fuel vs rpms

[hemioiler]

Just a general question. Let's say I'm towing in 7th gear at 65 mph, holding steady on a flat road. I feel the engine is lugging at that speed (just a feeling of course), so I want to drop a gear to raise the rpms. I use the gear limiter only to lock out 7th, and immediately the truck downshifts.

I was going straight, wind doesn't change, no difference in elevation, no difference in throttle input, all I did was use the computer to force a downshift.

Does my truck now use more gas at those slightly higher rpms to hold my speed?

 

[fsdork]

If we assume the engine is running at the stoichiometric ratio, yes. If the vehicle's tuning tells it to dump more fuel (run rich) under load, maybe.

 

[dadto2]

Yes. If for no other reason, there is more frictional loss in the engine at higher RPMs.

 

[JohnnyG]

Probably depends a lot on the vehicle design, but in my experience, you'll use about the same or less fuel. Lugging an engine wastes a lot more fuel than high rpm, within limits. Those limits would be IMO, whether your using EXCESSIVE rpm, so it's the gearing that makes a difference.
The Mercedes in my sig is a good example, 9 speeds, but will only go into 9th at high speed, light load. The computer map determines the downshift parameters. Consequently, I can carry over 38 MPG on a trip, not worrying about which gear I'm in.

 

[Deleted member 89374]

When you downshift to a lower gear, the engine's RPM increases, which means the engine is burning more fuel to maintain the same speed. This is because the engine is now operating at a higher RPM, which requires more fuel to keep up with the increased demand for power. However, the amount of fuel consumption increase will depend on the specific characteristics of your vehicle, such as the engine type, size, and fuel efficiency.

In general, if you downshift to a lower gear and maintain the same speed, you will consume more fuel than if you were in a higher gear. This is because the engine is working harder to maintain the same speed, which means it is burning more fuel. However, if you were feeling the engine lugging in 7th gear, then downshifting to a lower gear may actually improve your fuel efficiency, as the engine is now operating more efficiently in a higher RPM range.

It's worth noting that driving at higher RPMs can also increase wear and tear on your engine and transmission, so it's important to use downshifting judiciously and not over-rely on it as a means to compensate for a vehicle that is not ideally matched to a particular driving condition.

 

[hemioiler]

Probably depends a lot on the vehicle design, but in my experience, you'll use about the same or less fuel. Lugging an engine wastes a lot more fuel than high rpm, within limits. Those limits would be IMO, whether your using EXCESSIVE rpm, so it's the gearing that makes a difference.
The Mercedes in my sig is a good example, 9 speeds, but will only go into 9th at high speed, light load. The computer map determines the downshift parameters. Consequently, I can carry over 38 MPG on a trip, not worrying about which gear I'm in.

At 65 mph, my truck in 7th gear is running 1796 rpms (7th and 8th are over drives and its a pretty tall highway gear in the rear end). After a downshift at the same speed it will be running 2191, an increase of 395 rpms.

 

[hemioiler]

When you downshift to a lower gear, the engine's RPM increases, which means the engine is burning more fuel to maintain the same speed. This is because the engine is now operating at a higher RPM, which requires more fuel to keep up with the increased demand for power. However, the amount of fuel consumption increase will depend on the specific characteristics of your vehicle, such as the engine type, size, and fuel efficiency.

In general, if you downshift to a lower gear and maintain the same speed, you will consume more fuel than if you were in a higher gear. This is because the engine is working harder to maintain the same speed, which means it is burning more fuel. However, if you were feeling the engine lugging in 7th gear, then downshifting to a lower gear may actually improve your fuel efficiency, as the engine is now operating more efficiently in a higher RPM range.

I don't necessarily agree with all of this. What does "working harder" mean to you, because to me lugging an engine is working harder. Lugging builds heat. The truck has seen no increase in power demand, it's not working harder like it would if you add more weight/wind/elevation etc.

Note also that the truck is much more responsive at those rpms. When towing in 7th there is no extra "oomph" in the pedal. If you hit a slight slope you push the pedal to maintain speed and there is no reaction. However in 6th it is far more lively, you push the pedal down at the same slope and the truck can easily accelerate. To me that means its working less hard in 6th, even though the rpms are slightly higher.

