Engine Braking

[kennybobby]

Originally Posted by wobombat

I don't think the point of highest torque always lines up with the point of highest efficiency as seen on BSFC charts. For example, my brother's civic has a peak torque at 7000 RPMs. My car has a peak torque at 4750RPMs. Yet the common knowledge around here seems to be that operating your car over about 3000 rpms tends to be very fuel inefficient. What could account for the difference, or is there actually no difference and these car models really do produce power the most efficiently at these numbers.

My point about using the RPM speed of the torque peak as the shift point was meant as guidance for running the engine up into a high(er) efficiency region for acceleration in order to minimize fuel consumption.

To do this you will spend less time accelerating overall and do it at a high enough power level to be operating the engine in it's most fuel-efficient region.

The Bosch Automotive Handbook has a section of empirical graphs and formula for internal combustion engines both gasoline and diesel. The torque-power-speed curve for gasoline engines shows the peak torque occurring at the minimum specific fuel consumption point at WOT. In addition the formula for peak torque is engine displacement times the peak cylinder pressure (BMEP).

The speed of maximum volumetric efficiency occurs when the airflow has the least restrictions and the cylinder filling is greatest and the pressure thereby peaks. Hence the torque peak occurs at the point of maximum volumetric efficiency.

Most of the BSFC charts that i have seen seem to show a tiny island of the absolute minimum bsfc slightly below the WOT line and slightly to the left of the torque peak-- but the point to be made here is to get up into those lower bsfc bands at the higher torque regions during those times when you accelerate (the "pulse" phase). You can never operate your car when cruising down the road (the "glide" phase) in that tiny zone of lowest bsfc--the power level is too high for gliding...

[serialk11r]

Originally Posted by kennybobby

Most of the BSFC charts that i have seen seem to show a tiny island of the absolute minimum bsfc slightly below the WOT line and slightly to the right of the torque peak-- but the point to be made here is to get up into those lower bsfc bands at the higher torque regions during those times when you accelerate (the "pulse" phase). You can never operate your car when cruising down the road (the "glide" phase) in that tiny zone of lowest bsfc--the power level is too high for gliding...

BSFC charts are few and far between. The point of highest volumetric efficiency pretty much dictates the torque peak as I said, and volumetric efficiency and thermal efficiency are very very different. As you noticed, minimum BSFC is usually below the WOT line. This is because running at WOT has the lowest throttling loss, and also the highest mechanical efficiency, but the last bit of power comes at a greater cost because the extra heat can't be extracted efficiently by the piston.

The reason you're seeing lowest specific fuel consumption close to the torque peak is because most engines are made "street friendly" with a small rev range and low-medium torque peak usually in the 3000-5000rpm range. Coincidentally, the size of almost all street engines (per cylinder displacement range on most engines is like in the 0.35L to 0.7L range and bore/stroke has a similar range) makes it so that their peak efficiency occurs in the 2000-3500rpm range. However a torque peak in the 4000s is not uncommon at all, especially for smaller cars, so you really can't go by torque peak.

Again there's very few BSFC charts out there but the ideal shift point is probably usually in the low-mid 2000s to low 3000s, depending on engine. If the efficiency drops a lot below 2000 then shifting from 1st to 2nd gear should probably happen at a higher engine speed to avoid dropping too low.

[wobombat]

Okay that makes a lot of sense. Thanks.
Personally, I base my shift point less on the RPMs and more on speed. I find the lowest speeds I can go in each gear without lugging and shift into that gear at that speed. That way I can operate the car at almost WOT and low rpm's for more of my acceleration time than normal.
Also, BSFC charts are few and far between, but is there any way that we can determine BSFC charts for our own engine? For example, run some tests and figure out fuel consumption at certain points of throttle and rpm, then plot them and come up with a rough bfsc chart where we can roughly determine the sweet spot to shoot for. Has anyone done this before, or is there no way to do this as an individual?

[serialk11r]

Originally Posted by wobombat

Okay that makes a lot of sense. Thanks.
Personally, I base my shift point less on the RPMs and more on speed. I find the lowest speeds I can go in each gear without lugging and shift into that gear at that speed. That way I can operate the car at almost WOT and low rpm's for more of my acceleration time than normal.
Also, BSFC charts are few and far between, but is there any way that we can determine BSFC charts for our own engine? For example, run some tests and figure out fuel consumption at certain points of throttle and rpm, then plot them and come up with a rough bfsc chart where we can roughly determine the sweet spot to shoot for. Has anyone done this before, or is there no way to do this as an individual?

Need a dyno that can put a variable load on wheels. If you have the engine out you need less parts but it's still hard to do. Then you also need something that accurately measures the fuel flow.

