of air through the engine.
Scenario 3 demonstrates total energy losses with WOT otherwise identical to 2. Now you are moving the maximum amount of air through the engine.
Scenario 4 demonstrates total losses without suction or compression losses which are all the other relevant losses inherent in IC engines.

Which scenario will give you distance travelled that is closest to neutral coast which is your base measurement?

Now we can debate the point to infinity.

This test can be done with some effort and it will demonstrate what I tried to explain in my previous post.

If your knowledge is so superior that you can state wrong or recommend reading material, maybe you can predict the results without having to actually do the experiment.

i know the results, like to guess why.

regards
gary

I find real data to be the true test. Have you ever conducted such a scientific experiment?

It may be your book has flawed data or it was not completely understood.

Since you seem to believe suction losses exceed compression losses, I can only assume you would think scenario 2 would be less distance than scenario 1 since you assume it takes more work to create vacuum than compression.
Scenario 2 has no suction losses since you have WOT. You do however have maximum compression. While scenario 3 will eliminate almost all of your compression and suction losses you will experience the losses due to reciprocation.

It's 3 separate losses, and this test is a way to separate those individual losses, which allows them to be estimated separately.

You might just learn something about losses that are relevant and the relative percentages.

Monroe your conclusion about WOT operation assumes the BSFC map for a VX is identical to a map for a non lean burn engine, which is a false assumption.

You also assume that WOT is preferrable to 70% which is more than enough throttle position to fill the cylinders (check it with a vacuum guage if you wish to not believe this) at low engine speeds when WOT is not necessary. Check your BSFC map at 50 % compared to higher throttle opening positions AT LOW ENGINE SPEEDS where it is relevant to our discussion (although your reference map is not relevant).

You also choose to ignore the fact that if you maintain lean burn you are using about 60% of the fuel you would use at WOT (check AF ratios of WOT and lean burn).

This is why your conclusion is basically flawed, the map you are using as a reference is not a VX engine map.

The basic design of the VX lean burn system is to increase efficiency at throttle positions that would be less efficient in a normal 1.5 liter non lean burn engine.

Thats why I use less than 50% throttle position, because I can pulse quite a bit longer for the same amount of fuel which increases the total distance of one P&G cycle.

If you want to reply try to be somewhat civilized gentlemen. I find communication flourishes when it is conducted with civility.

I use the sil indicator to determine throttle position when I accelerate. It takes me 13 seconds in 5th gear to accelerate from 50 to 65 MPH, so you can duplicate that time and know exactly the amount of throttle I am using.

I only pulse in 4th gear when my initial speed is below 30 MPH and the light never comes on during the pulse.

Last but not least is the mileage figures I achieve, and remember I am not using engine off coasting, which would increase those figures by about 15%.
I made it clear that I don't feel comfortable using EOC in my situation. That certainly does not mean I dont recognize the additional benefit of EOC. I have used it under very controlled situations without traffic to worry about, and I feel that it would affect my situational awareness and increase the chances of getting in an accident.

regards
gary

"Scenario 2 has no suction losses since you have WOT."

Earlier you said this: "3. Decelerate using same shift points as 2 with accelerator at WOT." I think you're getting your numbers mixed up. That's one reason I'm finding your example hard to follow, but it's not the only reason.

Anyway, I think your example reduces to the following much simpler comparison.

A) Take an engine that's not running, and spin the crankshaft, with the throttle closed.
B) Take an engine that's not running, and spin the crankshaft, with the throttle open.

I think you're claiming that A takes less work than B. Really?

"your conclusion about WOT operation assumes the BSFC map for a VX is identical to a map for a non lean burn engine"

No, I'm not claiming it's identical. I'm just claiming that there's enough similarity to make for a worthwhile comparison.

"the map you are using as a reference is not a VX engine map"

True. But I think it's helpful, as compared with using no map at all. If you have a map or reference that you think is superior, I hope you will give us a chance to see it.

I think it's clear that it's possible to save gas by utilizing lean burn, and it's possible to save gas by minimizing pumping losses. The interesting thing is that it's just not possible to do both of those things at the same time. Lean burn is generally associated with relatively small throttle settings. Trouble is, small throttle settings are generally associated with large pumping losses. Therefore it's necessary to make a choice. And the right choice probably depends on the immediate situation.