Higher RPMs don't necessarily mean increased fuel either, because we know Ram uses engine breaking to cut off fuel when rolling down hill. The truck sees the nose dive, knows you hit the brakes, it detects all of this and cuts fuel and drops you a gear or two to slow you down.

It's worth noting that driving at higher RPMs can also increase wear and tear on your engine and transmission, so it's important to use downshifting judiciously and not over-rely on it as a means to compensate for a vehicle that is not ideally matched to a particular driving condition.

It's just 400 rpms. One can argue that lugging builds more heat and stress on your engine. Though obviously if the truck can pull fine at 6th and you drop to 3rd just because you like redline, then yes I'd agree with you that that is silly and going to cause unneeded wear and tear.

 

[MRC01]

All else equal, fuel burn rate is proportional to power output. If you're towing the same load at the same speed, that is the same power output.

However, all else is not equal. Power is torque * rpm, so you can make that power with high torque and low RPM, or low torque and high RPM. Usually, running at lower RPM with higher torque (BMEP) is more efficient than higher RPM at lower BMEP. But not always, and only within the "normal" range of RPM for the engine. If your RPM is below this normal range we call that "lugging" and it becomes less efficient.

In short, your efficiency could go up, or down, as you shift gears. It depends on the power level and conditions. For max efficiency you want to be in a tall gear, but not too tall.

 

[hemioiler]

Yes. If for no other reason, there is more frictional loss in the engine at higher RPMs.

Is it that simple though? I'm assuming you're right about frictional losses, but I'm less certain that that is the entire story of what's going on.

Take my example above, where the truck has no power left in reserve at 7th, but does feel far more lively in 6th. When you need to climb a slight slope, which gear would require less fuel to climb... a truck that's lugging and dumping more fuel in and getting no where (possibly even causing a downshift to 6th eventually anyway), or putting slightly less pedal down in 6th and easily climbing the slope?

 

[hemioiler]

However, all else is not equal. Power is torque * rpm, so you can make that power with high torque and low RPM, or low torque and high RPM. Usually, running at lower RPM with higher torque (BMEP) is more efficient than higher RPM at lower BMEP. But not always, and only within the "normal" range of RPM for the engine. If your RPM is below this normal range we call that "lugging" and it becomes less efficient.

In my scenario, both torque and rpms increase. At least I'm assuming so, I haven't seen a torque curve for the 5.7 hemi but assuming it follows a standard N/A curve which would be a somewhat linear increase in torque until 4500 rpms or so anyway.

However, though the torque and rpms increased (and so would power if I read you correctly), the truck isn't actually moving any faster. So is it actually putting more power down? And if not, why would it need more fuel to simply run at higher rpms?

The stoichiometric ratio mentioned by @fsdork may be a clue but that is beyond my paygrade.

 

[Tman220]

So many variables!! Higher friction at higher rpm. possibly higher or lower bsfc #'s at higher or lower RPM.
In general you will use more fuel at higher rpm just due to friction alone. The only time that may not be the case is if the load and rpm puts you in a better efficiency area of the bsfc map

Or, at the lower rpm the ecm is being forced to constantly pull timing to keep out of detonation thus lowering efficiency, where at a higher rpm and less load the engine can take all the timing the ecm wants to throw at it.

Since it seems you are driving a hemi and the manual recommends you use 89 octane while towing it's possible you are pulling a lot of timing at lower rpm if you're running 87

 

[SC Maintenance]

The answer is no one here will know.

You could attempt to monitor OBD2 calculated load, which most the most abbreviated simplification - is the amount of fuel used at a specific RPM to calculate how much of the theoretical max power from the engine is being derived at that speed - based on how much fuel its burning. Its represented as a %.

However, different engines have different efficiencies at different RPM - and I don't mean different amount of fuel used - I mean how much of that heat can be turned into motive power at that specific RPM. So calculated load may or may not be all that meaningful.

The very short answer - in your case - it likely doesn't matter that much, a few percentage points at best. Also, I don't like lugging an engine - seems foolish and more likely to cause wear than running at a little higher RPM.

 

[MRC01]

In my scenario, both torque and rpms increase. At least I'm assuming so, I haven't seen a torque curve for the 5.7 hemi but assuming it follows a standard N/A curve which would be a somewhat linear increase in torque until 4500 rpms or so anyway.