I think ultimately, there's no need to worry too much about how you're accelerating, because on the road it's more important to pay attention to traffic and stuff. The best we can do is try to shift up whenever we don't need power and try to strike a compromise between ideal load/rpm and ease of controlling power. Small engines probably aren't super efficient under 2000rpm in general but being slightly less efficient is okay if the lower amount of power available makes it easier to fine tune your acceleration in traffic and reduce energy wasted to braking. At least that's how I drive.

[wobombat]

There is some truth to that. For example, I never do P&G in traffic jams as that helps to stop up traffic and is extremely hard not to brake more. However, as traffic permits, I really do think it best to operate the engine as efficiently as possible. As long as you need the power and it wont be converted into brake heat, it doesn't significantly matter how long or in what matter you accelerate, but rather how efficiently the power is produced.

And concerning BSFC charts, it would be smart for dyno centers to offer to produce BSFC charts for vehicles they test. I bet it would cost a lot though.

[Uncle Dave]

Have retired, and have moved from Boston to Colorado. I have also gone from a TDI to a Chevy Cobalt. With all the steep hills around here, I have a hard time with this Cobalt. The engine will not hold me back, and I have been nailed for doing 12 over. Let me go but is watching. In Boston, down shifting, I went 250 K miles with out a brake change, Would tend to hold the speed down in town.

So what is new?

[serialk11r]

Originally Posted by wobombat

There is some truth to that. For example, I never do P&G in traffic jams as that helps to stop up traffic and is extremely hard not to brake more. However, as traffic permits, I really do think it best to operate the engine as efficiently as possible. As long as you need the power and it wont be converted into brake heat, it doesn't significantly matter how long or in what matter you accelerate, but rather how efficiently the power is produced.

And concerning BSFC charts, it would be smart for dyno centers to offer to produce BSFC charts for vehicles they test. I bet it would cost a lot though.

Right, but you can never predict what's going to happen, and the miniscule gains you make from staying in this gear instead of that gear for 0.5 seconds longer can easily be wiped out, so I think it's best to not sweat it too much.

Producing BSFC charts isn't just about measuring the power accurately, you also need to measure the fuel flow rate accurately! That's not that easy. 3 gallons per hour fuel flow rate for example is only 0.05 gallons per minute which is under 250 mL a minute, or ~4mL per second. Can't just run the engine for like 20 seconds at a time, because then you'd be sitting on that dyno for quite a while, as you'd have hundreds of points to test.

[Uncle Dave]

I have a local hill. 27 miles of hill and grade. I took a run, than put 45 lbs in the tires, then a dropped them to 25 lbs. Used Cruse Control to maintain the Posted. It was a Proof of Concept. Altitude is about 7,500 ft.

The other day I let a fellow get on I-25; flashed, and pulled over to make room. Just as I got there, I saw the DEER lying in my High Speed lane. Hit the lights, brakes, and swerved to straddle the line. I always drive with the Canadian Day Time Lights.

I always use the seat belts. If you don't, you walk. No Talk. You walk.

Bottom line, by the Grace of God, I am telling you about it.

[wobombat]

Nice story. And you bring up a point that I would like to emphasize, although it isn't about engine efficiency.Daytime running lights. I tend to keep my lights on all the time as I feel it makes people slightly more aware of my car.

My understanding of Alternators is that they constantly draw power from the engine and constantly produce a certain amount of amps. If you go over that amp amount, then you'll slowly lose power and your battery will die, but otherwise, electricity is free because whether you use 1 amp or 50 amps you aren't changing the load the alternator draws from the engine. Yet some threads here seem to think that your electricity use effects your fuel economy. I am pretty sure that's wrong and I've seen tests done running the engine without any electrical things going, then running it with everything full blast, and there wasnt any noticable difference between the two in fuel use. So to me there's no reason to not have my lights on all the time because electricity in cars is essentially free.

[Mustang Dave]

Originally Posted by wobombat

My understanding of Alternators is that they constantly draw power from the engine and constantly produce a certain amount of amps. If you go over that amp amount, then you'll slowly lose power and your battery will die, but otherwise, electricity is free because whether you use 1 amp or 50 amps you aren't changing the load the alternator draws from the engine. Yet some threads here seem to think that your electricity use effects your fuel economy. I am pretty sure that's wrong and I've seen tests done running the engine without any electrical things going, then running it with everything full blast, and there wasnt any noticable difference between the two in fuel use. So to me there's no reason to not have my lights on all the time because electricity in cars is essentially free.

Your "understanding" violates the laws of physics. The alternator generates enough power to maintain system voltage around 13.8 V with the ECU, radio, and whatever other electrical accessories operating. The power to drive the alternator is proportional to the electrical load placed on it. ANY additional electrical load places a mechanical load on the engine.
I actually DO have a degree in electrical engineering.