WAN Show April 12 - fuel efficient acceleration - a misconception

[Freeman]

In WAN show on 12th April at around 3rd hour, Linus and Luke talk about light acceleration being more efficient than stronger acceleration. This is a very common misconception, that I've encountered way to often, when working in this field.

 

Engines work optimally at optimal load, which, for ICE engines, is close to full load. For electric motors, it varies greatly based on design.

 

For optimal efficiency and best fuel mileage, you'd want to accelerate at near full load in highest gear, until reaching the revs where the friction of the piston against the the cylinder causes too much drag and a speed where the wind resistance causes too much drag.

 

In fuel mileage competitions, when a driver reaches this speed, they shut off the gas and let the kinetic energy turn the engine, or they just switch the engine off and shift into neutral, if they can save more fuel than what it takes to start the engine.

 

I'm not saying anybody outside a competition should drive like this, that would be horrible, but don't feather that gas pedal either. Keeping the engine at minimal load all the time won't help it and you are not winning on efficiency either.

[IkeaGnome]

12 minutes ago, Freeman said:

Engines work optimally at optimal load,

Really?

 

15 minutes ago, Freeman said:

For optimal efficiency and best fuel mileage, you'd want to accelerate at near full load in highest gear, until reaching the revs where the friction of the piston against the the cylinder causes too much drag and a speed where the wind resistance causes too much drag.

Also not always true. Just as an example it's been found that the 5.9 Cummins does best at 1300-1500 RPM. Your logic says I should drive it much closer to the 2500 rpm it's governed to...

[BiotechBen]

1 hour ago, IkeaGnome said:

Really?

 

Also not always true. Just as an example it's been found that the 5.9 Cummins does best at 1300-1500 RPM. Your logic says I should drive it much closer to the 2500 rpm it's governed to...

Will personally vouch that if I gas my Jetta to 3k rpm and shift manually, I get about 26 city, 34 highway. If I stay between 1300-2200rpm I get about 32 city, 42 highway. On a poorly geard vehicle, doesn't make much of a difference: my 2008 RAV4 driving back and forth to school got 15mpg average whether you drove it like an F1 car or a Prius.

 

Efficiency changes when you talk about acceleration at highway cruising speed, where you may be dumping more fuel per firing if you are in 10th gear at 1200rpm vs 7th gear at 2400rpm where your power band is and you can accelerate for a significantly shorter period.

[Sharkyx1]

In a purely mechanical sense you're correct OP. Having an open throttle reduces suction losses in the engine, but given the realities of vehicle design and automatic transmission software things are much more complicated 

 

 

It also has to be taken into account that in real every day driving slow acceleration can be much more efficient due to the simple nature than if the person in front of you is going to stop eventually it's more efficient to never reach your target speed 

[Sharkyx1]

8 hours ago, IkeaGnome said:

Really?

 

Also not always true. Just as an example it's been found that the 5.9 Cummins does best at 1300-1500 RPM. Your logic says I should drive it much closer to the 2500 rpm it's governed to...

Not OP but,I think there's some technical simplifications that might make the point here hard to track 

 

For one Diesel motors do not have a restriction based throttle and that is the primary efficiency loss that OPs post is talking about the nature of.

 

 

On a gasoline engine if you're accelerating at 20% throttle, you will incur pretty significant efficiency losses as a fraction of the power the engine is producing is used to pull air by the throttle plates.

 

If you were instead in 3rd gear at 1500rpm and full throttle, there are other issues involved, but you would not suffer from efficiency losses resulting from the throttle plate 

 

The actual RPM that is absolute most efficient is the torque maximum if you could have infinitely adjustable gearing as that is the RPM that the most energy is extracted from a single stroke. But that's only if the engine is held at full throttle and the load and gearing is sufficient to not let the engine accelerate 

[Hypocee]

ICEs are complicated and there are a lot of maxima, but OP did go out of ens way to specify "highest gear", i.e. low revs.

 

Coincidentally this is also actually useful IRL as the procedure for seating new ICE piston rings. I first ran across it in the world of motorcycles, whose owners are more likely to do this kind of maintenace/repair themselves. The rings are intentionally made out of a softer metal than the cylinder sleeve and a touch oversize, and the cylinder is intentionally left slightly rough. The cylinder walls act as files to machine the rings down to exactly match their cylinders' dimensions and shape at the engine's operating temperature and mechanical loads, until the hydrodynamics of the oil film starts to keep them apart as designed. When you put new rings in, you take the vehicle out and do a lot of grunting, early-upshift, throttle open acceleration runs to hit the maximum lateral load on the pistons and put that metal in the oil at a predictable time - then change the oil.