No, that is only for full throttle operation. But you're not at full throttle. To produce the same power at higher RPM, the engine produces less torque.

However, though the torque and rpms increased (and so would power if I read you correctly), the truck isn't actually moving any faster. So is it actually putting more power down? And if not, why would it need more fuel to simply run at higher rpms?

It's not, because your first assumption is incorrect. When you downshift while traveling at the same speed, power remains the same, RPM goes up, torque goes down.

 

[hemioiler]

So many variables!! Higher friction at higher rpm. possibly higher or lower bsfc #'s at higher or lower RPM.
In general you will use more fuel at higher rpm just due to friction alone. The only time that may not be the case is if the load and rpm puts you in a better efficiency area of the bsfc map

Or, at the lower rpm the ecm is being forced to constantly pull timing to keep out of detonation thus lowering efficiency, where at a higher rpm and less load the engine can take all the timing the ecm wants to throw at it.

Since it seems you are driving a hemi and the manual recommends you use 89 octane while towing it's possible you are pulling a lot of timing at lower rpm if you're running 87

89 or 91 octane for this lad at all times

 

[hemioiler]

No, that is only for full throttle operation. But you're not at full throttle. To produce the same power at higher RPM, the engine produces less torque.

It's not, because your first assumption is incorrect. When you downshift while traveling at the same speed, power remains the same, RPM goes up, torque goes down.

Learned something new, thanks.

 

[thastinger]

It is just BTUs, takes the same amount of energy to move weight regardless of how you get it.
I would say its a wash though if you hit 100% load in the higher gear though, because your ECU probably goes to open loop over a certain % of calculated load. In the lower gear and higher RPMs, the calculated load may be less so the ECU would call for 14.7:1 instead of 12.5:1 but there would be a bit more parasitic loss from the higher RPMs.

 

[vwmaniaman]

I can't say because you need to know what RPM your motor achieves peak torque. That is the point with the maximum efficiency.

 

[twX]

Generally, for a given power output, engines are more efficient at higher load and lower rpm. This is why small engines get better fuel efficiency in normal driving. Here is a pretty typical example of a brake specific fuel consumption map for an engine. The engine is most efficient at wide open throttle at around 3000 rpm. This most efficient point usually comes before the rpm where the engine makes peak torque.

If you compare two points of constant power at different rpm, the engine will almost always be more efficient at the lower rpm (at least until revs get down to around 1000 rpm). For example, if the engine represented here is operating at 4000 rpm and 40 Nm of torque, fuel consumption is around 360 g/kWh. At the same power level at lower rpm, 2000rpm and 80 Nm of torque, fuel consumption is 265 g/kWh, a 26% reduction.

 

[PandaBear]

It all depends on the engine design and tuning.

From what I learned in the 90s, typically the engine valving, air fuel ratio, spark timing, etc are designed to provide a good torque and efficient band and you want to stay close to it and downsize the engine to be just right for best fuel efficiency. Lugging an engine shouldn't really happen in today's automatic as the ECU will decide what is the best for itself and the fuel efficiency. It may not be comfortable for you to not have the throttle response at low rpm if you step on it, but it is the whole point as that means it is "efficiently using the power while minimizing pumping lost". Instant throttle response without hybrid or downshift typically means you are wasting pumping lost so you can have that throttle response when you want it: it is fun but it is wastful.

Assuming a normal engine with no fancy variable valving (unrealistic today), the efficiency band is typically 40% of redline. Engine will likely stay below 50% redline and "lugging" instead of staying at 70% redline and "not lugging" for efficiency. It would definitely downshift if it is at 25% redline burning rich to 40% redline burning close to optimal air fuel ratio.

 

[tnt_motorsports]

IMO it really depends on the tuning of the vehicle.

My 2012 Ram gets better mileage towing my TT in 5th gear @ 68MPH (2,200RPM) then it does in 6th gear @ 68PMH (1,600RPM). The load on my CTS2 shows significantly different between the 2 driving styles as well. My truck is a diesel so there is a difference for sure, but I do know the torque curve starts at about 1,600 and goes to 2,400. When in 5th gear it works much better on the transmission (no downshifting) and my fuel economy is better.

Just my $0.02