What you need to know about Brake Specific Fuel Consumption

[drmiller100]

Originally Posted by serialk11r

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A good way to measure pumping losses would be to compare the manifold temperature to the temperature that air would be at if adiabatically expanded to the manifold pressure. The difference in thermal energy is the pumping loss.

but to make it into horsepower energy you have to measure CFM moving.

if there is no air moving, then there is no work being done. The more CFM, the more work being done.

[drmiller100]

Originally Posted by serialk11r

You're right, it takes energy to maintain a pressure differential as gas moves across it. But you're calculating the amount of power it takes incorrectly.

Maybe I am calculating it wrong.

How would you calculate it????? Engine displacement is one of the variables.

I THINK I have identified all the variables.

??????

[serialk11r]

I know that...do you see why pressure drop * cfm moved is a close number?

Say the manifold vacuum were 7psi (excuse me, I'll use 50kPa for easier calculations). Say we have a 2L engine, 1000rpm idle. 2L engine draws 1L every revolution. So you have 1 cubic meter of air each minute, drawn against 50kPa pressure. That's 50000J per minute, or 833W, slightly over 1 horsepower.

[drmiller100]

Originally Posted by mort

Oh Yeah! Congratulations, you're both wrong. Throttling is adiabatic, but not reversible.

-mort

You are suggesting air is not an ideal gas in normal atmospheric conditions?????

Now you are just being silly.

[Olympiadis]

Why all the arguing over throttle suctioning loss, as it is minuscule in practical application ?

Because the inefficiency caused by the throttle plate itself is actually a % based on mass of air-flow over a time period, the only time it becomes significant would be in a condition where the RPM is high and the throttle position is very low, which most of us do not often encounter in our normal driving routine.

For this reason, doing coast-down tests from high RPM in gear would actually not be very revealing in terms of predicting actual efficiency loss during normal driving.

You would be creating an unusually low level of pressure in the intake ports, while at the same time moving a relatively large mass of air past the throttle over a given time period. This would boost the % of inefficiency by an order of magnitude over what you could expect during normal cruising.

Another thing I must point out if you're thinking about reducing pumping loss while in DFCO mode is that one of the thresholds for DFCO operation is manifold pressure. If you raise the manifold pressure while in DFCO, then you are likely to cause the system to kick out of DFCO mode and inject fuel again, the ECM thinking that you may have opened the throttle.

In the past people have tried to reduce throttling loss during DFCO by using the IAC throttle-follower routine in the ECM, where IAC steps are added when coasting, or they have tried to initiate EGR operation during coasting, but both methods have failed in improving efficiency, and often have caused other problems like the engine dying at low loads, and the above mentioned kicking out of DFCO.

At the end of the day the engine is going to have move X-amount of mass from the atmosphere into the cylinder to make X-amount of power. You have to initiate a pressure drop to move this mass, and it's either going to be primarily at the intake valve, or it's going to be a combination of intake valves and throttle valve. As you open the throttle and reduce the pressure differential there locally, the efficiency loss due to the throttle decreases, and at the same time pressure differential increases at the intake valve and the efficiency drop at the intake valve increases, in sort of a proportional trade-off. Either way, both are very minuscule losses during normal driving conditions where you must control the engine's VE due to the type of fuel you are using.

No matter where or how you are getting your mechanically created pressure differential, it is going to cost you some efficiency that you will not be able to recover. The good news is that what little is lost at the throttle is more than made up for by the throttle providing very good resolution for the driver to use to better match engine VE to the driving conditions. We all know that a good driver can use throttle control to improve FE.

Also, FYI, an analysis of cylinder pressure in your average non-running engine will not be representative of actual pressure differentials in a running engine. There are a few reasons for this. The first is that the pressure measured in the chamber is subject to adjacent pressure changes in neighboring cylinders via both the shared intake plenum and via a shared exhaust manifold. There is actually a high level of back-pumping or reversion into the intake manifold during very low RPM operation, such as cranking speeds. This is observable by watching a vacuum gauge or MAP sensor output during the operation. This source of error could have been eliminated by performing a test on a single cylinder engine.

The next issue is that extreme pulsing in the ports (compression waves) combined with reversion in a running engine greatly effects localized pressure differentials, such as the one that was trying to be analyzed in the compression test by using the average results measured in the chamber. Just as the actual pumping loss in a running cylinder for removing the exhaust charge is much less than what simple calculations would predict, the same is true for the intake cycle, but for different reasons that change depending on varying driving conditions, none of which a non-running compression test does a good job of emulating.

What it (the compression test) does show is simply the driver's ability to control the VE of the engine via throttle modulation. The calculated energy lost from the throttle being closed could only be applied correctly to a non-running engine being cranked, or similarly to a non-combustion type of device, like electric or hand-operated pump. Once combustion starts, the conditions of operation are changed dramatically.

All of this is getting further off-track from the BSFC subject.
I has yet to be mentioned here, but a big aspect of what determines where the BSFC falls on an RPM chart is the particular tune parameters of the engine, the spark advance having far more impact than the AFR alone. Given this fact, the better match between the charge burn-speed and the corresponding piston speed will have at least as much affect on BSFC as the restricted VE and/or the thermal rejection rates.
Simply put, the burn-speed needs to closely match the piston speed in order to achieve best efficiency. That's what tuning is all about.