[IkeaGnome]

7 hours ago, Sharkyx1 said:

For one Diesel motors do not have a restriction based throttle and that is the primary efficiency loss that OPs post is talking about the nature of.

They never said gas. They were talking about ICE in general. The only time they said gas they used it as the word “Fuel”. 

 

15 hours ago, Freeman said:

Engines work optimally at optimal load, which, for ICE engines, is close to full load.

 

[Sharkyx1]

28 minutes ago, IkeaGnome said:

They never said gas. They were talking about ICE in general. The only time they said gas they used it as the word “Fuel”. 

 

 

That isn't what my comment was about. Motors have Specific Efficiency curves, a gasoline motors suffer much more than a diesel in part throttle operation, there still is an advantage to being on the throttle in a diesel,  also  only if gearing permits 

 

 

The vehicles I drive for example, "elongate" half the cylinders under "medium loads" in order to advance the throttle,  example : 3 cylinders and half throttle open, instead of 6 cylinders at 1/4 throttle, by disabling the valves on one bank of the v6

[Freeman]

Wow, I'm very happy to see lively conversation. I haven't really expected anyone to be interested, so I've tried to simplify and offer the simple advice at the end.

 

To provide some background, I've spend several years organizing racetrack events, riding schools, sale event that included closed circuit demonstration and took part in organizing some emergency services driving courses. This included hypermiling competitions and test drives as part of vehicle development. In case of both diesel and gasoline powered vehicles, the best results were always achieved by the technique described in OP. Perhaps the most significant effect can be observed in motorcycles, which was also my primary domain.

 

As to explain why I care, we would get clients that would feather the throttle, thinking they are being efficient and helping the engine. They would do this in situations like turning onto a main road or merging on a motorway, where reaching the speed of other traffic and avoiding a rear end is paramount and there is an outsized vulnerability of motorcyclists. One such accident happened to a client riding to our event. Luckily, there were no long term injuries, but it is the main reason, why I'm invested in this topic.

 

As for automatic transmission and hybrid vehicles, I've never observed them in a hypermiling exercise, but would certainly be interested in exploring optimization techniques, especially comparing the ECO mode to manual modes and switching between EV, battery recharge, gas only and combined modes!

 

As for the engines themselves, while there is a lot of variation, there are also a lot of physical effects, that will always be present, if you want to provide the efficiencies that current ICE engines achieve. You need to deliver the fuel to the combustion chamber with minimal drag, you need to mix it with air before the compression cycle is finished, with gas engine, you need to assure that the ignition spreads evenly through the chamber, you need to maintain the pressure on the piston throughout the cycle (this is why oversquare engine can't achieve high RPM, the piston would move too fast for the gas expansion to maintain pressure).

 

This is why engine load, not the RPM is the main thing to watch. If the torque is the rotational power of the crankshaft the engine load is the force that acts to stop this rotational movement. If the load exceeds the torque, the engine stalls. If the manufacturer targets a specific power and torque at a displacement, they need to optimize the effect from previous paragraph at the load, where these values can be achieved. That is why I've never seen a modern (2000+) car, van or motorcycle engine, that would have peak efficiency under 60% load. More sophisticated injection, variable valve timing and the switch from fuel maps to using real-time input from the various sensors could most likely widen this efficiency window and complicate the results, again, something I'd love to discuss.

 

While there is a maximal torque reached at a specific RPM, the drag force acting against the movement of piston relative to the cylinder is the main factor of efficiency when RPMs are concerned, because the drag force grows with the square of the relative speed, there are RPMs that will never be efficient, so the relevance of peak torque is very much engine dependent, with some high torque undersquare engines with small cylinders having awful efficiency at peak torque.

[05032-Mendicant-Bias]

It's a technique called Pulse and Glide and it does work to save fuel. As far as I know, in some countries it's not allowed because the uneven speed makes it harder to drive in a motorway, with people constantly coasting and accelerating.

 

 

Efficiency just depends on the engine and operating conditions of the engine. The chart above tells you that this particular engine has an optimal RPM/Torque at which it gives the best efficiency from chemical potential energy to mechanical energy. The further away, the lower the efficiency.

 

So rather than driving at a constant speed maybe in the yellow region, it's more efficient to operate the engine in a burst in the red region, then coast, then burst again and repeat.

 

This chart also tells you the best efficiency you get 40H to 80HP, and if you want the best acceleration instead you can operate at the peak where the engine is giving you 124HP, which is where you are at high RPM, and the engine is giving you high torque. (Power = torque*rpm*conversion constant)

[Needfuldoer]

17 hours ago, Freeman said:

As for automatic transmission and hybrid vehicles, I've never observed them in a hypermiling exercise, but would certainly be interested in exploring optimization techniques, especially comparing the ECO mode to manual modes and switching between EV, battery recharge, gas only and combined modes!

