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Old 05-28-2008, 01:52 PM   #1
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With all this dicussion of "minimizing pumping losses"

I see people arguing that they should use more throttle because it can reduce pumping losses, which I think really means that because so little air can get in, the engine is consuming energy in getting air/fuel into the engine. This is similar to the situation where you only have 2 valves per cylinder and the engine has to spend energy pushing out the exhaust. So when you have VTEC-E, it makes the exhaust stroke easier by having two valves open while at the same time restricting the intake valve.

So what is it, do we want to make air/fuel easier to come into the engine so that it doesn't strain to breathe, or do we want to restrict it so that it doesn't consume more fuel? With people saying that we need to use WOT or at least more throttle instead of starving the engine of air/fuel so it doesn't have to strain, why don't you guys install turbos or superchargers then?

Maybe what we should do is install a super/turbo charger while at the same time use a hot air intake, that way you have an easier time filling the cylinders while at the same time not filling it up with more fuel/air than usual.



Though I think my question/answer is starting to show my lack of knowledge in this field. This brings me the next question, why does the butterfly valve on a petrol engine regulate Engine speed and fuel consumption but not on a diesel? When you open the plate, you let more air in but how does that increase the engine speed? My reasoning is that opening the butterfly valve at 10% just means it takes longer for air to get in opposed to 100%.

I guess I'm just in over my head.
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Old 05-28-2008, 03:19 PM   #2
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its logical thinking. kinda like a diesel, WOT all the time. I know on my manual protege the TPS has 2 settings, 0% and 100%. SO in that case its in my best interest to floor the petal to 2400rpms and shift everytime, maximizes the efficiency of the motor and FE. The Grand Prix however has a variable TPS, and adjusts the PW of the injectors accordingly, so flooring it actually wastes more fuel than a light throttle.

A lot of this depends on the vehicle and the ecu.
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Old 05-28-2008, 04:14 PM   #3
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Diesels operate by welcoming and inviting detonation. COMPLETELY different.
Some of the engines have a special area offset of the main cylinder to start preignition. Forgot its name I've been out of school and out of the industry. Indirect ignition is what i think it is
Diesels operate at 1:1 or 20:1 a/f ratio. Your foot regulates the fuel going in, which will always ignite and push piston. The fuel ignites because of the pressure and heat already there, no spark. More fuel more power no matter what. When there is an oil leak and it gets into the cylinder you can have a runaway engine that revs and revs itself to death. Locomotives do this and id not want to be anywhere near that. The gas engine you are operating an air plate, that the computer injects fuel to hit that 14.7:1 stoich nonsense.

Gas engines that are turboed are always on the brink of detonation. Hot air is the last thing a turbocharged gas engine wants. The turbine already does a great job heating the air--enough to need air to air intercoolers. In the mid 1980s ford turbocharged the pinto 2.3 engine. It ended up in the mustang GT, a special thunderbird turbocoupe and the merKur xr4ti. I have a merkur. It came with no intercooler, which meant all the hot air went straight into the engine. All it did was not let me tune past 13psi. If I had a bad tank of gas I was lucky to keep up with a 10:1 compression engine of the same displacement. The ECU of all turbocharged cars are very conservative on the ignition timing so they wont blow up with the introduction of more air. If you start doing sensor manipulation with your car you will find your mpg's come up when you can KEEP as much timing as possible. Bump your timing add some water injection and you will have power AND economy. With an intercooler, you can crank on the boost and add timing past what the fuel injectors can deliver.
Anyway the economy of that ford 2.3 engine was no better than the N/A counterpart used in the ford ranger and mustang. The power in stock form was almost as nice as the 5.0 v8 engine, but the mpg's werent double like you want to see. Another example is my other car, the dodge srt-4 old epa 22/30. There is no neon 2.4L but in the PT cruiser its better than the turboed one, but advertized as the same. I would rather see you convert to propane and turbocharge that, as propane is a gas when introduced so there are no "hot spots" and the effective octane rivals racing gasoline so when set up right it should be safe for the engine.

I just reread your post before hitting submit. (lightbulb) CVT transmission on the 2006 altima 2.5 rental I had ...steady 4500rpm when you mash gas and it holds it there as you accelerate. When you are part throttle and behaving its constantly hunting for sweet spot(s) depending on the terrain.
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Old 05-28-2008, 04:46 PM   #4
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GDI (Gasoline Direct Injection) engines don't necessarily need a throttle, and can be designed more like diesels with no throttle restriction plate (butterfly valve), just WOT all the time. Like a diesel, engine speed/power is controlled by the amount of fuel injected, and the a/f ratio can vary.

The point of turbo/superchargers is to cram more air and fuel in. The point of minimizing pumping losses is NOT to do that; it's to reduce the energy expended while moving the same amount of air/fuel and making the same amount of power. If you do it right, you're not moving more air than you otherwise would; the strategy of WOT for reduced pumping loss requires lower shift points for the same amount of acceleration.

