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Old 02-03-2020, 11:57 AM   #11 (permalink)
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To my knowledge, recent naturally aspirated Hondas (last 20 years or so) do not go rich. However, their compression is high enough that, in order to not have detonation issues, they retard ignition timing significantly at high load. Running rich is an alternate tactic which allows a few degrees of ignition timing to be kept, increasing the energy extracted from combustion at high load.

High compression is double-edged. It provides really huge gains in efficiency at part throttle on any fuel, but can actually decrease power and efficiency at higher loads. Toyota's hybrid Miller cycle engines are basically a tactic to never let their engines run at more than ~70% load, thus allowing sky-high compression while never going into zones where they would need to pull ignition timing or run a rich fuel mixture.

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Old 02-04-2020, 10:38 PM   #12 (permalink)
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That would appear to be a thermal efficiency chart, which does indeed have the same shape as a BSFC chart.

Toyota claim a peak thermal efficiency of 40% for one version of this engine and 41% for the other. I suspect that chart is for the 40% version.

They have actually chopped quite a bit of the the top of the curve as the 40% version has a peak torque of 205 Nm at 4800 rpm whereas that thermal efficiency chart is only showing a peak value of approx 155 Nm.
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Old 02-05-2020, 12:43 AM   #13 (permalink)
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The OP is probably gone already, but I thought I would point out that that Dynamic Force 2.0L chart is definitely not complete. That engine produces around 200Nm of torque, not only 150. The new Toyota engines avoid enrichment up to a much higher load and speed so you can pretty much mash the pedal hard while driving.

A bit of history on best BSFC load:
In the late 90s and early 2000s, it was common for engines to run stoichiometric at all times below ~4000rpm. On these engines, near maximum load is efficient, with knock limiting efficiency at full throttle.

Then cars started running slightly rich at full load for all rpms to get more torque, and probably to help prolong catalytic converter life. On these cars, you want to avoid going rich, so you're limited to below 80% load or so.

In the latest engines, cooled EGR with better combustion speed/higher lean or EGR dilution limit has actually brought peak BSFC load down. I think this is a combination of pumping losses being negligible down to a much lower load, as well as the EGR gas increasing heat capacity (and decreasing cooling losses).
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Old 02-05-2020, 10:21 AM   #14 (permalink)
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When I had a saturn SC1, I got into hypermiling. I was shifting at 2000rpm, lugging around all the time. Being a car guy it came to me that engines are most efficient at peak torque. So, I started shifting at 4k and saw no drop in mpg. I tried not to go over half throttle though, because of fuel enrichment. Sure made driving a little more fun.
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Old 03-29-2021, 05:42 PM   #15 (permalink)
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In the hybrid Avalon I've noticed a huge drop in fuel mileage above 65mph (obviously air drag) (about 35mpg).

At 55mph to 65mph I don't notice much help from hypermiling. (about 40mpg)

Below 45-50mph I start getting worse fuel mileage unless I hypermile (drive with the power meter right at the line between ECO and PWR then let off the throttle and coast with engine off), Then I get better fuel mileage than around 55mph (or about 45 to 50mpg).

Ideally we'd have much smaller engines that output an average amount of power, locked at the most efficient RPM and Load, and the fluctuations of needed power would be administered from a highly efficient hybrid system.

Last edited by Isaac Zachary; 03-29-2021 at 05:48 PM..
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Old 04-09-2022, 08:46 AM   #16 (permalink)
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Originally Posted by #SaveTheManuals View Post
Of course I'll still be flying blind without the BSFC map, unless the torque/RPM/MPG map published by Toyota is (accurate and) more or less equivalent. In that case, per my annotations below, it sure looks like the most efficient region is between 2,200 - 3,100 RPM, with load between ~76% - ~95%. The sweet spot is 2,700 RPM at 88% load:

76% - 95% load is higher than I expected based on the general consensus around the forums, but I suppose there are always outliers. At some point I'd like to verify or debunk whether I can really drive my car around at 90% load for peak efficiency. That seems a little like having my cake and eating it too. Therefore I'm skeptical of this chart.
That chart should be accurate. Toyota published a similar one for the sister 2.5L which the EPA tested & verified.

You can convert that thermodynamic efficiency chart to a classic BSFC chart by taking 81.8 and dividing it by the efficiency. E.g. 81.8/.39 = 210 in the peak efficiency island. 81.8/.35 in the "expanded" zone = 234 (which is still quite efficient).

You can convert Nm torque to horsepower with the formula:

Horsepower = Nm x RPM / 7127

or vice versa:
Nm = HP x 7127 / RPM

So if you need to make 20 horsepower to cruise on the freeway at 2000 rpm, that would be ~71 Nm which your engine makes with ~ 35% efficiency at 2000 rpm.
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Old 04-13-2022, 12:35 AM   #17 (permalink)
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Remember that "efficiency" can mean many different things!

The BSFC graphs show the most efficient way for your engine to make power, not specifically the most efficient way to get from point A to point B!

Generally, we say "80% load" is the most efficient--but most vehicles are accelerating at that load. You really don't want to accelerate continuously from the start of your journey to the end; you have to bleed off lots of speed at the end and you wind up wasting all of that energy! And that ignores the potentially HUGE impact of drag at higher speed.

Pulse-and-glide, or pulse-and-coast, allows you to take advantage of the high-BSFC regions of the graph to gain speed, and then use little or no fuel when coasting to a lower speed.

The upshot is that hitting peak BSFC is the most efficient way to accelerate, but usually not the most efficient way to drive overall. You have to do more stuff than just that.


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