I can usually get 120 MPGe (just under 4 miles per kilowatt-hour) or 40 MPG mixed city/highway out of my Chevy Volt, and I'm not a diligent hypermiler. 

 

Just city, I've seen it touch 150 MPGe (almost 5 miles per kilowatt-hour), reading almost 70 miles on the EV range guess-o-meter.

 

My worst tank ever was about 34 MPG combined, but that was in the dead of winter and running the engine to defrost in the driveway a lot.

 

I have the dash set so I get a total power rating of how much is going into and coming back from the wheels (regen), and I treat that like a vacuum gauge. The gamified "efficiency score" screen seems to be happiest when I keep that between +30kW and -30kW, but that might just be an arbitrary goalpost in the programming.

 

On the highway, if I'm careful to just maintain speed but not accelerate, the power output reading is something like 15 kilowatts.

 

Even in "Hold" mode (which maintains the state of charge read when it's engaged) or "Mountain" mode (which stores an extra buffer above "empty"), the ECU is still in complete control of the engine's speed. No matter what, there's no direct correlation between the position of your foot and the position of the butterfly valve in the throttle body. I've never plugged an OBD2 scanner in to read it, but anecdotally it seems like the controller tries to keep the engine at a fast idle on average, or at least within the range of an ICE car at cruising speed, and it will run lower longer instead of working as hard as possible to minimize run time.  I don't know if that's what's actually required, or if it was done for "emulate an ICE car" purposes (like transmission creep), or if it was just for NVH.

 

[Sharkyx1]

On 4/17/2024 at 7:41 AM, Needfuldoer said:

I can usually get 120 MPGe (just under 4 miles per kilowatt-hour) or 40 MPG mixed city/highway out of my Chevy Volt, and I'm not a diligent hypermiler. 

 

Just city, I've seen it touch 150 MPGe (almost 5 miles per kilowatt-hour), reading almost 70 miles on the EV range guess-o-meter.

 

My worst tank ever was about 34 MPG combined, but that was in the dead of winter and running the engine to defrost in the driveway a lot.

 

I have the dash set so I get a total power rating of how much is going into and coming back from the wheels (regen), and I treat that like a vacuum gauge. The gamified "efficiency score" screen seems to be happiest when I keep that between +30kW and -30kW, but that might just be an arbitrary goalpost in the programming.

 

On the highway, if I'm careful to just maintain speed but not accelerate, the power output reading is something like 15 kilowatts.

 

Even in "Hold" mode (which maintains the state of charge read when it's engaged) or "Mountain" mode (which stores an extra buffer above "empty"), the ECU is still in complete control of the engine's speed. No matter what, there's no direct correlation between the position of your foot and the position of the butterfly valve in the throttle body. I've never plugged an OBD2 scanner in to read it, but anecdotally it seems like the controller tries to keep the engine at a fast idle on average, or at least within the range of an ICE car at cruising speed, and it will run lower longer instead of working as hard as possible to minimize run time.  I don't know if that's what's actually required, or if it was done for "emulate an ICE car" purposes (like transmission creep), or if it was just for NVH.

 

What year is your Volt? I was under the impression that the engines just stayed At About 3000rpm when in generator mode and never really fluctuates. At least in the first gen a lot of reviews talked of this 

[Needfuldoer]

6 minutes ago, Sharkyx1 said:

What year is your Volt? I was under the impression that the engines just stayed At About 3000rpm when in generator mode and never really fluctuates. At least in the first gen a lot of reviews talked of this 

2018, so second-gen. The engine's different from the first-gen; it's a 1.5L aluminum block that runs on 87 octane instead of a 1.4L iron block that requires premium.

[Sharkyx1]

On 4/17/2024 at 6:49 AM, 05032-Mendicant-Bias said:

 

 

 

Ideally the most advantageous thing to do given the realities of actual driving is variable displacement technology i would think, assuming the technology eventually becomes practical and reliable 

 

I drive two dual displacement vehicles a 3.9L V6 than can operate as a 1.95L 3 cylinder and a 6.0L V8 than can operate as a 3.0 4cyl

 

The technology involved in GMs variable displacement tech relies on engines being of a Cam In Block design, which is a type of engine that isn't really utilized anymore. In those designs function  depends on completely disabling the valves of the "off" cylinders. Engines that feature dual or single overhead cams don't generally have the space to do this.

 

The other type of variable displacement is a design infiniti has been working on where the connecting rod length is varied. An interesting concept but I'm unsure if this will be ready for prime time. This design can be combined with variable valve profile to significantly reduce the stroke of a engine, live, on a load and rpm and boost basis

 

 

Just some neat stuff.