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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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