P&G is a technique that relies on relatively large throttle settings. And there seems to be lots of evidence that P&G works (and I think it works primarily by minimizing pumping losses). Trouble is, P&G is basically incompatible with lean burn, because in the pulse phase, you're accelerating. It's generally possible to maintain lean burn while at a steady speed, but not while accelerating. So we're back to that choice again, to either minimize pumping losses, or maximize lean burn. I have a hunch that the former strategy is more effective, when driving conditions allow for P&G (ideally in the range of 25-45 mph, I think). When driving conditions require a steady speed (especially at higher speeds, like 50+), then my hunch is that it makes sense to maximize lean burn. I'm not sure about this, and I'm open to evidence that I'm wrong. But I just haven't seen that evidence yet.

"Check your BSFC map at 50 % compared to higher throttle opening positions AT LOW ENGINE SPEEDS"

I have. It shows that WOT is more efficient than 50%. Not always by a wide margin, but nevertheless the margin is positive. Therefore it's probably worth taking. It also shows that at 1000 rpm, the margin is actually quite large. And 1000 rpm/WOT is a state I use quite frequently.

"You also assume that WOT is preferrable to 70%"

True, I do. I don't believe the margin is always large, but I believe the margin is always positive. That's what I see on the graph we're discussing. On the other hand, I think you assume that 70% is always preferable to WOT. I'm open to believing this, but so far I've seen no proof.

"You also choose to ignore the fact that if you maintain lean burn you are using about 60% of the fuel you would use at WOT"

I think the number might be even less than 60%. But what's more important than the air/fuel ratio is the work/fuel ratio. In other words, it's important to approach the analysis in terms of BSFC, not absolute fuel consumption.

"Thats why I use less than 50% throttle position, because I can pulse quite a bit longer for the same amount of fuel which increases the total distance of one P&G cycle."

If I understand correctly, you're using P&G, and you're claiming that you're in lean burn during the pulse phase. May I ask how you know? My experience using a DMM to monitor my O2 sensor leads me to believe that lean burn generally does not occur during acceleration, even mild acceleration. And this is consistent with the comments of others who have monitored lean burn in this manner. How are you monitoring lean burn?

"I use the sil indicator to determine throttle position when I accelerate. It takes me 13 seconds in 5th gear to accelerate from 50 to 65 MPH, so you can duplicate that time and know exactly the amount of throttle I am using."

I'm very confused by that paragraph, because the SIL is off 100% of the time that you're in 5th gear, regardless of vehicle speed, engine speed, or throttle position.

"I only pulse in 4th gear when my initial speed is below 30 MPH and the light never comes on during the pulse."

If you are in any gear other than 5th, and using a throttle setting large enough to keep the SIL turned off, then you are almost certainly not in lean burn.

Gary, I think you might be reading more feeling in Monroe's posts than he intends. Just a guess.

Everybody, please use the built-in quote feature. It's much easier to follow, as it does a better job separating and highlighting text. It's not hard to trim quotes and respond to individual points, you just have to put in ['quote'] and ['/quote'] tags like the ones that are automatically inserted when you click "Quote".

I'm avoiding open loop. What I've read is that when you are at WOT, the ECU goes into open loop mode, ignoring the post-cat o2 sensors (if that makes sense) and using pre-set fuel tables instead of metering fuel based on actual measured conditions. In more simple terms (for anybody reading this who is more of a layman than me), it just dumps fuel into the engine rather than trying to be efficient.

Making things more complicated for me, my VW has a Drive By Wire system, so the gas pedal is not necessarily a throttle control. I am not sure that 50% of the gas pedal == 50% throttle opening, or that 100% of the gas pedal == WOT.

Now, that said, I'd really like a way to know in real-time whether I'm in open loop or closed loop. I think I may be able to floor the pedal and still not be in open loop at low RPMs. I started using a Vag-Com cable to monitor it on a laptop, but I can't tell if it's lagging a few seconds in its readings or if it really doesn't go into open loop easily. It takes until like 3500 or 4000 rpm before it shows open loop, then stays open loop a couple seconds after I take my foot off the gas.