I've been monitoring my fuel injector duty cycle, and in my VW I've found that gas pedal on the floor actually uses less gas than at 90%. I'm not saying merely that it uses less gas per unit of work done; I'm saying it actually uses less gas per unit of time. I don't think I'm at WOT when the pedal is down though -- the pedal isn't connected to the throttle, rather it's connected to the computer. Still it's counterintuitive that less fuel would be consumed if I floor it.
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Old 05-28-2008, 05:42 PM   #5
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Originally Posted by theholycow View Post
GDI (Gasoline Direct Injection) engines don't necessarily need a throttle, and can be designed more like diesels with no throttle restriction plate (butterfly valve), just WOT all the time. Like a diesel, engine speed/power is controlled by the amount of fuel injected, and the a/f ratio can vary.

The point of turbo/superchargers is to cram more air and fuel in. The point of minimizing pumping losses is NOT to do that; it's to reduce the energy expended while moving the same amount of air/fuel and making the same amount of power. If you do it right, you're not moving more air than you otherwise would; the strategy of WOT for reduced pumping loss requires lower shift points for the same amount of acceleration.

I've been monitoring my fuel injector duty cycle, and in my VW I've found that gas pedal on the floor actually uses less gas than at 90%. I'm not saying merely that it uses less gas per unit of work done; I'm saying it actually uses less gas per unit of time. I don't think I'm at WOT when the pedal is down though -- the pedal isn't connected to the throttle, rather it's connected to the computer. Still it's counterintuitive that less fuel would be consumed if I floor it.
Well I was just thinking that a turbo with no boost, basically heating up the intake air would mean that less fuel is burned because less air is available but at the same time would mean more throttle in order to accelerate. However what a turbo does is force so much more air and fuel into the cylinder that the temp of the air is negated since more overall air/fuel is in the cylinder than under atmospheric pressure with lower intake temps. Wouldn't a low boost turbo act as an assistance to the air intake without increasing the amount of fuel being burned?


Also I'm just wondering how it would be possible to increase the fuel economy of cruising speed over pulse and glide as it isn't practical nor a good idea for some vehicles.
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Old 05-28-2008, 06:31 PM   #6
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Think of it as a "cylinder filling event"

In order to fill the cylinder for every combustion pulse you must allow all the air in that the cylinder needs to fill completely.

Using a vacuum guage at 1200 RPM in high gear slowly depress the gas pedal until the vacuum drops to a point then levels off. That is basically where your "cylinder filling event" has occured. Adding throttle really makes little difference after the vacuum has dropped to its lowest point. Going beyond this can use more fuel in some systems.

Pumping losses are the essental components of cylinder filling events. Decrease the throttle and you starve the engine for air. It responds by loosing power.

This is because you are reducing the effective compression by allowing a vacuum to exist in the manifold.

Less compression means less power, and less fuel, as well as less efficiency (generally speaking).

The problem is you cant maintain "cylinder filling events" because the engine is too large, and the highest gear isn't high enough.

There are 4 ways you can solve this (and probably more).

Diesel-always a CFE (cylinder filling event)

More gears in tranny-would allow you to cruise at less than 1000 rpm possible CFE (not quite if engine is too large)

Smaller engine+more gears to the point where you would have to downshift to accelerate even on a slight incline-CFE

The 4th is lean burn. This is where it gets controversial.

I am reducing the compression by restricting the intake air. To compensate for this I create a whirlwind in the intake system and reduce the ratio of air to fuel from 14 to 1 to 22 to 1.

Effective I have created a variable "EFFECTIVE DISPLACEMENT" engine that can create ample power to sustain velocity by using less total air and fuel, but especially less fuel volume for the same amount of air than a non lean burn engine.

In a lean burn situation it takes more energy to pull the air past the restriction, but it is offset by the fact that you can have combustion with significantly less fuel (as well as less work to compress the mixture). The limit is how lean you can go without creating too much NOX, which is a product of high combustion chamber temperatures.

Higher combustion temperatures create more effective pressure to convert into work. They also tend to creat spark "knock" in many cases, which is usually controlled by a knock sensor.

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Old 05-28-2008, 06:38 PM   #7
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Quote:
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Well I was just thinking that a turbo with no boost, basically heating up the intake air would mean that less fuel is burned because less air is available but at the same time would mean more throttle in order to accelerate.
That's the description of a Warm Air Intake, a common DIY mod discussed here.

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However what a turbo does is force so much more air and fuel into the cylinder that the temp of the air is negated since more overall air/fuel is in the cylinder than under atmospheric pressure with lower intake temps.
Turbos are not at all about the temperature of the intake air, they're solely about the volume of air. A turbo also does not force more fuel into the cylinder. Instead, it merely forces air in and the fuel system puts in as much fuel as is necessary for the desired air/fuel ratio. People using them for performance generally try to use the coldest air they can, so that its density results in additional air mass in the cylinders.