The OBD1 and OBD2 ECM & PCMs do NOT contain an adaptive spark logic that adjusts to find maximum combustion speed, fuel efficiency, or best BSFC. The logic is programmed to limit the range of spark advance to a particular window of operation that, along with EGR function, is designed to reduce combustion speed, temp, and efficiency, and in the process compromises BSFC, and shifts best BSFC to a relatively higher RPM range, which is also not good for fuel efficiency.

So there you have it. The EPA regulations have far more impact in the BSFC you can legally achieve than other types of issues such as throttle suctioning losses.

[serialk11r]

Originally Posted by drmiller100

You are suggesting air is not an ideal gas in normal atmospheric conditions?????

Now you are just being silly.

Go read my previous post.

Aside from that, no he is not suggesting air doesn't behave like an ideal gas. Drawing air past a throttle is adiabatic since there is no heat exchange. That does not mean the work it takes to draw air past the throttle has anything to do with energy required for adiabatic compression. I just computed slightly over 1hp average power consumption on the intake stroke, and that doesn't account for the small amount of energy returned to the piston during compression. True idle pumping loss should be around 1hp with a 1000rpm idle on 2L engine, 7psi manifold pressure.

Seriously, go read a physics textbook.

[Olympiadis]

Originally Posted by drmiller100

look up lean burn. the basic idea is you are cruising down the freeway at 60 mph. lean the mixture down to 20 to 1, requiring more throttle to keep the flames lit. at the same time, advance the timing a BUNCH (30 degrees plus).

do it all right, EGT's drop, economy goes up. Do it wrong, you get detonation.

Lean burn in the sense you described can only be used in certain types of engines and used efficiently only in a certain window of conditions. It does work (increases FE) to an extent, but that is primarily because a 20:1 AFR requires ~35% less fuel to maintain. The efficiency of the engine doesn't really increase so much by way of a wider throttle opening.

The lean burn by way of power reduction in a window of operation also has the effect of increasing throttle (VE) resolution, thus making it a bit easier to drive efficiently.
A very steady cruise on a flat surface is the ideal situation for this type of reduced power operation.

[rmay635703]

Originally Posted by mort

Ugh, I hate doing this. Usually I can abide by the old saw that if you can't say anything nice don't say anything. But this thread has gone way stupid lately and I feel compelled to comment. So I'll get the nasty part out of the way first.

There are pumping losses, that is there is power needed to pull the charge into the cylinder and that power, which could have been used to propel the car, is instead wasted in suction across the throttle. And second, a loss of efficiency due to the charge in the cylinder being sparse, the sparse charge results in lower pre-ignition temperature and just like running an engine with a lower compression ratio, the thermodynamic efficiency suffers. My claim is that the thermodynamic efficiency drop swamps pumping power. I will call this view the "reality position."

...a bunch of stuff that eventually gets to the big picture

-mort

Diesel technology: report of the Technology Panel of the Diesel Impacts ... - National Research Council (U.S.). Diesel Impacts Study Committee. Technology Panel - Google Books

So your position agrees with the article above that a motor that does not have a throttle can provide up to double the fuel economy under low power conditions as compared to a vehicle with a throttle being operated under low power conditions?

In other words you left each loss that results from the throttle existing separate although in reality the combined effect is rather large and the subsequent effects are a result of the throttle being used.

Precision can be deceptive.

[drmiller100]

Originally Posted by serialk11r

Go read my previous post.

Aside from that, no he is not suggesting air doesn't behave like an ideal gas.

Seriously, go read a physics textbook.

you are saying a process is adiabatic, but not reversible, only true with non idea gasses.

I have degrees in physics and math, but I spent more time on tensors and relativity then the engineering crap.

and you????

[drmiller100]

Originally Posted by serialk11r

I know that...do you see why pressure drop * cfm moved is a close number?

Say the manifold vacuum were 7psi (excuse me, I'll use 50kPa for easier calculations). Say we have a 2L engine, 1000rpm idle. 2L engine draws 1L every revolution. So you have 1 cubic meter of air each minute, drawn against 50kPa pressure. That's 50000J per minute, or 833W, slightly over 1 horsepower.

1.1 horsepower.

what kind of mileage does that get you at idle stopped in traffic?

Lets look at cruising down the road at 2000 rpm. that means 2.2 horsepower is used in throttling losses.

That means in an hour, we are burning about .20 gallons of gasoline to maintain the vacuum inside the intake manifold, assuming 30 percent efficiency or so.

That is nothing.

Heck, if we are getting 50 mpg at 50 mph, the difference is only 20 percent in our mileage.

If we could go to aerocivic guy, and ask him if he would like another 20 percent mileage, think he'd be up for it?????

FUNDAMENTALLY, this is the EXACT reason smaller displacement engines get better mileage then large engines.

Large engines have much higher throttling losses BECAUSE THEY ARE BIGGER.

This also TOTALLY explains why taller gears get better mileage then shorter gears - because the RPM is lower. And the RELATIONSHIP IS LINEAR.

Plus, a smaller engine can run at higher absolute manifold pressure, for even less throttling losses.