Since this thread discusses P&G quite a bit, I might as well mention this: I have been tried P&G (neutral, not EOC) and it looks promising. Based on the mileage and position of my gas gauge I'd guess I've gained at least 2 mpg over my previous record, but it's hard to tell (harder still because of a minor technical difficulty I'm having with my digital LCD odometer reading in km instead of mi).

The source of my assumptions is what I read in this forum, that doesn't make it a fact any more than everything you read in any book makes it a fact. Its all hypothetical until real testing proves otherwise. Try the throttle closed versus open test that I suggested. If you coast down distance is lower with throttle open I am correct (more resistance at WOT). If it is lower with throttle closed you are correct (more resistance at closed throttle). The reason I think I am correct is because compression is greater at WOT, while suction losses are insignificant at WOT. A simple test that proves my point.

I have done thousands of compression tests and the conclusion I have reached is the cranking speed is lower with WOT. My reasoning to support that is simple. You are pumping more air through the engine at WOT, because the compression is lower if you don't open the throttle. Since suction losses are practically nil at WOT I can't rationalize any other conclusion.

Please let me know the results of your test.

Think about the Jake brake on big rigs, they are using trapped compression, not vacuum, obviously they can't use vacuum. The difference in compression versus vacuum is directly proportional to the compression ratio. No closed throttle will produce a perfect vacuum, but in my previous example with 185 pounds of compression you have over ten times the resistance that 14.7 negative pounds of vacuum would ever produce.

I wish I could prove when lean burn is engaged. Since that is not possible I rely on those contributors to this forum who have developed a means of proving that it is engaged when climbing a slight grade in 2nd, 3rd, 4th, and 5th gears. In fact in the links Tom O proivided there is information that justifies that assumption. The contributor was one of the hypermilers that was involved with Wayne Gerdes, who made the statement that lean burn was possible in lower gears and climbing grades.

"You can climb a slight grade and still stay in lean burn, even in lower gears".
The use of lower gears would indicate to me that steeper grades would still allow lean burn as long as you used lower gears.

Think of it this way, we know the VtecE closes one valve almost completely to create swirl and improve low speed fuel air mixture (below about 2500 RPM). Homogenous Charge Compression Ignition relies on close to perfectly even distribution of the fuel particles in the fuel air charge. Testing has shown that this creates the potential for compression ignition of gasoline with a 25% improvement in efficiency.

You can check that by googling HCCI. The problem is its presently not possible to accomplish HCCI over all the loads and ranges of normal engine speed. I takes some time for the mixture to completely emulsify which brings on the problem of having a fairly significant amount of fuel air mix in the manifold, which would be a scary thing if you had a backfire. One solution is ultra high injector pressures, but it may be that it will not be possible with fuel injection occuring soo close to the intake valves. We may actually have to go back to a form of throttle body injection with mechanical turbulence and preheating of both air and fuel to allow better fuel particle distribution and to offset the fact that when vaporized fuel tends to become cooler. In the carb era without preheating of the air in cold temps you had carb icing where the fuel would not suffeciently atomize. The result was hesitation to the point of stalling. Its the same principle as warm air intake which while reducing the mass of air going in the engine also allows for better atomization of the fuel particles. Preheating the fuel would also help, but when the fuel is heated you risk vaporizing the fuel in the lines which causes serious problems.

My position is lean burn requires better atomization in order to function at 22-1 AF ratios. While it is not true HCCI at certain speeds and loads it is at least a significant step towards HCCI.

We disagree on when lean burn is actually occuring. I am at a disadvantage with my old time pre computer education which ended in the late 60's. The shift indicator light is there to tell you when to shift for best economy. To me it makes no sense that the light would be on when lean burn was working but off when it wasn't. The light uses several imputs, but basically it is telling you your vacuum is to high for efficient operation. Thats why I believe (can't prove it other than my mileages) that lean burn occurs if you stay just above the point where the light comes on and below 2500 rpm. Of course the light does not come on in 5th gear, but you can still use the same throttle position percentage. It doesnt take much throttle to accelerate from 50-65 in 5th gear in 13 seconds. The coasting time is about 17 seconds,and it may be better to just forget P&G at those speeds. Other forum members have also stated that it is difficult to achieve the percentages of improvement in average mileage with a VX, compared to the same year vehicle without lean burn. Thats easy to understand when you look at the difference in combined mileage for both vehicles with the difference about 33%.