Quote:
Wouldn't a low boost turbo act as an assistance to the air intake without increasing the amount of fuel being burned?
Yes, but it would be a net loss, or if it was a perfect machine with no friction, it would be a net draw. The pumping energy that is saved on intake by having the turbo push it in would be lost again by having to push it out to operate the turbo. You are effectively using an air pump to drive a fan.

Now, if a low-boost blower could be powered by scavenged energy, such as steam pressure from exhaust heat or regenerative braking, I suppose it could reduce pumping losses.

Quote:
Also I'm just wondering how it would be possible to increase the fuel economy of cruising speed over pulse and glide as it isn't practical nor a good idea for some vehicles.
Such vehicles would need to be driven slower, or would need major mechanical or design changes. My VW is a bit impractical for P&G at highway speeds because of its low gearing and sometimes unpredictable drive-by-wire system, combined with my crappy rev-matching skills. P&G on the highway destroyed the benefit I had from P&G on slower roads. It does seem to work with a modified P&G strategy where I don't use neutral, just letting the engine braking happen (which at least uses DFCO); I gained 1.5mpg over the previous attempt I described above with the highway difficulty.
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Old 05-28-2008, 08:17 PM   #8
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Originally Posted by R.I.D.E. View Post
Think of it as a "cylinder filling event"

In order to fill the cylinder for every combustion pulse you must allow all the air in that the cylinder needs to fill completely.

Using a vacuum guage at 1200 RPM in high gear slowly depress the gas pedal until the vacuum drops to a point then levels off. That is basically where your "cylinder filling event" has occured. Adding throttle really makes little difference after the vacuum has dropped to its lowest point. Going beyond this can use more fuel in some systems.

Pumping losses are the essental components of cylinder filling events. Decrease the throttle and you starve the engine for air. It responds by loosing power.

gary
So how do you know if the engine will use more fuel past that point? Does the OBD provide this sort of vacuum information and whatnot?

It's interesting to see everybody advocate P&G yet isn't it conventional wisdom that cruise control saves gas? I mean don't most people let the throttle get away(there is a word for this) and when they realize they've increase speed, they let off the throttle, only to do this again? I remember reading some article somewhere saying how cruise control can save you fuel. Also it is said that because it requires more energy to increase speed than maintaining momentum that P&G technically shouldn't be saving but wasting more gas.

Are these people that say these things going mostly off of theory and not in practice like we all do?

Also has anybody considered using a stirling engine to power a compressor for an airconditioner? That air conditioner would cool the engine coolant and then the engine could allow for hotter temperatures and so on? Can you reduce NOX by reducing the temps of the coolant?
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Old 05-28-2008, 08:38 PM   #9
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It takes X amount of fuel to maintain speed. Say 8 horsepower at 50 MPH.
You have a 100 HP engine. To maintain 8 hp you have to choke off most of the air to keep the power level that low.

Here is where it gets hard to understand. If you ask for 20 more HP, it only takes 50% more fuel.

You use the 20 more hp to store inertia in your speed, then coast with engine off or idling.

My car uses about .2 gallons per hour idling. If I start to coast at 50 MPH and use .2GPH I am getting 250 MPG while I coast. It gradually drops as my speed decreases but even at 40 MPH I am still getting 200 MPG.

In summary I use some more fuel to increase my speed, but the mileage goes sky high when I coast. Since I used only 50% more to increase my speed, and 80% less when coasting my average is better than you can get with cruise control, or just driving at a steady speed.

The average for me is 57.55 MPG over the last 2600 miles driving a car that was originally EPA rated at less than that for the highway.

Theory is good but proof is absolute.

Imagine you have gas rationing, you get 10 gallons a week, but you can't drive normally and make it on 10 gallons. Pulse and glide and lower overall speeds are your only solution.

The first time I read about P&G was 1970. An Opel Kadett Wagon got 124 MPG, averaging 26 MPH.

regards
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Old 05-28-2008, 09:20 PM   #10
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It's interesting to see everybody advocate P&G yet isn't it conventional wisdom that cruise control saves gas?
P&G is not the same thing as random speed variations, which is what a cruise control is designed to minimize..

P&G is more along the lines of how some hybrids operate, where the engine only runs part of the time at full power for just long enough to recharge the batteries/pump up the accumulator/whatever..

If you look at the BSFC map of a Prius engine below, you will see that at high loads, above 70 Nm or so the engine uses less than half the fuel per kilowatt hour than it does at low loads of 15 Nm at the same 2500 rpms. 230 g/KWh versus 500 g/KWh.

If you don't understand what you are seeing looking at the BSFC map below, you really need to do more studying before we can properly relate the concepts that some here are trying to explain to you.. There are a lot of things in life that are counterintuitive, and this is one of them..

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