What you need to know about Brake Specific Fuel Consumption

[serialk11r]

drmiller, you're not compressing air though...that calculator gives the amount of power it takes to compress a certain amount of air from atmospheric pressure to 5psi gauge pressure. 5psi gauge pressure means 5psi above atmospheric. This does not describe what is happening with the throttle.

What happens at the throttle is there is atmospheric pressure on one side, and air is flowing across to the manifold with lower pressure. The pressure drop is going to be atmospheric pressure minus manifold absolute pressure, aka vacuum. The air escapes through the throttle opening, gains velocity, and then has all that energy dissipated into the air itself as heat.

A simple estimate for pumping loss is to consider the piston moving against a constant amount of pressure. The manifold pressure is lower than atmospheric, so it takes at most the amount of air displaced by the piston * the pressure acting on the piston to pull the air in, as pressure * volume = work (more accurately it's \int PdV, but whatever). But now that the piston is filled with below atmospheric pressure air, some of the work used to pull the air in is returned to the piston as it travels up. Previous posters have used this upper bound estimate to calculate a number for pumping loss. Admittedly pumping loss is significant, but not as great as friction or even cooling losses. For example, if we look at BMW's Valvetronic engines, BMW claims a 10% overall fuel efficiency increase. Since the EPA tests run at low loads, the amount of throttling loss saved should be significant, yet they only saw a 10% increase. Now of course 10% is nothing to scoff at, but considering that the part load efficiency is still pretty bad, there's clearly a lot of other things at work. At idle I believe throttling accounts for about 60% of power usage, since the throttle is used to consume the engine's excess output, but you don't need very much load before the throttle is second to friction.

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.

[drmiller100]

it is an adiabatic process.
this means it takes the same energy to add 5 psi or remove 5 psi.

[E4ODnut]

I'm assuming that the graphs posted by Old Tele Man show results for an engine tuned with A/F ratios for best power, ~ 13:1 or so. It would be interesting to see what would happen with a leaner mix, say ~ 16:1. Obviously power loss would likely be significant, but I'm thinking that FE should improve at lower loads where a greater throttle opening would be required to generate the same power as with a richer mix.

Comments?

[mort]

Originally Posted by drmiller100

ok, bear with me.

One cubic foot is 1728 cubic inches.

so, we have
.05 hp * 1000 rpm * 228 cubic inches / 1728

is 6.5 horsepower. Which passes the big picture bs test.

Duh.

Hi drmiller100,
The errors in this calculation are,
1. taking the displacement of the engine as the volume of pumped air. The volume of air at 1 atm will be less than the displacement by approximately the volumetric efficiency.
2. one displacement occurs for 2 revs of the engine in a 4 stroke.

If VE at idle is 20% and divide by 2 then pumping power = 0.65 hp.

Which really does pass the big picture bs test.

I'm OK with 1000 rpm and 5 psi, but I will note that many engines idle slower. The standard idle manifold vacuum is about 17 to 22 in hg, 1 atm is 29.9 in hg so a good idle manifold pressure is about 9.5 psi. The suction across the throttle is then 14.7 - 9.5 = 5.2, so excellent guess.

-mort

[serialk11r]

Originally Posted by drmiller100

it is an adiabatic process.
this means it takes the same energy to add 5 psi or remove 5 psi.

EDIT: my bad, it's adiabatic since there's no heat transfer, but adiabatic does not mean same energy to add 5 psi or remove 5 psi.

E4ODnut, I think under full load conditions, ~1.1 fuel equivalence (16:1 AFR or so) gives highest thermal efficiency, ~0.9 (13:1) gives greatest power, but under partial load you can go much much higher with AFR since you don't need the power, and drawing in extra air is not as bad for efficiency as restricting the airflow.

[mort]

Originally Posted by drmiller100

it is an adiabatic process.
this means it takes the same energy to add 5 psi or remove 5 psi.

Originally Posted by serialk11r

Wrong, drawing air past a throttle is converting kinetic energy to thermal energy. It is by definition not adiabatic.

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

-mort

[drmiller100]

Originally Posted by E4ODnut

I'm assuming that the graphs posted by Old Tele Man show results for an engine tuned with A/F ratios for best power, ~ 13:1 or so. It would be interesting to see what would happen with a leaner mix, say ~ 16:1. Obviously power loss would likely be significant, but I'm thinking that FE should improve at lower loads where a greater throttle opening would be required to generate the same power as with a richer mix.

Comments?

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.

[serialk11r]

Originally Posted by mort

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

-mort

Mah bad...got mixed up with isentropic for some reason. Terribly sorry about that. Need more sleep.

[drmiller100]

Originally Posted by mort

Hi drmiller100,
The errors in this calculation are,
1. taking the displacement of the engine as the volume of pumped air. The volume of air at 1 atm will be less than the displacement by approximately the volumetric efficiency.
2. one displacement occurs for 2 revs of the engine in a 4 stroke.

If VE at idle is 20% and divide by 2 then pumping power = 0.65 hp.

Which really does pass the big picture bs test.

I'm OK with 1000 rpm and 5 psi, but I will note that many engines idle slower. The standard idle manifold vacuum is about 17 to 22 in hg, 1 atm is 29.9 in hg so a good idle manifold pressure is about 9.5 psi. The suction across the throttle is then 14.7 - 9.5 = 5.2, so excellent guess.

-mort

So I thought about this some more. You are right about the 4 stroke, but the same calcs are also valid for cruising down the freeway.

VE at low rpms is pretty easy to get over 90 percent. and I am not aware of any variable displacement engines.

My premise is the displacement is the displacement. And it takes energy to maintain a pressure differential.

[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.