I enjoy civilized debate and hope to learn from our interaction. It certainly is possible that your position is correct and mine is less correct.

I don't think a "I am totally right and you are totally wrong" position would produce any learning.

regards
gary

My rationale is based on this statement by a fellow hypermiler who apparently has a means of verification.

"I can maintain lean burn up to 70 MPH"

TomO stated that lean burn is a function of several imputs one of which is the load placed on the engine.

If I use the same throttle position as I would at a constant 70 mph, I should be able to stay in lean burn. At lower speeds that means you would be accelerating since total energy losses are lower at lower speeds maintaining the same load would allow for acceleration.

If the potential for lean burn did not exist, I would only disagree with you on the throttle position. In that case I would use 75% because at WOT you are creating enrichment that doesn't exist at 75%.

If you looked at a BSFC map for the VX (even though I don't know where on is or I would provide a link) it would be two maps.

One in lean burn

One outside of lean burn

Combining the two would be confusing to say the least.

This is what I believe. Mr Honda was a pioneer in lean burn technology, going back to the stratified charge prechamber CVCC engines in the 70's. He died in 1991 the year the first VX was produced. He was not a highly educated man, but his intelligence is unquestionable. After his death the driving force behind lean burn technology seems to have been gradually lost. Combined with increasingly stringent emissions requirements lean burn fell out of favor, principally due to NOX emissions.

Lean burn was a way of compensating for the poor BSFC of normal engines when there is any significant throttle restriction ,and it was a practical solution that proved to be more economical.

Try one more test.

Hook up a vacuum guage and see how little throttle opening you can use to achieve the lowest possible vacuum reading at low engine speeds. I beleive the throttle opening percentage would be below 25%, because you are only running the engine at about 1/5 of maximum speed (give or take).

I believe the sil light uses vacuum as its principle imput (not the only imput), to show the driver when upshifting will create a greater load and lower vacuum to produce the same work, which I think we can agree is more efficient.

regards
gary

I think this brings up a good point. Note that the SFC chart indicates when the engine is operating at maximum efficiency. This is not the same as operating at minimum rate of fuel flow. This explains why at certain load+RPM combinations (e.g. optimum efficiency on the BSFC graph), the engine is burning more fuel than at idle (note comments by Pale in other threads about fuel use at idle).

I understand the concept of SFC, but after a few tanks of gas and use of P&G but no ridiculous improvements in fuel mileage, I was wondering what the hell was going on. Then it occured to me that the isopleths on the BSFC graph (that I was trying to emulate in my P&G technique) are of fuel flow rate per hp produced. Which I had read but didn't "compute" in my mind.

I can't directly determine what the BMEP of my engine is at any given time, so I goto the SFC vs RPM chart with the load curves. I was able to find a dyno run for a 4 cylinder Camry (believe it or not), so to get an idea of fuel flow rate I plugged in the hp number for given RPM numbers for different load lines. Lo and behold that at less-than-ideal RPM and throttle settings, I can easily reduce the rate of fuel flow. In other words, at 2,000 RPM on the 50% load line I can use less fuel per hour than at 2,500 RPM at 100% load.

Therein lies the point - to minimize fuel use. I can maximize efficiency by operating in certain load+RPM combinations, but in doing so can actually use more fuel in total. And the point is to use the least amount of fuel for the most amount of miles - this is why idling (which is less efficient) can improve you rmileage.

I'm not sure I can hit crazy MPG numbers by worrying about throttle angle and RPM alone - the other variable is the HP created. This defines the fuel used.

<sigh>

I think I need a scangauge.

Unfortunately, as I found out in another thread, the SG doesn't actually measure fuel flow and some of the other important variables it displays. Instead, it calculates them. Be aware of that as you consider buying it for precision measurement of such things.

As for the leanburn vs P&G argument...


To me, the answer seems simple. Leanburn is obviously there to compensate for poor engine efficiency at light throttle by leaning the engine, which reduces power, which requires you to give it a bit more throttle to maintain power... which reduces pumping losses in addition to being a slightly more efficient burn. But, it also seems obvious that the engine produces more output energy per lb of fuel at high load (low rpm obviously)

So, the answer is to use them according to driving conditions. If you can P&G, DO IT! If you're not in a position to do so, you may need to DWL for a while, and leanburn is better for that than non-leanburn.

Honestly, thermal efficiency is by far the biggest player in all of this.

At part throttle there isn't much air in the chamber. That small amount of air doesn't have a lot of heat in it when you ignite it so more of the heat goes to the cylinder walls instead of pushing the piston back down, it burns slower because it isn't as dense making for timing advances that rob the engine of efficiency and at part throttle it's working against a vacuum, a partial parasitic loss only if there is enough vacuum in the chamber left to pull the piston up on the bottom portion of the compression stroke.

At WOT you have way more air in the chamber. The larger amount of air has much more heat that would have to be absorbed into the engine, it does get absorbed but the ratio of heat created to amount absorbed plummets as engine speed and throttle increases. Since there is more heat in the chamber the pressure stays higher longer and can actually perform work. The air is very dense when you get close to the top of the compression stroke and results in a faster, better burn requiring less timing advance and increasing efficiency. At WOT you have MUCH less loss from pumping on the intake stroke but you end up with more losses on the compression stroke as has been mentioned, however, when you are compressing the gases you are building potential for the event happening in the power stroke. Not just throwing it away.

Fair enough, but the next logical question: what device out there actually measures fuel flow in passenger cars?

I think you're good guesser.

Sorry, I know I'm a major offender in this regard. I should realize it's probably annoying. I'm just so used to doing it the old-fashioned way, but I'll give it a try.

OK, that's what I figured. Thanks for explaining; I think you explained it very well.

I think one of the interesting things about open-loop is that it's something my car generally doesn't do, as far as I can tell. I think a characteristic of wideband-sensor systems is that open loop occurs only when the engine is cold, or when there's some kind of a sensor failure.

But I understand why avoiding WOT makes sense in your situation.

Interesting question. I don't know what that lag is about. With my DMM, I can monitor my O2 sensor directly, and the response is immediate. That is, I can see AFR changing right away, as I move the throttle. But you're reading info indirectly, with Vag-Com getting the data from your car's computer in OBD format (I think that's how it's working; I could be wrong). That's a more complicated process, so it's harder to tell where the delay is being introduced.

Coming out of open loop, I could see how your car's brain (ECU or ECM or whatever VW calls it) could choose to lag and hold open loop for a few extra seconds, for good reasons. But I would think that going into open loop would be immediate, when you floor it under certain conditions. After all, the idea is to give you good throttle response, and provide the power you're demanding. Also to prevent an overly lean mixture from hurting your motor. So it should happen without a lag, I think.

I think coasting in neutral is a very powerful technique. I think it's easy to underestimate the drag created by coasting in gear, and I think this drag probably overwhelms the benefit of DFCO.

EOC is great, but it requires very specific driving conditions.

You're claiming, literally, that there's "more resistance at WOT." I have a lot of trouble grasping how we create "more resistance" when we remove a restriction that is preventing air from entering the engine.

Yes, a diesel can't create engine braking with vacuum, because it has no throttle. Engine braking in a gas engine is the result of the engine fighting the vacuum created by a closed throttle.

If you were correct that there is "more resistance at WOT," then it would be possible to maximize engine braking on a gas engine by using WOT (with the injectors off, of course). With current drive-by-wire systems, this could be easily accomplished. I wonder why no one has ever built an engine that works this way. Then again, maybe you know of one?

Aside from all that, I have no idea what point you're trying to make by mentioning diesels and Jake brakes.

For some reason you're comparing the compression cycle to the intake cycle, but I have no idea why. As dk pointed out, the energy put into compression isn't wasted. It's like compressing a spring. We get it back a moment later (aside from a bit of frictional loss) on the power stroke.

On the other hand, the energy that goes into fighting the throttle restriction is waste. We don't get it back.

But it is possible. Thanks to the helpful instructions posted by TomO and others, in various places, it's not hard to do. You just need to put a DMM across D14 and D16. At Harbor Freight, DMMs start at $4.

If you monitored your lean burn, I imagine that you would notice things that would be helpful to the rest of us. Since you're not monitoring your lean burn, I have a hunch that there are quite a few moments when you think you're in lean burn, but you're not. It's very, very sensitive to throttle position; you can rapidly move from lean to rich with a very small increase in throttle angle.

Yes, you can climb a hill and maintain lean burn. Provided it's not too steep and you're not going too fast. And yes, you can cruise at fairly high speed (60+) and maintain lean burn. But it's very hard to maintain lean burn in any gear, if you are conducting any form of acceleration. Unless the acceleration is exceedingly moderate.

Yes. But a difference worth noting is that I've monitored it directly with an instrument, whereas you're essentially making a bunch of guesses based on imprecise statements you've read in various places.

Because lean burn is so sensitive to small throttle movements, I don't think you can count on your ability to maintain lean burn based on the idea that your foot is able to remember the exact angle it was holding a few seconds ago. If you had an instrument reading your Throttle Position Sensor, that would be a different story.

Somewhere I saw a photo of someone who rigged a hand throttle, for this purpose, essentially. You would need that kind of precise control to really be able keep the throttle in exactly the best position to maintain lean burn.

It might make no sense to you, but that's still what the light does (under certain conditions). If the light goes on, and then you increase the throttle setting by a certain amount, the light will go off. Even though you haven't obeyed the light and upshifted. And at this moment, lean burn is off and so is the SIL. Go figure.

I think you're oversimplifying the relationship between lean-burn behavior and SIL behavior.

Yes. In 1980 I bought a '78 Civic CVCC. A great car. It looked a lot like this: https://www.hondaclassiccars.co.uk/images/Dcp_1052.jpg

I'd like to understand what you did, but I'm having trouble following this.

I think these statements, and the rest of the comments from which they are taken, do a really nice job of summarizing what we're talking about.

SuperMID. And this: https://forum.ecomodder.com/forumdisp...mputer-26.html

Not much else, that I'm aware of.

You have the links to the SFC and BMEP isopleth charts? The links are on this board, Pale originally posted them, I reposted in other threads.

You will note on the SFC charts that there are a number of iso curves, each iso curve represents a particular engine load - 25%, 50%, etc. The curves show that each load has a different efficiency, and that 100% load is most efficient.

The next step is to realize that the units of SFC are lb/hp-hr. So if you know what SFC your engine is operating at and how much hp is being produced, you can calculate the fuel flow rate.

I used the SFC chart for a "typical 2-valve 4 cylinder engine" and was able to find a dyno run graph for my car. Multiplying the hp number from the dyno chart with the SFC number that corresponds with a particular engine RPM yielded fuel flow rate. I calculated points on each iso curve to compare flow rates.

That's under heavy discussion in the "realtime monitoring tools for pre-OBD cars" thread.

Thanks, it's much easier for me (and I hope everyone else) to follow.

Why wouldn't it happen when you go WOT at high RPM?

So, if I tap a wideband O2 into my exhaust, I can hook it up to a DMM and have a realtime AFR gauge? I don't think I'd risk it in my leased VW but I'll do it in the vehicles I own.

It might be using OBD, or it might be using VW's proprietary protocol, I'm not sure which one it's using. You're right, with these systems being effectively black boxes to me, I can't tell where the delay is.

Well said! It never seemed like much drag, but in neutral it becomes obvious that it was. Now I use DFCO when I'm planning on slowing anyway, and neutral when I coast. The fuel cost to idle is less than the momentum lost using DFCO.

My commute has a few places perfect for it. I tried it once and got three angry beeps from my car, now I'm afraid of how it could bite me in the *** in the event of a warranty claim, so I won't do it again until I can find out what the three beeps meant. Unfortunately, when I asked on a VW forum, all I got was moral judgement and hate; they were more interested in why I did that than why it beeped. There's another bigger forum where I'm not a member, but I might join to ask.

Hmmm now that's an idea... that or the MAP reading...

Thanks for the heads-up. I'll check it out.

I can see what you guys mean about more resistance at WOT than with a closed throttle. We know that a vacuum pump will only give so much resistance, a compressor can continue to increase in the amount of resistance offered depending on the amount of pressure it needs to put out.

It makes sense that there would be more 'loss' in an engine at WOT if and only if we are talking about an engine that isn't running at all but being turned by an outside source. If the engine is running it's completely different. We all know that raising compression makes for better fuel economy and more power overall. It's the main reason diesels are so efficient.

As for making a throttle-by-wire system that engine brakes at WOT, they won't do that because all that cool air in the exhaust would cool the catalytic converter(s).

...and does anyone have any suggestions where I could get wideband O2 sensors cheap? Searching Google Products and eBay it looks like it will cost me ~$70 shipped.

OK, thanks for explaining, and I understand all that. But I still don't understand something you said earlier: "In other words, at 2,000 RPM on the 50% load line I can use less fuel per hour than at 2,500 RPM at 100% load."

Trouble is, your fuel consumption (per hour) has decreased, but your engine's output has decreased even more, which means you achieved less speed and/or distance.

It's always possible to save fuel by reducing speed and/or distance. But when we optimize BSFC, we can save fuel without reducing speed and/or distance. So I'm trying to understand why you decided that 50% throttle is superior to 100% throttle.

Because in a wideband-sensor system, the sensor is able to continue to provide the ECU with useful info even in those kinds of extreme operating conditions. Therefore there is no need for the ECU to ignore the sensor (and that's what open-loop means).

Don't get me wrong; WOT/high RPM in a wideband-sensor system is still going to mean a rich mixture. It's just that it's a rich mixture arrived at by an ECU that's reading the O2 sensor, rather than an ECU that's reading values from a stored internal map (that's what happens in open-loop mode).

Yes. But keep in mind there are a bunch of commercial products directed at people who are asking the question you just asked. So you should look around and see what's out there before you reinvent the wheel. Try this google: wideband gauge afr.

You're not likely to hurt the vehicle. Aside from making a hole in the exhaust system to mount the sensor.

Exactly. If you really want to monitor AFR in realtime, there are more direct ways of doing it. On the other hand, you might discover that your current monitoring system has no major lag, and your ECU is itself inducing the lag, for whatever good or bad reasons.

Exactly. I think people tend to misunderstand that, and treat DFCO as some sort of secret magic trick, and they overuse it. DFCO is to be used only when your environment is forcing you to slow down. If you're in a situation where you would not use brakes, then you shouldn't be using DFCO, either. Rather, you should be maximizing your glide, via either coasting in neutral, or EOC. (Obviously I'm using the term DFCO as shorthand for "coasting in gear;" it actually stands for Deceleration Fuel Cutoff.)

Interesting. At what point did you hear the beeps? What was the position of the ignition key at that moment? Was the engine running? Car in motion? Just curious.

'Cheap' is a relative concept. The wideband sensor required by the VX is $334 at Autozone, some much higher amount at the dealer, and usually about $150 on eBay.

There's a cheaper wideband sensor made by Bosch. I think that's what you're seeing for $70. I would be surprised if you ever see a wideband sensor for much less than that (aside from used ones with a questionable life expectancy).

I realize I brought up the idea of monitoring TPS. But it would be more direct to monitor AFR. And something more direct than monitoring AFR (and also more direct than monitoring MAP, for that matter) is monitoring actual instantaneous fuel use.

A certain throttle setting will lead to a certain AFR, which, in turn, will lead to a certain level of flow through the injectors, which, in turn, will lead to a certain level of money flowing out of your wallet. There are lots of different things we can monitor, but some are more relevant than others.

I was being a bit ironic when I hinted that Gary could use a TPS monitor. What would probably be more useful (in his particular situation, given his interest in maximizing lean burn) is an AFR monitor, which he could achieve for about $4.

Assuming that the engine is being turned by an outside force, why is there more loss at WOT? I think you and Gary have both tried to explain this, but I understand neither explanation.

But if WOT somehow creates more loss (which I don't comprehend), doesn't that mean that less cool air, not more, would be leaving the engine?